Talk:Relativity of simultaneity/Archive 2

Archive 1

Archive 2

Train and lightning flashes

Einstein's example of a train and embankment and lightening flashes, which is in his popular exposition "Relativity: the special and general theory" but not the 1905 paper, is in my opinion a better illustration than the thought experiment in the article. In Einstein's example lightening hits each end of the train when each end of the train coincides with opposite ends of the emabankment. An observer on the embankment, mid-way between the ends of the embankment, sees the flashes simulataneously, and therefore calculates that the lightening strikes were simultaneous (each lightening strike occured at an earlier time than the moment he sees it of course). An observer in the middle of the train sees the lightening strike the front of the train before the strike on the rear of the train, and concludes the events were not simultaneous (he calculates each happened at an earlier time than the moment he saw them, of course). Each observer is equi-distance from the events and therefore does not have to calculate time of travel of the image from the event to his eye to know if they were simultaneous or not. E4mmacro 07:57, 12 September 2006 (UTC)

I don't see where any more claculation is needed in the given example than in Einstein's. Also, the current text explicitly connects the movement of the train wrt the platform to the differences in the perception of simultaneity. That is the key point, which is clearly stated and well illustrated. It is not Einstein's example, but I see little need to slave Wikipedia to the historical demonstration of the relativity of simultaneity. This approach works very well, and I call for it to be retained. --EMS | Talk 14:48, 12 September 2006 (UTC)

One problem with the explanation as given on the main page is this: if someone who knew little about relativity read it they might naturally think, "How does the guy in the train know that the light flashes reached the ends of the train simultaneously? He wasn't at the ends of train when the flashes reached those ends. He just said they arrived simultaneously, because he assumed the light speed was the same in either direction, so it looks merely like a definition. And how do we know it isn't a wrong definition?" The answer might be that he waits for the image of the flashes to get back to him (in the middle of the train) and finds they do indeed reach him simultaneously (we know this will happen from the MM experiement, don't we?). But this answer isn't given.

It could be that Einstein's example, which considers only the return journey of the light (from the ends to the middle) suffers in the same way, but it seems less obvious.

I think maybe Cleonis was right: it seems difficult to justify what looks merely like a convention, without mentioning something more, like length contraction and time dilation. E4mmacro 05:22, 13 September 2006 (UTC)

I call for adoption of Einstein's example: the current one has a logical flaw as you point out (relativity of simultaneity comes in earlier), and it should not be up to the reader or editors to unnecessarily consider the correctness and value of other examples that differ from the published and well known ones - especially not in an encyclopedia.

Harald88 07:14, 13 September 2006 (UTC)

The first job of an encyclopedia is to communicate. If a novel exercise, wording, and/or illustration can do so more effectively than other established means, it should be used. Note that WP:OR applies to concepts not wordings. In fact, the use of established wordings can violate copyright laws and rules! So as long as we are documenting an established concept, we have a fair amount of latitiude on how we do so.

My suggestion is to precede the current exercise with a description of clock synchronization and time determination in relativity. After all, how an event's time is determined actually is done by a convention! (Given that c is universally the same for all observers, $t=t_{obs}-d/c$ where d is the distance to the event and $t_{obs}$ is the time the event was observed is one way of setting the time of an event.)

It also may be an idea to also use Einstein's exercise, but I assure everyone that it will be easier to understand when placed after the current example and its explanation. --EMS | Talk 15:45, 13 September 2006 (UTC)

I still think that his version, even if slightly truncated /simplified, would be easier to digest as it's easier verifiable. For that matter, we discovered a minor error in your presntation which is a direct consequence of the different variant that you present. The problem is similar as with the Twin paradox: every new variant that is kooked up on the spot requires new verification by other editors and readers alike.

However, a lot of dust has settled since the last time that I read Einstein's illustration/exercise; thus it's time for me to have a fresh read of it before I can express my opinion about how difficult it is. Harald88 21:05, 13 September 2006 (UTC)

I just looked at the exercise (from "Relativity: The special and general theory" by A. Einstein, chapter 8). Personally I think that is needs to be much better illustrated than was done in that book. I also do not think that it is as clear or effective on its ovn as the current example is, but its differences make it an effective way of drumming the basic point into the reader when used in conjunction with the current example. --EMS | Talk 21:53, 13 September 2006 (UTC)

I Now also looked at it. His discussion with its illustration is a perfect follow-up of the earlier dicussions with illustrations by Poincare and just as easy to understand; I don't see a justification to invent our own variant. That doesn't mean that we can't use our own words of course.

To recall for others, the essence of his example is - and I quote:

Are two events (e.g. the two strokes of lightning A and B) which are simultaneous with reference to the railway embankment also simultaneous relatively to the train?

Einstein's example is in fact simpler and less tricky than the following variant that EMS chose:

A flash of light is given off at the center of the train

The observation-at-a-distance problem is not well explained in that version. Harald88 18:00, 16 September 2006 (UTC)

I just corrected that version to the encyclopedic version when incompatible space-time diagrams were put in. To keep it simple I now park the fully corrected version here, and changed it into a compromise in the article. Harald88 11:50, 1 October 2006 (UTC)

Einstein's thought experiment:

Lightning flashes strike two ends of the train just when the two observers pass each other. According to the observer on the platform, the flashes occurred simultaneously because he receives the flashes at the same time. This is based on the assumption (or, as Einstein emphasized, the stipulation) that light speed is the same in all directions. On the other hand, the observer in the train is moving toward the point at which the front flash was given off, and moving away from the rear flash. This means that the light flash coming from the front will have less distance to cover toward the train observer compared to the distance the flash needs to cover that is coming from the back of the train. Consequently, the flashes do not reach the train observer simultaneously. Assuming that the speed of light is a constant relative to the train, the train observer must conclude that the lightning flashes did not strike the ends of the train simultaneously. Harald88 11:50, 1 October 2006 (UTC)

From the beginning, I have not understood how this train experiment is logical. The person on the train knows that he is moving. Therefore, he knows when he sees the lights that his perceptions of when they originated are inaccurate. On the other hand, the man on the ground is not moving. If light travels constantly, and he perceives the light striking his eyes at the exact same time, then the lightning strikes must have originated at the same time.66.57.27.39 01:24, 4 April 2007 (UTC)

How does the person on the train know that he is moving? Or, what is equivalent, how does the person on the platform know that he is not moving? (This is a trick question; if we pick the sun as a stationary point, we will find that the he is moving around quite a bit.) The basic axiom of special relativity is that there is no way to distinguish between inertial frames of reference; there is no one thing we can point to and say, "That thing is stationary." There is only relative motion. The perceptions of the man on the train are not inaccurate, because, as the Michelson-Morley experiment demonstrates, any observer who attempts to measure the speed of light, in any direction, will find it to be c. This is why we can't say the observer on the train is "wrong". --Ian Maxwell (talk) 00:45, 10 December 2008 (UTC)

More removed material

I'm guessing that this section is supposed to be talking about the Bell inequality? This is not a good summary of the Bell inequality - it's misleading enough that without citations for which scientists, exactly, believe what, I'm removing it to the talk page.

A number of scientists have pointed out that some of the predicted consequences of quantum mechanics appear to be incompatible with relativity of simultaneity — and even causality — because of the nonlocality of quantum measurement. This issue is not yet understood by physicists.

No, the Bell inequality is only related to it. It's talking about the consequence of quantum non-locality. As mentioned above, causality isn't necessarily related so I'll delete that part. I'll also include the reference that I mentioned above on this talk page, as it gives a good overview of opinions. Harald88 22:55, 12 September 2006 (UTC)

Development of the concept ROS

I will now put back the more relevant parts of the removed history sections. While abbreviating the old material, I came across the following statement:

"In particular Poincaré pointed out that if the rate of time is in some way dependent on location in space or dependent on velocity relative to the luminiferous ether, then the classical assumption that an newtonian absolute simultaneity is accessable to measurement, would have to be abandoned. "

That would be very interesting to put in the article if indeed it can be verified. Does anyone if indeed (and where!) Poincare pointed that out?

Harald88 12:53, 1 October 2006 (UTC)

2. The constancy of the speed of light detected in empty space, independent of the relative motion of its source.

Is this really one of the postulates of Special Relativity? I was taught that, whatever Einstein said at the time, the constancy of the speed of light was a consequence of Maxwell's equations and a result in the Michelson Morley experiment. The actual (i.e. significant) postulate of Special Relativity is that nothing can travel faster than the speed of light. —The preceding unsigned comment was added by Sante Sangre (talk • contribs) 23:28, 13 March 2007 (UTC).

Connection with set theory?

See Talk:History_of_special_relativity#Connection_with_set_theory? --Pjacobi 16:23, 23 April 2007 (UTC)

Train Thought Experiment Description Seems Cloudy

I'm sure this is well-trodden ground, but as the examination in the article makes reference at the outset to special relativity, it seems that the description of the famous train thought experiment (in its modified form) loses the essence of the argument and perhaps needs clarification. I saw some mention previously of time dilation, etc., so I'm probably beating a dead horse here, but I would reword the example as follows per my understanding of Einstein's original intent:

---

A popular picture for understanding this idea is provided by Einstein's thought experiment consisting of a moving train with one observer midway in the train, and another observer midway on the platform as the train moves past. Here we will present a slightly modified version.

A flash of light is given off at the center of the train just when the two observers pass each other. The observer on the train sees the front and back of the train at fixed distances away from the source of the light flash, and these points are at rest with respect to this train-bound observer (that is, front, back, and train observer are all in the same inertial frame). According to this observer, the light flashes reach the front and back of the train at precisely the same instant of time — that is, simultaneously.

On the other hand, the observer on the platform sees the back of the train moving toward the point at which the flash was given off, and the front of the train moving away from it. This means that the light flash going toward the back of the train will have less distance to cover than the light flash going to the front. As the speed of light is the same in any direction relative to the platform (regardless of the motion of its source), the flashes will not strike the ends of the train simultaneously.

This is the key image in understanding this concept, so let's move through it in more detail. A strobe light in the middle of the train pulses once, emitting light in all directions. Some of this light begins its journey towards the back of the train, and some of it heads towards the front, but the source is the same. To the platform observer, the back of the train closes some distance between the original source point (not the current position of the source device itself, which will move) before the light strikes the back of the train. To the train observer, however, the light appears to have traveled the exact distance from the source to the back of the train, and no less. Conversely, the platform observer sees light cover a greater distance to the front of the train, in less time but at the same speed observed by the train observer. For both observers, the speed at which the light traveled is constant, but the distance traveled (and thus the time consumed in covering the distance) varies depending on the relative motion of the observer.

For the train observer, light took the same amount of time to move to the front as it did to the back (thus, light struck front and back simultaneously). For the platform observer, light moved more slowly toward the back than towards the front; thus, the light strike events could not have occurred simultaneously.

Now, we might try to decide if one observer is right and the other wrong. However, Einstein's other assumption is that the two inertial frames (the platform vs. the moving train) must operate under equivalent physical laws. This means that neither one can be shown wrong, and that the concept of simultaneity is not relevant outside one's own inertial frame.

---

Some notes - - removed phrases that seems to doubt the constancy of the speed of light - Extended example to broach the time dilation issues, as this stuff (RoS) really doesn't make much sense without it. - "Einstein's other assumption is that the two observers are equivalent" - I think you mean "two inertial frames obey the same laws of physics," so changing thus - To say that "simultaneity is simply relative" is imprecise. Rewording. Still not very happy with it - "It is also important to note..." - no, if we assume a single light source (thus the strobe in the center) - Removing up/down comments, as they serve to muddy the waters -- this line of thinking has its own problems (light traveling straight for one observer vs. diagonally for the other, etc) —The preceding unsigned comment was added by MxHyway (talk • contribs) 06:32, 26 April 2007 (UTC).

Einstein's example is better -reprise

I know this has been over before, but Einstein's version of the train experiment is much better, in my view. And you can draw the space-time diagrams for it if you want. The lightening hits the ends of the train, and the ends of the platform, leaving charring evidence on train and platform. The images reach the platform observer simulataneously, but reach the train observer non-simultaneously. Since the speed of the signals is the same for each observer, the lightening strikes were simultaneous for the platform observer, non-simultaneous for the train observer.

That should be enough for anyone to see for themselves that simultaneous events for one observer are not necessarily simultaneous for another, provided each knows or assumes that the speed of light as isotropic in her or his own reference frame. But one can go on, since the new-comer might suspect there is something wrong with the assertion that the speed of light is the same for each observer, or suspect this is a definitional thing (i.e. take this as a demonstration that the signal-speed is not the same for each observer). E4mmacro 09:17, 11 May 2007 (UTC)

The only slight problem is that you have to start the description from the platform POV, but this is exactly the way a new-comer, someone trying to learn this, thinks. So it has the benefit of pointing out a consequence of that thinking.

How about this:

First imagine that everywhere along the platform and along the train there are detectors which will only activate if they receive the image of (signal from) the lightening flashes from both directions simultaneously. Thus we can see absolutely the point at which the two images arrive simultaeously, marked by one platform detector and one train detector which both fire simultaneously, at the moment both signals reach them, the moment both detectors are at the same point in space. For later reference, call the moment the detectors fire (the moment the signals coincide) the zero moment (t = 0).

Before time zero, the centre of the train and the centre of the platform coincide. The two observers pass each other. A short time later (when the centre of the train no longer conicides with the centre of the platform) the signals reach the centre of the platform simultaneously (time zero). Both observers agree that the signals reached the centre of the platform simultaneously, and also reached a point off-centre, towards the rear of the train, at the same moment.

The platform guy says the lightning strikes must have been simultaneous, and occured at time t = -L/c, i.e. before he saw them (L is half the length of the plaform).

The train guy calculates that the lightning struck the rear of the train at time -(L-D)/c, where D is the distance off center of the detctor, and L is the half-length of the train. He calculates that the lightning struck the front of the train at time t = -(L + D)/c. The front strike was first. The strikes were non-simultaneous, from his POV.

Now you could go on the explain that L and D are measured in the rest frames of each obsever, and show what the t values are, and that the Lorentz transformation resolves any seeming paradox, but the above is RoS demonstrated, very clearly, in my view. E4mmacro 21:43, 7 May 2007 (UTC)

Hi I have not changed my view that this encyclopedia should stick to the most standard descriptions as found in the literature when there is no good excuse to do otherwise. I think that here no excuse exists to strongly deviate from Einstein's well-known example. The fact that most people have heard of it in the past will make it easier for most readers to follow it, as refreshing the memory is easier than replacing it with something that could confuse them. Your version looks OK to me, but I haven't followed te discussions for a long time. Is there any reasonable argument against it?

Harald88 21:27, 9 May 2007 (UTC)

Hi. I am not sure if you think it should be changed or left as it is. I haven't been following the discussions. E4mmacro 20:17, 11 May 2007 (UTC)

I thought that Einstein didn't say absolute simultaneity did not exist but only that it could not be measured. From this it follows that anything that can be observed by two observers located in the same place at the same (relative) time are absolutely simultaneous regardless of their relative velocities. And I've seen that disputed in some discussions in this matter. WFPMWFPM (talk) 16

18, 2 October 2008 (UTC)

spurious claims

I removed the following spurious passages from the end of the article:

"Lorentz(1920) was clear that, after the formulation of special relativity there was no need to invoke any reference to an aether to explain simultaneity and other results of relativity theory and that relativity theory was the correct interpretation of these phenomena. and simultaneity is due to Minkowski space and there is no property of the aether that needs to be taken into account. "

Clarification: Lorentz stated explicitly that he kept valuing the ether concept, and also Einstein came back to some kind of an ether concept. Thus the above remark is misleading. Similarly, the claims that "simultanetity is due to Minkowski space" can be an opinion of certain people that may be cited in the article on that topic; and the claim that "aether has no property that needs to be taken into account" is at odds with the opinions of for example Lorentz, Einstein and Langevin. Such expressions of faith about a metaphysical topic are inappropriate for Wikipedia, and it is not essential for this article. Harald88 20:37, 14 June 2007 (UTC)

I have partly reverted the phrase with a few minor corrections. The section is about the evolution and history of the concept of relativity of simultaneity. In this regard the section ends where special relativity provides the framework in which the aether is finally abandoned. You stopped it early.

About your phrase "Lorentz stated explicitly that he kept valuing the aether concept":

In 1927 Lorentz wrote:

"As to the aether (to return to it once more), though the conception of it has certain advantages, it must be admitted that if Einstein had maintained it he certainly would not have given us his theory, and so we are very grateful to him for not having gone along the old-fashioned roads." -- H. A. Lorentz, "Problems of Modern Physics; a course of lectures delivered in the California Institute of Technology," Edited by H. Bateman, Ginn_, 1927.

About your phrase "Einstein came back to some kind of an aether concept.":

In 1934 Einstein wrote:

"Physical space and the aether are only different terms for the same thing: fields are physical states of space. If no particular state of motion can be ascribed to the aether, there do not seem to be any grounds for introducing it as an entity of a special sort alongside space." -- Einstein in "Mein Weltbild" (Amsterdam, Querido, 1934)

DVdm 21:32, 14 June 2007 (UTC)

That section was simply about the historical development of the term "relativity of simultaneity". The 1927 reference confirms "once more" that for Lorentz the ether concept had certain advantages. Evidently your interpretation of what that sentence means differs from mine. One of the advantages of the ether concept is that it explains relativity of simultaneity but he did not discuss that topic there. Further, the topic was not relativity of simultaneity but most probably he meant GRT. Furtheron it is eroneous to claim that Minkowski space has no ether property since its main properties such as preference for inertial frames and the wave nature of light are conserved in Minkowski space.

That is highlighted by Einstein's 1934 quote which is excellent and matches his explanations of 1920 - it shows how much the opinions of Lorentz and Einstein had converged.

Thus I'll replace your thoughts about what Lorentz thought by your quotation of Einstein 1934. Harald88 08:46, 17 June 2007 (UTC)

Okay, no problem. I also added Lorentz' quote since it clearly shows how the aether was already regarded as "old-fashioned" some 80 years ago by a notable leading physicist who at first had been advocating the concept and its physical significance so intensively.

DVdm 10:11, 17 June 2007 (UTC)

He did not state that the concept is old-fashioned; anyway, adding the exact quote looks fine to me. :-)

Harald88 10:31, 17 June 2007 (UTC)

Michelson-Morely was the determinant of the "subluminiferous ether", not SR. 72.228.150.44 (talk) 13:13, 28 January 2009 (UTC)

XKCD links here!

http://www.xkcd.com/514/ Finally Wikipedia is getting famous. --Fixman^{Praise me} 04:09, 10 December 2008 (UTC)

simultaneity and quantum entanglement

There's an article in the current issue of SciAm exploring the implications of quantum entanglement and simultaneity in relation to special relativity. It might prove useful in adding material to this article. 76.66.201.179 (talk) 14:08, 11 March 2009 (UTC)

Great article!

Congrats to those who contributed to this article...it really does a great job of including all the complexities, but a the same time making it easy to understand. --1sneakers6 (talk) 09:55, 19 April 2009 (UTC)

Yes, I agree. --D.H (talk) 16:11, 24 April 2009 (UTC)

simultaneous hyperplane

One of the invariants of a Lorentz transformation is the relation between a time-line and the spatial hyperplane of simultaneous events associated with the zero of the time-line. A LT moves the time-line to a new one and at the same time moves the simultaneous hyperplane to a new hyperplane of events simultaneous with the zero of the new time-line. This property of LTs is as important as is its preservation of spacetime interval lengths. To show relativity as an advance of science one must show that the old order broken down is replaced by an improved model. Especially in reference to simultaneity there is a challenge to demonstate a better picture with relativity. So far in WP there are indications of the new structure at Minkowski space#Causal structure and hyperbolic orthogonality. Congradulatory remarks aside, this article does not yet present relativity as an improved viewpoint with reference to the modern understanding of simultaneity. Some bold editing is necessary to bring the science into this article.Rgdboer (talk) 23:12, 6 September 2009 (UTC)

Simultaneity is currently a disambiguation page. That would be a great place to discuss the various concepts of simultaneity, with links from summaries here to there. Paradoctor (talk) 12:39, 22 November 2009 (UTC)

Relativity of Simultaneity (Experiment)

Let a remotely controlled spaceship is moving close to the speed of light [say 0.9c] relative to stationary observer on asteroid. Onboard observer sends a signal (pulse) with the help of a remote control device from the back of ship to it's front Thus

For onboard observer: A pulse has already arrived at the front and a spaceship has stopped just in front of asteriod while

For asteriod's observer: A signal (pulse) will still be moving inside the moving ship due to its high speed and yet to arrived at the front

Thus its little confusing

As spaceship has already stopped in front of asteriod for onboard observer but for asteriod's observer the stopping signal is not yet arrived at the front therefore would a ship be still moving relative to observer on asteriod? [Special:Contributions/68.147.41.231|68.147.41.231]] (talk) 01:29, 30 March 2011 (UTC)Khattak#1-420

simultaneous hyperplane

One of the invariants of a Lorentz transformation is the relation between a time-line and the spatial hyperplane of simultaneous events associated with the zero of the time-line. A LT moves the time-line to a new one and at the same time moves the simultaneous hyperplane to a new hyperplane of events simultaneous with the zero of the new time-line. This property of LTs is as important as is its preservation of spacetime interval lengths. To show relativity as an advance of science one must show that the old order broken down is replaced by an improved model. Especially in reference to simultaneity there is a challenge to demonstate a better picture with relativity. So far in WP there are indications of the new structure at Minkowski space#Causal structure and hyperbolic orthogonality. Congradulatory remarks aside, this article does not yet present relativity as an improved viewpoint with reference to the modern understanding of simultaneity. Some bold editing is necessary to bring the science into this article.Rgdboer (talk) 23:12, 6 September 2009 (UTC)

Simultaneity is currently a disambiguation page. That would be a great place to discuss the various concepts of simultaneity, with links from summaries here to there. Paradoctor (talk) 12:39, 22 November 2009 (UTC)

Relativity of Simultaneity (Experiment)

Let a remotely controlled spaceship is moving close to the speed of light [say 0.9c] relative to stationary observer on asteroid. Onboard observer sends a signal (pulse) with the help of a remote control device from the back of ship to it's front Thus

For onboard observer: A pulse has already arrived at the front and a spaceship has stopped just in front of asteriod while

For asteriod's observer: A signal (pulse) will still be moving inside the moving ship due to its high speed and yet to arrived at the front

Thus its little confusing

As spaceship has already stopped in front of asteriod for onboard observer but for asteriod's observer the stopping signal is not yet arrived at the front therefore would a ship be still moving relative to observer on asteriod? [Special:Contributions/68.147.41.231|68.147.41.231]] (talk) 01:29, 30 March 2011 (UTC)Khattak#1-420

Readability suggestions

Hi. It seems to me that the phrase "Einstein's train thought experiment", while not incorrect in any way, is somehow awkward. Honestly I can't recall the last time I saw three words in a row that are simultaneously nouns and verbs. For ease of reading, can we improve it? This is the sort of thing hyphens were invented for, but a reordering might suffice: how about "Thought experiment: Einstein's train"? Rschwieb (talk) 17:42, 13 April 2011 (UTC)

Which relativity do we mean?

The following are not the same. The relativity in the Intro does not involve motion, rather, it has ancient Greek origins:

According to the special theory of relativity(??), it is impossible to say in an absolute
sense whether two events occur at the same time if those events are separated in space.

The Caption and the rest of the entry is special relativity, as in On the Electrodynamics of Moving Bodies:

Events A, B, and C occur in different order depending on the motion of the observer. 

BlueMist (talk) 11:38, 16 October 2011 (UTC)

This being a physics article —not a philosophy (or other) related one— it uses relativity in the sense of what is explained in the article Special relativity. That is the reason why, in the lead, the string "special theory of relativity" is wikilinked to that particular article. - Cheers - DVdm (talk) 11:51, 16 October 2011 (UTC)

Hi. Philosophy has many types of relativisms, some (but not all) of which you can read about in the IEP article http://www.iep.utm.edu/cog-rel/ .

Physics has three types of relativity: due to relative 1)position 2)velocity (change of position over time) 3)acceleration (change of velocity over time).

If you reread the Intro carefully, you'll see that it talks about 1), which is correct, but is not properly referenced from any physics book. The rest of the entry is about 2) and Einstein's Special Relativistic extension to Galilean-Newtonian relativity. There are also 3) General Relativistic effects, due to relative acceleration. BlueMist (talk) 13:29, 16 October 2011 (UTC)

If you have a careful look at General relativity, you'll notice that general relativistic effects are due to gravitation, not to relative acceleration. The latter can perfectly be described within the framework of special relativy — see, for instance, Time dilation#Time dilation at constant acceleration and this little example. Anyway, this is off-topic on this article's talk page. DVdm (talk) 14:57, 16 October 2011 (UTC)

This is indeed a very confusing point. Some textbooks are misleading. It ought to be explained somewhere. Roger (talk) 15:14, 16 October 2011 (UTC)

Position of observers

I find the statements on the position of the observers to be unhelpful in this article. For the thought experiment to be valid, the observer on the train does not need to be in the centre of the carriage, nor does the the flash need to be emitted the moment the train passes the observer on the platform. Observations of time differences between two events depends on relative motion of observers, not on their exact position at any given time.

I would remove these statements about observers positions. They tempt the reader to start thinking about visual perception of the events (thinking about time it takes photons to get from events to them, which is _not_ part of the argument) rather than the _observation_ of events which is independent of location. I made this edit, but it was reverted - "Undid revision 447223181 by 137.111.13.200 (talk) undid removal of essential information) (undo)" — Preceding unsigned comment added by 60.225.54.95 (talk • contribs)

The text says "The observer onboard the train sees the front and back of the traincar at fixed distances from the source of light and as such, according to this observer, the light will reach the front and back of the traincar at the same time." That can only happen if this observer is midway inside a speeding traincar, which is a word that you removed in your edit. The other part that you removed ("... just as the two observers pass each other") is also essential in the setup. This thought experment is set up, precisely to show what happens when the two observers assume that light speed is isotropic for both of them. As such you had removed essential information. Note that this setup is extensively described in the literature (see, for instance here), and changing something to that setup would not only amount to inducing erroneous information to the article, but also, and perhaps even more importantly, to wp:original research. DVdm (talk) 12:12, 31 October 2011 (UTC)

I believe you are making a common misunderstanding about the meaning of an observers perception of events. The observer on the train 'sees' the two ends of the train at fixed distance from the light source wherever the observer stands. These distances are of course not changed if the observer stands in a different place in the car. In your statements you seem to considering the visual perception of the scene by the observer (involving photons coming to his eyes from events - these consideration play no part in this argument (nor any other dilation/contraction experiments) as can be seen by the fact that in the train diagrams that depict the two frames, there is no indication of where the observers are, only an understanding of their relative motion that sets the frame in which light travels isotropically out at c for each case. In this article there are several unfortunate reference to observers 'seeing' things. In better texts there is a more careful language of 'observations' that try to avoid making people think of photons travelling from events to the observer. "This thought experment is set up, precisely to show what happens when the two observers assume that light speed is isotropic for both of them" - this is correct, but still not dependent on the 'location' of the observers, just their relative velocity.

The page you refer to here is a slightly different thought experiment in which the specified location of the observers is used to simplify the argument (but is not essential to the physics).

I'll make another argument to support this view. Think of there being several observers on the train, at different places on the train. They are all in the same inertial frame, and so they all agree that the light hits the ends of the train at the same time, as they agree on the time of all events. Just consider the Lorentz transformation with v=0 if you disagree with this statement. If you still disagree, then I think you are confusing time of visual perception of events including light travel time from the events to the observer, with the 'observed' time of events, for which this is accounted for. (Observers at different distances from a lightening strike see and hear the lightening and thunder at different times, but they all agree that the events themselves were simultaneous.) So, all observers on the train agree the events are simultaneous wherever they are standing. All observers on the platform agree that the events are not simultaneous wherever they are standing, and they agree on the delta-t between the events. Thus is cannot matter which specific observers we choose for our thought experiment, provided one is on the train, and one on the platform. Therefore, we need not specify the observers' locations.

A related argument from Lorentz transformations. Take two events, and use the Lorentz transforms to transform the measured time interval between the events from one inertial frame to another to get

${\begin{aligned}\delta t'&=\gamma \left(\delta t-v\delta x/c^{2}\right)\\\end{aligned}}$

Note that the transformation of the time interval between the frames depends on the separation of the events in the unprimed frame, but not on the absolute positions of the events which is the thing that changes when we change the position of the unprimed observer. Only the relative velocity of the two observers and the separation of the events determines the disagreement in elapsed time between event. Relative position of the observers is not a factor.

A last point. This wiki page is not alone in making misleading statements that I am arguing against. Many web sites, and many text books (mostly at high school level) also make these errors, probably from a misguided attempt to simplify what is a hard subject to grasp. Higher level texts such as at university level are mostly more precise, and I'd suggest that such texts are the only place to check our understanding, rather than other webpages or wikis. I'd recommend the text book Griffiths, Introduction to Electrodynamics. For the record, I'm a university lecturer teaching Special Relativity at second year undergraduate level. — Preceding unsigned comment added by 137.111.13.200 (talk • contribs)

Please sign your talk page messages with four tildes (~~~~). Thanks.

Sure, indeed in Griffiths' intro to electrodynamics "the lamp is equidistant from the two ends", and the "observers" could be anywhere, one somewhere on the train and riding along with the lamp, another standing on the ground, or if you like, looking at figures 12.4 and 12.5 on page 484 (in my edition). In Einstein's quoted text, which lies at the basis of the text in this article, the train observer (M') coincides with the lamp, whereas the ground observer (M) is a person standing at the place where the light flash is emitted. Here you see the difference between the modern "observer" concept as a reference frame, and the common day-to-day "observer" as a person making observations — see also our article Observer (special relativity). The day-to-day way is how the original thought experiment was described, and also the way it was done in this text, to, as you say, "simplify the argument". It is after all, as the article says (with proper and relevant references), "a popular picture", and as such I don't see anything wrong with that. DVdm (talk) 23:18, 5 November 2011 (UTC)

I actually said that in the slightly-different Einstein thought experiment here the observers are put centred to simplify the example. But that is a different thought experiment to the one presented on this page, a fact acknowledged in the History section of the present article, and so we should not be blindly including constraints from that experiment here if they are not required. In the thought experiment on the present page, which is a valid variation of Einstein's original, observer locations are not part of the logic, and it is a mistaken attempt to simplify the experiment by introducing unnecessary constraints that actually encourages people to misunderstand the nature of the simultaneity issue - which is a physical reality, and not just an issue of visual perception. As you say, Griffiths, in his wisdom, in his presentation of this thought experiment correctly does not specify locations for the observers, just frames; I'm not sure why the fact that Griffiths does this does not convince you that Wiki can and should omit it too.

Look at the spacetime diagrams for the thought experiment adjacent to the Lorentz Transformation section. These show the light flash occurring at an arbitrary positive x (and an arbitrary positive t for that matter), showing that the x-position of the flash in each frame (i.e. the location of the flash relative to each observer) is of no consequence. It then seems strange to specify a specific observer location in the preceeding section when a different and arbitrary location is used in these diagrams.

I accept that there is a distinction between the modern 'reference frame' observer, and the day-to-day concept. However, it is implicitly the reference frame observer that is invoked in this version of the thought experiment (e.g. no observer, or photons travelling from events to observers, are drawn in the train diagrams; spacetime diagrams are used that are implicitly reference frame tools), whereas it is the day-to-day observer that was used in Einstein's original version (with explicit consideration of light travel time from events to observers). This is exactly why the Einstein version requires the locations of the observers to be specified, and this version does not. Since the locations are not required here, it can only confuse to include them.

Thanks for chatting, and apologies for neglecting to sign the previous posts. If I haven't convinced you (or others?) yet, I think I will never succeed, so I will leave it at that. Congrats anyhow on an (otherwise) excellent treatment of SR in general.

137.111.13.200 (talk) 03:10, 6 November 2011 (UTC)

I agree. The thought experminent in the text differs from the diagrams'. I objected to your edit because it changed one version to the other while leaving the referenced sources untouched, which is what we call wp:original research. I wouldn't mind if you would rework the text (and/or the diagrams), but the sources should reflect the changes — or rather, the new text (and/or diagrams) should reflect the content of the new sources. Cheers - DVdm (talk) 11:00, 6 November 2011 (UTC)

OK. It's just a referencing problem. There are actually no sources quoted in the text right now for this thought experiment, just refs 1, 2, and 3 that are stated to be similar experiments (but not identical and so we might expect differences). I suggest that the solution is to also reference a text book with this exact thought experiment (e.g. Griffiths), as well as these current references. This should ensure that everything is suitably verifiable. 60.225.54.95 (talk) 21:14, 7 November 2011 (UTC)

Further Reading

I'd like to suggest adding a "Further Reading" section to this entry. One proposed reading would be the following:

• Craig, William Lane and Smith, Quentin (Editors) (2008): Einstein, Relativity and Absolute Simultaneity. Routledge, London and New York. ISBN13: 978-0-415-70174-7.

However, being a relatively inexperienced editor, I'm not sure of the correct way to do this. Any help from more experienced hands would be appreciated. Thanks. JCNSmith (talk) 02:45, 8 March 2012 (UTC)

I don't think this is a good candidate for further reading, as it is mainly philosophical, and rather fringy. For instance, on page 5 it says: "Apart from leaving unaddressed the epistemolocial and semantic presuppositions of STR, there is an even stronger factor behind physicists' unwillingness to abandon the Special Theory. [...] In fact, there is a theory that is not merely observationally equivalent to the Special Theory, but also observationally superior to it, namely Lorentzian or neo-Lorentzian theory..." On page 6: "It may be concluded that the main reasons why many physicists still hold to Special Relativity are (1) an insuficient awareness of the epistemological and other philosophical problems with Special Relativity; [...]" - etc... There's also an essay by fringe scientist Tom Van Flandern. - DVdm (talk) 08:22, 8 March 2012 (UTC)

Thanks for your comments. I agree that the quality of essays in the book I recommended is uneven, but I found enough of interest in it to think it might be worthwhile for those interested in the topic. Regardless of whether the book I mentioned is included in a "Further Reading" section or not, however, I certainly think the topic merits having a Further Reading section of its own! Perhaps you could recommend some other, more worthy readings to populate such a section? Thanks. JCNSmith (talk) 20:21, 8 March 2012 (UTC)

I had a look at the other related articles on consequences of special relativity (Time dilation, Length contraction, Velocity-addition formula, Mass–energy equivalence) to see how it's done overthere. It looks like none of them has a further reading section. I guess the reason is that in each case the references section is pretty rich already and provides ample reading material. Having had a look at the guideline Wikipedia:Further reading, I think the reason is indeed clear. As these subjects are pretty narrow, it's hard to find book material that is sufficiently topical (per subsection 1.1). The Craig collection doesn't qualify on topicality either, as there's more on quantum stuff than on this article's subject. Furthermore, as becomes quite clear from the introduction of the book, the authors have an agenda based on a fringe view on special relativity, which certainly makes it insufficiently balanced (per subsection 1.3). I guess, if you're looking for good reading, there's some really good pointers in the ref list. - DVdm (talk) 21:32, 8 March 2012 (UTC)

Thanks for the guidance. As noted, I'm relatively inexperienced at this, so appreciate your insight and pointers to Wiki policy/guidance on the use of Further Reading sections. JCNSmith (talk) 03:33, 9 March 2012 (UTC)

2nd Paragraph Illustration

Something is wrong with this sentence (from 2nd para. of article): "Where an event occurs in a single place–for example, a car crash–all observers will agree that both cars arrived at the point of impact at the same time." First of all, it is postulated that "an event occurs," and since you can't attribute the property of simultaneity to a single event (at least two events are required for the property of simultaneity to come into being), we're off on the wrong foot. We then are meant to visualize a car crash, which again is a single event, but which *by definition* requires two things in the same place at the same time, i.e., the fact that there was an impact *requires* there to have been the simultaneous arrival of two things at the same place and time (overlapping space and time: hmm, yep, requires simultaneity already). It is thus a tautological endorsement because the postulation of a wreck requires there to have been two things in the same place and time for all reference frames. In other words, it begs the question about simultaneity being relative, and therefore shouldn't be used to illustrate relative simultaneity.

It seems to me one would somehow have to illustrate two different things happening (in a physics sense involving states of particles and fields, etc.) simultaneously at the same point to evoke the desired comparison to the space-separated case, but I'm not sure anyone can come up with a proper example of such a thing. Perhaps there is a quantum mechanical angle here. I'm out of my depth. I do hold to the above (logical, not physics) objection, though. Chafe66 (talk) 07:41, 14 May 2012 (UTC)

Yes, it looks like this is caused by a tendency to identify an event with its definition. If one defines event_A with definition_A ("car_A arrives at crossroads X") and event_B with definition_B ("car_B arrives at crossroads X"), then we have in fact two different definitions for one event, and we have that event_A = event_B, although definition_A ≠ definition_B. We can even add a third definition: "car_A and car_B crash at crossroads X". Tricky. Perhaps we could add one word ("distinct") in the preceding sentence: ... whether two distinct events occur at the same time..., and then remove the car crash sentence altogether. I also think there's a style problem with the sentence: the car crash is a dashed example, but the main sentence uses the cars from that example. - DVdm (talk) 08:52, 14 May 2012 (UTC)

Sounds good. But I don't really think the issue has to do with definitions and reference. Consider thinking of "car_A arrives at crossroads X" as simply naming an event rather than defining it. Now from philosophy of language, we know that the quoted phrase has a sense and a reference. The reference is the event itself, and the sense (the "meaning") of the phrase has to do with an automotive type object appearing in reference to an intersection of paths, etc. These are different things (one involves mass and energy, etc., the other is a linguistic entity) and aren't really being confused here (in my opinion). To use your terminology, my objection is this: the meaning of "Car_A arrived at CRs X at the same time as car_B arrived at CRs X" logically entails that there was a crash and the meaning of "there was a car crash between car_A and car_B at CRs X" logically entails the former phrase. They have the same reference, but, as in your example, different meanings. So either 1) if one event happened then two more events happened or 2) if two events happened then a third event happened. I'm sure the list goes on. "Metal was bent in such and such a way" could be added to probably an infinite list. But they all describe a single event--in a certain sense. And if we pick the single event as the canonical representation of "what happened" then it begs the question about simultaneity at a single location. But that has me wondering if one can even conceive of examples where two distinct things (there's your word) could happen at the same place and time. What kind of example would it be if not something like the car crash example?Chafe66 (talk) 09:38, 14 May 2012 (UTC)

I don't really care about the difference between, like you say, "naming an event rather than defining it". In this context the difference is not important. Let's just get rid of the sentence, unless of course we have a proper source for it. Lacking such a source we can dump it as someone's original research. I'd go ahead and wp:boldly do it. - DVdm (talk) 10:13, 14 May 2012 (UTC)

I would, but I honestly wouldn't know what to change it to. I can't think of an example involving a single location that makes sense as an illustration of possible non-simultaneity! I.e., in order for the example to work, the events described at a single location would have to be conceivably not simultaneous, and events happening successively at a particular location already assumes they're not simultaneous...sigh.Chafe66 (talk) 18:22, 14 May 2012 (UTC)

I don't think we have to change it. Let's just dump it. Ok, I went ahead and dit it. We'll see where it gets us. Good catch, by the way! - DVdm (talk) 18:33, 14 May 2012 (UTC)

:) Cheers. Thanks for all your work on this article, and being so open to discussion. Chafe66 (talk) 18:46, 14 May 2012 (UTC)

"Where an event occurs in a single place–for example, a car crash–all observers will agree that both cars arrived at the point of impact at the same time." ~ There are two distinct kinds of simultaneity of position: 1) causal, and 2) observational. A crash is causal simultaneity. Observational simultaneity requires separation of events that are observed to be simultaneous from some locations but not from others. Location, relative velocity, and relative acceleration of the observer determine what is (actually! if you really think about it, is) simultaneous. BlueMist (talk) 08:59, 14 May 2012 (UTC)

But this example has to do with, first, the fact that an event happened *at* a single location, and the notion of where observers are is invoked afterward. What is first postulated is observerless. Something happened independently of whether anyone observed it at all, let alone their relative locations. My point has to do with the logical entailing of each other of "crash" and "car_A arriving at X at the same time as car_B." The postulation of a crash requires (unless one thinks a crash can have occurred wrt one reference frame and not another, which breaks some accepted law I'm sure) that this alleged 'simultaneous' event of two things arriving at the same place and time also occurred. So it can't have been otherwise if there was a crash; so I think "crash" should be left out of it altogether. But that raises another issue. See above...Chafe66 (talk) 09:38, 14 May 2012 (UTC)

Here's another way of putting it: the very notion of simultaneity requires the notion of sameness of time and place, particularly now that relativity has shown how we need to think about temporality when one event happens in location A and another event happens in distinct location B. If we can't talk about being in the same place at a given time, then we don't even know what "two events being simultaneous" could possibly mean. Therefore, we can't use any single event at a given place as an example of simultaneity (either causal or observational). Why not? Because if a given thing happening at a given time means something else happened at that same time and place, then those things could not have been unsimultaneous, by the definition of whatever that event is. And that doesn't help in terms of what's needed in order to contrast with the coming example (in the article) explaining simultaneity's being relative.Chafe66 (talk) 10:11, 14 May 2012 (UTC)

No, it doesn't. A crash is a single event without separation, so it looks the same from everywhere. But two separated events may or may not, seemingly and actually, *be* simultaneous depending on the location, velocity, and acceleration of the observational frame of reference. This concept is extremely counter-intuitive. Space-time is not a fixed Newtonian construct. Simultaneity of two events is relative, see [1]. ~ BlueMist (talk) 00:13, 16 May 2012 (UTC)

We're talking about different things. My subject has to do with logic and the example of a single event as being helpful or meaningful. You are talking about principles of physics. You're taking the example as given, and moving from there. I'm not taking the example as given, i.e., not accepting it as saying anything other than what is required by logic (it's true "by definition"), hence it (example of crash) is not a case in point of a physical principle. It's required definitionally, and nothing can be concluded from such statements.Chafe66 (talk) 07:34, 16 May 2012 (UTC)

Aristotelian logic of either-or cannot be applied to relativity of simultaneity. The same pair of events separated in space are both simultaneous and not simultaneous. Even at the same time, if time is properly adjusted by the speed of light. This is not an illusion, but a demonstrable reality. Again, see wikibooks for a more detailed explanation[2]. The point is that Aristotelian logic fails in all cases of physical relativity.

The single event example is used in contrast. But if a pair of events are gradually merged, then the single event becomes just a special case of a more general law. As far as I am concerned, you can take it or leave it. ~ BlueMist (talk) 11:20, 16 May 2012 (UTC)

Yes, let's leave it, unless there's something notable, interesting and properly sourced to add to the article. Otherwise this is just off-topic chat per our wp:talk page guidelines. - DVdm (talk) 11:31, 16 May 2012 (UTC)

Lead promotes a philosophical bias

"In physics, the relativity of simultaneity is the concept that distant simultaneity – whether two spatially separated events occur at the same time – is not absolute, but depends on the observer's reference frame. According to the special theory of relativity, it is impossible to say in an absolute sense whether two distinct events occur at the same time if those events are separated in space, such as a car crash in London and another in New York. The question of whether the events are simultaneous is relative: in some reference frames the two accidents may happen at the same time, in other frames (in a different state of motion relative to the events) the crash in London may occur first, and in still other frames the New York crash may occur first."

The bias is that simultaneity "depends on the observer's reference frame." I will give an example where that is clearly not the case. A split-beam laser is fired at two targets on the Moon, separated by whatever distance. The beams leave the laser at the same instant when it is fired and travel at the same velocity (c), so therefore they arrive at the two targets simultaneously. That (thought experimental)fact is established. Now introduce two observational frames, passing through the Earth/Moon system. Each will observe the targets to be struck by the beams at different times. The point is that the above assumed philosophy, that simultaneity depends on the observers' reference frames does not change the established simultaneity of strikes hitting the two targets, separated by whatever distance on the moon's surface. The assumption upon which the lead statements are based endorses a philosophy which absolutely precludes the possibility of events happening simultaneously independently from when they are observed from different frames. My example illustrates that such an assumption is false. LCcritic (talk) 21:19, 1 December 2013 (UTC)

Alas, on article talk pages we are not allowed to discuss our own philosophical, psychological, or even scientific views and musings about the subjects of our articles — see wp:talk page guidelines. This was explained to you before in the discussions at

• Talk:Length contraction#Relevance of "Philosophical Realism" to the "Reality of Length Contraction" subsection
• Wikipedia talk:WikiProject Physics/Taskforces/Relativity#Criticism of relativity not allowed
• Wikipedia:Requests for comment/Request board#Non-mainstream views of relativity
• Wikipedia:Reference desk/Archives/Science/2013 October 31#Contracted Earth diameter and atmosphere depth
• Talk:Length contraction#Physical?

I have now left a 3rd level warning on your talk page. DVdm (talk) 08:02, 2 December 2013 (UTC)

Effects

Since the length of an object is the distance from head to tail at one simultaneous moment, it follows that if two observers disagree about what events are simultaneous then they will also disagree on the length of objects.

If a line of clocks appear synchronized to a stationary observer and appear to be out of sync to that same observer after accelerating to a certain velocity then it follows that during the acceleration the clocks ran at different speeds. Some may even run backwards. This line of reasoning leads to general relativity.

Just granpa (talk) 21:00, 4 April 2014 (UTC)

Please note that article talk pages are for discussions about the article, not about the subject. See wp:talk page guidelines. - DVdm (talk) 09:06, 5 April 2014 (UTC)

Unqualified statements about relative simultaneity

Precession of judgements about reality in the name of a theory should be verified by the encyclopaedia of the global reach.

The popular statement that events simultaneous in one inertial frame are not simultaneous in a frame moving relatively is generally false. Infinite number of events occurring on every possible plane perpendicular to the velocity vector of the moving system will be simultaneous in both inertial systems where simultaneous means: occurring at the same time. This results from elementary calculation using Lorentz transformations. I have modified the article to reflect the truth based on mathematical framework of Special Relativity to which this article applies Andrew Wutke. — Preceding unsigned comment added by 58.111.76.118 (talk) 05:03, 18 April 2015 (UTC)

Please put new talk page messages at the bottom of talk pages and sign your messages with four tildes (~~~~). Thanks.

I have reverted your edit. This is not a matter of interpretations of special relativity. Everyone knows this, and there is no discussion about it. The popular statement merely restricts chosen events to happen on the common axis along the relative velocity vector. Simultaneity of events which are not on that axis still depend on the observer. Exceptions for events on a perpendicular plane don't change the lack of absoluteness. Indeed, like you say, "two events occurring on the same plane perpendicular to the direction of the relative motion of two inertial systems will be simultaneous in both systems", but these events are non-similtaneous for still other observers in relative motion along different directions. In other words, simultaneity still depends (and not merely may depend) on the observer's frame. - DVdm (talk) 08:52, 18 April 2015 (UTC)

History

In the paragraph entitled "History", the word "he" might mean Lorentz or Poincare. I have put the tag "vague" in it. It seems to refer to Poincare. — Preceding unsigned comment added by 81.141.100.104 (talk) 11:50, 11 December 2015 (UTC)

The train-and-platform thought experiment

A naive question. This thought experiment assumes that observers in both the train car frame and the platform frame agree that the flash of light is emitted at the midpoint of the train car. But why should this be true?

For example, if in the platform frame, the light flash occurs at a point (1-β)/2 behind the front of the car (assuming the car has length 1 in the platform frame and β is the dimensionless velocity v/c) then in the platform frame the light flashes will arrive simultaneously at each end of the car, just as in the car frame.

I don't understand how the postulates of special relativity (equivalence of physical laws and light speed in all frames) prohibit this possibility. Jeffrw (talk) 07:32, 27 July 2014 (UTC)

The middlepoints are taken because it makes the calculation trivial, and to precisely demonstrate the point of ROS—see the literature. If you don't understand something about it, the place to ask is our wp:reference desk/science. Here, per wp:talk page guidelines, we are supposed to discuss the article, not the content. Good luck at the refdesk. - DVdm (talk) 09:27, 27 July 2014 (UTC)

I don't see what's so big thing in this. The light hits the ends of the car at different time also in the moving frame. The traveler in the car just doesn't see it because what he sees is the reflection of beams hitting the ends of the car. The reflection of the light hitting earlier from the rear end takes more time to travel back to the observer as he is traveling away from it, and accordingly the light reflecting from the front end comes back faster as the observer is traveling toward it. Those differencies compensate each other and so by the observers point of view, the events appear to be simultaneous though they are not. — Preceding unsigned comment added by 85.76.137.124 (talk) 12:41, 24 December 2015 (UTC)

Something Missing from or glossed over in the description. The significance of Rest Frames.

A good topic well written, but easily leading to a gross misconception. Take for example the first diagram where the Green frame is at rest, while the blue frame and the red frame are moving relative to the green frame. Using this representation for Einstein's train, the Embankment is naturally viewed as the Green frame - at rest - while the train would be represented by the Red frame (a moving frame) with simultaneity measured in the green frame. However, it is just as valid to take the train as the Green frame and the embankment as the Blue frame (a frame moving in the opposite direction to the first example), with simultaneity again measured in the green frame. NB. This was surely the point of Einstein's so often overlooked comment in his chapter 9 of

Is that not what the Relativity of Simultaneity means? That it is relative to the reference frame it is viewed from: that observer on the Embankment and the observer on the train will each measure the lightning strikes as simultaneous, from their own frame - it being at rest - but that they cannot be simultaneous from a moving frame...

D1d4 (talk) 11:45, 29 October 2016 (UTC)

References

Clarification requested: how to observe (non-)simultaneity?

The section "Explanation" says

For example, a car crash in London and another in New York, which appear to happen at the same time to an observer on the earth, will appear to have occurred at slightly different times to an observer on an airplane flying between London and New York.

But how can an observer on Earth, or one in the airplane, decide if the events were simultaneous in his frame of reference, since at least one event occurred at a distance from the observer? Does he receive speed-of-light communication from the two locations? Even with that, deciding which event occurred first must be problematic, at least for the observer on the airplane.

Similar wording appears in the section on the train experiment. I can understand that the midpoint observer on the train might receive reflections from mirrors at the two endpoints; but how could the observer on the platform decide which occurred first? Loraof (talk) 04:43, 15 January 2017 (UTC)

Excellent point. The time measurements of this particular setup would need the airplane observer to send out some kind of electromagnetic signals that just happen to arrive at both crash events, and that get reflected to the observer. They could then compare the event times, each calculated as the average of the emission and echo reception times. Of course the plane should be at sufficient altitude to have both locations in sight. And the rotation of the Earth should be taken into account. And the presence of the atmosphere. And etcetera. For a ground observer the measurements are somewhat less problematic, as in practice it could be possible to arrange for properly synchronized clocks at the two locations, on which the local event times could be recorded and read off. Of course, obtaining these clocks would be very difficult indeed.

So all in all, i.m.o. this is a rather silly example. Perhaps, in order to avoid puzzlement or confusion, it should be amended to something like this:

For example, a car crash in London and another in New York, which appear to happen at the same time to an observer on the earth, would, provided if it would be practically possible to accurately measure the crash times, appear to have occurred at slightly different times to an observer on an airplane flying between London and New York.

Feel free. - DVdm (talk) 10:32, 15 January 2017 (UTC)

I don't think that the average of the emission and echo reception times works for the observer in the airplane. To check this he would have to use the math of relativity, so this could not be used to test the prediction of relativity. Do the references have anything to say about this? Loraof (talk) 16:05, 15 January 2017 (UTC)

But this is not about testing a prediction of relativity. It's just about relativity. - DVdm (talk) 16:42, 15 January 2017 (UTC)

But something that's not testable is vacuous of content. Since I doubt that something as well-known as the subject of this article is vacuous, I think that I (and the article) must be missing something here. Loraof (talk) 23:50, 15 January 2017 (UTC)

Most of the sometimes silly but often cited "every-day examples of relativity consequences for the lay person" with cars, trains, airplanes and rockets are hardly testable in practice, but testable in principle, and therefore falsifiable, because "it is possible to conceive of an observation or an argument which negates the statement in question." The article was probably written with the lay person in mind, and the delicate subtleties of the measuring procedures for testing the theory seem to have been beyond the scope of this article. - DVdm (talk) 08:09, 16 January 2017 (UTC)

Relativity of simultaneity

The following discussion is closed. Please do not modify it. Subsequent comments should be made on the appropriate discussion page. No further edits should be made to this discussion.

---

"Since experimental evidence, it is well known that a desynchronization of clocks appears between different altitudes on earth (simultaneity is relative). However, simultaneity (absolute for the sky) of the position of the sun or the moon seen relative to altitudes (since million years for example) questions the concept of relativity."

Why not putting it on the wiki page "relativity of simultaneity"?????? — Preceding unsigned comment added by N738139 (talk • contribs) 22:09, 21 May 2017 (UTC)

Because "the sky" does not exist and simultaneity does not apply to it. This article is about physics, not fringe ramblings. Kleuske (talk) 22:13, 21 May 2017 (UTC)

Every position of planet at a given time are the sky!!! We can also discuss simultaneity relative to "general relativity" and gravitational potentialsN738139 (talk) 22:17, 21 May 2017 (UTC)

Positions of planets at a given time is just that, the position of a planet at a given time. "The sky" is an illusion. Planets do not move at relativistic speeds, so relativity does not play a role (except for Mercury, due to the gravity of the Sun, but that's another matter). We are discussing the inane bit of prose you edit-warred over, don't sidestep the issue. Kleuske (talk) 22:28, 21 May 2017 (UTC)

I propose you to learn a little bit more about https://en.wikipedia.org/wiki/General_relativity or https://en.wikipedia.org/wiki/Time_dilation to understand the effects of gravitational potentials (but I know you already have done it and you just don't want the truth that Einstein got wrong). I am not talking about speeds of planet, but about seen position from earth like when you watch the sky!!! Following frame-dragging (Kerr metric), our skies shouldn't be simultane. — Preceding unsigned comment added by N738139 (talk • contribs) 22:33, 21 May 2017 (UTC)

Sources, please. Kleuske (talk) 22:38, 21 May 2017 (UTC)

https://en.wikipedia.org/wiki/Frame-dragging Do you mean our skies are simultane or not??? Please answer.N738139 (talk) 22:44, 21 May 2017 (UTC)

Do you mean there is no frame-dragging effect on earth??? After 1 second?? After 1 million years?N738139 (talk) 22:44, 21 May 2017 (UTC)

I asked for sources. I have made no claims regarding frame-dragging. I propose you read WP:RS and find out why Wikipedia does not consider Wikipedia to be a reliable source. Kleuske (talk) 22:49, 21 May 2017 (UTC)

Sorry, but source about what??? Please explain! N738139 (talk) 22:56, 21 May 2017 (UTC)

Specifically, I want proper sources for the claim you wished to introduce into the article, additionally, I want sources for all the claims you made above. Kleuske (talk) 22:58, 21 May 2017 (UTC)

1. "Since experimental evidence, it is well known that a desynchronization of clocks appears between different altitudes on earth (simultaneity is relative)." Does this sentence need a source in a wiki page related to relativistic physics???N738139 (talk) 23:05, 21 May 2017 (UTC)

2."However, simultaneity (absolute for the sky) of the sun or the moon (since million years for example) questions the concept of relativity." Does this sentence need a source? Isn't it obvious when you are at 0m and at 5000m altitudes that you are seeing the planets as absolute simultaneity???N738139 (talk) 23:05, 21 May 2017 (UTC)

Your claims need sources regardless of how "obvious" they may seem to you. They also need clarity as what you are writing is largely gibberish. "Since experimental evidence, it is well known that..." is not proper grammar and it is meaningless. Justeditingtoday (talk) 23:07, 21 May 2017 (UTC)

Are you kinding me? The synchronization of a rising sun needs a source for you??? #Franklin N738139 (talk) 23:09, 21 May 2017 (UTC)

Then can I write: "Desynchronization of clocks appears between different altitudes on earth (simultaneity is relative). However, simultaneity of a rising sun (after million years for example) analysed from different altitudes point of view questions the concept of relativity."???

I just checked the first source you provided and found no trace of an article with that title. Please don't provide bogus sources, you're only wasting everybody's time. Also, the bit about gravitational time dilation is not what I'm objecting to. Kleuske (talk) 23:24, 21 May 2017 (UTC)

What are you objecting??? the link works http://iopscience.iop.org/article/10.1088/0143-0807/37/3/035602/meta

Sorry, I repeat myself: does the synchronization of a rising sun needs a source for you??? www.jstor.org/stable/20780005N738139 (talk) 23:33, 21 May 2017 (UTC)

Yes. It most definitely needs a source. Your source titled "Liberty to Communicate" does not exist. Kleuske (talk) 23:36, 21 May 2017 (UTC)

www.jstor.org/stable/20780005 isn't that the best source we can have??? — Preceding unsigned comment added by N738139 (talk • contribs) 23:39, 21 May 2017 (UTC)

It's a fine source for an article on Benjamin Franklin. As a source for "relativity of simultaneity" it's ludicrous. Kleuske (talk) 23:44, 21 May 2017 (UTC)

And this one??? http://www.sciencedirect.com/science/article/pii/S1384107613000110 — Preceding unsigned comment added by N738139 (talk • contribs) 23:48, 21 May 2017 (UTC)

The relation between sunspot numbers and radio flux? What has that got do do with your claim? Throwing random articles around is not "providing sources". You're wasting my time. Kleuske (talk) 23:54, 21 May 2017 (UTC)

"It is well known that synchronization of the Sun is one key aspect for understanding the origin and evolution of active regions on the Sun" http://www.sciencedirect.com/science/article/pii/S1384107613000110

This one is better: http://link.springer.com/article/10.1007/s11207-007-0405-5 "Historical sunspot records were analyzed by means of nonlinear tools to find synchronization phenomena at different time scales on the Sun"N738139 (talk) 00:01, 22 May 2017 (UTC)

reverse it and you got it "phenomena on earth are synchronized" — Preceding unsigned comment added by N738139 (talk • contribs) 00:05, 22 May 2017 (UTC)

I'm being very charitable pointing out WP:SYNTH, but honestly, "reversing it" is poppycock. Kleuske (talk) 00:11, 22 May 2017 (UTC)

Why is it poppycock??? (I think you are waisting my time!!!)N738139 (talk) 00:21, 22 May 2017 (UTC)

You are creating WP:SYNTH by making a personal interpretation of sources where the sources do not explicitly makes the claims you are ascribing to them. To be blunt, the "information" you want to add to this article is original research hinging on fake sources and personal interpretation of real sources that don't say what you want them to say. Wikipedia works on consensus. So far exactly nobody agrees with you that adding that information makes the article better. Arguing longer and becoming more aggressive will not change that fact. Please just let it go. Justeditingtoday (talk) 00:53, 22 May 2017 (UTC)

It is not an original research, it is just a claim that need a source. Where is the fake here? — Preceding unsigned comment added by 2A02:120B:C3C4:29C0:B1DF:EA9D:1999:A19E (talk) 01:00, 22 May 2017 (UTC)

This does not belong in the article unless you can produce a rigorous, reliable source that makes the exceptional claim that relativity predicts that the sky/sunrise should look different than it does. You are performing an original synthesis in a way that introduces a gross error - the theory of relativity doesn't make the prediction that you seem to think it does. VQuakr (talk) 01:18, 22 May 2017 (UTC)

References

The discussion above is closed. Please do not modify it. Subsequent comments should be made on the appropriate discussion page. No further edits should be made to this discussion.

Second diagram from top

It states "Event B is simultaneous with A in the green reference frame, but it occurred before in the blue frame, and will occur later in the red frame"

The grey area is the light cone of the observer. Just wondering ct is perpendicular to the x-axis but x’ and x” are not perpendicular to the ct’ and ct” respectively. Since there are 3 time-axis (ct,ct’,ct”) and 3 space-axis (x,x’,x”) therefore shouldn’t be there 3 light cones?2001:56A:739C:6D00:E413:4836:6E71:8037 (talk) 00:49, 8 September 2016 (UTC)eek

Note that the light cone is defined by taking lines that bisect the axes—remember that the light signal lines through the origins are represented by x = ±ct, x' = ±ct', x" = ±ct".

In most drawings of spacetime diagrams there is of course that special (x,ct) system for which the axes happen to be drawn perpendicularly, because that system is just the one in which we—looking at it—are at rest. In that system the light lines make of course 45 degrees angles with the axes. Note that the axes in the (x',ct') and (x",ct") systems are actually perpendicar too, but only for the observers at rest to them respectively. For further questions you should probably contact the wp:Reference desk/Science. - DVdm (talk) 06:39, 8 September 2016 (UTC)

I understand but elaboration for those who have not reached the crux of the above statement

The tilting/rotation or squeezing of the space-time diagram depends upon the speed of observer relative to the observer who thinks s/he is at rest.

There are 3 time-axis (ct,ct’,ct”), 3 space-axis (x,x’,x”) and hence 3- light cones.

Construct the 3-light cones such that 2 of the light cones are tilted relative to the observer who is at rest. Let

A is surface of the base of a light cone of x-ct

B is surface of the base of a light cone of x’-ct’

C is surface of the base of a light cone of x”-ct”

At Apex (common point)

All observers would be able to see each other when they are at rest

Beyond Apex

Relative to the observer on A, observers on B and C are either above or below A.

Above is for future while below is for past. Since observer on A can’t see things either in future or past, therefore, none of the observers would be able to see each other as soon as 2 of the observers starts moving with different velocities due to tilting of the surface of basses of light-cones. In a nutshell, observers can’t see each other if their speed is not the same – is it possible? — Preceding unsigned comment added by 2001:56A:739C:D300:6928:3C97:8B7F:2A65 (talk) 02:59, 23 April 2019 (UTC)