slowing light down?


by 1MileCrash
Tags: light, slowing
1MileCrash
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Nov18-10, 05:22 PM
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I was just checking out an article that talked about scientists who were able to slow light to 38 mph. they went on to say that "einstien said that light could only travel at 3x10^8, but he never said it couldnt go slower!"

this seems plainly wrong to me. in my understanding, current theory would be violated if something were able to "cross the threshold" something that could travel the speed of light, or below it, or above it. if a particle were found that traveled faster than the speed of light, theory would still be applicable unless that same particle were able to slow down to lower than the speed of light (if i am incorrect here, please educate me)

even when we say "light travels slower through air than through a vacuum" this is essentially because refraction causes the light to travel further, thus making it appear to travel more slowly.

so how is this experiment explained? is it the same case as traveling through air?
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bcrowell
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Nov18-10, 05:26 PM
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It's the speed of light in a vacuum that is fixed according to relativity, not the speed of light in matter. Light goes about 2x10^8 m/s in water, but that has nothing to do with relativity. The following FAQ may also be relevant.

FAQ: Is the c in relativity the speed of light?

Not really. The modern way of looking at this is that c is the maximum speed of cause and effect. Einstein originally worked out special relativity from a set of postulates that assumed a constant speed of light, but from a modern point of view that isn't the most logical foundation, because light is just one particular classical field -- it just happened to be the only classical field theory that was known at the time. For derivations of the Lorentz transformation that don't take a constant c as a postulate, see, e.g., Morin or Rindler.

One way of seeing that it's not fundamentally right to think of relativity's c as the speed of light is that we don't even know for sure that light travels at c. We used to think that neutrinos traveled at c, but then we found out that they had nonvanishing rest masses, so they must travel at less than c. The same could happen with the photon; see Lakes (1998).

Morin, Introduction to Classical Mechanics, Cambridge, 1st ed., 2008

Rindler, Essential Relativity: Special, General, and Cosmological, 1979, p. 51

R.S. Lakes, "Experimental limits on the photon mass and cosmic magnetic vector potential", Physical Review Letters 80 (1998) 1826, http://silver.neep.wisc.edu/~lakes/mu.html
mathman
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Nov19-10, 03:36 PM
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Although neutrinos go slower than the speed of light, photons do not (in a vacuum)..

bcrowell
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Nov19-10, 04:07 PM
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slowing light down?


Quote Quote by mathman View Post
Although neutrinos go slower than the speed of light, photons do not (in a vacuum)..
I assume this is in reply to "The same could happen with the photon"? In that case, take a look at the Lakes paper, which is about setting an upper limit on the mass of the photon.
michelcolman
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Nov20-10, 01:19 PM
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The material absorbs and re-emits photons, which takes a while, therefore the light appears to move more slowly through the material. The photons that come out are not really the same ones that came in (although that may be a philosophically debatable issue).


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