Wikipedia:Reference desk/Archives/Science/2013 March 17

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March 17

Holograms

I have read quite a bit on holography inc. bought books and here at Wikipedia but I cant find answers to these questions?

Here are my questions:

1. In addition to being required to interfere with the object beam, is the reference beams inclusion in the production stage simply a way of weaving a light source for viewing, into the production stage. It is a clever way of allowing a light source to illuminate the hologram for viewing without introducing new data which would destroy any hope of seeing the image? The reference beam being the purest form of light (monochromatic, coherent, collimated and with plane wavefronts) means it is the most reliably reproducible light source for viewing at a later date and venue?

2. Would the object beams' wavefronts expose the emulsion if the lasers' output was doubled (equalling the intensity of the peaks recorded with both an object and reference beam)?

3. I understand the speckles on the hologram are what have been recorded of the scene, so what are the swirling patterns we see in the emulsion?

Thanks in advance. — Preceding unsigned comment added by Eagle eyes 000 (talk • contribs) 14:32, 17 March 2013 (UTC)

The objective in holography is to recreate a three-dimensional light-field. The hologram itself is exposed on a single plane - which is a two-dimensional sample of the light-field from the original scene. To recreate a three-dimensional field, some additional light must be present. This is why a reference beam is used. The fact that we require monochromatic light, or laser light, is a practical limitation of the way we capture holograms in visible light. If we take the deep-dive into theoretical realms - or even if we consider acoustic tomography or holography, which operate on the same principle of reconstructing a full wavefield, we can see that the monochromatic reference beam is not a strict requirement, as long as we have the ability to reconstruct the reference with accurate phase. That process is possible - and easy - for, say, an ultrasonic scanner, but not easy for a visible light source, so we use a laser as a suitable source.

When you see wavy lines, you are seeing moire patterns. The actual "image" on the hologram's emulsion is a projection of the unfocused wavefield onto a plane. That projection is expected to have lots of periodicity for normal scenes, so it is expected to see moire patterns for the same reason they appear in other materials with similar patterns. Nimur (talk) 16:06, 17 March 2013 (UTC)

Moiré pattern   ←[correct link -74.60.29.141 (talk) 16:46, 17 March 2013 (UTC)]

Redirect created. Nil Einne (talk) 20:50, 17 March 2013 (UTC)

Usually redirects aren't created for article titles pluralized by appending letters—instead you write [[moire pattern]]s. But it doesn't matter since redirects are cheap. -- BenRG (talk) 02:13, 18 March 2013 (UTC)

While putting the pluralisation outside the wikilink may arguably be the preferred option in writing articles, this doesn't mean redirects from plurals shouldn't be created. In fact for frequent cases they should be because there is no expectation that wikilinks in the other way should be mass corrected particularly in comments, even Category:Redirects from plurals notes this ('do not replace these redirected links with a simpler link unless the page is updated for another reason'). There are also other advantages with the redirects existing, like in searching. While our search engine would generally end up suggesting the singular article as an option, a redirect will mean people automatically go to the article. Another advantage is external sites making linking errors will still find a target. In fact, Wikipedia:Naming conventions (plurals) notes this, 'Creating a redirect in cases like crayons is advisable as well, since third-party websites started adding automatic links to Wikipedia from their topics, and many of them follow the opposite convention, i.e., pluralization'. So from my experience redirects from plurals are very, very commonly created e.g. cats, humans, bananas, computers (all just random examples I didn't even both to test). Nil Einne (talk) 06:39, 18 March 2013 (UTC)

What do you mean by "To recreate a three-dimensional field, some additional light must be present"? -- BenRG (talk) 02:13, 18 March 2013 (UTC)

I was of course describing the reference illuminator, using somewhat coarse terminology.

When viewed, a hologram is not a static image. It is a light field, and it can be viewed from many different angles. It is a reconstruction of the original light field at all points in 3 dimensions, albeit it is an imperfect reconstruction, due to the limits of sampling and projection.

The exposed plane (the emulsion) is itself not three-dimensional. It is a 2-dimensional sample of the light-field at the plane of the exposure. To reconstruct the full light-field at all points, a source of illumination must be provided. Conventionally, that source is the "reference beam," a monochromatic source of the same type originally used to perform the exposure. The illumination scatters off the holographic emulsion, and the resulting light-field is a close approximation to the light-field of the original scene. This is how we can view the "scene" in three dimensions. The viewing-angles that are reconstructed - rather, the set of viewpoints where the reconstructed field is similar to the original field - are limited by the sampling aperture.

If we were modeling the wavefield, instead of synthesizing it in the real world, we could completely eliminate the reference illuminator. We could replace it with the "exploding reflector model" to cause the wavefield to "start emitting" with the sampling-plane as a boundary condition. In the real world, passive materials like photographic emulsions do not spontaneously "start emitting" wavefields - we must illuminate them. This numerical technique is used extensively in computed tomography to synthesize 3D imagery in a computer model, but we have no material that can synthesize such a wavefield, subject to the exposure-emulsion as a boundary condition, at optical frequencies, in the real world. Here is a reference on synthesizing an (acoustic) wavefield without re-injecting the source illuminator: the Exploding Reflectors model. BenRG, you may appreciate this approach, because it considers every possible wave interference interaction - including ordinary propagation in free space - as just a special case of Born scattering. I have seen this numerical technique (the "holography trick") applied to seismic imaging; positron tomography for medical imaging; ground-penetrating radar; imaging radar; and of course, wavefield optics modeling for computer-graphics. If you're incredibly interested, I can dig up papers for each one of these applications; but suffice to say, the wave physics and the math are all the same, even if the wave type is very different. Nimur (talk) 03:48, 18 March 2013 (UTC)

What you're calling the reference illuminator seems to be what the article calls the reconstruction beam. The original question was about the reference beam that's used when creating the hologram.

In lower-frequency cases you don't need the reference or reconstruction beams, even in real-world applications, because you can record and reproduce the waveform directly. At visible-light frequencies that's impossible (as far as I know) and the interference-pattern trick is a way around that. Even so, the emulsion doesn't record complete information about the light field in a plane (it only records the amplitude of the interference pattern) and the reproduced light field is not the same as the original, despite what a lot of presentations (including our article) seem to imply. This thesis seems to have a good mathematical treatment in sections 2 and 3.1. -- BenRG (talk) 17:21, 19 March 2013 (UTC)

I'm not sure if I understand your questions. Are you asking if the reference beam is really necessary? The answer to that is yes. There's no such thing as a film that records the phase of light; all film records only the intensity. The way you fake recording the phase is by combining the light reflected off the object with a reference beam from the same laser. If the reference beam has intensity A² and the object beam has intensity B² at a particular point on the film, then the combined intensity of the two is (by the law of cosines) A² + B² + 2 A B cos φ, where φ is the relative phase of the two beams at that point. (The absolute phase of both beams varies with time, but the relative phase doesn't, as long as the light is monochromatic and the scene isn't changing.) The reference beam intensity is constant across the whole film, and you can treat the object beam intensity as roughly constant also, so roughly speaking the intensity captured on the film depends only on φ. -- BenRG (talk) 02:13, 18 March 2013 (UTC)

Would methane hydrate extraction spill significant amounts of methane into the atmosphere?

Methane hydrates have recently been in the news as possibly a main fuel source of the future. However, environmentalist friends of mine are very negative about this development, mainly because they believe that the process would spill too much methane into the atmosphere. Is this likely to be true? B^e——C_ritical 17:17, 17 March 2013 (UTC)

Not sure, but note that, even if none spills and you get 100% combustion, you'd still produce carbon dioxide, which is also a greenhouse gas. StuRat (talk) 17:32, 17 March 2013 (UTC)

Methane would produce much less carbon dioxide as a fuel than other hydrocarbon sources. It's not carbon neutral unless carbon dioxide sequestration is used as part of the extraction process, but it give off much less CO2. Because it creates much less atmospheric CO2, the only question is whether methane emissions are a significant danger. B^e——C_ritical 17:40, 17 March 2013 (UTC)

Not more so than for "conventional" gas drilling (the techniques used in clathrate recovery are similar to those already widely practiced in conventional gas drilling and in enhanced oil recovery), and less so than for fracking of shale gas deposits. 24.23.196.85 (talk) 00:11, 18 March 2013 (UTC)

So the calculation would at least have to:

• Make some assumption of how much methane would be released during extraction.
• Take into consideration the amount of time that methane exists in the atmosphere (8-12 years?)
• Take into consideration the amount of CO2 which would not be released as a result of using methane instead of other soruces.

The guesses require knowledge of, for example, how much natural gas is lost from fracking in the ocean. And other knowledge. B^e——C_ritical 17:57, 17 March 2013 (UTC)

At the ocean bottom depths it may also be possible to form Carbon dioxide clathrate at pressures between 12 and 44 bars. (120 and 440 meters deep). So there could be a way to store this CO₂. Graeme Bartlett (talk) 10:35, 18 March 2013 (UTC)

Russian M4 Parka

Are there any wiki articles about Russian military clothing? I'm particularly interested in their winter clothing like the Russian M4 Parka--Tommythehook (talk) 17:24, 17 March 2013 (UTC)

I don't think we have much. However, you could start from, let's say, Afghanka, and see what you can find... By the way, it would be easier to answer if you gave a little more details: are you interested in military clothing of Russian Federation only? Or would Gymnasterka of Russian Empire and USSR count too? --Martynas Patasius (talk) 17:50, 17 March 2013 (UTC)

Or the Telogreika of World War 2 vintage? 24.23.196.85 (talk) 00:02, 18 March 2013 (UTC)

The god particle

From CBS news today "The Higgs boson is often called 'the God particle' because it's said to be what caused the 'Big Bang' that created our universe many years ago." That isn't the reason it is called that, is it? Bubba73 ^{You talkin' to me?} 17:50, 17 March 2013 (UTC)

No, that isn't the reason, see this NPR interview ([1], several other good links within) with Dick Teresi, who is reported to have coined the term. He discusses it at length, but the title of the piece is

“ The Man Who Coined 'The God Particle' Explains: It Was A Joke! ”

That's pretty clear to me. SemanticMantis (talk) 18:01, 17 March 2013 (UTC)

Thank you,

Resolved

. Bubba73 ^{You talkin' to me?} 18:17, 17 March 2013 (UTC)

I don't know... The Higgs Boson sure does look like the FSM... More evidence here. --Guy Macon (talk)

That's a picture of the long-lived products of one or more collisions in one of the LHC detectors. They all look like that. Even if that one is a Higgs candidate, the Higgs isn't visible in it, since (a) its lifetime is too short and (b) the same decays can happen without the Higgs, so the existence of the Higgs is inferred from an increase in the rate, not from any particular event. -- BenRG (talk) 21:49, 17 March 2013 (UTC)

According to the book itself, the name was invented by Lederman, not Teresi, and he originally called it "the goddamn particle" because it's been so hard to find. However it was Teresi who decided to use it in the title of the book, thus ensuring that idiot journalists would repeat it forever after. -- BenRG (talk) 21:49, 17 March 2013 (UTC)

I guess the thing you should always remember is that the Higgs boson is a proposed, and experimentally-testable incarnation of the Higgs mechanism. And, the Higgs mechanism is just a really really high-resolution explanation of mass. We already know that mass exists, and we already know that mass interacts gravitationally. The Higgs mechanism only arises because we want to be very precise when we say "mass," and when we say "exist," and so forth. By using incredibly high energy, we set up the scale lengths and scale time intervals in such a way that we can probe the interaction that causes gravitation.

This is much the same way that - prior to the discovery of the electon - we knew that there was electric charge in an atom. By carefully setting up experiments, we were able to clarify how electric charge is distributed in the atom. We were able to localize the position of that charge to a point particle, albeit subject to certain constraints on this localization. And, perhaps most importantly, we were able to use this knowledge to more thoroughly predict the way that charge interacts wih matter by correctly describing the role of the electron in atomic interactions: what we today call electrochemistry. From the macroscopic point of view, nothing changed because of this discovery: scientists had known for millenia that charge exists, and that atomic interactions are somehow mediated by electric charge carriers - but by completing the picture at a higher resolution, we improved our understanding and very probably expedited the experimental discovery of many new material properties, chemical reactions, semiconductivity, nuclear physics, and so on. So, discovering the Higgs boson probably won't cause the ground to fissure open; but these investigations help physicists make better predictions and design new experiments to further our understanding of fundamental interactions.

All these questions come down to a very existentialist question: why isn't the universe simple? If the universe were simple, we'd all be one giant isotropic homogeneous soup of uniform goop. There would be no electric charge; no mass; nothing that interacted with anything else; nothing would ever change; the universe would have always existed and would exist into perpetuity, without ever deviating from itsmsteady-state equilibrium; and yet we instead observe a universe full of different types of particles, with different charges and masses, that interact to form nucleons and atoms and molecules and sentient mammals. If we can only figure out the mechanism that caused that very first fundamental symmetry breaking, then all the electric charge and nuclear forces and ribosomes and koalas all follow from first principles, given enough time to evolve, and the universe makes a lot more sense. This is why theistic philosophers like to equate symmetry-breaking with what is effectively a dialect of Creationism. Actual physicists tend to be predictive and descriptive, rather than epistemological, so physicists don't usually care for that equation. Nimur (talk) 21:43, 17 March 2013 (UTC)

isn't your definition of simple subjective and or presumptuous? I mean relative to the cosmos? Maybe it is simple and we just dont know it yet.68.36.148.100 (talk) 01:10, 19 March 2013 (UTC)

Fair point; substitute the term "simple" with the term "symmetric," as that is the term that physicists actually use. Particle physicists have the tendency to assume symmetry is the "natural" state over a particular variable, until they discover otherwise, by experimentally showing a specific type of symmetry-breaking. Nimur (talk) 02:13, 19 March 2013 (UTC)

I don't think the Higgs interaction can be called "the interaction that causes gravitation". For one thing gravity is its own interaction, but also the Higgs mechanism is only responsible for ~1% of the mass of ordinary matter (see Proton#Quarks and the mass of the proton). It's true that nothing like the world as we know it would exist without the Higgs interaction, but the same can be said for many other parts of the standard model. -- BenRG (talk) 17:50, 19 March 2013 (UTC)

Charge Selectivity of Glomerular Barrier in Nephritis

Hello. Why can anionic proteins pass through the glomerular barrier more easily than neutral particles if both are not repelled by the basement membrane? A link to a journal article would be appreciated. Thanks a lot in advance. --Mayfare (talk) 22:00, 17 March 2013 (UTC)

A quick search for "Charge Selectivity of Glomerular Barrier in Nephritis" on Google Scholar:[2] yields "About 2,710 results".

This one looks promising:  The glomerular filtration barrier function: new concepts

~Good luck on your exam!  ;)  ~:74.60.29.141 (talk) 04:51, 18 March 2013 (UTC):~