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Travelling at the speed of light

  1. Jun 14, 2005 #1

    Art

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    I read somewhere that Einstein's interest in the properties of light stemmed from a question he posed to himself "What would light look like if you matched it for speed?" This then led to his theories of relativity. The question though is still valid, because although he determined that the speed of light in a vacuum is a universal constant, light moves slower through other mediums depending on the refractive index of the medium. In fact it is even possible to travel faster than light for example electrons can through water which has a refractive index of 1.33. So is it possible to answer Einstein's original question and if not why not ie is there a law in physics preventing such knowledge, is it only an apparent reduction in the speed of light as a consequence of it taking a longer path due to ricochets off molecules or is it just a matter of limitations in our technology preventing us from conducting such an experiment? IMHO I imagine it is only an apparent reduction due to a longer path taken. Can anyone confirm this?
     
    Last edited: Jun 14, 2005
  2. jcsd
  3. Jun 14, 2005 #2
    Please can you elaborate how can electrons travel faster than light? ... 'c' is the limit which relativity imposes on speed , and talking of technology , we have done experiments at speeds close to 'c' and all have always abided by the rules of relativity.
     
  4. Jun 14, 2005 #3
    He never said faster than light in a vacuum (c). He said faster than light, e.g. in water.
     
  5. Jun 14, 2005 #4

    JesseM

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    But the inspiration for his thought-experiment was the fact that Maxwell's laws say light always travels through a vacuum at the same speed, so implicitly I think it's clear he wasn't talking about travelling alongside light moving through a medium.
     
  6. Jun 14, 2005 #5

    Art

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    Apologies, I thought my post was pretty unambiguous but to clarify I was talking about light travelling through a medium. Electrons can travel faster than light when both are travelling through a medium such as water.
     
    Last edited: Jun 14, 2005
  7. Jun 14, 2005 #6

    JesseM

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    I understood that you were talking about light travelling through a medium, my point was that the context of Einstein's thought-experiment had to do specifically to do with light travelling through a vacuum, light travelling through a medium isn't really relevant to this thought-experiment. So, when you asked "So is it possible to answer Einstein's original question", the answer is still that it's impossible to travel alongside light travelling through a vacuum, which is all his question was about.
     
  8. Jun 14, 2005 #7

    JesseM

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    I think the classical explanation would not be that the incoming electromagnetic wave just slows down when it enters the medium, instead it'd be that the medium responds to the incoming wave with its own electromagnetic waves, and the sum of the original wave and the new waves is a wave moving slower than c. See the explanations on this page, for example, or on this sci.physics thread, where one poster gives a simplified picture of what's going on:
    At a quantum level, my guess would be that the individual photons are still travelling at c between the particles of the medium, but that they are periodically being absorbed and re-emitted, which slows their progress. That's just a guess though, I don't know much about quantum electrodynamics.
     
  9. Jun 14, 2005 #8
    If you were a light ray in a vacuum, you can definately say that 1. There would be no concept of mass 2. You would not 'interact' with anything: an interaction would either consist of a transition into a particle or a change in wavelegnth. 3. There would be no concept of space because you could not observe light emmitted by other matter.

    So we can conclude that mass, velocity along with spatial dimension is relative only to 'inertial observers' (ones with mass). They do not exist for any bosons, or entities that travel at c.
     
  10. Jun 14, 2005 #9

    Art

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    In the article I read, Einstein wondered what a light wave/particle would look like if you travelled alongside it at the same speed. Would it look like a frozen wave or a particle or whatever. Relativity then showed you could never achieve this situation of equal speeds in a vacuum however it is theoretically possible if light travels at less than c in other mediums and so my question is the same as Einstein's original with the proviso in a medium where the speed of light is less than c and so no longer a constraint on the experiment.

    However it appears from the first article you referenced light does not slow down in a medium with a refractive index greater than 1 but rather the photons that exit are not the same as the ones that entered and the 'swapping' times are what gives the appearance of the slowing of the speed of light. However the 2nd reference talks of a retarded wave which suggests it does slow down so now I'm confused again. :confused:
     
    Last edited: Jun 14, 2005
  11. Jun 14, 2005 #10

    Aether

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    How about in the case of gravitational refraction? Doesn't light actually slow down in a vacuum when the refractive index of the vacuum is greater than 1, and speed up when it is less than 1? And aren't the photons that exit an experiment the very same as the ones that entered?

    How to pace a photon: 1) Cause the photon to orbit within the event horizon of a black hole; 2) Orbit outside the event horizon of the black hole; 3) Send me a post card when you have some results, but before you have released them to the general public.
     
    Last edited: Jun 14, 2005
  12. Jun 14, 2005 #11

    Integral

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    No, the speed remains the same, the velocity is effected by the presence of a massive object. This means that we observe a direction change of light passing near a star or other massive object. So the photon does not speed up or slow down.. ever.
    Not necessarily, in fact probably not.

    There are many troubles with this though experiment. Not the least of is how do you know you are pacing a photon? It is impossible to get such information out of a black hole.
     
  13. Jun 14, 2005 #12

    Aether

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    The travel time of light varies as well as direction, so if the speed does not vary then it is the distance that varies?

    I meant in the case of gravitational refraction experiment, are photons discontinuous in free flight?
     
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