Talk:Special relativity/Page history

Absolute rest

Special relativity takes into account the fact that nothing can be accelerated to the speed of light in a vacuum. Does it also take into account the fact that nothing can be deccelerated to absolute rest in a vacuum?

GoldenBoar 21:36, 7 January 2006 (UTC)

There is nothing in special relativity that prevents an object from being at rest (in fact, in some reference frame, any given object will be at rest). It's the Heisenberg uncertainty principle that prevents you from simultaneously knowing both the position and momentum of an object (meaning that if you know an object is at rest, or any other specific velocity, relative to you, you have no idea where it is). That's an aspect of quantum mechanics, not special relativity.

What special relativity does is state that light always appears to travel at C no matter what the relative velocities of the emitter and the observer, and works through consequences of that axiom (which ends up giving time dilation, mass gain, and length contraction for relative velocities approaching C, and the C speed limit). --Christopher Thomas 22:55, 7 January 2006 (UTC)

According to the article, absolute zero, the laws of thermodynamics show that absolute zero can never be achieved. If this is true, wouldn't that invalidate the concept of rest energy?

GoldenBoar 23:29, 7 January 2006 (UTC)

Temperature is only defined for a collection of particles. A single particle can appear to have any velocity you please, depending on what frame you choose to look at it from. Furthermore, because the kinetic energy invested in temperature has a known, very simple, relation, you can easily calculate what the mass at zero temperature would be for a collection of particles from measurements of its mass at nonzero temperatures. In practice, there are sources of error for non-ideal solids and gases, but these aren't related to the argument you're referring to. --Christopher Thomas 00:52, 8 January 2006 (UTC)

nitpicking

at end of "Motivation for the theory..." "SR can correctly predict the behaviour of accelerating bodies in the presence of a constant or zero gravitational field" --- isn't saying "or zero" redundant? Luke Stodola 22:03, 14 Mar 2005 (UTC)

Is there anything physical moving in space-time?

This has more to do with general relativity but how can anything physical move in space-time? The conclusion is that the definitions of space and time and the equation of velocity $v=dx/dt$ forbids anything physical to move in spacetime[1]

Also there is no mathematical difference between the thing that we call 'spacetime' and the other thing that we call the gravitational field. -- Orionix 11:49, 17 Mar 2005 (UTC)

That site was done by someone who doesn't understand the basics of dimensional analysis. A time velocity component of 1 second per second is perfectly valid, and yes, it does look like a dimensionless constant on paper, as it is just as valid to specify it as one fortnight per fortnight. --Carnildo 18:52, 17 Mar 2005 (UTC)

True true. There is time in space-time. One way to look at it: in a time-relative minowski space (i.e. one with the the time origin (t=0) at the current time), as the current time changes at a constant rate, the points are consistently shifted "down" (or up, however the space is orientated) in the t direction, at a constant rate. In any case, that basic minkoski space, as I understand, represents the state of the system, from the pov of the local reference frame.

All bodies in space-time (minkowski space) have inertia. I.e. they "move", but they do not change energy levels; they conserve speed (velocity). Were a body to accelerate (it's energy level thus changing), a force would ipso facto be involved, and we would be discussing a bending of space. That's where things like general relativity would come into play. Kevin Baas^talk 19:02, 2005 Mar 17 (UTC)

HUH? Once again, in Eng, for the non-physicists? :) Trekphiler 13:55, 2 December 2005 (UTC)

NPOV problem

For the past 15 years the problems with both Special Relativity Theory (SRT) and General Relativity Theory (GRT) have been discussed in the Journal Galilean Electrodynamics ISSN 1047-4811 published by Space Time Analysis, Ltd. This contradicts the article sentence "However, at macroscopic scales and in the absence of strong gravitational fields, special relativity is now universally accepted by the physics community and experimental results which appear to contradict it are widely believed to be due to unreproducible experimental error.". In fact the the above journal was created to counteract the suppression of any questioning of Relativity.

There are much bigger concerns with NPOV, for example the corrections I made half a year ago to the biased POV about mass definitons have been vandalised and replaced by the same biased POV. Worse, misconceptions about the theory of Newton that I corrected and extended have been deleted and partly replaced by misinformation.

In the near future I'll reinsert the corrections, with some explanations and references on this page to the ignorant as well as to bystanders (isn't Wikipedia meant to REDUCE ignorance?!). Harald88 22:00, 10 October 2005 (UTC)

In the March/April 2005 issue of Galilean Electrodynamics page 23 is an article titled "First-Order Fiber-Interferometric Experiments for Crucial Test of Light-Speed Constancy". This article covers the experimental design requirements to test some of the features of relativity. The last sentence at the bottom of page 23 is "Therefore, the assertion that light speed is still c in a system moving translationally relative to Earth has not yet been verified.". Relativity is a theory that has not been adequately tested. The last three sentences in the above article are "Here, we challenge the relativistic physicists: please don't try to make the light-speed constancy un-definable. If you care to define that the speed of light is the same for any moving observer, we will design a GPS experiment to show it is not the truth. Give us a clear definition, and we will disprove it.".

Also "Special Relativity Theory (SRT)" not "special theory of relativity" is standard in the above article and journal. Twice when I changed it in the Wikipedia article, it has been put back. I guess it's whatever people want but it's not what people who are discussing the issues are using.

I have trouble consiering any journal to be "mainstream" or even "serious challenge to orthodoxy" when the journal website's address is "mywebpages.comcast.net". It shows they're not putting a lot of effort into being taken seriously. --Carnildo 05:00, 18 Mar 2005 (UTC)

Therefore, the assertion that light speed is still c in a system moving translationally relative to Earth has not yet been verified. Indeed?

I misread the claim somewhat. I guess the authors want a time measurement to be conducted on board a spaceship somewhere. Probably they believe in some variation of the old "Ether drag" hypothesis. I concede that such a measurement would be nice to have and I'm not aware that this has been performed yet, but I hasten to add that the results of the "cosmological source" experiments would be very hard to explain if the speed of light wasn't constant (in Earth's reference frame) over the entire journey. Aragorn2 19:34, 27 February 2006 (UTC)

The articles on Galilean Electrodynamics and Petr Beckmann are quite interesting, too. Seems like Beckmann et al had trouble getting their theses through peer review and into well-known scientific publications, that's why they started their own fringe journal. This inability is, of course, owed to the Vast Relativistic Conspiracy, and not due to the disinformation they seem eager to spread (see above). The mention of GPS is just hilarious. The entire functioning of GPS is based on relativity, and I seriously doubt that a theory that doesn't provide a model that closely approximates reality would have been sufficient in this undertaking. It's also safe to say that the evidence for SRT is a lot more solid that that for Newtonian (Galilean) physics.

Notwithstanding the above rant, I grant them the possibility of having a point somewhere, and don't want to discount their works as "unscientific" all too easily, so the word "almost" could be inserted between "now" and "universally". Aragorn2 18:59, 27 February 2006 (UTC)

I was curious as I didn't know Galilean Electrodynamics had a web site. In the 15 years that I've read it, I have never seen a web site mentioned. I carefully looked through the last two issues but no mention of a web site. So I did what you apparently did, I put Galilean Electrodynamics into Google and got the site you refer to. I looked through it and the best I could determine is that it is a personal web site of one of the editors as it contained an email address for authors to use for submissions. Anyway, thank you for the web site address. I used it as an external link in a Wikipedia article I created on Galilean Electrodynamics as the Wikipedia article on the founder Petr Beckmann mentioned it. While I was on Google, I scanned down the hits and noticed Symmetry or Simultaneity by Ronald R. Hatch a contributer to Galilean Electrodynamics. If you are interested his credentials are at credentials.

While on the topic of google, "Special theory of relativity" gets 84,000 google hits [2], whereas "Special relativity theory" gets 9,650 [3]. I'd say it's no contest, at least as far as contemporary usage is concerned. Terry 06:57, 20 Mar 2005 (UTC)

Archive created

I took the liberty of moving all talk from before this month to an archive, see Talk:Special_relativity/Archive1. It seemed things were getting a little bloated around here. Terry 06:57, 20 Mar 2005 (UTC)

• Also, I realized that there are a lot of SR pages floating around WP which are not really linked well to here (or to the Special relativity category). I am doing a bit of clean-up and linking, please excuse the flurry of minor edits. :-) Terry 05:14, 21 Mar 2005 (UTC)

• • OK, done with all that. Terry 07:10, 21 Mar 2005 (UTC)

Alternatives to SR

In the interests of NPOV and all, and given all the previous discussion on this talk page, I have opened up a section on alternatives to special relativity at Status_of_special_relativity#Alternatives_to_special_relativity. As you see it is extremely rudimentary at present, but perhaps some people here would have an interest in expanding it. I might also suggest that discussion of these theories (or anything else relating to the theoretical, experimental, or cultural status of special relativity) be sent to Talk:Status_of_special_relativity. Terry 07:10, 21 Mar 2005 (UTC)

Motivation section

I almost reverted the unexplained section blanking myself until I reread that section and decided that it wasn't currently adding anything to the article. I think that jumping to the postulates is a more appropriate way to organize it now, as the Motivation section, which is trying to put everything into historical perspective, is rather confusing the issue.

I think that in this instance, walking the reader through the many wrong turns physics has taken is not the best way to introduce a layperson to an already confusing and misunderstood field. I would vote for that section to be removed or moved to a later History section.--Laura Scudder | Talk 22:53, 20 Apr 2005 (UTC)

• I can see your point. Perhaps we can give a very short version of the history and motivation in this page, and spin off the longer discussion into a separate page, e.g. History of special relativity? That would be consistent with the way the rest of this page is organized, and help cut down the perennial problem of page bloat that we seem to have here. Terry 20:57, 21 Apr 2005 (UTC).

I think that's a very good idea. There's plenty of material that could go into a History of special relativity page that would just be bloat if put here. -- Fropuff 22:27, 2005 Apr 21 (UTC)

Well, I've spun things off, and left an abridged version on the main page. It's not the most ideal solution, but the best I can do on short notice. Presumably some further polishing is needed. Terry 17:18, 24 Apr 2005 (UTC)

I now read this, and I (blush) simply had not noticed that there was such a page - and obviously this is true for many! Somehow the reader is not really made attentive to the fact that there is an other article, despite the (too subtle) link at the start. OK next time maybe I or someone else can merge everything that has appeared here with the history page (plus a more visible pointer), and then the main article will go straight to the postulates. Harald88 19:28, 23 October 2005 (UTC)

• For trivia wanks (c'est moi?), the paper in which SRT was first publ was "On the Electrodynamics of Moving Bodies" (1905). Trekphiler 14:01, 2 December 2005 (UTC)

Quantum Physicists are just catching up with Relativity!

I understand quantum mechanics just enough, not to make any great discoveries in the field, but enough to understand the basics. Special Relativity is the same way. I found something interesting though, relativity implies both wave-particle duality and supersymmetry! A wave is a carrier of eneergy from place to place. A paritcle can be viewed as a carrier of mass from place to place. Relativity says energy is the fourth-dimensional extendsion of momentum (which is mass times velosity). This implies that waves are fourth-dimensional extendsion of particles! This also implies that the carriers of energy (Bosons) are extendsions of the carriers of mass (Fermions)! Wave-particle duality and supersymmetry. It seems so simple I'm surprized that this was overlooked for so many years.--SurrealWarrior 18:36, 20 Jun 2005 (UTC)

Quantum physics and special relativity were merged with a solid theoretical basis in 1928, by the Dirac equation. So I wouldn't say it was overlooked. One doesn't imply the other though. -- Tim Starling 02:14, Jun 21, 2005 (UTC)

Given mass-energy equivalence, doesn't that imply bosons are fermions? (Just call me Montgomery Scotch...) Trekphiler 14:07, 2 December 2005 (UTC)

No. Bosons and fermions differ because of the difference in spin; this results in different parity wavefunctions which results in different behaviour when positions are exchanged. Masud 21:23, 18 December 2005 (UTC)

Och, aye. Excuse me while I go change Planck's constant. =] Trekphiler 22:23, 18 December 2005 (UTC)

missing word(s)?

Seems to be something missing here:

The laws of physics are independent of location space or time.

Should it be, "...independent of location in space or time."? Or is punctuation needed? Otherwise, it sounds like we're discussing "location space," a phrase I don't understand. DavidH 1 July 2005 01:35 (UTC)

I think you're right, it's missing an in. --Laura Scudder | Talk 1 July 2005 02:01 (UTC)

Thanks Laura. I'm reluctant to make a change in a topic so out of my field. Anybody else want to weigh in before I make the change?

DavidH 1 July 2005 02:15 (UTC)

Ladder Paradox

Will anyone who understands special relativity please help us in our discussion of the Ladder paradox. We are going around and around in circles and maybe another viewpoint would help. Please don't muddy things - If you don't understand Minkowski diagrams its probably not a good idea, but if you do, we need your help. Thanks PAR 1 July 2005 03:44 (UTC)

Interesting

The universe seems to be arranged in hierarchies. There are various levels at which you can understand it. The level we are most famililar with is the level of everyday life. Going down we run into the levels of organs, cells, molecular biology, chemistry. Each level has its own laws which work in certian "special cases" with all violations at the "extreams". Complex Adaptive Swarms exhibit similar layered behavior also, at each layer the swarm is made of smaller complex adaptive systems. This layered behavior doesn't appear in swarms made of simple systems. This all seems to imply that either there is no bottom to the layers of the universe(and no TOE) or, the bottom layer is made of smart, adaptive particles.--SurrealWarrior 05:15, 10 July 2005 (UTC)

Actually, the universe only has one set of laws. It's just that under different conditions, different portions of the laws become the dominant force. Scientists tend to simplify things by only using and studying those portions of the laws that are dominant for any given set of conditions. This leads to the appearance that quantum physics, Newtonian physics, and general relativity are three different sets of laws. --Carnildo 03:41, 29 July 2005 (UTC)

New image and caption

First of all, my thanks to Cyp for that amazing image. It is quite marvelous and very, very much should be in this article. (It could also be on the Lorentz transformation article, but it must be on this one in any case. It has a lot of expanatory power and is a tremendous asset to this article.)

I have framed it and greatly expanded the caption. I hope that the expanded caption will help people to understand it and what is going on. If people can find a way to shorten the cation and/or make it clearer I would not mind, although I doubt that much cutting can be done without losing meaningful content. Similarly if people see something significant that I have not mentioned then please feel free to expand it. --EMS | Talk 21:37, 22 August 2005 (UTC)

Yes, it is a truly outstanding addition to this article. If you are still around Cyp, well done ! (did you make the animation yourself ?). It should definitely be here. The chunk of text in the caption is a little lengthy, but can't really change much there. However, if shortening the caption becomes an issue, then parts of the caption could refer to the main text, where the animation can be discussed further. ---Mpatel (talk) 15:46, August 23, 2005 (UTC)

Yes, I'm still around, and I did make it myself. (See the image description page for the source.) I made it very short, to keep it under ¾ MB. Wasn't sure how to explain the picture with a short caption, so I just dumped it on the page without a caption to let someone else worry about it. Thanks, I'm glad the image is liked. Κσυπ Cyp   11:13, 24 August 2005 (UTC)

the mass dispute

The concept of velocity dependent mass, relativistic mass, is examined and is found to be inconsistent with the geometrical formulation of special relativity. This is not a novel result; however, many continue to use this concept and some have even attempted to establish it as the basis for special relativity.

Actually, velocity dependent mass (although not relativistic mass) was at the basis of SRT, and even the first verified prediction of SRT. IOW, your(?) allegation is known to be historically at fault.

For half a century a large amount of textbooks and authorities on that subject used that concept - I learned it from Alonso&Finn and many others learned it from Feynman. Only in recent years there has been a push (or should I write Putch?!) by some influential people to abandon the concept, partly by spreading misinformation that the uninformed swallow (I call incorrect claims by experts of a certain age lies, because it's humanly almost impossible that they don't know the truth.) The cause can harldy be convention or lack of information, as the debate is too much an emotional dispute - instead it appears to be a clash of ideologies and ontologies, similar as the never fully solved difference between Lorentz and Einstein.

It is argued that the oft-held view that formulations of relativity with and without relativistic mass are equivalent is incorrect.

Hmm, there can be no doubt about them being *by definition* mathematically equivalent, but for reasons of consistency (as already pointed out on the main page) some prefer relativistic mass, while others prefer rest mass to be called "mass" for better compatibility with the now popular geometric reinterpretation.

Left as a heuristic device a preliminary study of first time learners suggest that misconceptions can develop when the concept is introduced without basis.

Indeed, that suggestion is misleading, it's an under-the-belt punch to suggest that people as Feynman had misconceptions about mass, or were stuck at "first-time learner level". I had not yet reflected on it, but now that you put my attention to it: this kind of dirty war is unacceptable for Wikipedia, I demand that phrase to be corrected to a neutral (NPOV) description of the disagreement (which BTW is already correctly indicated in the article).

In order to gauge the extent and nature of the use of relativistic mass a survey of the literature on relativity has been undertaken. The varied and at times self-contradicting use of this concept points to the lack of clear consensus on the formulation of relativity. As geometry lies at the heart of all modern representations of relativity, it is urged, once again, that the use of the concept at all levels be abandoned. PACS numbers: 01.40.Gm, 03.30.+p http://arxiv.org/abs/physics/0504110

Hmm, who is Anonymous @bellsouth.net?

Yes I know that people try to impose their POV and even demand the suppression of differing ontologie. I trust that such improper behaviour will have no chance in Wikipedia, as succombing to such propaganda is against the rules. But am I right to notice that the person who copy-pasted the text of that paper doesn't understand NPOV? At least he/she could have presented the two sides of the debate, or indicated that he/she didn't. For example, an internal report of the CERN (publicly accessible) shows that relativistic mass is actually used and accepted by a number of physicists (not a peer reviewed publication, but neither is the above Arxiv doc!). I could just as well give a similarly biased presentation which largely agrees with that overview, but disagrees on the crucial points, mainly by correcting the errors of that paper and by adding my own errors that push the contrary conclusion. (It's a common propaganda method, also used in religions, to "provide a survey" that suggests a NPOV, only to reach one's own POV as "conclusion"). Furthermore, the writer above is at odds with Einstein who admitted that his 1905 paper inconsistently based dynamics on kinematics, while the used instruments on which the units for kinematical derivation are based ("length", "time"), depend themselves on dynamics.

Clearly, the debate must be mentioned in Wikipedia, and IMO it's now reasonably balanced; the most popular(?) stand is in the main text and the threatened POV confined inside a separate paragraph. Harald88 23:18, 19 October 2005 (UTC) and 12:22, 20 October 2005 (UTC)

I now found the likely cause of much misunderstanding: In a paper by Okun in Physics Today (June 1989 p.31, "The concept of mass"), he may have caused confusion between "relativistic mass" and the "transverse"/"longitudual" mass concepts that were used around 1900:

"These are the very expressions with which Lorentz introduced the two masses. Together with the "relativistic mass" in the relation p = m_r v, where m_r = E/c^2 (which is equal to m_t when m =/= 0, but which had a more general meaning applicable also in the case of photons), these masses formed the basis of the language physicists used at the beginning of the century."

People can easily confuse the different concepts from reading that paper (or a later one of him, if he continued obfuscating them), and this misunderstanding may have played a significant role in the crusade against "relativistic mass". In particular, it may have caused the misunderstanding that relativistic mass is another word for transverse mass, or that F=ma is supposed to be generally valid with relativistic mass. However, that same paper has useful references, for example that relativistic mass" was called "the mass" by Tolman in 1912 - to be checked! Harald88 20:17, 23 October 2005 (UTC)

Errors in "second postulate"

It read: "Second postulate (invariance of c) The speed of light in vacuum, commonly denoted c, is the same to all inertial observers, is the same in all directions".

NOT so: SRT is a theory of physics, and Einstein made sure in 1905 to *not* include isotropy of light speed in the postulate, but in the following *operational definitions*, as it can't be verified by experiments, by definition. In SRT the OWLS is freely *chosen* to be isotropical by the experimenters. See Einstein's definition of "light speed" in his 1905 paper (html); also the discussion yesterday and today in sci.physics.relativity (search in Google groups for OWLS in subject and tom roberts , 19 Oct). Harald88 22:39, 20 October 2005 (UTC). Note also that misunderstanding about this point apparently played a role in this article not being a Featured Article in the past, without thorough verification. What made Einstein's approach appealing, was how he derived the LT from a *minimal* number of assumptions. For example: that light speed will look isotropic when clocks are synchronized by means of clock transport, follows from the *first* postulate.

BTW, there is still more wrong with that postulate paragraph: SRT is in particular *not* a theory about metaphysics. Unfounded metaphysical claims by editors have no place in SRT! Instead of that, the postulates should be phrased in a manner that closely follows Einstein 1905, for he chose his definitions with care, he himself closely following what others had concluded before him. Harald88 23:21, 20 October 2005 (UTC)

The actual words that Einstein used in his second postulate (in the English translation) are:

"Any ray of light moves in the "stationary" system of co-ordinates with the determined velocity c, whether the ray be emitted by a stationary or by a moving body. Hence velocity = (light path) / (time interval) where time interval is to be taken in the sense of the definition in §1."

"§1" refers to the definition of what we now call Einsteinian synchronisation using light signals. If Einstein were talking about "two-way light speed" (TWLS) then it would make no sense for him to have to explain what he meant by "time interval", as it would, in that case, just be the proper time recorded by a single clock. But he would have had to explain what he meant by "light path" being a there-and-back distance.

The second postulate should indeed be interpreted to refer to "one-way light speed" (OWLS). It contains within it the requirement, within Special Relativity, that clocks be synchronised via Einsteinian Synchronisation. Isotropy of light speed is more than just an "operational definition" for experimenters: it is an intrinsic feature of the theory. If you choose a different synchronisation method, you're not studying Special Relativity, you're studying another theory (for example, the version of Ether Theory by Mansouri & Sexl, which is mathematically equivalent to Special Relativity but has different definitions of coordinate time and velocity).--Dr Greg 18:14, 31 January 2006 (UTC)

I see what you mean: you do have a point about Einstein's presentation. However, principles of physics are based on observation, not on speculation or mere convention or freely chosen procedures (in fact, that timing convention was already in use at that time).

To do a re-take: your above quote is from paragraph 2, after he defined the coordinate "time" which is used in your above quote. He defined light speed operationally as follows:

"In agreement with experience we further assume the quantity:

2AB/(t_A'-t_A) = c

to be a universal constant--the velocity of light in empty space."

From this together with the freely chosen coordinate time follows that also one-way light speed is determined as c, which is what he wanted to get for convenience.

Of course, how to present it is a matter of taste: it's possible to write it in the mathematically simpler way as you propose and as Einstein arrived at presenting it, but it makes the full formulation of the second postulate more complex, without adding anything useful. In that formulation of the light speed postulate, the time interval that is used for defining light speed, is itself defined by a procedure that is based on two-way light speed (c). That definition needs to be mentioned and explained. Eliminating the circularity brings you back to the definition with one clock...

Harald88 19:34, 31 January 2006 (UTC)

Triangle of Velocities proves that invariant speed is mathematical error

The article at http://www.masstheory.org/triangle_of_velocities.pdf presents detailed analysis of Lorentz transformation, it's derivation procedure and formal mathematical proof that introducing "invariant" speed is indeed nothing but mathematical error.

I suppose that you mean light speed, as defined in the second postulate. But what do you mean with "indeed"? I hope you didn't misunderstand my criticism on misrepresenting the second postulate as criticism on the second postulate itself, which as Einstein explained, was based on experimental results. Apart of that, I assume that the NOR rule implies that discussion of your paper on this page is not useful. Harald88 20:41, 28 October 2005 (UTC)

All history moved to History page

I merged the History page with the version that was still on the Special relativity page, and reedited it plus made some additions. In some cases I had to make a choice between different renderings; some confusing/erroneous sentences I deleted as well as some non-relevant material that just didn't fit in. In case I stepped on a sore toe by deleting something, just reinsert any lost phrase that you may consider essential. Harald88 13:47, 29 October 2005 (UTC)

Relativity and unifying Electromagnetism - error?

I think that the main article is practically ready to make it a featured article; but just now my eye fell on the last paragraph, and I suspect that it's flawed. For how can one completely make the magnetic field of a current loop disappear? I don't think so. Note: In the coming days I'll be too busy with other things, so if I'm not mistaken, please correct this without me. Harald88 21:32, 1 November 2005 (UTC)

Its not making the current in a loop disappear, its making the current in a straight wire disappear. To make the current in a loop disappear, you have to be in a spinning (i.e. non-inertial) frame of reference. But still, why would the magnetic field not disappear, and the loop become an electrostatically charged loop?PAR 22:01, 1 November 2005 (UTC)

OK, you already picked up one glitch in the description. But different from the straight-line motion of two parallel moving charges (Einstein 1905), in a wire we have relative motion of negative and positive charges. You can't make that disappear. Thus, in any frame there are moving charges. How could then the magnetic field disappear? My advice: either consult a good (I mean Very Good) textbook on this and use that as source (I saw no reference anyway), or stick to the simple case of Einstein 1905 (available on the web) and which also is not too complicated for an introduction page to SRT. IMO that last option would be really the best, as a current in a wire is more complicated than it looks. Harald88 22:56, 1 November 2005 (UTC)

Now it's almost 2 weeks later, and that a paragraph is still defect (questionable claims without backup from a reliable source). If this week still nothing is done about it, and if I find the time, I'll partly rewrite it using the simple case of Einstein 1905 (but if someone else likes to do that, please do!). Harald88 12:31, 12 November 2005 (UTC)

I'm starting to think that you are right. If the wire is electrostatically neutral with moving negative charges, but, say, equally dense non-moving positive charges, there is no moving coordinate system that will make the current go to zero. For a moving observer the wire will acquire an electrostatic net charge, but the current can't be made to go to zero. If the current is not zero, the magnetic field is not zero. PAR 15:52, 12 November 2005 (UTC)

From a mathematical point of view, the four-current is a four-vector J = (cρ, j). If the charge density ρ is zero, then no Lorentz transformation will annihilate the current j. Trying to use physics language: if the wire carries a current but it is electrostatically neutral, the charge density is zero, so the four-current is space-like. However, every four-current with zero current j is time-like, and it is impossible to transform a space-like vector into a time-like vector. So I agree, the current can't be made to go to zero. -- Jitse Niesen (talk) 18:01, 12 November 2005 (UTC)

I now rewrote it, but it may still be expanded (Wolfkeeper, the conclusion reflected the erroneous claim). Harald88 09:49, 13 November 2005 (UTC)

No.WolfKeeper 20:21, 14 November 2005 (UTC)

Need another diagram or two

The first half of the article is devoid of diagrams that's bad...

Also check out this awesome diagram:

http://origins.colorado.edu/~ajsh/sr/centre.html

It shows how two comoving objects can both be at the center of a lightcone at all times.

We need one of those... WolfKeeper 16:41, 2 November 2005 (UTC)

nominated a daughter article for deletion

I nominated a daughter article Special relativity for beginners for deletion. My gist is basically this should be merged, since forks are bad, and that the parent article needs this kind of language in the first place, and should not have its existence for an excuse for people to continue writing technical details in the current article without elaboration. This is the Wikipedia policy: include as much details as possible, but explain them in the main article in simpler language. Not the other way round, move the simple language off the main article and onto another one! It is discussed at Wikipedia:Articles for deletion/Special relativity for beginners -- Natalinasmpf 16:51, 17 November 2005 (UTC)

Yes, I don't like this separation. And the main page needs some of those diagrams- my feeling is that we need more Minkowski diagrams in the article to help those who are more visual. Readers shouldn't have to plug numbers into Lorentz equations, the diagrams should do it for them.WolfKeeper 17:58, 18 November 2005 (UTC)

The result of the debate on the votes for deletion placed on Special relativity for beginners was: merge (4) Not merge(7). The 7 votes broke down as: Keep(4) Transwiki(1) Rename(2). See discussion on the Special relativity for beginners page. loxley 10:37, 23 November 2005 (UTC)

I am against merger because it will disrupt the Special relativity page and because, in most textbooks the Special relativity for beginners page is summarised as 'according to SR the space-time interval is invariant therefore, as s^2=gxx ...' (ie: one sentence). I am also against merger because there are at least three ways of explaining SR: simultaneous linear equations with Einstein's assumptions, Minkowski/Noether invariance with a geometrical interpretation and advanced differential geometry (cf:general covariance). Should we include full guides to each of these approaches? All three approaches are actually deeply related but explaining the relationship will need a short course on maths. The advantage of using the second approach for explaining SR is that it is geometrical and, as Plato put it, understanding is largely geometrical. Lastly, I am also against merger because SR is widely taught in terms of Einstein's raw assumptions and if we portray it differently in the main article we will confuse intermediate students and not be thanked by their teachers. loxley 10:55, 23 November 2005 (UTC)

Hyperbolic Geometry and Special Relativity

The following got added to the article by an anonymous author and removed by me:

Special Relativity is a physical model of hyperbolic geometry. For example, on the Poincare Disk, set up a polar coordinate system. Then any point on the Poincare Disk can be identified with a uniform motion on a plane. The point (2, 30), for example, could represent an object travelling on a plane with a uniform rapidity of 2 in the direction of 30 degrees north of the polar axis. (The rapidity of an object is the arctanh of its speed as a proportion of the speed of light. An object travelling with rapidity 2, for example, would be going tanh(2) = 96.4% of the speed of light.) The Poincare distance between two points on the Poincare Disk can be identified with the relative speed between two objects travelling in uniform motion on the plane. So every theorem in hyperbolic geometry can be translated into a true statement in special relativity.

This is a very dense and misleading write-up. A constantly accelerating observer is undergoing what in special relativity is called hyperbolic motion, but this does not endow spacetime with a hyperbolic geometry. (I know that this is not what was being said or intended, but it was being implied.) The point being made is also very subtle and trivial. --EMS | Talk 04:59, 20 November 2005 (UTC)

I agree that the addition of hyperbolic geometry was too densely written, and somewhat misleading. (In the last sentence, perhaps the writer should have said something like "...every theorem in hyperbolic geometry can be translated into a theorem in relativistic kinematics, and conversely.") Still, the connection between hyperbolic geometry and relativistic kinematics is pretty cool (well, at least I think so, but then again I'm a geometer!) and do I think it belongs somewhere in Wikipedia. I'm just not sure where! Tuolumne 07:27, 20 November 2005 (UTC)

You indicate it yourself: it seems to be neatly fitting in an article about hyperbolic geometry. Harald88 12:26, 20 November 2005 (UTC)

change sequence?

The sequence of the different paragraphs is, I think, not optimal:

1 Postulates

2 Status

3 Consequences

4 Lack of an absolute reference frame

5 The Lorentz transformations of space and time

6 Addition of velocities

7 Mass, momentum, and energy

Why not introduce the subject matter earlier, thus for example:

1 Postulates

2 Lack of an absolute reference frame

3 Consequences

4 The Lorentz transformations of space and time

5 Addition of velocities

6 Mass, momentum, and energy

[almost at the end:] Status

Harald88 12:52, 20 November 2005 (UTC)

It might even be easier to read in the 4,2,1,3,6,7,status order; since that way you can show how the postulate agrees with lorentz equations and it all makes sense. That's even the historical way it happened. Personally I think starting with the postulates is unduly mystical, but not formally incorrect in any way.WolfKeeper 19:13, 20 November 2005 (UTC)

Historically it started with the impossibility to pinpoint an absolute reference frame, with the conclusion that the PoR seems to hold as if one is always at rest in an ether - from which follow the postulates. To start with the Lorentz transformations is IMO a bit heavy. Anyway, my main point is that "status" should be near the end, and I notice that you agree on that. Let's wait for more opinions. Harald88 20:50, 20 November 2005 (UTC)

I now did the minimum swaps about which we agreed; however, if you or someone else prefers to also swap 1."Postulates" with 2."No absolute ref.", I'm OK with that. Harald88 20:00, 22 November 2005 (UTC)

Isn't the lack of an absolute reference frame really a consequence of the postulates? Masud 17:09, 23 December 2005 (UTC)

diagram 1

It's a nice illustration, but the text can certainly be improved by removing jargon and confusion. Notably, two accelerations are confused, and I'm thinking about how to avoid that confusion without making it too long. And isn't the animation a bit too fast? Harald88 09:29, 26 November 2005 (UTC)

OK I'll try to improve it a little more, and we'll see. Harald88 19:38, 2 December 2005 (UTC)

Galilean relativity discussion

An anonymous author added this to the Consequences section:

However, applying Galilean relativity to electromagnetic phenomena leads to serious problems and was one of the starting points when Einstein laid down the principles of Special relativity. Rotors of electric motors and generators move with velocities far below the speed of light. Contrary to mechanical movement there is no low velocity approximation for electromagnetic induction.

I have removed it because:

• The text lists no consequences of SR that I can see.
• The first sentence is correct and is a part of the history of special relativity, but I cannot see the point the point of the rest of this write-up, or how it fits into a discussion of special relativity.

--EMS | Talk 22:45, 26 November 2005 (UTC)

While I agree with the removal, it did make me realize that there is nothing about the history in this article (except the one sentence refering to history of special relativity). What do you guys think about writing one or two paragraphs about this, and if you think that's a good idea, where in the article would this fit best? -- Jitse Niesen (talk) 10:46, 27 November 2005 (UTC)

I don't really care: I think that history is unnecessary for a first understanding of SRT. Harald88 20:36, 2 December 2005 (UTC)

A question

Dears contributors

I want to pose a question, and I hope that any of you can answer.

There is any plausibility on assert that all particles, and so the all the other bodies in Universe, travels to the future?? Can I say that the "light speed can be considered as the velocity by what an observer moves yourself towards future". This is (as long as I can translate adequately from Portuguese) an free translation from an quote excerpted from the Portuguese version of Wikipedia (http://pt.wikipedia.org/wiki/Espaço-tempo). This is an article to treat of Spacetime. Would any of you please answer me about this kind of reasoning? I would be happy if any commments can be send to eduardolauande@aol.com. Thank you all in advance.

Greetings

Note:Appologies on my "bad English writing"

EDULAU

a Brazilian (Portuguese speech) contributor of the portuguese Wikipedia

I don't think that would be a scientifically correct statement, but it does have intuitive appeal. In a Minkowski diagram, the set of spatial points as a function of time could be seen as a "movie" by moving the spatial axes along the corresponding time axes at the speed of light. I think that explaining the Minkowski diagram would be better. Then the idea would have some meaning, instead of being a vague and untestable idea. PAR 03:43, 27 November 2005 (UTC)

Principle of special relativity

I made a redirect for the principle of special relativity to this article. Is that appropriate? Would it be better if it were directed to postulates of special relativity or if it had its own article? -- Kjkolb 00:19, 28 November 2005 (UTC)

I think that's OK... however: I know the PoR, and that it later was called the Special principle of relativity by Einstein. But I never heard of the label "principle of special relativity". Harald88 21:44, 28 November 2005 (UTC)

Thanks. Another encyclopedia had the equivalent of a Wikipedia redirect under that name and we're trying to make sure that Wikipedia covers everything they do. The other encyclopedia has many inappropriate redirects, so I wanted to make sure. -- Kjkolb 08:35, 29 November 2005 (UTC)

Tensors in SR

I was thinking it'd be a good idea to have a small section of 4-vectors such as position, energy-momentum, etc. and the lorentz transformation tensor, EM tensor etc. and describe how we use the lorentz transformation tensor to transform these quantities from one inertial frame to the other. This would also help in linking from the Classical field theory page when that's done. Masud 05:56, 1 December 2005 (UTC)

I was writing an article for Classical field theory and felt it necessary to put in a section on relativistic invariance, but later realised that it probably belongs in the SR article. I would like to ask people to see if they think we should put it in (or at least in modified form). Masud 08:53, 23 December 2005 (UTC)

Looks like I'm just talking to myself; I assume no one has a problem with this. I'm assuming there are little or no objections. However, in order not to bloat the SR article, I may add a little subsection or a link to a new Tensors in Special relativity page. Masud 12:51, 23 December 2005 (UTC)

I think its a good idea - I had it in my mind to do it myself eventually. I think a section in the SR article is a good start. Then if it gets too big, we can move it. "Tensors in relativity" sounds like "Real numbers in Newtonian mechanics" - its not really the subject. Maybe "Spacetime physics" or "spacetime electrodynamics". Just thinking out loud. PAR 16:48, 23 December 2005 (UTC)

I have added the section. Lots of editing still to be done. My knowledge of various articles on wikipedia that we can link to is limitied, so I was hoping someone could help out by adding some in. There are invariably faults in my work, and the speedy remedy of these faults on an otherwise quality page would be much appreciated. Masud 17:47, 23 December 2005 (UTC)

Notation

Some editing is in order: somehow, the prime indicator of S-prime got deleted. Trekphiler 14:45, 2 December 2005 (UTC)

where? Harald88 20:32, 2 December 2005 (UTC)

Physics 101

Is it possible to rewrite somewhat? As is, the article seems to be aimed at physicists (or students of physics), not general readers. Fewer equations? More explication? Certainly, fewer assumptions of knowledge not (necessarily) in hand. Trekphiler 16:13, 2 December 2005 (UTC)

Your request looks almost opposite to that of Masudr here above. But if you do some suggestions or at least point out what could benefit from some more simple clarification, maybe someone here will try it. You could also try yourself of course. And what do you think of starting with the Lack of absolute frame paragraph, and next the Postulates? Harald88 20:23, 2 December 2005 (UTC)

PS What do you think of Special relativity for beginners, does that help? Harald88 20:29, 2 December 2005 (UTC)

Clock Comparisons

These effects are not merely appearances; the time in the different frame of references essentially do travel at different rates to each other and the lengths of objects really are physically changed whilst in relative motion.

a) Read it carefully and this sentence looks pretty garbled. b) In particular 'the time....do travel at different rates' actually, it's worse than the usual cliches make out. If Mrs Stripe is moving (inertially) at an appreciable fraction of c with respect to Mr Plain then after they have met, her clock runs slower than his on events in her immediate neighbourhood, and faster than his in his neighbourhood. Before they meet, exactly the opposite is the case. In short some processes observed by both run faster according to Ma Stripe, some run faster according to Daddy Plain, and a sufficiently convoluted and widespread process will look like a bit of both to both at different times. Clocks don't 'run at different rates' - that suggests that they are comparable. They're not. c) Even if Ma Stripe has a sufficiently powerful telescope to read Plain's clock, the moment she reads 4 am, most events she sees as simultaneous with that reading aren't simultaneous with it for Daddy Plain. The reading is meaningless. Some of the events Plain thinks happened at 4am are already past for her and some are in a far future.

Priority, ad nauseum

An anonymous editor using IP address 68.0.143.223 has changed this article and General relativity to stress prior contributions of Lorentz, Hilbert, and Poincare. Please be aware that exactly how to briefly describe the development of gtr and str is contentious. That is, there is substantial agreement among historians of science on the basis issues, but some cranky claims that Einstein "stole" from Hilbert, or his first wife Mileva, or whomever, have currency among certain individuals. For this reason, editors should not make such changes without discussion on the appropriate talk pages.---CH 05:35, 23 December 2005 (UTC)

CV is very right about how contentious it is, and he/she demonstrates it perfectly by trying to suggest that the view of some historians is that of "certain" "cranky" "individuals" -- a tendentious, POV approach. However, the Wikipedia policy can prevent problems: just stick to reliable sources, without ignoring notable differing opinions. Harald88 14:57, 2 January 2006 (UTC)

Related to this, what do you think of this edit by an anonymous contributor, suggesting "that the work of Carathéodory help shape some of Albert Einstein's theories". It has enough details to make me think that the fact are probably correct (though I didn't check anything). -- Jitse Niesen (talk) 16:48, 3 January 2006 (UTC)

It is out of character with the rest of the article, and reads "possible hoax" to me. My temptation is to move it to the talk page and call for citations backing the claim up. --EMS | Talk 02:51, 8 January 2006 (UTC)

Two way "calculating" velocities?

Why do we elismate(changed to "estimate"--HydrogenSu 12:02, 30 January 2006 (UTC)) phase velocity by ${\mathcal {E}}=mc^{2}$,and elismate group velocity by $E={\sqrt {P^{2}C^{2}+m_{0}^{2}C^{4}}}$?

Any differences?

I think that reasons may be a group velocity can represent a true body when it moves.(which might need to considerate both of rest-mass and move-mass ${\mathcal {(}}P^{2}C^{2})$ effect) But a phase velocity is just a S.H.O. vibrating.(which doesn't need to considerate of rest-mass effect) —The preceding unsigned comment was added by HydrogenSu (talk • contribs) 18:41, 26 January 2006.

??? elismating? What is that supposed to mean? First of all, this business of phase and group velocities is matter for quantum mechanics, as relativity is a classical theory isntead of a quantum one. Secondly, you are not even looking at the right equations for determining the energy and momentum of an object. ${\mathcal {E}}=mc^{2}$ is for the rest energy of an object (meaning that it is good only when v=0), while ${\mathcal {E}}={\sqrt {P^{2}C^{2}+m_{0}^{2}C^{4}}}$ is the relativistic energy-momentum relationship and is good at any velocity. --EMS | Talk 04:41, 27 January 2006 (UTC)

Maybe I spelt the word wrong. I thank you. (I'm a Taiwanese.)

I was then wondering of that question above. It's started on my book when I read. It said:

Find a relativistic particle's ${\mathcal {V}}g$ and its ${\mathcal {V}}p$.

My book solved them by those:

     ${\mathcal {V}}_{p}={\frac {\omega }{k}}={\frac {E}{p}}={\frac {mc^{2}}{mv}}={\frac {c^{2}}{v}}$
           which is possible greater than speed of light.
     ${\mathcal {V}}_{g}={\frac {d\omega }{dk}}={\frac {dE}{dp}}={\frac {d{\sqrt {P^{2}C^{2}+m_{0}^{2}C^{4}}}}{dP}}=v$
           which is impossible equal to speed of light,even greater than that.

My trouble is in their physical picture.I'm not quite sure still. --HydrogenSu 11:19, 27 January 2006 (UTC)

I still don't know what word you are trying for. The current version is reminiscent of "eliminating", but velocity is eliminated when something stops! As for the details: Yes, that is a standard quantum mechanical calculation. (However, you have your labels reversed as of 1/17/06. It is the first equation that gives superliminal values, not the second. Actually, the labels are correct but just plain confusing. "cannot" would work better than "possible" [parenthtical remarks revised by EMS | Talk 00:03, 28 January 2006 (UTC)]) In essense, the waves within an object are always superluminal, the the group (which defines the object) travels with the object (and it had better do so). Since the object/wave group itself cannot travel at c or greater, relativity is not violated by this. --EMS | Talk 17:58, 27 January 2006 (UTC)

"Eslimating" is not an english word either. "Estimating" perhaps? However that implies inaccuracy in the result, and there is none in classical equations. I strongly advise finding other words to use. Either that or use |the Chinese version of this article --EMS | Talk 00:03, 28 January 2006 (UTC)

Bingo! The word that you are after is "calculate"! Now your question makes sense. However, the answer is as I stated above: This involves quantum mechanics, and not special relativity in particular. You need to look in category:quantum mechanics for the overall answer. Remember that you are dealing with wave velocities here, and not just the particles. --EMS | Talk 00:10, 28 January 2006 (UTC)

Thank you. That truely was not my fault. (Beacause that word "Estimate" something about math-or-physics was taught by my professors to me and maybe misunderstand from we Taiwanese textbooks from America. (HHHeer...be humorous a monment...^^)

About everything about those questions of S.R.,I need to think alone for a long time. I've been learning Einstein.

(We Chinese cultures do not actually clarify "Calcualte" and "Estimate". Sorry but that makes you confused.><")By the way,thx

--HydrogenSu 08:20, 28 January 2006 (UTC)

In a few days,I proposed my new opinions as the followings.

The "phase" velocity means that particles as sets of waves(Q.M.'s wave-pocket) and vibrates in Y axis (represnts as heights) only. The famous English physist,P.A.M. Dirac,truely said on his book named of P.Q.M. :Waves with harmonic oscillators,and that of the opposite either. But he had never said this kind of concept already violated Special Theory of Relativity. Because of : the physical action which something vibrates in Y axis,it cannot represent as "the speed of wave-pocket travelling in X axis". Hence,alough my previous formulas proposed in a few days before which were got by ${\mathcal {E}}=mc^{2}$ so that phase velocities can be greater than those of light but still not violate S.R. And speaking another,I feel we don't need to use Q.M. to think about this question with a good acient Chinese words:"Killing smaller animals within worse weapons;killing gohsts within a pices of paper-golden-money". How do you think?.....

--HydrogenSu 13:15, 29 January 2006 (UTC)

To HydrogenSu - These questions should be asked at Wikipedia:Reference desk, not on the talk page of an article. PAR 20:36, 29 January 2006 (UTC)

All right. Thanks for reminding.

About previous talking of "velocities which eliminate each others" by EMS,I would say is we might consider two objects' some other phisical properties.

Like masses,accerelations,...etc. I just take the simplest for instance. If two particles' velocities eliminate exactly,that doesn't represent as "stopping". 'Cause the masses are not must the same exactly. If we use "Vector Methods" to analyse,it gives

${\mathcal {,}}m_{a}v_{a}+m_{b}v_{b}=0,$ when wanting objects "both" stop.

Only if the masses are the same value and the velocities are too,can they both stop. If not,there exists some momentum exchanging each others remainly.--HydrogenSu 12:34, 30 January 2006 (UTC)

Your conclusion is mistaken even in classical mechanics, and I see no good reason to clarify it here. You have wasted enough bandwidth on this business. This page is to discuss issues related to the article, not to educate someone in relativity. --EMS | Talk 01:13, 1 February 2006 (UTC)

Could you please tell me why my question need to be sloved by quantum mechanics but not relativity? I keep memory of an inserted question copied from my book's exercise that is:

"Find a relativistic particle's ${\mathcal {V}}g$ and its ${\mathcal {V}}p$."

Had already said of "a relativistic particle's...". By the way,I know that I truely made something about talking polite and was rude too much. I apology now. :) In fact,I'm glad to discuss with you. Wanting to read your again reply. (I didn't educate someone but expressed my opinions about phase and group velocities.)--HydrogenSu 20:52, 1 February 2006 (UTC)

Criticism of Relativity Theory

I'm not sure about this. I left the section in, because it seems there's a real NPOV here- there really are quite a lot of people who don't understand relativity and make honest but usually ignorant criticism of the theory. Perhaps we should move it into a separate article? (Having said this, I am quite reluctant to do this, but generally the experience in the wikipedia of this kind of thing has not been too bad at all, but nevertherless I am still a bit nervous.)WolfKeeper 01:26, 2 February 2006 (UTC)

The phrase "His 'proof' that Relativity theory is inconsistent has not been in any way supported by other scientists or experimental evidence." is incorrect, but it's a bit subtle. I plan to write a part about Dingle and reactions in twin paradox (that's what he focussed on), but I haven't come around doing that yet. Now I correct that phrase to "special relativity" and I turned the phrase around, as proof of absence is hard to give. That should make it quite correct IMO, and keeps it with the subject matter of this article. Harald88 11:45, 2 February 2006 (UTC)

The section that starts "The German scientist group g.o.mueller..." in the current revision is very odd indeed. It's also pretty incoherent. It looks like it's been machine translated from German (or possibly Italian) without any editing at all. It appears to be a POV rather than a serious attempt to add to a critical analysis of the issues; the suggested search for a link (and why not the link itelf?) at [4] times out, so I can't do a better analysis of this. The Google cache of this page is most peculiar. --Alex 21:20, 2 February 2006 (UTC)

Lorentz Transformations

Quote from this article:

Then the Lorentz transformation specifies that these coordinates are related in the following way:

$t'=\gamma \left(t-{\frac {vx}{c^{2}}}\right)$

Good day.I cannot be cleared for this formula. More details:why does $(t-{\frac {vx}{c^{2}}})$ (Especially in term $({\frac {vx}{c^{2}}})$.) Is it about the speed of light,messages transported?

I am going to go my professor's class about S.R. But he puts some important part near the end of the Modern Physics class. Hope someone can tell me that yo. (Yo is a Chinese pronunciation for using of expressing stronger,not "I" in Spanish.) --HydrogenSu 18:19, 2 February 2006 (UTC)

Picture two coordinate systems, one moving in the x-direction relative to the other. The two origins overlap at time t=t'=0.

Let's imagine that there's an observer at the origin of the unprimed frame and a clock that emits a flash of light every second at the origin of the primed frame. As the primed frame moves away from the unprimed origin, not only does it's clock appear to tick slower (the spacing between the flashes is larger because the clock is moving away), but it's time is also delayed because the flashes take a nonzero amount of time to travel from the clock to the observer. The first term accounts for the first effect, and the second term for the latter.

It's hard to describe well without diagrams, but try drawing it yourself with some sample clock-flash wavefronts. In fact, I found when studying this that it's impossible to understand without drawing a diagram at every important event. — Laura Scudder ☎ 18:36, 2 February 2006 (UTC)

The term that bugs you creates an effect called the relativity of simultaneity. Imagine a light clock: |<-------->|. At rest, the light will go back and forth between the bar characters (which act as mirrors in this discussion) in a time of d/c in each direction where c is the speed of light and d is the distance between the mirrors. Now let's put this light clock in motion: |<------->| --->. In our frame of reference, when the light moves to the left, it goes between the mirrors in $d/(c+v)$ (where v is the speed of the clock) since the clock is moving towards the light. Then when the light is going to the right, it goes between the mirrors in a time of $d'/(c-v)$. So the "tick" is not consistent, but instead alternates faster and slower. The difference in perception of how the clock operates is explained by "at the same time" being different in co-moving frames of reference. BTW, because of the Lorentz contraction, <math>d' = d / \sqrt{1 - v^2/c^2}</math> $d'=d{\sqrt {1-v^{2}/c^{2}}}$, and this makes it so that the moving light clock ticks at the same average rate (for a given speed) no matter how it is oriented. --EMS | Talk 21:43, 6 February 2006 (UTC)

No, the above is not correct. The error is in saying that the velocities add as c+v or c-v. Velocities do not add in this way. (See the article). All clocks keep steady time, with the same time between ticks, although that time may vary (to you) depending on the speed of the frame (with respect to you). PAR 02:22, 7 February 2006 (UTC)

Oh yes it is correct. Within a given frame of reference, velocities add linearly even in relativity. As seen in the observer's frame, the setup is moving at v. Within that same frame of reference, the light is moving at c (as it always must). So of course the time for the light to go between mirrors must be $d'/(c\pm v)$. Now if you want to know the speed of the light with respect to the mirrors in the frame of reference of the mirrors, then you use the relativistic addition of velocities (which you refer to), and as expected will get c in either case. The unsteady tick (based on each tick being when the light is reflected) is a very real phenomenon, and the $c\pm v$ business also appears in the relativity of simultaneity article. The point is that the midtime between reflections of the same mirror is not simultaneous with the reflection off of the other mirror when the setup is in motion. I advise you to think about that. --EMS | Talk 04:40, 7 February 2006 (UTC)

Ok, yes, you are correct, except for the statement $d'=d\gamma$. It should be $d=d'\gamma$, but I expect thats just a typo. I didn't account for the reflections correctly. I agree, the clock will have two different periods between ticks when viewed from the observer frame. Also, in your answer, saying that the velocities add linearly within a given frame of reference is not correct. The v+c term (for example) does enter into the calculation, but there is no physical object travelling with velocity v+c. PAR 02:50, 8 February 2006 (UTC)

As Einstein and others put it, their relative velocity is c-v (note that this is subtraction). Nowadays more commonly the jargon "closing speed" is used for that same concept. Harald88 18:49, 8 February 2006 (UTC)

Time Dilation

The sentence is not correct:

"Similarly, in the equation for time t', t is multiplied by gamma in the second non comoving frame. This may be interpreted as time proceeding more slowly when an object is moving relative to another frame of reference."

because then, it has to be a time "shortening" because 1 sec in rest frame would be more than 1 sec in moving frame.

That's exactly what time dilation is. Let's say that in my frame it takes 3 minutes to boil an egg, and x=0. From the point of view of a moving frame with a gamma of two; the elapsed time is 6 minutes. Hence time dilation. It's as simple as that.WolfKeeper 22:23, 30 March 2006 (UTC)

No: t' is the time-coordinate of the moving frame. t is yours. you are in t. t' is another frame in which you are not. In your picture a person in t' would have cooked two eggs, in 6 minutes (one after the other) while you have only time for one in 3 minutes. You can only compare the rest frame with the moving frame by using the equation. you cannot use the equation to go from t' to t by inverting it.

So if the moving frame thinks that it takes six minutes to boil your egg, then does the moving frame think your water is colder, or that the fundamental properties of eggs have changed? --Carnildo 05:32, 31 March 2006 (UTC)

Frame's don't think. People think. People think that time is dilated :-), but also the mass of the molecules in the egg will be higher; so in a sense the fundamental properties of eggs have changed. The temperature has gone down from the point of view of the moving frame though.WolfKeeper 13:52, 31 March 2006 (UTC)

It`s not possible to argue like that, because not the speed of particles defines their temperature but their momentum. It is very dangerous to argue in words.

the correct calculation would be:

$t'=\gamma (t-{\frac {vx}{c^{2}}})$

$t'=\gamma (t-{\frac {v\cdot vt}{c^{2}}})$

$t'=\gamma (1-\beta ^{2})t$

$t'={\sqrt {1-\beta ^{2}}}t$

$t'={\frac {1}{\gamma }}t$

The point is to express x in terms of v*t, then the equation contains not gamma but 1/gamma, so 1 sec in rest-frame is less than 1 sec in moving frame. --84.152.247.197 17:19, 14 February 2006 (UTC)

I think you've swapped around what t and t' refer to. Your maths looks correct to me from a quick glance at it; it's just more complex than it needs to be.

Thank you for your answer. You cannot swap around t and t' to go to the inverse transformation. You have to go from v -> -v for the inverse transformation. You are always in the rest frame t and the moving frame is always t'. So from every point of view you see the time running more slowly in the other frame. the only thing that I like to say is that you cannot "read" the equation for the time as the equation for x, because then you would make the wrong conclusions (when you watch the moving frame, then there time seems to go faster). You have first to use the insertion above.

SR, mass and E=MC²

I recently enjoyed actually reading Einstein's fourth 1905 paper ("does the inertia of a body depend upon its energy content?" from [5]). In that paper, Einstein says

"If a body gives off the energy L in the form of radiation, its mass diminishes by L/c2. The fact that the energy withdrawn from the body becomes energy of radiation evidently makes no difference, so that we are led to the more general conclusion that The mass of a body is a measure of its energy-content; if the energy changes by L, the mass changes in the same sense by L/9 × 10²⁰, the energy being measured in ergs, and the mass in grammes."

(emphasis mine).

This - the idea that mass and energy are the same thing - has always seemed to me to be one of the key and unique insights of special relativity - but the article here, and other articles on the subject, seem to talk only about the "mass change of radiation" when discussing E=mc² - the "consequences" section of this article doesn't mention it at all.

Is there a reason that I don't see why this generalized equivalence shouldn't be more strongly featured in the articles talking about special relativity? --Alvestrand 09:21, 18 February 2006 (UTC)

First of all, mass and energy are according to Einstein not exactly the same thing, and I agree. Energy and mass are different concepts, but mass = energy/c^2. When normalizing to c=1 they become numerically equal.

Secondly, that equation is not SRT; Einstein showed that SRT can be used to derive it. I have also seen derivations without SRT. Thus it looks appropriate to me that in this article it's not much discussed.

However, I saw somewhere the suggestion to write a separate page about the history of E=mc2. That would be a very good idea, IMO.

Cheers, Harald88 07:27, 2 March 2006 (UTC)

Redunant (sic) second postulate - edits by EnormousDude

This article is not a place to state a personal opinion. Not everyone agrees that the second postulate follows from the first. A separate section discussing the fact that some believe the 2nd postulate to be redundant is more appropriate than stating an opinion as fact. Alfred Centauri 02:52, 16 March 2006 (UTC)

On the other hand, the second postulate is frequently misstated. I edited the "Postulates of special relativity" after reading this article on PhysOrg.com

http://www.physorg.com/news11829.html

One of the points that Baierlein makes, is that relativity is generally taught using a version of the second postulate which is far less intuitively plausible than the second postulate as originally formulated by Einstein. I hence replaced the second postulate text with an exact quote and added comments.

Minor Crank 12:42, 25 March 2006 (UTC) (aka 67.163.106.133 before I created a user name)

The exact quote you contributed comes from the introduction. Later, Einstein writes "These two principles we define as follows: -" and "2. Any ray of light moves in the “stationary” system of co-ordinates with the determined velocity c, whether the ray be emitted by a stationary or by a moving body." Contrast this with his earlier statement that "light is always propagated in empty space with a definite velocity c which is independent of the state of motion of the emitting body". See the difference? In the former, he refers to a co-ordinate system and a determined speed while in the later, he refers to empty space and a definite speed.

Earlier, Einstein writes "Let us take a system of co-ordinates in which the equations of Newtonian mechanics hold good. In order to render our presentation more precise and to distinguish this system of co-ordinates verbally from others which will be introduced hereafter, we call it the stationary system."

IMHO, Einstein here identifies 'stationary system' with what we now call an 'inertial system'. Taken together with his 2nd principle, as he defined it above, Einstein does appear to assume that the measured speed of light in an inertial system of co-ordinates is c.

I referred to [6] and [7] for the quotations. Alfred Centauri 16:25, 25 March 2006 (UTC)

The actual demonstration of the fact that the measured speed of light in the "moving" system of coordinates is c does not take place until Section 3, "Theory of the Transformation of Co-ordinates and Times from a Stationary System to another System in Uniform Motion of Translation Relatively to the Former", where Einstein writes: "We now have to prove that any ray of light, measured in the moving system, is propagated with the velocity c, if, as we have assumed, this is the case in the stationary system; for we have not as yet furnished the proof that the principle of the constancy of the velocity of light is compatible with the principle of relativity."

Einstein follows this statement with a demonstration that a spherical wave emitted in the "stationary" system with velocity c at time t = τ = 0, when the origins of the two coordinate systems coincide, appears also to be a spherical wave with velocity c in the "moving" system.

Clearly, Einstein did not include invariance of the velocity of light as measured by any inertial observer as part of his starting assumptions.

Minor Crank 00:58, 26 March 2006 (UTC)

Please re-read the very quote you have provided. Einstein assumed that the 2 principles were true but had yet to prove that they were not contradictory. That is, it is not at all clear at the outset that there is a co-ordinate transformation that is compatible with the two principles that he defined earlier. I quote from section 3: "With the help of this result we easily determine the quantities $\xi ,\eta ,\zeta$ by expressing in equations that light (as required by the principle of the constancy of the velocity of light, in combination with the principle of relativity) is also propagated with velocity c when measured in the moving system" (emphasis is mine). Einstein states quite clearly here that the notion that light is progagated at the velocity of c in the moving system is required by the combination of the 2 principles. Thus, IMHO, section 3 proves only that there is a transformation that is compatible with the 2 principles. Alfred Centauri 06:20, 26 March 2006 (UTC)

Let us look at Einstein's use of the second postulate.

1) We are agreed that the introductory statement did not include any mention of invariance of c as measured by the "moving" observer.

2) In Section 2, Einstein re-states what he regards to be the principle of the constancy of the velocity of light: "Any ray of light moves in the “stationary” system of co-ordinates with the determined velocity c, whether the ray be emitted by a stationary or by a moving body." His arguments in this section use only this principle.

3) In Section 3, he continues using only this limited assumption: "...by inserting the arguments of the function tau and applying the principle of the constancy of the velocity of light in the stationary system" (italics mine). He does not assume correctness of the principle in the moving system.

4) Continuing in Section 3, Einstein writes, "With the help of this result we easily determine the quantities xi, eta, zeta by expressing in equations that light (as required by the principle of the constancy of the velocity of light, in combination with the principle of relativity) is also propagated with velocity c when measured in the moving system." Einstein continues carefully to distinguish what he terms the "principle of the constancy of the velocity of light" from the more general notion that light might be propagated at c in the moving system. Rather, he states that the following paragraphs must establish that fact.

5) Einstein states: "We now have to prove that any ray of light, measured in the moving system, is propagated with the velocity c, if, as we have assumed, this is the case in the stationary system..." Einstein repeats that his starting assumptions included only constancy of c in the stationary system. Einstein proposes to show that if c is constant in the stationary system, then c must be constant in the moving system.

6) "...for we have not as yet furnished the proof that the principle of the constancy of the velocity of light is compatible with the principle of relativity." Einstein continues to use the term "principle of the constancy of the velocity of light" in exactly the sense in which he expressed it in the introduction.

7) Einstein shows that

x^2 + y^2 + z^2 = (c^2)(t^2)

transforms to

xi^2 + eta^2 + zeta^2 = (c^2)(tau^2)

8) The demonstration that light travels at c in the moving system "...shows that our two fundamental principles are compatible", by which Einstein means the principle of relativity and the principle of the constancy of the velocity of light, still referring to the second principle in the original sense in which he stated it.

9) Concerning Section 3, you wrote: "Einstein states quite clearly here that the notion that light is progagated at the velocity of c in the moving system is required by the combination of the 2 principles." I agree perfectly, the two starting principles being the principle of relativity and the principle of the constancy of the speed of light in the stationary system. The notion that light is propagated at the velocity of c in the moving system is derived from the combination of the two starting principles.

Minor Crank 10:46, 26 March 2006 (UTC)

10) You wrote: "IMHO, Einstein here identifies 'stationary system' with what we now call an 'inertial system'."

I disagree. Einstein very deliberately chose as his definition of 'stationary system' one with which any of his (almost universally aetherist) contemporaries would be comfortable, namely "a system of co-ordinates in which the equations of Newtonian mechanics hold good."

Obviously, a 'moving system' would be one in which the equations of Newtonian mechanics might possibly be violated. To an aetherist, such a scenario would make perfect sense, and many experiments at the time were directed towards detecting such violations. On the other hand, if you are at rest with respect to the aether, no motion of an emitter body could possibly alter your measurement of the velocity of light being radiated. Einstein's statement of the second law was carefully worded to be completely acceptable to his aetherist contemporaries.

Minor Crank 16:15, 26 March 2006 (UTC)

If, as you claim, Einstein derived the result that light is propagated at the velocity of c in the moving system, then the assumption that this result holds cannot be used in the derivation of this result, right? Yet, in section 3, immediately after establishing the form of the transformation for the time coordinate τ in the moving system, Einstein writes "For a ray of light emitted at the time τ =0 in the direction of the increasing ξ

$\xi =c\tau$..."

Let's summarize step by step where we are at this point:

(a) Einstein assumes that the ray of light is emitted from the origin of the moving system of coordinates (τ, ξ, η, ζ)

(b) By applying the 2nd principle only, that the speed of the light ray is c in the stationary system of coordinates (x, y, z, t), he derives the form of the transformation τ(x',y,z,t).

(c) By applying the 1st principle only to (b), that the speed of the light ray must also be c in the moving system of coordinates, he derives the transformations for ξ η and &zeta.

So, at this point, Einstein has established the form of the transformations from the coordinates of the stationary system to the coordinates of the moving system based on the assumption that a ray of light emitted by a stationary object in the moving system propagates at c in both systems. I suppose one could say that this assumption is derived from the 2 principles but, IMHO, that is a stretch. Look at it this way, this is a simple case of "If A AND B THEN C" - if the 1st principle is true and if the 2nd principle is true then then the speed of light is c in the stationary and moving coordinate systems. Case closed - nothing else needs to be said. The result has been 'derived' and no further proof is needed as long as one assumes that the 2 very reasonable principles are true. So what is the rest of the paper for?

Clearly, Einstein needs to show that the the two principles are not contradictory. To show this, he first assumes the 'derived' result that the speed of light is c in both coordinate systems. Thus, contrary to your point (8) above, he cannot show this result as he assumes it to be true from the outset. Instead, what he shows is that the coordinate transformation derived from this assumption is consistent thus establishing that the 2 principles are compatible.

Once again, I think it is quite clear that Einstein assumed the constancy of the speed of light in both systems in order to derive the coordinate transformations which he then showed to be consistent proving that the two principles are compatible. On the other hand, time dilation, length contraction, etc. are quite clearly derived results. That is, there were no starting assumptions with respect to these results. Alfred Centauri 18:05, 26 March 2006 (UTC)

We seem to be in some danger of arguing in circles here...

I've just emailed Prof. Baierlein, who was the subject of the article in PhysOrg that I cited above http://www.physorg.com/news11829.html

His email address (slightly modified to deter spambots) is Ralph.Baierlein AT nau.edu

If we BOTH write him, maybe Prof. Baierlein would step in and offer his insights, at least on a private basis. Could you do so? Thanks!

If I have a chance, I'll try to get to the university library to obtain a copy of Prof. Baierlein's article in AJP, and I'll figure some way of getting you a copy.

This is what I wrote:

Dear Dr. Baierlein:

Are you familiar with Wikipedia, the online freely editable community encyclopedia?

Please refer to the Wikipedia article on special relativity:

http://en.wikipedia.org/w/index.php?title=Special_relativity

In response to your article in AJP, I changed the discussion of the second postulate in the above article from

Second postulate (invariance of c): Light is always propagated in empty space with a definite velocity c that is independent of the state of motion of the emitting body or on the state of motion of observer measuring it.

In other words: The speed of light in vacuum, commonly denoted c, is the same to all inertial observers, and does not depend on the velocity of the object emitting the light. An observer attempting to measure the speed of light's propagation will get the same answer no matter how the observer or the system's components are moving.

to

Second postulate (invariance of c): Light is always propagated in empty space with a definite velocity c that is independent of the state of motion of the emitting body.

Most current textbooks mistakenly include a major derived result, that the speed of light is independent of the state of motion of the observer measuring it, as part of the second postulate. A careful reading of Einstein's 1905 paper on this subject shows that, in fact, he made no such assumption. The power of Einstein's argument stems from the manner in which he derived startling and seemingly implausible results from two simple and completely reasonable starting assumptions.

One of the most highly counterintuitive of these results (and, as stated above, commonly included in statements of the second postulate), is that the speed of light in vacuum, commonly denoted c, is the same to all inertial observers. An observer attempting to measure the speed of light's propagation will get the same answer no matter how the observer or the system's components are moving.

As "Minor Crank", I am currently engaged in debate with another member of the Wikipedia user community, "Alfred Centauri", over whether my change was a proper edit. We are reading the same 1905 article and coming to opposing conclusions.

I was wondering if you could spare a few minutes to enter the discussion and offer your insights? "Alfred Centauri" and I are currently engaged in discussion topic 36.

Thank you very much!

<signed with real name>

Minor Crank 18:31, 26 March 2006 (UTC)

One e-mail should be sufficient. If not, I will e-mail him also.

BTW, I don't really consider this a debate over whether your edit is appropriate. If I truly thought it to be inappropriate, I would have reverted your edit first and then explained why on the talk page. In any event, the historical wording versus the modern wording of the 2nd postulate is certainly an appropriate topic for this article.

Where we disagree appears to me to be with the use of the word 'derive' and what constitues a 'startling' result. If we assume that the speed of light is c in a coordinate system in which Newtonian mechanics hold regardless of the state of motion of the emitting body AND if we assume that in uniformly moving coordinate systems, Newtonian mechanics hold, it follows that the speed of light is c in uniformly moving coordinate systems. This is an immediate and purely logical conclusion - and not a particularly startling one, IMHO.

I suppose the reason I'm pushing back here is that I associate the word 'derive' (in this context) with some mathematical process as in the way the transformation equations are derived. Thus, I would prefer that the entry on the 2nd postulate say something more along the lines of "That the speed of light is measured to be c by all inertial observers is a logical consequence of the combination of the principle of relativity and the principle of the constancy of the velocity of light". Alfred Centauri 19:26, 26 March 2006 (UTC)

From your modern perspective, I don't doubt that the constancy of the speed of light in moving coordinate systems seems immediate and obvious. Historically, however, this was not so, and many experiments were conducted in an attempt to detect the consequences of movement through the aether, by Roentgen, Eichenwald, Bradley, Wilson, Arago, Fizeau, Airy, Michelson and Morley, Trouton and Noble, etc.

The conviction that there should be measurable consequences of movement through an aether was great, and Einstein was careful to frame his starting postulates in such a way that his paper would not suffer immediate rejection. Little by little, through inexorable logic, Einstein leads the reader to an abyss, either to accept or reject his conclusions. To early 20th century physicists, the constancy of c in all inertial frames as a logical consequence of seemingly innocuous starting assumptions was indeed one of many startling results in Einstein's paper.

However, it has been over a century since Einstein's seminal publication. To what extent should we retain the historical perspective? From a modern standpoint, Einstein's approach is very "clunky". Several alternative derivations of far greater elegance exist.

For example, starting from the first postulate alone, and understanding that any reasonable set of transformations must form a group, then one finds that there exist only two sets of transformation laws that satisfy the principle of relativity. These are the Galilean and Lorentz groups. If the universe follows Galilean relativity, then there is no upper speed limit to the universe. If the universe follows Lorentzian relativity, then there must exist an upper speed limit. Experimentally, we observe that an upper speed limit exists; therefore the universe must follow Lorentzian relativity. The role of the second law is extremely limited in this development of SR. Basically, the second law exists only to establish the value of the upper speed limit. SR is purely a geometric theory in this development, and light has no special role in its derivation. The velocity of light is observed to correspond rather closely with the upper speed limit, but that's merely a consequence of photons' being either massless or very nearly so.

So, should we abandon Einstein's primitive, clunky derivation in favor of one of the modern approaches? Perhaps I am being a Luddite, but I don't think so...

Minor Crank 22:54, 26 March 2006 (UTC)

Good points that you make here. This [8] is an interesting read. I found this particularly interesting:

"Thus they [Poincare and Lorentz] believed that the speed of light was actually isotropic only with respect to one single inertial frame of reference, and it merely appeared to be isotropic with respect to all the others. Of course, Poincare realized full well (and indeed was the first to point out) that the Lorentz transformations form a group, and the symmetry of this group makes it impossible, even in principle, to single out one particular frame of reference as the true absolute frame (in which light actually does propagate isotropically). Nevertheless, he and Lorentz both argued that there was value in maintaining the belief in a true absolute rest frame, and this point of view has continued to find adherents down to the present day."

Yes, it is hard to shed long held notions. Alfred Centauri 00:15, 27 March 2006 (UTC)

I received two emails from Prof. Baierlein including a pdf of a proof version of his AJP article. However, he was very explicit in stating that I should not redistribute the pdf because of copyright restrictions (Sorry). I've uploaded the two emails that he sent me to my daughter's web site http://rosemarysgallery.home.comcast.net . Scroll down four inches below the list of images, and you will see two underscores "_ _". Each of these is a link to one of Prof. Baierlein's emails. After you've accessed the emails, let me know so I can delete the links.

It is evident from the AJP article that Prof. Baierlein would say that I had an incorrect emphasis in my wiki edit. The second postulate as commonly stated today is not wrong; rather, it has shifted in meaning since Einstein's day.

I believe I should be able to quote a few paragraphs of the AJP article without violating the bounds of fair use:

Today, the primary meaning of the phrase is that, given a specific burst of light, the burst's speed is measured to have the same numerical value in all inertial frames. That is, the speed is constant with respect to changes in the reference frame in which it is observed.

A secondary meaning also exists: in any given frame, bursts of light from sources with different velocities all have the same speed. That is, the speed of light is constant with respect to changes in the source’s velocity.

When the typical contemporary textbook uses the phrase, "the constancy of the speed of light," it intends that both the primary and the secondary meaning apply.

In the years immediately preceding 1905 and in Einstein's seminal paper, the phrase, "the constancy of the speed of light," meant only that the speed of light is independent of the source's velocity.

. . . .

To derive the Lorentz transformation, Einstein used only the principle that the speed of light is independent of the state of motion of the emitting (or reflecting) body and the relativity principle (the laws of physics are the same in all inertial frames).

. . . .

To take as a postulate that the speed of light is constant relative to changes in reference frame is to assume an apparent absurdity. . . . No wonder, thinks a student, that we can derive other absurdities, such as time dilation and length contraction, from the premises. Far better to start much closer to where Einstein started and to derive the logical consequence that the speed of any given light pulse has the same value in all inertial frames.

It's a bit late right now for me to make changes to reflect Prof. Baierlein's critique, but maybe tomorrow I'll have time to revise my edit, and you can check it over? Thanks!

Minor Crank 05:56, 28 March 2006 (UTC)

BTW, the MathPages indeed have some of the most thoughtful commentary on relativity available on the internet. The author of the MathPages is an executive of a major corporation and prizes his anonymity. Back a few years ago, when I was helping Don Koks find a backup editor for the Usenet Physics FAQ http://math.ucr.edu/home/baez/physics/ , I was able to do a whois on the site to identify the owner, and unsuccessfully tried to recruit him for FAQ duties. Shortly afterwards, it no longer was possible to get any useful whois information...

Minor Crank 06:13, 28 March 2006 (UTC)

The pdf that Prof. Baierlein sent me is watermarked, and if his proof copy "got out into the wild" there would be absolutely no doubt that he was the source. But if I make a pdf from a photocopy, that should protect him from any liability.

Minor Crank 12:24, 28 March 2006 (UTC)

Thanks! I've saved the e-mails and will now take a look at them. Alfred Centauri 14:30, 28 March 2006 (UTC)

OK, I downloaded Prof. Baierleins' paper. Having reflected on our previous discussions and then quickly reading this paper, I believe I can better summarize my position on this. I agree that Einstein phrased the 2nd postulate in a way that is more restricted than the modern phrasing. And, I agree that the modern phrasing of the postulate is actually is a logical consequence of the original 1st and 2nd postulates.

However, I would like to point out that the following statement in Prof. Baierleins' paper:

"Second, Einstein wrote, 'Now, we have to prove [my italics] that, measured in the moving system, every light ray propagates with the speed V [we would write c]...'".

cannot be used as a evidence that Einstein was here proving the modern form of the 2nd postulate. As I have pointed out above, it is my opinion that here, Einstein is showing that the derived transformation equations are consistent. That is, in deriving the transformation equations, he writes:

$\xi =c\tau$..."

Recall that these are the coordinates of the moving system. Thus, Einstein is explicitly saying that the propagation velocity of light is c in both the stationary and moving coordinate systems. Since this is 'built in' to the transformation equations, why would he later need to prove this?

In any event, there is no need to 'prove' the modern form of the 2nd postulate in his paper at all as it is an immediate and logical consequence of the original 1st and 2nd postulates. Instead, Einstein needs to show that these postulates are not contradictory by demonstrating that there is a non-trivial transformation between the coordinate systems that is consistent with the postulates. Do you see my point? Alfred Centauri 15:23, 28 March 2006 (UTC)

Relativistically Rotating objects.

Suppose you take a large spoked wheel and spin it up so that the rim is moving at a high relativistic speed (say .9c). What would you see if you looked down at it from above? The rim would appear length contracted, but the spokes would still have the same lengths and angles to each other. What would happen here? 12.37.33.3 23:04, 17 March 2006 (UTC)

Because the parts of the wheel aren't moving in interial frames (free-fall), but are instead undergoing acceleration, you have to use general relativity to figure out what happens. --Christopher Thomas 18:04, 22 March 2006 (UTC)

No GRT is needed for this analysis: SRT has no problems with describing the behaviour of moving objects in a single inertial frame! What happens in practice is that the rim expands due to inertial forces. But if we forget about those forces, the rim would very slightly Lorentz contract and push against the spokes. As the spokes push back with a negligible force (compared to the rim), this relativistic effect is really very easy to calculate.

But note that such questions are more properly forwarded in a newsgroup, such as sci.physics.relativity. Harald88 20:20, 22 March 2006 (UTC)

Harald, the rim is being accelerated radially so that it can follow a curved path. This causes its time dilation to be other than what SR predicts, as any given point in the rim is not in an inertial frame. If memory serves, the Einstein lectures explicitly cover this case as an example of where it's hard to apply SR. --Christopher Thomas 22:13, 22 March 2006 (UTC)

Well, as I remember it, Einstein uses the SR relations to show that accleration changes geometry (or "curves space"), the ratio of radius to circumfernce (pi) is changed. So you can use the SR relations on these accelerating objects in some sense. I will look for a reference where Lorentz deals with this using GR. E4mmacro 22:26, 22 March 2006 (UTC)

A good example of time dilation with a curved path can be found in Einstein's 1905 SRT paper; that paper is online and linked from this article. BTW, the first verified prediction of SRT was on accelerating electrons...

Using GRT/SRT, Lorentz provided a more complex calculation (because it doesn't have a simplification as with spokes) about a rotating disc, in Nature vol.106, p.793-795 (1921). Harald88 11:03, 23 March 2006 (UTC)

Ives, H. E. "Theory fo the double Fizeau toothed wheel" Journal of the optical Society of America, v29, p472-478 thinks the disk must bend (curve into a cup shape) when it rotates. The cruvature of the disk depends on the speed. The ratio of radial distance along the curved shape to the circumference depends on the rotation speed. It appears he has not considered (on top of that) the deformation due to the stresses which supply the centripetal forces. (real disks have a limited rotation speed before the material yiled stress is reached). Ives's analysis is all from an interial frame POV using the length contractions. No GR involved. E4mmacro 23:46, 22 March 2006 (UTC)

Two Small Questions

One: Could someone show me where mass is mentioned in any original papers on special relativity?

Look at section 10 of "On the electrodynamics of moving bodies".

Two: Could someone tell me why, when two bodys are experiencing motion relative to one another, time compresses for one clock, but not for another? That is, why, after the bodies become at rest with respect to one another, is one clock objectively slower than the other? SJCstudent 01:23, 6 April 2006 (UTC)

Actually, if both bodies decelerate identically until there is no relative motion between them, their clocks will be synchronized (assuming they were synchronized when they were instantaneously co-located). A moving clock appears to run slow. Accelerated clocks actually do run slow. The twin 'paradox' occurs because one twin experiences acceleration (the one in the spaceship) while the other does not. In GR terminology, geodesics of spacetime are paths of maximum elapsed proper time. Alfred Centauri 02:12, 6 April 2006 (UTC)

But this is exactly the source of my confusion. In Einstein's 1905 paper, he never mentions acceleration to account for this difference. Yet people did not reject that theory outright. What, within Einstein's SR, backs this up? SJCstudent 03:40, 6 April 2006 (UTC)

See Twin paradox, it's "all" there (you'll notice that you're right that acceleration doesn't account for the difference). Harald88 22:16, 6 April 2006 (UTC)

You sound like a smart person so do your own research. There's plenty of scientific, pseudo-scientific, and non-scientific (historical) articles written about SR on the web. Have fun. Alfred Centauri 03:53, 6 April 2006 (UTC)

Uh, ok... Thanks Harald88. This helps, but I'm still not sure where anything about inertial reference frames is mentioned in SR proposed by Einstein. Look, I'm not interested in joining the list of whackos attempting to disprove relativity. I am simply interested in moving past a particular impasse in the course of my studies. I understand, Alfred, that I ought to do research, and that is exactly what I am doing by asking these questions to those that are so knowledgable that they wrote this article. Sorry for all the trouble. SJCstudent 03:16, 10 April 2006 (UTC)

2nd postulate update

The current version of the special relativity "Postulates" section contains the following statement: "An observer attempting to measure the speed of light's propagation will get the same answer no matter how the observer or the system's components are moving."

Apparently, that particular sentence was given by User:Christopher Thomas, who is currently on intermittent sabbatical. (As far as I could see, here is when he added said sentence: Thomas link - the page as of 16:44, 23 November 2005)

Here is the problem with the given sentence: "Measure" means "To ascertain the quantity of, using standard measurement instruments." Since the round-trip light speed case was closed prior to special relativity, this left only the one-way case for postulation; however, it is currently impossible to measure the one-way speed of light. Specifically, it's not possible (currently) to measure light's speed between two clocks.

Here is a suggested replacement for the second postulate section:

2. Second postulate - In empty space, light's round-trip, one-clock speed is c per experiment, but no one has yet measured light's one-way, two-clock speed due to the lack of absolute clock synchronization; therefore, the one-way speed of light was simply defined to be c to follow Einstein's assumption that inertial frames should not be distinguishable.

Since even Einstein agreed that light's one-way speed would vary given (absolute) synchronization, his other (hidden) assumption was that (absolute) synchronization is impossible, although he did not prove this. Indeed, since one cannot prove such a negative, the one-way case remains open, and can only be closed if and when (truly) synchronous clocks are used to actually measure light's one-way speed. Cadwgan Gedrych 18:31, 10 April 2006 (UTC)

In the "Status" section it is mentioned that the speed of light is understood two be the two-way speed. I think there should be no change in the "Postlates" section, because only two-way speed can be measured, there is no way to conduct "absolute synchronization". Icek 18:16, 11 April 2006 (UTC)

As I mentioned, at the time of relativity's creation, there could not have been any postulation regarding light's round-trip speed (because that case was closed via experiment); therefore, the second postulate could not apply to the round-trip speed of light, but could only apply to the one-way speed.

As I also mentioned, the one-way case remains open due to the possibility of absolute clock synchronization. (It remains possible because no one can prove that it is impossible.)

To reiterate, Einstein did not postulate regarding the round-trip case, but simply accepted the null result as an experimental fact. This left only the one-way case, and his postulate in that case was "Since my [Einstein's] rule is null results always, I postulate that clocks should be set to obtain a one-way null result, even if it means that they are not absolutely synchronous." Cadwgan Gedrych 18:48, 12 April 2006 (UTC)

Being experimentally tested and not falsified does not contradict being used as a theory's postulate - the experimental evidence was the basis of the theory. The first postulate is also well-supported by experiments. In Einstein's original publication, simultaneity is actually defined by synchronization by light signals (emitted in the same reference frame, as I understand it). So I agree partly, the definition of simultaneity should be in the article. But I don't think of it as a hidden assumption but rather as a mere definition and as such cannot be tested. If someone finds a way for instantaneous communication and thus absolute synchronization, it would, if one accepts Einstein's definition, allow for communication backward in time in certain reference frames. Icek 23:49, 12 April 2006 (UTC)

Thank you, Icek, for your comments. Just for the sake of the argument, let's say that you are correct about the round-trip case, i.e., that it was a part of the 2nd postulate at the creation of special relativity; this still leaves the problem of the one-way case.

By "the one-way case," I mean (as did Einstein) the specific case where light's one-way speed is measured by two clocks which are neither rotated nor transported.

In other words, the part of the second postulate that addresses light's one-way speed says that whenever two mutually-at-rest clocks are used to measure this speed, it will be c (in all frames and in all directions).

Icek, would you be so kind as to tell us how this one-way experiment could be done. You can even use the ideal clocks and rulers of theoretical physics. Please show each step, starting with two unstarted clocks. 66.147.55.213 18:39, 13 April 2006 (UTC)

The relevant sentence from Einstein's paper:

We have not defined a common "time" for A and B, for the latter cannot be defined at all unless we establish by definition that the "time" required by light to travel from A to B equals the "time" it requires to travel from B to A.

Einstein did not explicitely state that the light should be emitted in the rest frame, but I think this is implied since many physicists in 1905 still thought that velocities simply add up.

Taking into account the definition of simultaneity, a one-way experiment can actually be done:

• Leave one clock (A) at the coordinate origin (which shall be at the starting point of the clocks). Take the other clock (B) to distance x, as measured by ruler.
• Now both clocks are at rest. Start clock A and at the same time send a light signal toward clock B. When it arives at clock B, set clock B to time x/c and start it. By definition, the clocks are now synchronized. It should be noted that the light signals where emitted in the rest frame.
• Now let a spaceship fly by clock A with velocity v. When it is at the position of clock A, it shall send a light signal toward clock B. Record the sending and arrival times and subtract one from the other.
• Divide x by the time difference to get c(v), the one-way light time.

Icek 00:56, 14 April 2006 (UTC)

Now let's click our heels, and go back to Kansas! ;-) Nice try, Icek! (Seriously!) But now that we are back in Kansas, let's focus on your word "experiment"; by definition, an experiment is an attempt to discover the nature of Nature, so there can be no rigging of the result by man; however, in the case you cited above, man rigged the result ("c invariance/isotropy") by forcing the clocks to obtain "c" (in the form of using the rigged "time" "x/c").

Yes, I know that you used a different source for the second light ray, but since experiment says that light is source-independent, this matters not.

It also matters not what the 2nd postulate says about the round-trip case because that case was closed experimentally prior to special relativity. (That is, special relativity has no unique position re this case because every theory must have a null result for it.) It is only the one-way case that really matters. It is only regarding that case that special relativity (SR) can take a unique position.

And that unique SR position is exactly (and only) what the Wiki SR story should tell.

What, specifically, does SR say about the only case that matters?

What, specifically, does SR say about the case where a light ray's speed is experimentally measured between two same-frame clocks sans rigging?

For example, does the Wiki SR article say that the one-way case is still open?

For another example, does the article state that no one has ever actually used two same-frame clocks to measure light's one-way speed?

For yet another example, does the Wiki article say that Galileo and Newton and Lorentz have not been proved wrong in the only case that matters?

And here is yet another example: Does the article state that one-way light speed invariance has not been shown, and cannot be shown (experimentally)?

Cadwgan Gedrych 01:53, 15 April 2006 (UTC)

I agree that the article should be changed.

There is indeed the hidden assumption that light propagation is isotropic and c is invariant in at least one reference frame when the light is emitted in this same reference frame. Else the definition of time could lead to contradictory results. But if we accept that, time is no longer "rigged", it's just a definition. Maybe we should include the hidden assumption in the article. Icek 15:13, 15 April 2006 (UTC)

You've confused me with your word "invariant"; normally, "invariant" means "the same for all frames," and yet you mentioned only one frame.

You also confused me by reverting to your prior position of bringing up whichever light source emits the light, as if that makes any difference.

You have further confused me by your statement that "the definition of time could lead to contradictory results"; please explain this.

If Einstein's time is just a definition, then what has it got to do with the physics of space and time? Why not explicitly state that it is merely a definition given by man in lieu of absolute synchronization, which we (Einstein) cannot obtain.

Can Einstein claim that the results a mere definition are laws of physics? (I am talking about the standard SR results such as c invariance, the transformation equations, and the composition of velocities theorem.)

Why does the Wiki 2nd postulate section not say that it is currently impossible to experimentally measure light's one-way speed?

Why does the Wiki 2nd postulate section not say that the only way to correctly measure light's one-way speed is to use absolutely synchronous clocks (but Einstein does not have such clocks)?

Cadwgan Gedrych 19:20, 17 April 2006 (UTC)

I should clarify a few things:

• When I answered at 18:16, 11 April 2006, I had not taken into acount a proper definition of time, so let's ignore that answer, since I have thought more about the problem afterwards.
• "Invariant" is probably not the right word in my last answer, a better word would be constant. The statement means that there exists (at least) one reference frame (which I will subsequently call the rest frame) for which the two-way light trip time only depends on the distance, provided that the light is emitted in the rest frame.
• It should be clear now that if the two-way light trip time does e. g. also depend on the direction of the light beam there could be a different times at clock B depending on whether you synchronize it directly with a light signal from A to B or you synchronize it via a "relais" clock C which does not lie one the line passing through A and B.
• Every physical concept is "just a definition". Definitions aid us in describing the world. The definition of a quantity should include a method for measuring that quantity. How would one define "absolute time" or "absolutely synchronous clocks"?
• Definitions may rely on assumptions as I showed above for the case of relativistic time.
• Definitions cannot be disproved, but underlying assumptions can be, and this would make the definition unuseable.
• In relativity, only synchronization is defined, but not local time (we are speaking of "clocks" without specifying how they measure time). In physics, (local) time is currently defined by a resonant frequency of the caesium atom.
• The definition of time in relativity allows us to measure one-way light trip times for light emitted in other reference frames than the rest frame.
• In contrast to the definition of time, the postulates of special relativity can be tested.

Icek 12:10, 18 April 2006 (UTC)

Let's touch on three of your just-given points.

• The definition of time in relativity allows us to measure one-way light trip times for light emitted in other reference frames than the rest frame.

Please remember that "measure" does not include rigging the result, and as we all know, Einstein's definition of "time" certainly does that. (It does it by presetting (rigging) the clocks to read the prechosen time "x/c" - as I have already mentioned.)

• In contrast to the definition of time, the postulates of special relativity can be tested.

So tell us how we can test Einstein's second postulate. (That is, how can we experimentally test whether or not light's one-way speed between two same-frame clocks is invariant? Please show all steps, starting with two unstarted clocks.)

• Every physical concept is "just a definition". Definitions aid us in describing the world. The definition of a quantity should include a method for measuring that quantity. How would one define "absolute time" or "absolutely synchronous clocks"?

You seem to think that there is no definition of absolute time or absolute clock synchronization. Einstein himself gave two very good definitions of absolute synchronization, to wit:

[Quoting Einstein:] "This is what is meant when we say that the time of classical physics is absolute: The simultaneity of two definite events with reference to one inertial system involves the simultaneity of these events in reference to all inertial systems." [Appendix V of Relativity] [sentence order reversed and colon used]

[Quoting Einstein:] "w is the required velocity of light with respect to the carriage, and we have

w = c - v.

The velocity of propagation of a ray of light relative to the carriage thus comes out smaller than c.

But this result comes into conflict with the principle of relativity...." http://www.bartleby.com/173/7.html

Sure, Einstein (baselessly) rejected the result as being wrong, but the point is, he did derive it, so it could happen, at least on paper, and by this simple example Einstein told us that absolutely synchronous clocks would find that light's one-way speed varies with frame velocity, thereby giving us a definition of absolutely synchronous clocks.

Cadwgan Gedrych 18:45, 18 April 2006 (UTC)

On absolute synchronization: I don't see a definition, i. e. how would you theoretically "absolutely synchronize" two clocks which are not at the same location?

On the other two points you picked: I've answered these questions above. If there is a logical flaw in my answers, I don't see it, so please tell me exactly why you are not happy with my answers. Icek 20:13, 18 April 2006 (UTC)

Re absolute synchronization: You're confusing a definition with a procedure.

I see nothing wrong with either of Einstein's definitions. Do you?

The logical flaws were given above, but it seems that I must now repeat them.

You wrote:

• The definition of time in relativity allows us to measure one-way light trip times for light emitted in other reference frames than the rest frame.

Please cite the date and persons involved when light's one-way speed was measured between two same-frame clocks. I maintain that this experiment not only has not been done, but cannot be done today.

You wrote:

• In contrast to the definition of time, the postulates of special relativity can be tested.

Please back this claim by showing step-by-step how the 2nd postulate can be tested. As we know, the 2nd postulate pertains to the one-way speed of light (saying that light's one-way speed per two same-frame clocks is invariant and isotropic), so this is essentially the same problem as above where I asked you to supply the date and experimenters.

Tell us how the 2nd postulate applies to anything in physics.

Cadwgan Gedrych 20:44, 19 April 2006 (UTC)

I partly disagree: as Einstein pointed out, both postulates were based on experience. Moreover, the one-way speed of light can indeed not be determined except by convention, while physics theories are about what can be determined experimentally. Thus the second postulate is primarily concerned with the (experimental) round speed of light. It sets the constant c in the LT. Harald88 20:56, 19 April 2006 (UTC)

The elapsed time required for light to propagate along a closed path can be measured by a single clock where synchronization is not an issue. It is my understanding that experiments show that the length of the closed light path divided by the elapsed time is the constant speed c. Now, consider an experiment whereby light is sent along a path towards a reflector oriented to reflect the light back along the incident path. Is it not true that the electromagnetic traveling waves that are the incident and reflected light superpose to produce a familiar standing wave pattern of stationary nodes and anti-nodes? But, if the incident light propagates at a different speed than the reflected light, the nodes and anti-nodes will not be stationary but will instead move in the direction of the 'faster' light path, right? The speed with which the nodes move, in conjuction with the result that the closed path average light speed is c, can be used to calculate the one-way speed of light for the incident and reflected paths. Of course, if the nodes are stationary for all orientations and states of uniform motion of the apparatus, then one-way light speed must be c which would imply that the notion of absolute synchronization of spatially separated clocks is without meaning. Alfred Centauri 00:47, 20 April 2006 (UTC)

Alfred, I also thought so once, but after thorough calculation it turned out to be not so. I don't remember clearly what the error in your "nodes" picture, but probably it is that the long and short waves together reproduce the same result. One can't invalidate the Lorentz transformations with such simple tricks. ;-) Harald88 11:15, 25 April 2006 (UTC)

What turns out not be so - that one cannot calculate the OWSL in this way? Long and short waves from what perspective? Alfred Centauri 13:48, 25 April 2006 (UTC)

One can of course calculate it it, but not more than with any other method: you get out what you first put in. What look like equally long waves in the co-moving frame, look like short and long waves in all other frames - but they add up in a way that everyone agrees on what the light pattern is. Just try it for yourself (I did). Or (to get back to what this page is supposed to be about!), give a reference to a peer reviewed paper that found another result. 20:45, 25 April 2006 (UTC)

Thanks for the input, Harald88 and Alfred, but we are losing focus here, and I would hate to see this simple discussion drag on forever!

Surely you admit that special relativity (SR) pertains to the case of the one-way light speed per two same-frame clocks. Surely you admit that SR states that this speed must be experimentally measured as c in all frames. Surely you admit that if this is not the case, then SR falls, regardless of round-trip nullness or node nullness or whatever.

However, as I have tried to get across, it is currently physically impossible to experimentally measure light's one-way speed between two same-frame clocks. (This is why the one-way Michelson-Morley experiment has never been performed.)

Also (as I have also tried to get across), even Einstein admitted that two absolutely synchronous clocks would disprove SR by finding a variable one-way light speed, regardless of round-trip nullness or node nullness.

[RE-Quoting Einstein:] "w is the required velocity of light with respect to the carriage, and we have

w = c - v.

The velocity of propagation of a ray of light relative to the carriage thus comes out smaller than c."

Einstein did not disprove his w = c - v. He merely discarded it because he disliked it.

Even though SR must have one-way invariance, it cannot get it without forcing it to happen (by rigging the one-way time on the clocks), so SR's one-way invariance cannot exist experimentally. On the other hand, one-way variance can exist experimentally, if absolutely synchronous clocks are used to measure light's one-way speed.

So where does this leave the 2nd postulate?

It leaves it saying the following:

Since I, Einstein, firmly (but, to be honest, baselessly) believe in "all null results," I believe that clocks will not be properly related unless they are forced by definition to obtain "c" for light's one-way speed. Furthermore, I must assume that absolute synchronization is not possible because it can quickly overturn my theory.

Translation (again speaking as Einstein): Although my clocks are admittedly not correctly related (i.e., they are not truly (or absolutely) synchronous), we must nevertheless accept their results (e.g., the transformation equations, the composition of velocities theorem, one-way light speed invariance) as meaningful parts of physics. But since I cannot prove a negative, I have to admit that it may be possible to absolutely synchronize clocks, thereby totally disproving special relativity, despite round-trip nullness or node nullness or whatever. Cadwgan Gedrych 14:25, 20 April 2006 (UTC)

continued in 2nd Postulate Update II

Expansion of explanation?

I really like that this article has both the basics and the details/math. However, this section left me wondering:

Because of the freedom one has to select how one defines units of length and time in physics, it is possible to make one of the two postulates of relativity a tautological consequence of the definitions, but one cannot do this for both postulates simultaneously, as when combined they have consequences which are independent of one's choice of definition of length and time.

Could someone please expand it a bit? Or if it is immediately apparent to everyone (and I'm just a dumbass), can someone please explain this in more mathematical detail for me?

Thanks OhSoCurious 18:44, 14 April 2006 (UTC)

Clean Up

Firstly, this article if pretty difficult to understand. I don't have the know-how to tune it up, but it needs simpler wording. Also, near the bottom their is alot of bolded, bright red HTML coding, which shouldn't be there. Thanks, Theonlyedge 22:47, 19 April 2006 (UTC)

Agreed. Can any expert in this field attempt to make this article a bit more simple to read? --Siva1979^{Talk to me} 21:10, 25 April 2006 (UTC)

The special relativity article is not understandable because critical facts were omitted, such as the simple fact that Einstein's clocks are not synchronous. Cadwgan Gedrych 13:50, 26 April 2006 (UTC)

2nd Postulate Update II

continued from 2nd postulate update

Cadwgan Gedrych, here's why I am not happy with the definition of absolute time: "Simultaneity" is not defined (presumably you don't accept the light signal definition, because then the absolute time just defined does not exist; that has been shown e. g. by GPS satellites). You have yet to explain how any (simpler) alternative to special relativity explains Alfred Centauri's standing wave example; this example is really a better way to measure one-way light speed, voiding the "time-rigging" argument. It is a way to test the second postulate. You did not point out any logical flaws in my arguments, you only urged me to show you that the experiments have already been done. However, I found a possible flaw myself: Given my previously stated hidden assumption, we don't know which frame is the "rest frame". However, Alfred Centauri's approach does not have this problem. Icek 17:50, 20 April 2006 (UTC) Cadwgan, without speaking to the merits (or lack thereof) of any of your arguments, I remind you that Wikipedia talk pages are not intended to be forums for advancing a particular view over any other view. "Wikipedians generally oppose the use of talk pages just for the purpose of partisan talk about the main subject. Wikipedia is not a soapbox; it's an encyclopedia. In other words, talk about the article, not about the subject." Talk_page#Basic_rules_for_all_talk_pages So, please go ahead and make whatever edits you feel are appropriate to the main article. Of couse, as with all contributions, your edits are subject to modification and, if they are sufficiently controversial, being reverted. So, perhaps you should create a new article entitled "Cadwgans' intepretation of what Einstein REALLY said" Alfred Centauri 18:05, 20 April 2006 (UTC)

I qoute Cadwgan: "Surely you admit that if this is not the case, then SR falls, regardless of round-trip nullness or node nullness or whatever." It is straightforward to show that, assuming only the homogeniety and isotropy of space and time and the principle of relativity, the most general space and time coordinate transformation mixes space and time coordinates. Further, this transformation has a scaling that depends on an invariant speed and the relative velocity between the coordinate systems. If we allow this invariant speed to go to infinity, the space and time components decouple to yield the Galilean transformation where the notion of absolute time and absolutely synchronized clocks actually means something. In this context, the 2nd postulate is simply the assertion that the invariant speed of this general coordinate transformation is c. Now, it has been claimed to be shown that, by experiment, the invariant speed of the universe can be determined. It occurs to me that although this experiment may not give the exact result (due to experimental error), it certainly can determine if the invariant speed is finite. If this is true, it follows that if this experiment gives a finite result, the notion of absolute time and thus, absolutely synchronized clocks, is without meaning and, rather than breaking SR, would simply change the invariant speed of the transformation equations. Alfred Centauri 21:05, 20 April 2006 (UTC)

Cadwgan Gedrych, here's why I am not happy with the definition of absolute time: "Simultaneity" is not defined (presumably you don't accept the light signal definition, because then the absolute time just defined does not exist; that has been shown e. g. by GPS satellites). You have yet to explain how any (simpler) alternative to special relativity explains Alfred Centauri's standing wave example; this example is really a better way to measure one-way light speed, voiding the "time-rigging" argument. It is a way to test the second postulate. You did not point out any logical flaws in my arguments, you only urged me to show you that the experiments have already been done. However, I found a possible flaw myself: Given my previously stated hidden assumption, we don't know which frame is the "rest frame". However, Alfred Centauri's approach does not have this problem. Icek 17:50, 20 April 2006 (UTC) Cadwgan, without speaking to the merits (or lack thereof) of any of your arguments, I remind you that Wikipedia talk pages are not intended to be forums for advancing a particular view over any other view.

"Wikipedians generally oppose the use of talk pages just for the purpose of partisan talk about the main subject. Wikipedia is not a soapbox; it's an encyclopedia. In other words, talk about the article, not about the subject." Talk_page#Basic_rules_for_all_talk_pages

So, please go ahead and make whatever edits you feel are appropriate to the main article. Of couse, as with all contributions, your edits are subject to modification and, if they are sufficiently controversial, being reverted. So, perhaps you should create a new article entitled "Cadwgans' intepretation of what Einstein REALLY said" Alfred Centauri 18:05, 20 April 2006 (UTC)

I qoute Cadwgan: "Surely you admit that if this is not the case, then SR falls, regardless of round-trip nullness or node nullness or whatever."

It is straightforward to show that, assuming only the homogeniety and isotropy of space and time and the principle of relativity, the most general space and time coordinate transformation mixes space and time coordinates. Further, this transformation has a scaling that depends on an invariant speed and the relative velocity between the coordinate systems. If we allow this invariant speed to go to infinity, the space and time components decouple to yield the Galilean transformation where the notion of absolute time and absolutely synchronized clocks actually means something. In this context, the 2nd postulate is simply the assertion that the invariant speed of this general coordinate transformation is c. Now, it has been claimed to be shown that, by experiment, the invariant speed of the universe can be determined. It occurs to me that although this experiment may not give the exact result (due to experimental error), it certainly can determine if the invariant speed is finite. If this is true, it follows that if this experiment gives a finite result, the notion of absolute time and thus, absolutely synchronized clocks, is without meaning and, rather than breaking SR, would simply change the invariant speed of the transformation equations. Alfred Centauri 21:05, 20 April 2006 (UTC)

It has been said that I am pushing some sort of purely personal view; hopefully, the following will refute this:

Here is why only the two-clock version of the 2nd postulate is the only version that's relevant:

Only the 2-clock, one-way version (as opposed to all other versions, e.g., the round-trip, one-clock version, the nodes version, etc.) contains Einstein's clock-setting procedure, and this unique procedure controls all of the two-clock results of special relativity (SR).

For example, it controls all one-way speed values in SR, including the most critical one, the one-way speed of light.

For another example, it controls the relativistic transformation equations.

For yet another example, it controls the composition of velocities in SR.

It causes Einstein's relativity of simultaneity.

It causes Einstein's "time dilation."

It causes Einstein's "length contraction."

It causes Einstein's "momentum variance."

However, as critical as the one-way, 2-clock case is, this specific experiment has never been performed, not even on paper.

That is, c invariance has never been shown, nor can it be shown!

From this it follows that Galileo's one-way light speed variance has not been disproved.

Here are two important things to be asked of any article that purports to pertain to the 2nd postulate:

1. Does the article tell what the 2nd postulate says about the one-way, 2-clock light speed case?

2. Does the article dare to mention that the one-way, 2-clock light speed case remains open?

3. Does the article say that the one-way, 2-clock light speed case controls SR's results such as the Einsteinian transformation equations?

4. Does the article state clearly that no one has ever measured light's speed from Clock A to Clock B?

5. Does the article admit that Einstein's clocks are incorrectly related, so all of SR's 2-clock results are incorrect?

6. Does the article admit that no one has ever disproved the Galilean transformation equations (taking into account clock slowing and length contraction, which were added by Lorentz)?

7 Does said article explicitly state that light's one-way, 2-clock speed will vary with frame velocity if absolutely synchronous clocks were used instead of Einstein's absolutely asynchronous clocks?

I do not want to create some silly new article called "Cadwgan's Intepretation of what Einstein REALLY Said"; I DO want some sort of consensus here in TALK:SR about the current article's dismal failures.

Cadwgan Gedrych 19:54, 24 April 2006 (UTC)

Referring to the collaborative work of others as a 'dismal failure' is an inspired approach to achieving the consensus you desire. Please Don't be a dick Alfred Centauri 23:53, 24 April 2006 (UTC)

About the above points,

- Point 1: It was set by definition - in fact that convention was already commonplace, as Poincare indicated in an earlier article. A procedural definition isn't a postulate about physics, eventhough it appears that Einstein suggested so in his 1905 paper.

- Point 2: Regretfully the sources are either disagreeing or unclear. As this is an encyclopdia, we can't (not allowed!) do better. Still, according to http://arxiv.org/abs/gr-qc/0409105, Einstein reiterated in 1907 that "clocks can be adjusted in such a way that [OWLS] becomes [..] c". That's obviously not a postulate but a procedure, and it would be helpful if the article cites that.

- Point 3: If you mean the Lorentz transformations: they are indeed dependent on that convention, but SRT's results are not (read for example the above arxiv article to understand why).

- Point 4: covered by point 2.

- Point 5: "incorrect" is a misnomer for a definition.

- Point 6. That Lorentz developed the LT based on the GT can be found in the corresponding links.

- Point 7: It may be enlightening to mention the fact that in GPS the speed of radio waves relative to the (moving) satellite is not c, and that that in no way contradicts SRT.

Regards, Harald88 11:04, 25 April 2006 (UTC)

To Alfred Centauri:

If the facts show (and they do) that the article is a dismal failure, then I have every right (indeed, a duty) to state just that, so it is you who is being the dick, not to mention the snide remark previously made about me making my own little article.

Here is a clear-cut example of a failure:

Even though the 2nd postulate fully controls how Einstein's clocks are related temporally, and even though this given temporal relationship must control all two-clock measurements in SR (including light's one-way, two-clock speed), and even though the only difference between Lorentz and Einstein is how their clocks are related, the article does not tell us the difference between Lorentz's (and Galileo's) clocks and Einstein's. Can you, Alfred Centuri, do this? Or will you cop out by changing the subject?

To Harald88:

Thanks for trying, but no cigar.

Do you mean to flatly contradict Einstein's own mathematical statement (given above by me) that the absolutely synchronous clocks of classical physics yield a variable one-way speed for light?

RE your "Point 5: "incorrect" is a misnomer for a definition.":

Look, it doesn't matter if Einstein's clocks are set by definition or by a herd of howler monkeys; if his clocks are asynchronous, then they are asynchronous, and this renders all of their results incorrect, including light's one-way, two-clock speed, the special relativity transformation equations, the Einsteinian composition of velocities theorem, the relativity of simultaneity, etc., as I have mentioned over and over.

Can you prove that Einstein's clocks are absolutely synchronous? If not, then they are absolutely asynchronous, and all of special relativity's two-clock results are incorrect.

You can't get correct results from incorrectly related clocks.

Regards Cadwgan Gedrych 13:33, 26 April 2006 (UTC)

Please read the article on Relativity of simultaneity for an introduction to this subject. Next, in 1905 Poincare demonstrated that according to the new mechanics no "absolute speed" can be measured, from which follows that his light speed convention is the simplest solution. Harald88 20:01, 27 April 2006 (UTC)

Re "the difference between Lorentz's (and Galileo's) clocks and Einstein's": As I mentioned above, neither theory says anything about how the clocks themselves work locally.

Re "RE your "Point 5: "incorrect" is a misnomer for a definition."": You do not seem to understand that it's not determined a priori what simultaneity is for spacially displaced clocks.

Re "Can you prove that Einstein's clocks are absolutely synchronous? If not, then they are absolutely asynchronous": see above. "absolutely synchronous" does not have an inherent meaning.

I'm getting tired of this discussion (but I'm still interested in your explanation for the standing wave example). Icek 14:36, 26 April 2006 (UTC)

Icek, all will be clear only after you show how Einstein derived his simple equation w = c - v.

(This equation was the cause of special relativity, and it has zero to do with a standing wave.) Good luck! (And please note that I did not say that Einstein said that the equation was correct, just that he derived it.)

Cadwgan Gedrych 18:39, 27 April 2006 (UTC)

See above; and if you here refer to the closing speed formula, that is a straightforward and fundamental measurement rule (vector addition of velocities). IOW, there is nothing to "derive", relative velocities are defined as such in physics. Harald88 20:01, 27 April 2006 (UTC)

To repeat myself again:

[Quoting Einstein:] "w is the required velocity of light with respect to the carriage, and we have

w = c - v.

The velocity of propagation of a ray of light relative to the carriage thus comes out smaller than c.

But this result comes into conflict with the principle of relativity...." http://www.bartleby.com/173/7.html

Surely, Harald88, you have seen this Einsteinian statement, and just as surely, Harald88, you know that it is not a closing velocity, but is simply a direct, two-clock measurement of light's one-way speed, given in the context of a single inertial frame in classical (Galilean/Newtonian) physics.

Please, Harald88, do us all a favor, and show how Einstein derived that simple equation "w = c - v" if you can.

The payoff for such a small act would be huge: You will then fully understand Einstein's 2nd postulate!

Cadwgan Gedrych 19:01, 28 April 2006 (UTC)

I have already tried to explain to you here above that that relative velocity is a two clock measurement in which the speed of light has been "set" to be uniformly c relative to the frame of choice; and I do understand Einstein's 1905 derivation (there is a minor glitch of no relevance in it, which was discussed about one year ago (or two?!)) on sci.physiocs.relativity. And that is the place for such opinion discussions, not here. Harald88 22:09, 28 April 2006 (UTC)

Einstein, in VI of "Relativity" applies the (then standard) addition of velocities theorem to a man walking (with speed w) inside a train car (moving with speed v) to calculate the speed of the man relative to the embankment: W=v+w. Then, in the next chapter entitled "The apparent incompatibility of the law of propagation of light with the principle of relativity", Einstein employs the same theorem to light propagating at c relative to the embankment: w = c - v. (hmmmmmm - I'm still waiting for my huge payoff..................... nope - no payoff here). Look's like Einstein straightforwardly applied the Galilean addition of velocities theorem.

He then says that this is in conflict the the (restricted) principle of relativity. Why? Because, at the beginning of the same chapter, he refers to the 'simple' law of physics that light propagates at c in the vacuum. He also says this: "The assumption that the velocity of propagation is dependent on the direction 'in space' is in itself improbable." So, in 1916 when Einstein wrote this "popular exposition" as "A clear explanation theat anyone can understand", he non-rigorously explains that (a) the Galilean addition of velocities theorem yields a speed of light that can be essentially any value, (b) EM theory says the speed of light is c, (c) physical observations indicate this speed does not depend on frequency (color) or the motion of the emitting body, and (d) that the notion this speed is direction dependent is improbable. Do you believe that this 'laymans' relativity explanation should reveal to us that the 2nd postulate is whatever it is you think it is? Alfred Centauri 21:30, 28 April 2006 (UTC)

If I might throw my two cents into this debate, I might point out that it is possible to "measure" the speed of light in two different frames without needing synchronized clocks or such. According to the first postulate, "...the same laws of electrodynamics and optics will be valid for all frames of reference for which the equations of mechanics hold good", or more coloqiually all the laws of physics are valid in all inertial frames, including as insinuated in the above quote, the laws of electromagnetism, usch as Maxwell's Equations. By testing the strength of the electric and magnetic interactions, one can get (or confirm or whatever) the values of $\mu _{0}$ and $\epsilon _{0}$, the permeability and permittivity of free space, respectively. The speed of light in vacuum, however, is $c={\frac {1}{\sqrt {\mu _{0}\epsilon _{0}}}}$. This measurement does not require any two-way trips (or much of any trip whatsoever). Thus the speed of light is constant for two observers in inertial frames of reference, no matter how measured. Indeed, this is the reason why some people consider the second postulate redundant - it's included in some sense in the first one. DAG 23:10, 29 April 2006 (UTC)

None of you found the payoff because none of you told how the clocks were set during Einstein's derivation of his w = c - v. I, too, am getting tired of this discussion - it is not worth the effort - let Wiki remain clueless re the truth of the 2nd postulate.

Cadwgan Gedrych 01:07, 30 April 2006 (UTC)

Isn't it well known that the Galilean addition of velocities theorem is based on absolute, universal, no kidding, don't even think about changin, time? Is this the big payoff you've been alluding to? Bummer! I was hoping you actually had something of merit to bring to the discussion. Alfred Centauri 03:46, 30 April 2006 (UTC)

Pjacobi, Centauri: Neither of you hit the target, so the payoff eluded you both.

There is only one way to hit the target, and that is by simply telling us how Einstein's clocks are temporally related. A bonus can be won by then going on to tell us how Galileo's clocks' temporal relationship compares with Einstein's. Cadwgan Gedrych 19:14, 1 May 2006 (UTC)

You can read that below. If something is unclear, you can comment on that Talk page; and I'll add a link to it in this article. Harald88 20:13, 1 May 2006 (UTC)

Where below? Which Talk page?

Cadwgan Gedrych 13:19, 2 May 2006 (UTC)

Einstein synchronisation

Talk:Einstein synchronisation

Icek 21:24, 2 May 2006 (UTC)

Wouldn't it be much, much, much, much simpler if you simply copied and pasted the thing you claim is there to here? I read the article, and I did not see anything at all about how Einstein's and Galileo's clocks differ temporally in any given frame. And if you do find it, then it should be put into the 2nd postulate section because the 2nd postulate is precisely and only about how Einstein's time for two clocks differs from the classical 2-clock time. Cadwgan Gedrych 15:22, 3 May 2006 (UTC)

What are "Galileo's clocks"? Classical physics does not care about clock synchronization. Icek 02:06, 4 May 2006 (UTC)

To Icek: How do you think the Galilean transformation was derived unless Galileo's clocks were used with some sort of clock synchronization? This is elementary. Cadwgan Gedrych 18:48, 4 May 2006 (UTC)

As far as I know, Galileo did not really think of this to be a problem and implicitely assumed that information transmission speed is unlimited (though he thought about measuring the speed of light). Icek 20:11, 4 May 2006 (UTC)

To Icek: Did not think of what as a problem? Cadwgan Gedrych 18:08, 5 May 2006 (UTC)

Synchronization of locally displaced clocks. Icek 07:00, 6 May 2006 (UTC)

To Icek: Please tell us how clocks are temporally related when they are used to derive the Galilean transformation equations. Or, alternatively, tell us how Einstein's clocks are related when the SR transformations are derived. Then, and only then, will you see what is meant by "Galileo's clocks." Cadwgan Gedrych 19:36, 8 May 2006 (UTC)

Why should I tell you the obvious again? Because one cannot answer my simple question in a way that would be advantageous to your point of view? The only thing I see from this is that you are not able to, you don't want to, or you pretend to not understand the problem. As others pointed out, this is not the right place to talk about this. End of discussion. Icek 22:08, 8 May 2006 (UTC)

Einstein synchronisation

Please note that there is a small article, especially covering the problem of synchronization of clocks at different places and the related 1-way-LS vs 2-way-LS issue: Einstein synchronisation. It follows the treatment by Hans Reichenbach (obviously heavily condensed -- and not for the note reagrding GPS). A very thorough discussion can be found in his book "The philosophy of space and time", §19 and §20. --Pjacobi 12:37, 1 May 2006 (UTC)

The debate over second postulate

I was skimming through the long debate above over the wording of the second postulate. I feel it arises from a misinterpretation of how Wikipedia works. The wording of the second postulate in the article does not depend on which postulate is "right", but which postulate is sourced. WP:V, WP:NOR and WP:CITE tell us that we must attribute statements of controversial points to reputable sources. You can say "Einstein stated the second postulate thus, but recently researcher X disputed this in a paper published in a peer-reviewd journal(citation here)". You can not say "Einstein stated the second postulate thus, but editors after debating in the talk page of this article have arrived at the conclusion that Einstein was wrong." The debate can only be a purely academic one without any consequences to the article.

On the other hand, if a researcher has claimed a different version of the second postlate in a paper, then the claim should be included as a claim with proper citations irrespective of whether his claim can be said to be valid or not. Wikipedia is not the place to judge the merits of a scientific theory, we must only judge whether we cna provide sources for the theory. I hope the dispute is either resolved or continued with the understanding that its results will not have a bearing on the article. Loom91 06:01, 2 May 2006 (UTC)

Dear "Mr. Loom91," the debate has indeed been long, but it need not have been.

Anyway, since you seem to care a little, let's look back at the existing article's version of the postulate:

[quoting the article:]

"Most current textbooks mistakenly include a major derived result, that the speed of light is independent of the state of motion of the observer measuring it, as part of the second postulate. A careful reading of Einstein's 1905 paper on this subject shows that, in fact, he made no such assumption. The power of Einstein's argument stems from the manner in which he derived startling and seemingly implausible results from two simple assumptions."

Note that it begins by countering most current textbooks. So much for your point about using proper sources! Note further that it says that "A careful reading of Einstein's 1905 paper ...." Whose "careful reading" was this? Was it some Wiki editor's? What does this do to the cited source? Moreover, note that the word "assumptions" was used in relation to the second postulate, whereas Einstein stated that he was not making an assumption about the nature of light, but was merely giving a definition.

[quoting the article:]

"One of the most highly counterintuitive of these results (and, as stated above, commonly included in statements of the second postulate), is that the speed of light in vacuum, commonly denoted c, is the same to all inertial observers. An observer attempting to measure the speed of light's propagation will get the same answer no matter how the observer or the system's components are moving."

This omits the most important part of the second postulate. Einstein's version does not omit it. That important part is the fact that the speed of light between two clocks is given by definition, not measured.

Why would a given-up-front-by-definition "result" be "counterintuitive"?

Why does the Wiki article say "An observer attempting to measure the speed of light" when the speed was given up front by Einstein's definition?

Since Einstein's definition is the only difference between Lorentz's theory and Einstein's, it seems that no article about the second postulate (and special relativity) could rightly omit a direct comparison of Lorentz's and Einstein's clocks. How do they differ? It is impossible to fully understand either special relativity or the second postulate until one knows the physical difference between Einstein's clocks and Galileo's/Newton's/Lorentz's clocks (in the critical case of two same-frame clocks).

It may surprise you to hear that Einstein explicitly admitted that he could not measure time (in the case of two same-frame clocks), whereas Newton's/Galileo's/Lorentz's clocks could measure such time. But I saw no mention of this "perfectly-valid-source" fact in the Wiki article.

Cadwgan Gedrych 14:21, 2 May 2006 (UTC)

Cadwgan, I disagree with much of what you claim to be true above. Consider:

(1) The synchronization of clocks in a frame in which Newtonian mechanics is valid such that the speed of light is measured to be isotropic in this frame does not set the speed of light to c.

(2) The observed one-clock result that the closed path average speed of light is c is simply that, an observed result that suggests a principle.

(3) The postulate that the speed of light is c independent of the speed of the emitter when measured by the clocks in the frame so described in (1), while fixing the speed of light as c in this frame, does not imply that the speed of light is c in any other frame. However, it does imply (2) in this frame.

(4) The additional postulate that the changes of state in physical systems are not affected, whether these changes of state be referred to the one or the other of two systems of co-ordinates in uniform translatory motion, now fixes the speed of light at c in all coordinate systems that are related to the system defined in (1) by uniform translatory motion which clearly requires that clocks belonging to any of these other frames are synchronized according to (1)

(5) We now know that any synchronization procedure that is consistent with an invariant one-clock closed path average speed of light in any inertial coordinate system necessarily results in transformation equations that belong to a continous set of transformations with the Lorentz transformation at one end of the spectrum (OWLS absolute, simultaneaty relative) and the Selleri transformation at the other end (OWLS relative, simultaneaty absolute).

(6) Although the choice in synchronization procedure cannot affect the underlying physics, the Einstein synchrony choice (by design) leaves the form of Maxwell's equations unchanged in any inertial frame.

(7) There is no objective answer to the question of whether such information as in (5) and (6) rates a prominent position in this SR article but, IMHO, a link to a separate article is appropriate.

(8) Wikipedia doesn't need armchair editors. Make your changes already or go on about your mission elsewhere. Alfred Centauri 21:52, 2 May 2006 (UTC)

TO ALFRED: I am not being an armchair editor.

Your above is utterly useless, as far as I can see; for example, it failed to address these two important questions, which I now must repeat, thereby adding unnecessarily to the length of this discussion:

[1] Why would a given-up-front-by-definition "result" be "counterintuitive"? [2]Why does the Wiki article say "An observer attempting to measure the speed of light" when the speed was given up front by Einstein's definition?

Also, the article fails to tell us the difference between Einstein's and Newton's clocks, which is what the 2nd postulate is all about.

Moreover, you made no mention of this critical omission by the article: Einstein's explicit admission that he could correctly measure any two-clock times, including light's one-way, two-clock speed.

And why the heck are you talking for Loom91, anyway??? Where did he go!!!

And as for me going ahead and changing the article, don't you think that that would be silly with absolutely nothing having been resolved here? Cadwgan Gedrych 15:15, 3 May 2006 (UTC)

Cadwgan said "[1] Why would a given-up-front-by-definition "result" be "counterintuitive"?"

As I explained in my comments above, the result is not given by definition. This is our disagreement. You claim it is given by definition - I claim you are wrong.

Cadwgan said "[2]Why does the Wiki article say "An observer attempting to measure the speed of light" when the speed was given up front by Einstein's definition?"

You are repeating yourself so I will repeat myself. You are claiming it is given by definition and I claim that you are wrong.

Further, I claim that your are wrong when you say "...the difference between Einstein's and Newton's clocks, which is what the 2nd postulate is all about."

If you are waiting for a consensus before editing the article to reflect your claims above, then, IMHO, you might as well be on your way. I will revert any edit to this article that makes the claims you make above for the reason that these claims are POV.

If you wish to speak with Loom91 privately, use his or her talk page. Alfred Centauri 16:15, 3 May 2006 (UTC)

First, I will state my personal views regarding this debate. Then I shall hive my opinion about this article.

As I understand it, the statement that the propagation velocity of electro-magnetic waves is the same to all inertial observers is equivalent to the statement that it is the same irrespective of the motion of the emitting body, because a trivial Lorentz tramsformation of the inertial reference frame switches between the two. A textbook wouldn't be wrong unless it included BOTH these as postulates.

I also don't feel that Einstein DEFINED the velocity of light. He simply assumed the existence of one such velocity c invariant under inertial reference frame changes and then used this assumption to derive the mathematical expresssions for the Lorentz transformations. That this constant velocity is indeed the velocity of light comes from Maxwell's electro-magnetism. I do not understand what you mean by saying Einstein defined lights velocity and it can not be measured. The velocity of light can not be measured in meters or any units derived from meter because meter is defined in terms of c, but that has nothing to do with SR. Einstein does assume c to be a constant, but that doesn't stp an observer from performing experiments to actually verify that assumption. See Speed of light (FA).

Now for the article. I think we are to take On the Electrodynamics of Moving Bodies as the primary source of our information on the postulates of SR. Anything creative must be attributed. No saying second postulate is redundant or textbooks are mistakes without sourcing. I dug up an English translation of the paper and Einstein begins the paper by saying that c is indepent of the motion of the emitting bodies. We can't call the textbooks wrong, simply mention that they do not present the postulate in the form used by Einstein. Weasel words such as "a careful reading shows" have to go. If the second postulate is redundant, then that stement has to be attributed. In any case, what are the prefered versions of either party? Loom91 06:43, 4 May 2006 (UTC)

To Alfred Centauri:

I asked "Why does the Wiki article say 'An observer attempting to measure the speed of light' when the speed was given up front by Einstein's definition?"

You replied "You are claiming it is given by definition and I claim that you are wrong."

It is not my claim, it is Einstein's statement. Here, for your reference,

is that statement:

[From Section 1 of Einstein's 1905 special relativity paper:]

If at the point A of space there is a clock, an observer at A can determine the time values of events in the immediate proximity of A by finding the positions of the hands which are simultaneous with these events. If there is at the point B of space another clock in all respects resembling the one at A, it is possible for an observer at B to determine the time values of events in the immediate neighbourhood of B. But it is not possible without further assumption to compare, in respect of time, an event at A with an event at B. We have so far defined only an "A time" and a "B time." We have not defined a common "time" for A and B, for the latter cannot be defined at all unless we establish by definition that the "time" required by light to travel from A to B equals the "time" it requires to travel from B to A. Let a ray of light start at the "A time" ta from A towards B, let it at the "B time" tb be reflected at B in the direction of A, and arrive again at A at the "A time" t'a.

In accordance with definition the two clocks synchronize if

tb - ta = t'a - tb.

Did you see that not only is light's one-way, two-clock speed controlled by and given by the definition, but so are all other two-clock measurements in SR?

Here is Einstein's subsequent statement of his 2nd postulate, which shows how it is directly affected by the definition:

2. Any ray of light moves in the "stationary" system of co-ordinates with the determined velocity c, whether the ray be emitted by a stationary or by a moving body. Hence

velocity = light path/time interval

where time interval is to be taken in the sense of the definition in Section 1.

Did you notice that the time interval for light's one-way, two-clock speed in special relativity is controlled by the definition in Section 1?

Cadwgan Gedrych 18:27, 4 May 2006 (UTC)

To Loom91:

If light's one-way speed invariance were given by anything in nature, such as Maxwell's equations, then Einstein would never had had to use a definition to make it so.

Here is the relevant part of your cited source:

"One consequence of the laws of electromagnetism (such as Maxwell's equations) is that the speed c of electromagnetic radiation does not depend on the velocity of the object emitting the radiation; thus for instance the light emitted from a rapidly moving light source would travel at the same speed as the light coming from a stationary light source (although the colour, frequency, energy, and momentum of the light will be shifted, which is called the relativistic Doppler effect). If one combines this observation with the principle of relativity, one concludes that all observers will measure the speed of light in vacuum as being the same, regardless of the reference frame of the observer or the velocity of the object emitting the light."

Maxwell's c is not the c of the 2nd postulate because Maxwell did not use two clocks to measure light's speed, and yet the 2nd postulate pertains solely to light's speed between two same-frame clocks. Since this cannot be done sans some definition of clock synchronization, Einstein was forced to provide such a definition, and he did, as I pointed out above to Alfred Centauri.

Yes, as your source says, it is of course true that the principle of relativity implies that all frames will find Maxwell's speed to be c, but, again, this has nothing to do with the 2nd postulate, which, as Einstein said, requires a synchronization definition. Cadwgan Gedrych 18:41, 4 May 2006 (UTC)

Cadwgan: I've already explained why your claims are wrong. Restating your claims with quotes and all is unnecessary because I already understand why we disagree. Your claims are illogical and your conclusions do not follow from the quotes you've given. I've already stated why you wrong point by point based on the very same quotes you have used. This is the basis of our fundamental disagreement and there is no reason for any further discussion between us regarding this. Alfred Centauri 23:32, 4 May 2006 (UTC)

If I might throw another two cents in, after going back and reading On the Electrodynamics of Moving Bodies, I have to say that Einstein both stated that light "is always propogated in empty space with a definite velocity c which is independent of the state of motion of the emitting body", and apparently defined c in terms of the round trip speed. In regards to the first of Einstein's statements, it is this that he calls his second postulate. In regards to the second of his statements, regarding the round-trip speed of light, the last sentence in the sub-section taken up by it ("It is essential to have time defined by means of stationary clocks in the stationary system, and the time now defined being appropriate to the stationary system we call it `the time of the stationary system.'") that he was using the round-trip not to define the speed of light, but to use the already constant speed of light to define time intervals, and not the other way around. Basically, I believe the second postulate simply is the assumption that to all observers, no matter how it is measured, the speed of light is always equal to c, whether measured via Maxwell's equations, using two clocks, or using one clock and a reflected beam. This assumption was then plugged into the round-tip scenario to define time intervals for stationary clocks. At least, that's how I read it. DAG 15:26, 5 May 2006 (UTC)

You read it correctly. In fact, since the "two clock" experiment cannot be done without first synchronizing the clocks using a two-way exchange of signals, a case can be made that this "one-way two-clock" experiment is impossible to do (and ironically Cadwgan notes that impossibility below). However, scientific proof and/or refutation cannot occur on the basis of an experiment that cannot be done. So Cadwgan's concern is moot. --EMS | Talk 19:57, 5 May 2006 (UTC)

Although the round-trip, one-clock light speed case was closed experimentally (for all practical purposes) decades ago, the one-way, two-clock light speed experiment (with both clocks in the same frame, nonrotating, and not moving relative to each other in order to avoid any possible clock slowing differences) has never been performed, not even on paper. Unless and until you know exacly why this is so, you will simply never understand either SR or its postulates. Sorry, but I tells it the way it is! Cadwgan Gedrych 18:18, 5 May 2006 (UTC)

Is there any dispute regarding the article?

I see there is much debate on many fine theoritical points that frankly escape me. But is there any dispute as to what content should be in the article? Loom91 08:24, 5 May 2006 (UTC)

Let me put it to you this way: Cadwgan Gedrych wrote

If light's one-way speed invariance were given by anything in nature, such as Maxwell's equations, then Einstein would never had had to use a definition to make it so.

This demonstrates a total lack of understanding of special relativity and even of how theoretical physics works on the part of Cadwgan. After all, Maxwell's equations do suggest the invariance of the speed of light (c). Given that suggestion, one can construct a theory based on it, and that is what Einstein did. Cadwgan acts like Einstein created something unacceptably artificial, but in reality what Einstein did was to say "let us assume the principle of relativity and the invariance of c as emitted as a rule of physics, and see where it leads". The result is a theory that is highly consistent with observation, such that those observations prove the invariance of c.

For that reason, there is a consensus of editors that Cadwgan's concerns are unfounded and that the article is not in need of his proposed modifications, which is why he is belly-aching here. As for "disputes over what the content of this article should be": This is Wikipedia and there will always be someone who wants some content added or removed for whatever reason. In fact, I recently revised the "Physics in spacetime" section and am also wondering if that material shouldn't be elsewhere as it is higly technical and makes the overall article less accessible. None the less, the immediate issue is Cadwgan's complaints, and there is no question that his viewpoint is not appropriate for this article. --EMS | Talk 15:44, 5 May 2006 (UTC)

Cadwgan Gedrych is a well known crackpot/troll on the newsgroup sci.physics.relativity. His real name is Brian D. Jones. During the last 10 years he has been using the following false names:

• Cadwgan Gedrych
• 2ndPostulateDude
• SRdude
• Edward Travis
• Ron Aikas
• Roy Royce
• John Reid
• Martin Miller

Currently he is trolling the newsgroup under the name

• kk (Kurt Kingston).

To get an idea of his intelligence, see for instance: [9] —Preceding unsigned comment added by DVdm (talk • contribs) 16:23, 5 May 2006

In the first place, there is no proof of any of the above newsgroup-related claims, so there is no place for it here.

In the second place, my personal reputation matters not in any case; all that matters is the condition of the Wiki SR article, especially the second postulate part.

In the third place, I have already won one major case against the article's given condition, which involved removing the claim that the second postulate had been experimentally confirmed.

In the fourth place, as I just wrote in another section here, no one in Wiki or anywhere else can possibly understand the second postulate until and unless they can explain why the one-way, two-clock experiment has never been performed even though the round-trip, one-clock case was closed decades ago, along with the Maxwellian light-speed case.

In the fifth place, I have just proved that EMS is dead wrong about my understanding of SR by my just-given paragraph. (He/she wrote above: "This demonstrates a total lack of understanding of special relativity and even of how theoretical physics works on the part of Cadwgan.")

[To explain, because I know that an explanation is needed: The facts that the two other light speed cases (the round-trip, one-clock case, and the Maxwellian case) are closed whereas the one-way, two-clock experiment cannot be performed even on paper shows clearly that the latter case differs fundamentally from the other two, so only if this important fact is understood can anyone understand special relativity and its postulates.]

In the sixth place, let's please settle this simple second postulate issue so I can move on to another one, the case of the false claim that special relativity predicted the famous equation E = mc^2. Cadwgan Gedrych 18:43, 5 May 2006 (UTC)

I must agree that the above allegations against Cadwgan are not germane to this discussion, and even raise issues under WP:CIVIL. I hope that my point was pretty much made in my previous posting in this thread. However, on what Cadwgan said "in the sixth place", I rest my case. --EMS | Talk 19:39, 5 May 2006 (UTC)

Thanks for the fairness re the allegations issue, EMS. (FWIW, you can rest assured that I am not on any "troll" or "crank" list in the Newsgroups, and you can also take it to the bank that I have not lost a single argument re SR in the Newsgroups. You may also like to know that only one person - Dirk - is involved in the Newsgroup allegations.)

Now, as regards your above statement re my sixth place item, since it consisted of two parts, I am left to wonder to which of them you referred. I hope that it was the E = mc^2 part, because that is the only one that really matters here. Assuming that's the case, I would like you to prove that this equation cannot be derived sans special relativity. Cadwgan Gedrych 18:39, 6 May 2006 (UTC)

Above, EMS wrote

"After all, Maxwell's equations do suggest the invariance of the speed of light (c). Given that suggestion, one can construct a theory based on it, and that is what Einstein did."

It is demonstrably false that Maxwell's equations "suggest" one-way or even two-way (round-trip) light speed invariance or isotropy. The proof lies in the simple fact that even Maxwell himself, along with all other physicists of the day, firmly believed in a positive result for the Michelson-Morley experiment - despite the existence of Maxwell's equations. In other words, these equations have nothing to do with coordinate (ruler/clock) measurements of light's speed. Cadwgan Gedrych 18:57, 6 May 2006 (UTC)

Please stop feeding the ...

From SRdude profile on google groups: cadwgan_gedr...@yahoo.com

From SRdude on sci.physics, Dec 16, 2004:

.................Simple Math Derivation Needed..................

Even after deliberately disregarding all preceding and subsequent words in the book, the fact remains that Einstein at one point definitely derived the very simple equation w = c - v.

Here are his own words: "w is the required velocity of light with respect to the carriage, and we have w = c - v. The velocity of propagation of a ray of light relative to the carriage thus comes out smaller than c."

---

http://www.bartleby.com/173/7.html---------------------

All I am looking for is one brave soul who will show this derivation.

From Cadwgan Gedrych in this talk page:

[Quoting Einstein:] "w is the required velocity of light with respect to the carriage, and we have

w = c - v.

The velocity of propagation of a ray of light relative to the carriage thus comes out smaller than c.

But this result comes into conflict with the principle of relativity...." http://www.bartleby.com/173/7.html

From kk on sci.physics Feb 2, 2006:

Einstein said that the clocks of classical physics would obtain the value w = c - v for light's one-way speed (departing light ray). Einstein's clocks, on the other hand, obtain the value c. Therefore, the two sets of clocks differ.

How do Einstein's clocks differ from those of Newton and Galileo?

You cannot understand the basics of SR without knowing the answer to the above question.

From Cadwgan Gedrych in this talk page:

Since Einstein's definition is the only difference between Lorentz's theory and Einstein's, it seems that no article about the second postulate (and special relativity) could rightly omit a direct comparison of Lorentz's and Einstein's clocks. How do they differ? It is impossible to fully understand either special relativity or the second postulate until one knows the physical difference between Einstein's clocks and Galileo's/Newton's/Lorentz's clocks (in the critical case of two same-frame clocks).

From Cadwgan Gedrych on sci.physics.relativity Mar 31, 2003:

One-way light speed invariance cannot be a law of nature. Thus, one-way light speed invariance cannot be a scientific postulate. And since SR is based solely upon Einstein's "postulate" of one-way invariance, SR is not a scientific theory.

(My emphasis added).

I dunno EMS, I'm of the opinion that Dirk's assertion regarding Mr. Gedrych, in conjuction with the above, are germane to this discussion. I for one cannot assume good faith in the case of Cadwgan Gedrych. I do not believe that Mr. Gedrych is interesting in improving this article but is instead interesting in disproving the subject of this article. Alfred Centauri 20:40, 5 May 2006 (UTC)

(Note: This response was added after this page was archived.)

I don't see that Dirk's comments are needed to reach the conclusion that Cadwgan is trying to disprove the subject. Cadwgan's comments in the talk page (now archived) more than suffice to make that point. --EMS | Talk 03:00, 7 May 2006 (UTC)

In this context, I'll add a link in the references to the article (apparently published in Foundation of Physics) [10]; that saves a long story as it can be freely downloaded by everyone. Harald88 00:20, 6 May 2006 (UTC)

This article was indeed published in the December 2004 issue of 'Foundations of Physics' (Vol. 34, No. 12). Alfred Centauri 02:15, 6 May 2006 (UTC)

Reply to Alfred Centauri:

Why not invite Dirk into this Wiki discussion? If you would look a little closer at his Newsgroup posts, you would see that he has rarely discussed physics, but merely feeds his obsession for fighting anyone who he feels does not see SR as he does, which is merely mathematically, and not physically.

Contrary to Centauri's belief, I am working in perfectly good faith here, just as I did in the Newsgroups. And that is why I have never once lost a single SR argument there, probably much to Dirk's chagrin.

My good faith re the article demands that I properly present my case to Alfred Centauri. To that end, I proffer the following challenge to him/her:

I had written somewhere above:

Why does the Wiki article say "An observer attempting to measure the speed of light" when the speed was given up front by Einstein's definition?

Here is my challenge:

Will Centauri please show how an observer can attempt to measure light's one-way speed between two clocks that are in the same frame?

(Please start from scratch, using two ideal atomic clocks still in their shipping crates, and show all pertinent steps. Thanks!) Cadwgan Gedrych 19:19, 6 May 2006 (UTC)