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Freezing light contradicts Einstein?

  1. Oct 23, 2006 #1
    In "Frozen Light" an article written by Lene Vestegaard Hau, it talks about Lene's experiments with, literally, slowing down light by way of passing it through "ultra-cold clouds of atoms." The experiment was undertaken at Cambridge and Lene and a group of researchers were able to get light's speed down to 60 kilometers per hour.

    ~Einstein says that the speed of light is a universal constant.

    If all it takes is a temperature of near zero and a cluster of "frozen" atoms to slow light's speed, then how can light be a constant?

    Wouldn't that mean the cold regions, those reaching near zero temperatures, of space would constrict the speed of light and contradict Einstein’s theory of general relativity?

    And if light is not a constant, how can we be sure about the measurements of celestial bodies. Those are measured by the constant of light?
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  3. Oct 23, 2006 #2


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    All it takes is a piece of glass to "slow light down". The speed c is the speed of light in a vacuum, and it is that speed that is invariant in special relativity.
  4. Oct 23, 2006 #3
    "Roemer was able to calculate a value for the speed of light. The number he came up with was about 186,000 miles per second, or 300,000 kilometers per second." (http://www.colorado.edu/physics/2000/index.plthe [Broken]) This was from the measurement taken from Jupiter and its revolving partner Io. It was the discovery of the speed of light. Now, If cold areas in space interfere with light's speed, as cold atom clusters will slow down light, then how can we be sure of certain cosmological measurement?
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  5. Oct 23, 2006 #4


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    Empty space does not itself have a temperature, only stuff in that space can have a temperature, like hydrogen atoms or the photons that make up cosmic background radiation. Light slows through a medium because of interactions with the particles that make up the medium, and the particles in interstellar space (or even interplanetary space) are so diffuse that their effect on the speed of light would be negligible.
  6. Oct 23, 2006 #5


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    Those "frozen" atoms, as you call them, only occur under very special conditions. For one, they have to be much colder than the dust and gas between the planets. For another they have to be a lot closer together than the atoms in interplanetary space.(As an example, compare how tightly packed together people are in Times Square on New Years Eve, to the "Person density" on a typical street of a midwest town at 3:00 am on February 5th.)

    Just having atoms within a few degrees of 0°K isn't enough to have any measurable effect on the speed of light.
  7. Oct 24, 2006 #6
    In relativity under all conditions photons will travel at c.
    Of course we can setup experiments where photons are absorbed and emitted, a bit like a ball in a pinball machine taking more time to go down when it bounces off, that give a perception of light slowing down. :smile:
    Last edited: Oct 24, 2006
  8. Oct 24, 2006 #7


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    There's a subtlety here though, in the sense that a photon cannot be said to have a definite path between measurements, and if the medium is transparent the photons are mostly only interacting with the medium in a way that does not count as a measurement (equivalent to causing decoherence, I think), so you can't say the photon did or didn't collide with any particular atom, although in terms of something like the Feynman path integral you may be summing over a lot of individual paths where the photon was repeatedly aborbed and reemitted. There's an interesting discussion of this issue on this old thread from sci.physics.research.
  9. Oct 24, 2006 #8
    Of course you are right, that's why I started my sentence with "in relativity" :smile:
  10. Oct 24, 2006 #9
    The propagation of light in a medium can be considered as a succession of
    small steps of excitation and re-emission of the light by the individual atoms. Remember that light is an electromagnetic wave and its alternating electric field imposes oscillations of the electrons (of the individual atoms), which in turn will lead to the emission of photons (due to accelerated charge). One could say that in between two atoms, the photon moves at the speed of light (c as in vacuum) while time is "lost" at the atom locations. Using the expression that "light has been slown down" is thus misleading. It would be better to say that one has slown down the transmission of light through a medium.
  11. Oct 24, 2006 #10


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    Well, quantum electrodynamics is a relativistic quantum theory; and if you want to use classical electromagnetism, there are no individual photons to be absorbed and re-emitted, I think the classical explanation has to do with the incoming electromagnetic wave causing the charges in the medium to oscillate, which creates new electromagnetic waves, and somehow the sum of the original wave and the response waves would be a new wave whose phase velocity (or group velocity? some other velocity?) is lower than c.
  12. Oct 24, 2006 #11


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    Right. Now correct me if I'm wrong, but this would add a certain margin of error to the distance to Jupiter/Io and to the measurement of c - there would be some "blurring". Which is why no single measurement would suffice. (I don't mean literal blurring, I mean resolving power, or decimals of accuracy)

    Same goes for all other measurements of our universe, up to distant galaxies and quasars. But the errors cancel out, or at least cluster around a particular number, meaning we can state them with an extremely small margin of error - which we do.

    Perhaps someone can translate this into technical jargon?:rolleyes:
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