Rest frames in the early universe?

[Karl Coryat]

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?

 

[cristo]

As I understand it, there is a time in the early universe (t < 10^–12 s) when particles have not acquired mass.

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?

 

[Karl Coryat]

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?

 

[Chalnoth]

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.

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.

 

[sardar]

I can understand your confusion and frustration with this concept. The concept of rest frames in the early universe is a complex and often debated topic in physics. However, there are some theories and explanations that can help shed light on this concept.

Firstly, it is important to understand that the concept of rest frames is based on the idea that there is a preferred reference frame in the universe, which is a frame of reference in which the laws of physics appear to be the same. In the early universe, it is believed that there was no preferred reference frame, as the laws of physics were still evolving and changing. This means that there was no absolute rest frame in which all particles could be measured against.

However, this does not mean that there was no way to measure the spatial relations among particles in the early universe. In fact, the theory of inflation suggests that the universe underwent a period of rapid expansion in the first fractions of a second after the Big Bang. During this time, the universe was expanding at a rate faster than the speed of light, which means that different regions of the universe would have been in causal contact with each other. This means that there would have been a way to measure the relative positions of particles in the early universe, even without a preferred rest frame.

Additionally, the concept of rest frames is often used in the context of special relativity, which does not fully apply in the early universe. In the early universe, the laws of physics were still evolving and changing, and the concept of massless particles traveling at the speed of light may not fully apply. This means that the idea of a rest frame may not be applicable in this context.

In summary, while the concept of rest frames may not fully apply in the early universe, there are still ways to measure the spatial relations among particles and understand the unique configuration of the universe at that time. The concept of rest frames may need to be redefined or adapted in this context, but it does not invalidate the ability to understand the universe in its early stages.