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?