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SR and speed of light in matter

  1. Aug 7, 2012 #1

    ShayanJ

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    I know that lorentz transformations are the same in matter and vacuum
    My question is,when light is propagating in matter,it certainly has a speed smaller than c
    Does this mean that we should treat it differently from light in vacuum?
    I mean in a vacuum,we say that time doesn't pass for light and all lengths are zero for it and we don't consider a "light reference frame". Do these change when we are considering light in matter?
    thanks
     
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  3. Aug 7, 2012 #2

    Simon Bridge

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    It's the speed of light in a vacuum that counts for SR.
    Kinda suggests the possibility of catching up to photons travelling in a media doesn't it?

    There is a big thread somewhere about whether the speed of light "really" changes inside a solid or if it is just an emergent phenomena from interactions on the quantum scale.
     
  4. Aug 7, 2012 #3

    ShayanJ

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    Well,in SR always light is sth so special because it travels with speed c
    But while in matter,its speed decreases and so it shouldn't be special any more
    What I don't understand is how?
    How can we treat light like other things?
    I mean its strange to think about light in the light cone!

    Well its because of the magnetic and electric properties of matter
    Of course it has a reason and that doesn't make it unreal!!!
     
  5. Aug 8, 2012 #4

    Simon Bridge

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    You need to decide what you mean when you are talking about light - do you mean the classical light waves or do you mean photons?

    But that suggests that the idea of photons travelling between interactions at a speed other than c has something wrong with it. I failed to find the thread :(
    Feynman lectured that photons always travel at c between interactions and used QED to derive the classical behavior of light (reflection, snell's law etc). In that case, photon's are special even in a solid.

    I'm just being cautious because I haven't got either reference on hand and I don't want to get into a fight :)

    Oh - hang on, found a FAQ:
    https://www.physicsforums.com/showpost.php?p=899393 [Broken]

    But see:
    http://physics.stackexchange.com/questions/1898/do-photons-gain-mass-when-they-travel-through-glass
    http://physics.stackexchange.com/qu...-optics-phenomena-explained-in-qed-snells-law
    ... many objections in the FAQ are addressed in these.
     
    Last edited by a moderator: May 6, 2017
  6. Aug 8, 2012 #5

    ShayanJ

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    Einstein proposed SR based on maxwell's equations after explaining photoelectric effect!
    Doesn't lead us to understand which nature of light he had in mind
    But I guess its better to consider the particle nature just to be up to date
    Although I won't lose the explanation for wave nature

    By "that" you mean the last sentences in my previous post?
    Well I guess those weren't so relevant because they are based on classical electromagnetism and when I said "that doesn't make it unreal!!!",After reading the first thread you suggested,I understood I was wrong
    But is that a generally accepted theory?
    Can we apply it to fluids?(I read it can't be applied to opaque solids)
     
  7. Aug 9, 2012 #6

    Simon Bridge

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    Any interposing material will reflect, transmit and absorb some of the EM incident on it ("light" is just the short name for "electromagnetic radiation"). Materials absorb, reflect, etc different proportions of the EM at different wavelengths. The material is opaque if it does not transmit visible wavelengths... however, it may still be transparent to wavelengths which are not visible.

    Now... what was the question?

    Note: you should also look at the other links for a balancing POV.
    Try not to confuse "wave nature" of light with classical "light waves". The "wave" of the "wave nature" is a probability-amplitude function while the classical "light wave" is an energy function.

    You cannot get away from the photon models - not just to be up-to-date, but because they simplify a lot of what you are thinking about.
     
  8. Aug 9, 2012 #7

    Dale

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  9. Aug 9, 2012 #8

    ShayanJ

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    The space between atoms in a material,is a vacuum so there is no reason that photons travel with a speed different from c
    But there is a probability that they get absorbed and then re-emitted by atoms and that accounts for the decrease in speed.
    That seems reasonable but there is a problem with it
    Atoms can absorb a certain discrete set of frequencies and that means for other frequencies,light shouldn't slow down in the medium.Is that observed?
    And Thanks DaleSpam!
     
  10. Aug 9, 2012 #9

    DrGreg

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    Did you read the FAQ linked to in post #4?
     
  11. Aug 10, 2012 #10

    ShayanJ

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    Yeah,I did.

    You mean this part?
    I've read before that atoms don't absorb photons which have the "wrong" amount of energy
    I guess it should be the same for other discrete systems
     
  12. Aug 10, 2012 #11

    Simon Bridge

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    Also read the other links in post #4.
    Electrons can absorb photons in any frequency - they just won't make an atomic transition for some, instead releasing the photon pretty much right away. This is how you get the discrete spectra.

    Remember - this is a QED model here ... the electron does not know it is in an atomic orbital, all it knows is that it keeps interacting with photons coming from all over the place. The discrete orbital, in this model, is an emergent behavior over many interactions. The actual electron has no reason to discriminate over which photons to interact with.

    The last two links in post #4 cover the QED description for the refractive index and Snell's law in fairly lay language.

    The FAQ seems to be working off a more semi-classical approach where the inter-atomic space (inter-electron - whatever) is filled with electric fields changing the permittivity and permiability there. The square-root of the product of these gives you the speed of light.

    In QED, permittivity and permiability of space noted are argued to be emergent properties from, more fundamental, electron-photon interactions. The "speed of light in a material" is not the actual speed of photons between interactions within the material. If you go to youtube you can also find Feynman expounding on this.

    I don't want to get caught up in a fight over which view is more correct ... I am just puzzled that the PF FAQ post is being favored over the other descriptions.
     
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