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Rest frames in the early universe?

  1. Feb 11, 2012 #1
    Hi folks, I asked a form of this question in another forum and didn't get a satisfying answer.

    As I understand it, there is a time in the early universe (t < 10–12 s) when particles have not acquired mass. According to special relativity, massless particles travel at c. Also according to special relativity, a body traveling at c does not have a rest frame. Therefore it would seem that prior to 10–12 s, there is nothing in the universe against which a rest frame can be established.

    If that is true, my question is: In what precise sense can we speak of a unique configuration of the universe, with unique spatial relations among particles, etc., at a time when nothing in the universe is capable of measuring such relations?

    To put it another way: In physics we learn early on that a human observer is not necessary, that an electron can function as an observer. So how is that expressed in the case when the universe contains only radiation?
  2. jcsd
  3. Feb 11, 2012 #2


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    Is this true? For example, the inflationary epoch takes place much less than 10^-12 seconds, yet the inflaton is massive. Do you mean that the standard model particles have not acquired mass before this time?
  4. Feb 11, 2012 #3
    I didn't know that the inflaton is considered massive.

    Another apparent answer is that while no individual massless particle has a rest frame, two or more massless particles moving differently produce a center of nonzero rest mass and therefore a rest frame for the system.

    That's interesting, if two light rays with different orientations have a rest mass, but neither does individually. Does anyone know the theory behind this?
  5. Feb 12, 2012 #4


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    Well, I don't know about not having acquired mass, but certainly in the very early universe the temperatures were so high that all particles were traveling at very, very close to c. But yes, there is a universal rest frame: the frame in which the the radiation that filled the universe had the same temperature in any direction. Move with respect to this rest frame, and the radiation would be blue-shifted in the direction of motion, red-shifted opposite that direction.
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