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What happens to relativity when light is slowed down?

  1. Oct 24, 2003 #1
    I know that light slows down when it goes through certain substances. Does that mean that relativistic effects would be more visible in, say, water than in space? Or would it be possible to travel faster than light in those circumstances, though still less than c?

    I don't understand relativity very well, so please excuse me if I sound like an idiot.
  2. jcsd
  3. Oct 24, 2003 #2
    Relativistic effects are related to the speed of light in vacuum (c), so you won't see them more easily merely by slowing light down in a medium. It is possible for matter to travel faster than light in a medium, though less than c: when charged particles do so, they produce what is known as Cherenkov radiation (kind of a "sonic boom for light").
  4. Oct 24, 2003 #3


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    You aren't an idiot, its just a trick!!

    Light does not ever slow down. It always travels at exactly C.

    Unfortunately high school physics teaches that light slows down in a medium. That is true only as an oversimplification and leads to a lot of problems for people who go on to learn more about physics (such as relativity). Personally, I think that the basics of relativity must be taught when the behavior of light is discussed.

    Anyway, how refraction REALLY works is that light hits the PARTICLES in the medium and gets absorbed and then re-emitted. The time delay between absorption and re-emission causes the AVERAGE speed of transit through the medium to be slower than C even though when the light exists as light it is traveling at C.
  5. Oct 24, 2003 #4
    Re: Re: What happens to relativity when light is slowed down?

    Light slowing down in a medium is a perfectly viable -- and relativistically correct -- concept in both classical and quantum electrodynamics, if by the "speed of light" you mean the phase velocity of the wave (or wavefunction).
  6. Oct 24, 2003 #5
    Re: Re: What happens to relativity when light is slowed down?

    the index of a media is derived from ¦Å and ¦Ì. They are factors which reduce/enhance the effect of the electric field and magnetic field, so does it to the transmiting of radio wave.

    What if light is absorbed and emitted in medium? Emission is hard to maitain a uniform direction, how could u explain the direction of light in water?

    As we all know, a material can only absorb a photon which has certain frequency match with the energy gap between to energy step. how could u explain all light with different frequency can transmit in water, if the transmission mechanism is as what you've stated.

  7. Oct 24, 2003 #6


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    The direction of emitted photons is random, with a perhaps smidge of varition caused by the initial adsorption. Reflections appear where they do simply because it is the shortest time path between your eye and that object. I would recommend a book called QED By Richard Fynman. He addresses this topic prety well in laymen terms.

    For a photon to be adsorbed by a atom or molecule there only has to be a energy band corresponding to the energy of the photon. Water is a large molecule and has MANY availble orbitals for the electrons to move to.
  8. Oct 24, 2003 #7


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    I have always wondered about this:

    What about in BECs?
  9. Oct 25, 2003 #8
    So, what actually causes cherenkov radiation?
  10. Oct 25, 2003 #9
    See, for instance,


    Summary: a charged particle passing through the medium will polarize molecules in the medium as it travels, which depolarize after it has passed. The depolarization emits light waves. If the charged particle travels faster than light in the medium, then the waves emitted by depolarization form a coherent "shock" wavefront, like a sonic boom.
  11. Oct 26, 2003 #10
    Re: Re: What happens to relativity when light is slowed down?

    what is their speed in this radiation?
  12. Oct 26, 2003 #11
    Re: Re: Re: What happens to relativity when light is slowed down?

    What does "their" refer to?

    A charged particle that produces Cherenkov radiation travels at some speed between the speed of light in the medium, and the speed of light in vacuum. The Cherenkov radiation itself travels at the speed of light in the medium, as long as it is in the medium, as you would expect.
  13. Oct 26, 2003 #12


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    Re: Re: Re: What happens to relativity when light is slowed down?

    Conservation of linear momentum.
    You just described the mechanism. If something can't absorb a photon and hold its energy, it is unstable and quickly re-emits the photon. But it does absorb and re-emit the photon. This is part of the same oversimplification that I described before. Simply saying that a medium absorbs or transmits light is incomplete and leads to incorrect conclusions about what that means.
  14. Oct 27, 2003 #13
    Re: Re: Re: Re: What happens to relativity when light is slowed down?

    if the photons are obsorbed and re-emitted, can we consider this process as a two dimension collision case?

    if so, conservation of linear momentum does not guarantee the output particle has the same direction of input paticle. Part of energy of input particle(photon) may be shifted to molecule of media, and the freqency of photon may changed for it's energy is reduced..

    then the color of the light is changed...

    could u give me a clear/detailed explain?
  15. Oct 27, 2003 #14


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    Re: Re: Re: Re: Re: What happens to relativity when light is slowed down?

    Why do you suppose objects have a characteristic color?

    Emitted photons travel in nearly random directions. The emitted photons may or may not be of the same frequency as the adsorped, it depends on the decay rate and if there is a favored decay path for the electron. Again this is the mechanism which detemines the color of things.
  16. Oct 27, 2003 #15


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    Re: Re: Re: Re: Re: What happens to relativity when light is slowed down?

    What you are describing (for the most part) does happen some times. In the yellow lens on your turn signals for example. If the output particle (not exactly an accurate characterization of light, but I'll let it go) has a different energy then that means some of the energy was absorbed by the particle. Extra energy becomes heat.

    And certainly light can be relfected or refracted in random directions. For example, on a white piece of paper.
  17. Oct 31, 2003 #16

    Im a little confused here... what exactly DOES happen when photons stike an interface between 2 media? People mentioned that it is absorbed and transmitted and hence the slow down. But why the particular direction change... and how can we explain Snell's law on the basis of collisions.. and what is 'shortest time path'? Can I have a formal definition of that plzz?

    I thought the electron never gets excited at all... it only gets excited if it has a energy band. Plz correct me if im wrong.

    Also, how exactly is colour produced? For example.. I have been taught that in co-ordination compounds of d-block elements, the coloured ion arises from d-d electron transistions because of the splitting of the energy level of the d-orbitals under the effect of the attached ligands. I wonder.. if the colour arises due to preferential absorbtion of some wavelengths of light by the electron transition between d orbitals, then shouldnt the same wavelength be emitted when the electron falls back to the lower d orbital? Why arnt we able to see those photons?

    Integral pointed to QED by Feynman... I would like to know if there is some good online resource on QED as well.... possibly free :smile:

    Thanks for any help

  18. Nov 4, 2003 #17
    Re: Re: What happens to relativity when light is slowed down?

    just wondering: what governs whether a photon is emitted by a substance or absorbed (ie why does visible light pass through glass, but not through aluminium)

    oh, and how come light travels through glass in a straight line, if it constantly gets absorbed and reemitted?? why doesn't it get scattered all over the place??

  19. Nov 5, 2003 #18
    Why light is slowed in a medium is a question that has arisen before on these boards - and never answered with any proof other than assertions re absorption and re-emission. This notion may not be correct - it would seem that the individual atoms would appear to radiate at different frequences and in different directions - something not generally observed in most optical media. The mechanism may be entirely different - photons may be slowed as they pass near the fields of the electrons and nuclei - rather than being absorbed and re-emitted - it might even be related to close gravitational encounter with the particles that comprise the medium -while we don't think of G effects as being very strong on the scale of atoms - they could be if the photon passes very close because of the inverse dependence - sort of a Shapiro effect on a small scale multiplied by the billions of occurrances that would be experienced in passing through a solid or liquid, i.e., the photon path gets effectively lengthened as it passes by the many perturbing influences of the nearby atoms
  20. Nov 5, 2003 #19


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    Do you expect to be given a graduate level course in QM on these boards? What sort of "proof" do you require? If you had even a basic understanding of these matters you would see that all who are educated in QM say exactly the same thing. Your personal lack of understanding is not necessarily shared by the world at large. Perhaps you should be reading closely the posts of Chroot, Ambitwistor, Tom, Marcus and others who have knowledge of these things. What you have posted above is not based on anything other then your personal believe. I for one do not much like seeing this sort of thing posted in place of Current Physical theory. Please restrict your replies to that realm. Your beliefs can be posted in Theory Development.
  21. Nov 5, 2003 #20
    Integral - your posts reveal a total lack of appreciation for anything other than unfounded dogma - there are no predictive theories that explain the magnitude of the slowing based upon the idea of absorption and re-emission... if you cant tolerate new ideas you should have been a preacher - I have read many of your posts in the past - they are often plain wrong - you used to post information about some accident you had that affected your abilities - are you sure you have fully recovered??? If Einstein after many years of pondering the nature of the photon did not understand its nature - how do you claim such a talent???
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