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Alter the speed of light

  1. Oct 4, 2005 #1
    What does light travel through?

    I know sound travels through matter, like water or air.

    But what does light travel through?

    and if sound travels faster in denser objects

    then light must travel faster in "denser" space-time right?

    space-time? is that what is travels through?
  2. jcsd
  3. Oct 4, 2005 #2


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    There was a group of scientists that asked the same question in the early 1900's, when Einstein was in his glory. They called the theoretical substance 'ether' and they completed multiple experiments to try to detect its presence.

    For one of the experiments, they postulated that since light travels through space, space must be full of ether, and since the earth is moving through space, there must be some kind of 'ether wind'. All attempts to detect this 'wind' failed. Since nobody was ever able to detect it, the idea was given up.

    I'm not sure if anything recent has happened with this..
  4. Oct 4, 2005 #3


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    The only thing light has in common with sound is wave probogation. Sound is a longitudinal wave, light is transverse. Sound requieres a medium, light does not.

    There are very many threads on this board covering these questions. Have you tried using the search function? You might start with a search on the speed of light.
  5. Oct 4, 2005 #4
    Yes. In fact, a group of scientists (I have forgot the university/research lab) managed to get time going down to a speed very close to zero by making light pass through extremely dense mediums. Also, time is faster the higer you go up into the atmosphere. This is because the light has to pass through more atmosphereic gases and therefore it moves slower. As you know, the speed of time is based on the speed of light, so therefore time goes slower when it passes through more gasses.
  6. Oct 4, 2005 #5
    Wow! I think it was a collaboration of Zweistein, Dreistein, and Vierstein.

  7. Oct 4, 2005 #6
    I am not an expert on the subject, but I believe that when light is slowed down by something, in reality what is happening is that the light is being absorbed and re-radiated from the molecules of the material/substance. So technically, the speed is never truly changed, between the radiation and absorbtion, the light is still moving at c.

    As to your statements about time at the bottom, I am not quite sure what you mean...what you are thinking in your head may be correct, but how people take that could be very wrong.

    Time becomes dilated as matter moves faster or slower at a rate which is calculated using the speed of light.
  8. Oct 5, 2005 #7
    This is untrue. Time passes slower the closer you are to the earth's surface because of the strength of the gravitational field. This effect is known as gravitational time dilation. Time is not based on the speed of light in the way you are describing. You can check out this article for a very simple illustration of why this is the case. The basic idea is that the light is red shifted as it moves up in the atmosphere as it loses energy moving against the gravitational field. The frequency of the light decreases as this is what tells us that time is moving slower on the earth's surface than it is in orbit. This has been confirmed using Cesium clocks placed in jets flying around the earth and is factored into satellite systems (ex. GPS).

    Under circumstances I have studied, the frequency of light does not change when it moves into a material medium, only it's speed and wavelength do. There may be extreme circumstances where this does not hold, but I am personally not aware of them.
  9. Oct 5, 2005 #8

    Claude Bile

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    Only the group velocity is slowed in this fashion, which is distinct from the velocity of the actual wave itself (the phase velocity).

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