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Why is light travelling below c in different materials

  1. Dec 27, 2006 #1
    I know that in vacuum light travels with the speed c.
    But in different materials(eg water) the spedd is lower than c.
    I wanted to know what's happening that the speed decreases, and didn't came to a conclusion.

    given the energy of the photon ε=hf=√(m_0²c⁴ + p²c²)=pc=M·v·c
    since the rest mass m_0 is null, in a material the speed v it is equal to hf/Mc. However, M·v·c must be mc²(where M is the mass of the photon travelling the material, and m is the mass in vacuum), so, if the speed decreases the mass must increase, or the frequency must decrease(this is not possible, we're not speaking about the Compton effect, and the frequency of light characterize the source emitting the light, not the medium in which it travels)
    If the mass must increase, than by extrapolation, the rest mass must be ∞,which is not true.

    So my aproach of the problem is wrong, it must be. The speed of the photons in every material must be c, so in a material it must travel a greater distance than in vacuum. I mean, the photons could diffract multiple times, so this is causing them to travel a bigger distance, hence the slower light speed in materials. If diffraction is not the case, I cannot see why the speed of light should decrease.

    Excuse my bad english.
     
  2. jcsd
  3. Dec 27, 2006 #2
    You could think of it that way, OR you could think of it as TIME in the material must pass slower for the photon.
    BUT; As you are asking exactly how and why a photon seems to slow down in a medium (air, glass, water, etc) either way would require some way of explaining exactly how that could happen.
    IMO from the theories we have the best we can really say is we just don’t know other than to say “you can think of it as” something and make accurate predictions based on that. Longer distance, slower time, or Absorption & Re-emission none of these are currently justified IMO.

    One of the most popular explanations is the idea of “Absorption & Re-emission” where the photon is absorbed, held for a brief moment as if being bounced about by mirrors, and the released to continue on exactly the same path that it originally had. (Especially when thinking of solids, not water or air.)

    Although we have good theories on how atoms can absorb light of specific frequencies, and later re-emit the light at specific frequencies, sometimes at different wavelengths than the one first absorbed. But we only have good theories for how and why that works at selected wavelengths, plus the re-emission directions are not dependent on the direction of initial absorption.

    I don’t believe we have a good theory that explains how all light going through a medium seems to react and refract to some aspect of the medium to apparently slow down. Just as we do not have a good and complete explanation detailing exactly how and why the HUP works as it does. We just know that they both do work, and how to accurately predict results form them.
     
    Last edited: Dec 27, 2006
  4. Dec 27, 2006 #3

    Doc Al

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    Have a look at this entry from our Physics Forums FAQ: Do Photons Move Slower in a Solid Medium?
     
  5. Dec 27, 2006 #4
    Thanx for the comments.
    Especially the link, which is a verry good read. However, I have some other questions regarding this problem, but I must learn much more about what phonons are, and what are their properties(because i know almost nothing about them)

    Apparently, photons move slower in a gas or liquid medium also, and this, I presume, cannot be explained the same way as in a solid medium( I don't believe that phonons exists in a gas).

    sorry for the bad english.
     
  6. Dec 28, 2006 #5
    "All these years of conscious brooding about the nature of the photon have brought me no closer to the truth. Nowdays every Tom, Dick and Harry thinks he knows the answer, but he is mistaken"

    Albert Einstein - near his death in a letter to his lifelong friend Michaelangelo Besso

    There are several theories that are worth reading - I personally prefer the approach taken by JJ Thompson many years ago - it turns on the notion that the photon electic field interacts with the the electric field of the outer electrons of the atoms comprising the medium - the photon velocity is temporally reduced by the Inertia of the atom via the coupling between the outer electron and the nucleus. Feynman also developed a theory of why the effective velocity of photons is reduced in a thin sheet of transparent medium.
     
  7. Dec 28, 2006 #6

    HallsofIvy

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    "Phonons" are the "units" of sound- that's why they were referred to in terms of crystal binding. I'm not sure why they were even mentioned in a note on light. "Phonons" have nothing at all to do with "Photons", the "units" of light.
     
  8. Dec 29, 2006 #7

    Doc Al

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    Careful there. Yes, phonons are lattice vibrational modes. But solids can have both acoustic and optical phonon modes--photons can definitely interact with the latter. (Zz can give you more details.)
     
  9. Dec 29, 2006 #8

    ZapperZ

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    Phonons are extremely relevant here when dealing with optical transport through solid. It isn't by coincidence that one of the phonon branch is called the optical branch. This is the active mode when EM radiation passes through it. I believe that what I've written in the FAQ clearly indicates the relevance of phonons when photons are considered in solids.

    In liquids, what you have are the molecular vibrations mode due to the bonding between the atoms that make up the liquid (example between the two H's and O in water), and the weak inter-molecular bonding that forms that liquid. You have a similar situation in a gas also (without the inter-molecular bonding), depending on the gas. In air, you have plenty of different types of gasses for photons to intract. When you have only ONE type of gas, especially a noble gas, then you start to have discrete absorptions. In the slow-light experiment with the ultra-cold BE gas, you'll notice that only ONE particular frequency of light that is being slowed down or stopped. You do not get a spectrum of light that can undergo the same effect, because the "absorption" has been tuned to only interact at that particular frequency.

    Zz.
     
  10. Dec 30, 2006 #9
    I agree with you. “Phonons” seem to me just an abstraction to account for how energy can move through air or other mediums based on how the elements of the medium interact with each other. Moving as a wave, math can translate that into an artificial particle like thing that can be very useful mathematically. But that does not mean a phonon has individual reality. And I don’t see how the phonon or JJ Thompson explanations can account for the light slow down, except as a convent contrivance to satisfy until a correct explanation can be found. Both these theories logically should expect higher energy Photons to resist the slow down more effectively, thus slowing down less for a smaller diffraction. Although I suppose a contrivance of some kind could try to account for that as well. And as we all know it does not work that way, the shorter wavelength, higher energy photons bend (slow down) more.

    Students ask all the time if there is “some problem that still needs solving” that they could actually consider. This should be included as one that would be nice to find a good answer for. Just accepting incomplete solutions as good enough to discourage others form trying feels more like sour grapes of my older generation not wanting the new guys find something we could not (We can’t reach the grapes and if anyone does they are sour anyway, so stop trying).
    IMO they should be given more of a chance to try, by at least admitting we don’t have a solid solution to just how this works.
    And it likely means we are not understanding something at a fundamental level.
    It's stuff like this that should inspire thinking, the trick is to control the wild speculations, but spot a true inspiration when it comes.
     
  11. Dec 30, 2006 #10

    ZapperZ

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    I'm sorry, but this is a highly ignorant comment on an subject area that has been highly studied and, more importantly, used. You might as well claim that the 'glue' that holds a Cooper Pair that becomes part of one of the most tested and verified theory of all time is an "abstraction".

    Compare the following: your assertion versus the physics of phonons. What do FTIR, optical conductivity, neutron scattering, Raman experiments, etc. measure? Look at a photonic material and firgure out what exactly is the origin of the photonic band gap? It appears that this so-called abstraction is the ONLY mechanism that we have to explain ALL of the above.

    Furthermore, if you have a problem with phonons, then you should also lodge a protest to the whole of condensed matter physics (a subject area that is notorious for having phenomena with some of the highest degree of certainty) for propagating a whole slew of quantized excitation, ranging from quasiparticles to magnon, spinons, chargon, polaron, plasmons, etc...etc. But before you do that, ever wonder what type of experiments and physics that were used to derive the standard accepted values for "e" and "h"?

    The whole issue in this thread is that when you consider the propagation of light in solids, you have to consider not only the property of light, but the property of the material as well! I can explain why the index of refraction can be different not just between materials made up of the same atoms (carbon and diamond), but also why they are different in different crystallographic directions. Can you, without using any concept of phonons?

    Zz.
     
  12. Dec 30, 2006 #11
    If your describing those very real forces, well described by current theory, as being some kind physical “glue” fibrously stretching between them in a firm physical grip on both your darn right I’d call that an abstraction. Or do you have an observed physical description of that “glue” for us (get your patent papers in first).
    Never said I had a problem phonons read my post I said they can be very useful.
    Even you describe them a “Concept” (not physical things, right).
    So of course you can list off how the concept has been successful especially in matter physics what’s your point – we are talking about the physics of light and it’s refraction in various densities of matter of all types.
    BUT, You don’t speak to the missing part of the concept. Do you have an explanation for why high energy light bends more. Something that comes naturally from the principles of the phonon concept, not just shoehorned to fit with observation to make it that much better than the JJ Thompson approach.

    Talk about sour grapes, are you really that sensitive about your FAQ being the ‘end all be all’ on the subject. All I asked for is to allow people to look for a more complete solution.

    As to condensed matter issues - I know very little. But it only took me two minutes to find someone at MaxPlankInst that claims phonons on ‘optical conductivity’ come close but do not match observation.
     
  13. Dec 30, 2006 #12

    Gokul43201

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    Yikes, Randall. Where do you get your ideas of these "physical things" from?
     
  14. Dec 30, 2006 #13
    I'm the one saying they are not.
     
  15. Dec 30, 2006 #14

    Gokul43201

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    Exactly my point!
    Then really, is it wise of you to pass judgement on the state of the field?

    It does absolutely no such thing. I'm afraid, you'll have to spend a little more than two minutes to at least read the paper before you make completely erroneous claims about its content.
     
  16. Dec 30, 2006 #15

    ZapperZ

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    Yes, there ARE "observed physical descriptions" of that glue. I have measured one myself.

    In tunneling spectroscopy, by doing an McMillen-Rowel inversion of the the tunneling data, one can get EXACTLY the phonon spectrum in conventional superconductor. In optical spectroscopy on these material, the Drude peak matches exactly where the cut-off phonon spectrum is supposed to kick in. In the isotope effect, it matches exactly what one would expect the coupling constant to vary with the mass of the lattice ions. These are solid evidence of phonons being the glue for the Cooper Pairs that matches phonon description.

    But what ARE exactly "physical things", as if one can separate out our description with "physical things". Applying your argument, ALL of physics are nothing more than "abstraction". So why pick on phonons?

    This is very puzzling. Light bends? In a material? No kidding! There is no "bending". What you have is a KINK in the path when it encounters an INTERFACE between two different index of refraction at an ANGLE! Since we usually have a light beam of a finite width, different part of the beam front hits the interface at different times. So within a particular wavefront, the part that impinges on the interface first will get it its velocity changed earlier than the part that hits it later. This is why you get a change in direction! But it is the difference in speed that is the cause of this phenomenon, and it is the ORIGIN of this change in the apparent speed that is being addressed here, not refraction!

    It has nothing to do with being a "sour grapes". It has everything to do with not characterizing something that one doesn't know anything about, by your own admission. It is silly to dismiss the importance of the interaction of photons with matter since it is that matter that is causing the effects on the material. In fact, we USE such interactions to study the properties of matter. If the property of that matter has zero effect on the passage of light, then optical conductivity would not be such a useful tool in material science.

    And why don't you read a bit more on what kind of a MODEL most optical conductivity measurements try to fit - the Drude model based on typical Landau's quasiparticles! This is what does not fit! I can show you many "sum rule violation" of optical conductivity on more exotic material such as high-Tc superconductors. That has nothing to do with phonons not being there, or not part of the picture. For all material, the lattice vibration modes dictate the dominant property of the material. This is inescapable unless you live in a temperature below 1.4 K!

    Zz.
     
    Last edited: Dec 30, 2006
  17. Dec 30, 2006 #16
    You mean where it says "Although this goes in the right direction, it is not sufficient to describe experiment." Do you know some other meaning for "not sufficient"?

    Are you saying that sound travels though air and solids not by interactions of the particles that make up the molecules and atoms and that movement abstractly described as phonons. But that sound actually moves as real particles of phonons!? What are they actually made of? Same goes for Zz’s “glue”, not just a description of observations and calling it a "physical descriptions" like Zz does.. Molecules and atoms are made of electrons, neutrons & protons (up & dn quarks) and virtual photons interacting between them. What are you making these new particles of phonons and ‘Coooper Glue’ out of? If your going to call them real they need to be made of something.

    So what, we agree it the same thing, refraction comes from the change in speed, What I’m still missing is where your idea accounts for why the high energy photons slow down more than lower ones.

    Take your time I’ll check back next year.
    Have a good New Years.
     
  18. Dec 30, 2006 #17

    Gokul43201

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    You seem intent on proving that you've not read anything more than the abstract...and then misuderstood that too!

    What did you do in your two minutes? Google "phonon optical conductivity not sufficient", and then pick the first hit? I'm not going to dignify this discussion with a response to your question unless you first read the paper!
     
  19. Dec 30, 2006 #18

    ZapperZ

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    This is absurd.

    It is as if you have zero clue whatsoever on collective excitation and many-body physics. Oh wait, you haven't, by your own admission!

    For your information, none of us here are "making" these "new particles". They aren't new, and via QFT, these are as "real" as your photons. In fact, if you go by Phil Anderson's and Bob Laughlin's arguments, even your bare electrons, elementary particles, and your "photons" are emergent, many-body vacuum excitations. These are no different than the collective excitations of phonon, spinons, holons, etc. And the very fact that they have already described a whole zoo of phenomena from superconductivity to Fractional quantum Hall effect to the very nature of how we describe the semiconductor you use in your electronics are my evidence that these are as valid of a picture as anything you can come up with.

    Really! In what material does that happen? Show me a material with an infinite bandwidth for you to be able to claim that. If not, I can easily show you plenty of material in which the high energy photons not only did not get slow down, but it doesn't even get absorbed when compared to lower energy ones. Not only that, if I change the mass of the ions in the material, or simply change the crystal structure of it while keeping the atoms identical, I can tune in the index of refraction and also change what range of frequency gets transmitted and what range gets absorbed. Example: ordinary glass, fused silica, and quartz. This is a clear evidence that the material's property is essential in how any light interacts while going through it.

    The phonon structure in different materials can be very different, and the interaction of light in a metal and semiconductor can also be different. It depends on many aspects, such as the phonon spectrum of the solid, the density of conduction electrons, if any, the energy of the EM radiation, etc. It is why optical conductivity experiments can measure many different aspects of the property of a material, ranging from the charge carrier density, the phonon modes, and even the scattering rates of the quasiparticles in the material. This alone is ample proof that optical transport couples to many properties of the material. This is why optical conductivity is such a huge field of study, and graduates in this field are highly employable by industries!

    Read up on emergent phoenomena, and why your insistence that we do not introduce "new particles" is hystrical.

    Zz.
     
    Last edited: Dec 30, 2006
  20. Jan 2, 2007 #19

    WSB

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    multiple scattering ..


    This can be explained by considering multiple scattering. Multiple scattering has the effect as if the light would tavel slower. Scattering makes the path of light longer. So the classical description is an effective description which is correct but does not explain. Wanting to know more? Look at R. Feynman's book on quantum electrodynamics, a series of lectures given to the general public or see the chapter about refreactive index in the first or second book of Feynman's Physics lectures. - I myself was always surprized that no physics book does a good job in explaining this very important fact.
     
  21. Jan 2, 2007 #20

    ZapperZ

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    Actually, scattering, as in a collision of photons with heavy objects and causing it to change momentum, tends to be random and will result in a diffused, incoherent light. This is what you get when when shine a light into a fog, for example. So this explanation cannot account for the coherent light that emerges out of clear glass. This can only be explained via a mechanism that can somehow preserve both phase and momentum of the photon. Random scattering cannot do that.

    It is why, for example, the stopping of light in the Harvard experiment is so momentous. I mean, what's the big deal with stopping of light? Shine a flash light at a dark surface and voila! You've stopped light! What is different in that experiment is that the cold gasses preserve the both the phase coherence and the momentum of the light what was stopped, and then can "replay" it back exactly the way it came in. You can't do that when you stop light with a black paper, because the thermal vibrations that absorbed the light will cause a complete decoherence.

    Zz.
     
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