Speed of light in media other than vacuum

  • #1
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Main Question or Discussion Point

I don't know if this belongs in the Relativity forum or the Quantum Physics forum. Mentors, please move this if it is in the wrong forum.

Recently, I have read several posts on physicsforums claiming that photons always travel at c. The reason light (not photon) travels slower than c in media other than a vacuum is that the photons are absorbed and reemitted by atoms in the medium. I looked around on the wider web and found that this view is widespread. Yet there are several problems with it.

1. How do all the atoms in the medium conspire to delay the same amount of time from absorption to reemission?

2. How do the all the photons conspire so that each gets absorbed and reemitted the same number of times?

3. How do the atoms conspire to maintain the phase coherence of laser light?

4. How does the absorbing atom remember the direction that the photon was travelling before it was absorbed?

In addition, I have this question. In experimentally observed instances of absorption and reemission of photons, are the time delays compatible with this view? My guess (not scientific) is that the delay times are way too long to support this view of light in media. Does someone have data?

If the experimental delay times are too long, then in my opinion people are describing two different kinds of absorption-reemission mechanism. One of these has experimental evidence and the other does not. This speculative mechanism is then being applied in a rather classical way to a situation where quantum mechanics should prevail. In that regime, the more sensible the picture you draw, the more likely it is wrong.

And yet, the view is widespread. Am I the only person here who has problems with it?
 
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Answers and Replies

  • #2
russ_watters
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jimmysnyder said:
1. How do all the atoms in the medium conspire to delay the same amount of time from absorption to reemission?
2. How do the all the photons conspire so that each gets absorbed and reemitted the same number of times?
3. How do the atoms conspire to maintain the phase coherence of laser light?
[all 3 of these are the same question] They don't - if the medium is thin enough, substantial differences can appear. But remember, atoms are really small - a medium doesn't have to be very thick (fractions of a mm) before the difference between possible paths through it becomes infinitessimally small.
4. How does the absorbing atom remember the direction that the photon was travelling before it was absorbed?
The same way a nail "remembers" which direction to travel after a hammer hits it: conservation of momentum.
In addition, I have this question. In experimentally observed instances of absorption and reemission of photons, are the time delays compatible with this view? My guess (not scientific) is that the delay times are way too long to support this view of light in media. Does someone have data?
That's one of the more important parts of quantum mechanics (the behavior of light on the microscopic level), which is about the most successful scientific theory ever. But I'll let someone with more knowledge of it explain it better...
This speculative mechanism is then being applied in a rather classical way to a situation where quantum mechanics should prevail. In that regime, the more sensible the picture you draw, the more likely it is wrong.
And yet, the view is widespread. Am I the only person here who has problems with it?
No, you aren't the only person - the problem is that high school science classes mix the classical and quantum because they don't want to teach quantum mechanics. Now that you know about quantum mechanics, you think there are two conflicting explanations going on. There aren't: the one you learned in high school is wrong (an oversimplification).
 
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  • #3
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russ_watters said:
They don't - if the medium is thin enough, substantial differences can appear.
Like soap bubbles? What differences appear?

russ_watters said:
The same way a nail "remembers" which direction to travel after a hammer hits it: conservation of momentum.
Before the photon is absorbed, the atom has momentum and the photon has momentum. After it is absorbed, the atom has a different momentum and the photon no longer exists. How does the atom remember how much of it's current momentum to give up to the reemitted photon?
 
  • #4
russ_watters
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jimmysnyder said:
Like soap bubbles? What differences appear?
AFAIK, the reason you see the "swimming rainbows" is due to small differences in thickness. In addition, if it gets thin enough, there will be no interaction with a significant fraction of the incident light, and the bubbles become invisible.

http://www.newton.dep.anl.gov/askasci/phy00/phy00191.htm
Before the photon is absorbed, the atom has momentum and the photon has momentum. After it is absorbed, the atom has a different momentum and the photon no longer exists. How does the atom remember how much of it's current momentum to give up to the reemitted photon?
That, I'll need to leave for someone else...
 
  • #5
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russ_watters said:
AFAIK, the reason you see the "swimming rainbows" is due to small differences in thickness. In addition, if it gets thin enough, there will be no interaction with a significant fraction of the incident light, and the bubbles become invisible.
http://www.newton.dep.anl.gov/askasci/phy00/phy00191.htm
The issue in the url is not the number of atoms, but the thickness of the bubble. Assuming a 1 molecule thick bubble, it is the thinner and therefore denser bubble that is the invisible one, opposite of what the picture would have predicted.

I wrote the following before in a different thread. If the picture is correct, then we should see 'lucky' photons that don't get absorbed. They should travel at the speed of light. We should see some light outracing the speed of light in the medium. By the same token, we should see 'unlucky' photons emerging late. If such a phenomenon has ever been observed, then what form did it take and where was it written up?
 
  • #6
pervect
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jimmysnyder said:
I don't know if this belongs in the Relativity forum or the Quantum Physics forum. Mentors, please move this if it is in the wrong forum.
Recently, I have read several posts on physicsforums claiming that photons always travel at c. The reason light (not photon) travels slower than c in media other than a vacuum is that the photons are absorbed and reemitted by atoms in the medium. I looked around on the wider web and found that this view is widespread. Yet there are several problems with it.
To explain the apparent speed at which energy travels through a media via means of photons is a complex process. This is one reason I'm not as fond of this explanation as, say, Russ Watters is.

Basically, you have to use Feynman's ideas that photons take all possible paths, and interfere with themselves - aka Feynman's "path intergal" formulation.

This requires a certain amount of what seems like "conspiracy" to someone who is philosophically mostly a realist such as myself. But it is a logical consequence of using the "photon" to describe light.

The same class of issues arises in problems as simple as the reflection of a photon off of a mirror.

If you have a diffraction grating, the photon will be deflected by some angle that is dependent on the frequency of the photon.

If you have a solid mirror, the photon will be deflected by an angle where the angle of incidence is equal to the angle of refraction.

So the angle at which the photon is reflected depends on the global properties of all the atoms of which the refelctor is made. The absence of certain nearby atoms in the diffraction grating (as opposed to their presence in a solid mirror) can change the angle of reflection as compared to a solid mirror.

How did the photon "know" that the other atoms were missing, so as to "know" how to reflect? There is no more simple explanation that I'm aware of than the fact that it "knows" because it took all possible paths.

Note that this is discussed very well by Feynman in his book "QED", which is where the example of the diffraction grating vs mirror came from.
 
  • #7
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pervect said:
So the angle at which the photon is reflected depends on the global properties of all the atoms of which the refelctor is made.
I see that the absorption/reemission picture is widely accepted. But I can't find any experimental basis for this acceptance. I think the picture is too classical. In fact there is a site on the web that supports the picture, but calls it 'pinballing'. How evocative. I'm leary of evocative pictures of quantum processes. I would prefer to think of light as exhibiting a wave-particle duality rather than just being a particle. The very concept of the speed of a photon leaves me cold. The same goes for the trajectory of a photon. I prefer to think that the speed of light in a medium depends on the global properties of all the atoms of which the medium is made and of the light itself.
 
  • #8
Stingray
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jimmysnyder said:
I think the picture is too classical. [...] I'm leary of evocative pictures of quantum processes. I would prefer to think of light as exhibiting a wave-particle duality rather than just being a particle. The very concept of the speed of a photon leaves me cold. The same goes for the trajectory of a photon. I prefer to think that the speed of light in a medium depends on the global properties of all the atoms of which the medium is made and of the light itself.
I agree with most of this. Photons are not particles in the sense that grains of dust are. Trying to talk as though they were just leads to confusion.

For intuition, I think the classical wave picture is far more appropriate. Feynman's lectures have some details. Realistically, there are very few situations where QED is required to explain things properly, and it is not certainly not an easier theory to understand in either words or mathematics.

As for "lucky photons" and so on, there are actually many different concepts of speed that you can define for light. What we're talking about is called phase velocity. This can actually exceed c in some situations. Something called the group velocity is a little different, and is usually a better measure of actual signal speed. Still, there exist systems where this also fails to be a very useful concept. Again, thinking of light in terms of particles moving on well-defined trajectories is problematic. Even the apparently simple idea of speed is very difficult to pin down in any simple way.
 
  • #9
pervect
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One of the reasons I don't argue about this issue more is that many different approaches to QM can be made to work, and I'm not at all convinced I've found the "best" one.

But I do think it's definitely easier to explain certain aspects of the behavior of light using the wave model. In the wave model, the "speed of light" is not the speed of a photon (whatever that might be), but the actual speed at which energy is transmitted through the medium. This speed will be lower than 'c' in a media (and any theory will agree on this fact, including the photon/absorption model).
 
  • #10
JesseM
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Is it possible to explain why an electromagnetic wave would slow down in a medium using purely classical E&M? I seem to remember reading somewhere that you could, that you looked at how the incoming wave would induce oscillations in the charges in the medium, and that these oscillating charges would create their own electromagnetic waves which, when added together with the original wave, would look like a wave moving at a slower speed. Does anyone know more about this?
 
  • #11
russ_watters
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pervect said:
To explain the apparent speed at which energy travels through a media via means of photons is a complex process. This is one reason I'm not as fond of this explanation as, say, Russ Watters is.
Not being all that literate in QM, that's all I got. :grumpy: Yeah, I know, it gets you close, but not all the way there.
 
  • #12
pervect
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JesseM said:
Is it possible to explain why an electromagnetic wave would slow down in a medium using purely classical E&M? I seem to remember reading somewhere that you could, that you looked at how the incoming wave would induce oscillations in the charges in the medium, and that these oscillating charges would create their own electromagnetic waves which, when added together with the original wave, would look like a wave moving at a slower speed. Does anyone know more about this?
This is usually explained with an even simpler approach, which you've undoubtedly seen, whereby one includes the electromagnetic effect from dipoles directly into Maxwell's equations. To recap this very quickly, one defines the vector D = E + 4 Pi P, P being the field due to the polarization of dipoles in the media.

In linear media, P is proportional to E and acts to oppose it. Working through all the math, one finds that the new version of Maxwell's equations which apply to a media shows that light still propagates as a wave, obeying the wave equation, but its velocity is lower.

At high frequencies the electromagnetic field from dipoles "lags" a bit behind that of the applied field, having a different phase angle rather than being 180 degrees out of phase. This leads to a complex frequency dependent dielectric constant, and a lossy dispersive medium.
 
  • #13
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JJ Thompson developed a classical explanation for the slowing of light in a medium - I have a copy of the paper which I believe is still available on the internet - if anyone is interested i will hunt down the reference -

The whole idea of absorption and reemission is bothersome to many - including myself - what this idea depends from is an interpretation of SR which was not voiced by Einstein - at least not in connection with this problem - Einstein's concern in SR was with the constancy of the velocity of light in a vacuum - I don't know of any reference where he asserted photons could not be slowed by fields - we know they can be deflected and we know that em waves are slowed in a transmission line in accordance with the distributed properties of inductance per unit lenth and capacitance per unit length - and tis is true even where the transmission line comprises a vacuum coax
 
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  • #14
selfAdjoint
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yogi said:
JJ Thompson developed a classical explanation for the slowing of light in a medium - I have a copy of the paper which I believe is still available on the internet - if anyone is interested i will hunt down the reference -
The whole idea of absorption and reemission is bothersome to many - including myself - what this idea depends from is an interpretation of SR which was not voiced by Einstein - at least not in connection with this problem - Einstein's concern in SR was with the constancy of the velocity of light in a vacuum - I don't know of any reference where he asserted photons could not be slowed by fields - we know they can be deflected and we know that em waves are slowed in a transmission line in accordance with the distributed properties of inductance per unit lenth and capacitance per unit length - and tis is true even where the transmission line comprises a vacuum coax

Pardon me but this does not seem to be a productive way to learn physics. It is pointless to rely on quoting Einstein like some medieval scholastic quoting Aristotle to settle an argument. What Einstein did or didn't say about photons, or for that matter about relativity, is a historical question. What photons do is a question about modern understanding of physics, and you ought to address yourself to that.
 
  • #15
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yogi said:
JJ Thompson developed a classical explanation for the slowing of light in a medium - I have a copy of the paper which I believe is still available on the internet - if anyone is interested i will hunt down the reference -
The whole idea of absorption and reemission is bothersome to many - including myself - what this idea depends from is an interpretation of SR which was not voiced by Einstein - at least not in connection with this problem - Einstein's concern in SR was with the constancy of the velocity of light in a vacuum - I don't know of any reference where he asserted photons could not be slowed by fields - we know they can be deflected and we know that em waves are slowed in a transmission line in accordance with the distributed properties of inductance per unit lenth and capacitance per unit length - and tis is true even where the transmission line comprises a vacuum coax
I remember replying to you once and putting a lot of effort into explaining optical conductivity in matter in terms of both the conduction band effects (in metals) and the phonon interactions (in metals and dielectric). It appears that that was a total waste of time on my effort. Your continuing refusal to LEARN and to even look up a basic solid state physics text is appalling even AFTER you are told of sources to look for.

If you wish to simply continue in your ignorance on this matter, that's your choice. But if you wish to participate in such a discussion, please show at least SOME respect to the field of study and put some effort into learning stuff that has been WELL-ESTABLISHED.

Zz.
 
  • #16
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Well Zapper again - the Fritz Zwicky of the physics forums - I recall trying to explain the field theory approach to the slowing of light in a medium to you and you wouldn't listen because you are convinced that absorption and re-emission theory is sacrosanct -

Self-Adjoint: The whole notion of the emission/re-emission theory was contrived to rationalize the idea that photons must travel at c in the medium between the particles - this is an unreasonable extrapolation of what Einstein taught - its more important to read his papers that to read what others claim relativity to be

Why are relativist so sensitive to any questions that arise in connection with SR - if you want to pontificate - you should have taken up the cloth.
Einstein himself was doubtful that his own theories would survive the test of time - but if anyone on these forums raises an issue that calls some aspect of it into question - they are immediately insulted
 
  • #17
ZapperZ
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yogi said:
Well Zapper again - the Fritz Zwicky of the physics forums - I recall trying to explain the field theory approach to the slowing of light in a medium to you and you wouldn't listen because you are convinced that absorption and re-emission theory is sacrosanct -
Excuse me, but if you look in G.D.Mahan's text, the whole process of interaction with the phonons is described via field theoretic approach! And unlike your "field theory approach", I have tons of experimental verifications, even some that I've done MYSELF! Can you claim such a thing?

Yet, you continue to ignore the field of study such as FTIR and Raman scattering. Go look into it. You'll see that YOU are using the results of those experiments in your modern electronics.

Zz.
 
  • #18
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Quick short answer your probably looking for: Think of a prism... think of the sky, think of a lens... the effect of moving from one medium to another medium of different density distorts the passage of light, effects we see as magnification, refraction, diffusion.

I don't know much else on that, or have not bothered to think much on it... but I'd suspect increased electro magnetic field strengths and wave dissipation and interaction due to the increased number of atoms in the materials plays a part.
 
  • #19
DaveC426913
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Quick short answer your probably looking for:
After 5 years, he may have moved on...
 
  • #20
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After 5 years, he may have moved on...
Heh heh, There should probably be some kind of automatic warning from the server to someone who responds to an old, old post.

Anyway, I'd think this would be a topic that most everyone should be familiar with and interested in. It's treated fairly clearly and thoroughly in the initial chapters of Born and Wolf's "Principles of Optics" without getting deep into QM or Material Science. And yes, J. J. Thompson's mastery of the subject far preceded that of Born and Wolf.
 

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