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It’s known that the fundamental laws of physics are time symmetric. In electromagnetism $t$ can be changed to $-t$ and time reversal is also valid in particle physics, as there is the CPT symmetry.

Yet the following are still regarded as problems:

Why do we see time ‘flowing’ in one direction? Why do we see a dropped cup smash into thousands of pieces, but never see the pieces reassemble into a cup?

Why is there more matter than antimatter?

The first has caused scientists to wonder about different ‘arrows of time’, physical, psychological and thermodynamic. The matter/antimatter asymmetry has been a worry for cosmologists.

But can time be so fundamentally symmetrical that these issues cease to be a problem? Can time symmetry cause the apparent matter/antimatter asymmetry as described below? Is there any recommended work existing on this approach?

enter image description here

Let’s consider two universes A and B.

Universe A starts in a hot dense state, as is believed to exist at the moment of the Big Bang. There is no life and no psychological arrow of time. Just a dense ‘soup’ of randomly moving particles.

Universe A expands and cools. Atoms and stars form. Life evolves on one of the planets. The lifeform (people) invent cups. Someone drops one and it smashes into a thousand pieces. We wonder why we never see time flow the other way…

enter image description here

Universe B starts in a hot dense state, as is believed to exist at the moment of the Big Bang.

Time is flowing in the opposite direction to the time in universe A. Compared to Universe A there is a CPT transformation - (the motion of all particles is reversed, the charge of each particle is changed and there is a change in parity).

There is no life and no psychological arrow of time. Just a dense ‘soup’ of randomly moving particles. Let’s see what happens.

Universe B expands and cools. Atoms and stars form. Life evolves on one of the planets. The lifeform invent cups. One is dropped and it smashes into a thousand pieces. The lifeform wonders why they never see time flow the other way…


So, the point is:

If the laws of physics are truly time symmetric, then it would not be possible to decide which universe A or B is ‘our’ universe. Also it’s not possible to say there is more matter than antimatter, or that time is flowing ‘forward’. It’s equally valid to regard us as living in an antimatter universe with time flowing backwards.

For both universes A and B, time would appear to go forward and there would naturally appear to be more matter than antimatter.

Is this a valid argument? Can time symmetry cause the apparent matter/antimatter asymmetry?

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  • $\begingroup$ I think it's valid, but I don't know if it's true. ;) FWIW, here's a question I answered about a mirror universe resolving the antimatter abundance problem: physics.stackexchange.com/q/487301/123208 (but I don't think your question's a duplicate of that one). $\endgroup$
    – PM 2Ring
    Jun 30, 2021 at 9:08
  • $\begingroup$ The mirror universe looks interesting, thanks. The time symmetry of this question combines the two universes into one, where either direction is ok and it's not possible to tell which it is for the universe. $\endgroup$ Jun 30, 2021 at 21:12
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    $\begingroup$ Particle physics is not, in fact, time reversal symmetric. It is CPT invariant as you say, but CPT invariance does not imply T invariance. In fact, CP violation was a major experimental observation in the 60s (or 70s, can't quite recall). $\endgroup$ Jul 9, 2021 at 17:25
  • $\begingroup$ @ Richard Myers Yes, true, it just meant with the other transformations i.e. CPT invariance $\endgroup$ Jul 9, 2021 at 17:39
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    $\begingroup$ @ Andrew With the proposal there is no longer a matter-antimatter asymmetry and also no longer a time asymmetry. It's equally valid to say we live in an antimatter universe with time moving backwards. It actually removes an assumption that we usually make that time is moving forwards, so Occam's razor would favour it. $\endgroup$ Jul 12, 2021 at 8:31

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Your underlying assumption that "the fundamental laws of physics are time symmetric" is incorrect. T-symmetry violation has been observed in experiments.

Furthermore, CPT symmetry is the only combination of C, P, and T that is an exact and fundamental symmetry of all four fundamental forces. And since there is overwhelming evidence of CP violation by the weak force, T-violation is required in order to maintain CPT symmetry. Thus, experimental observation of T-violation is consistent with our theories.

With respect to matter-antimatter asymmetry, this is a current open question in physics and there are many ongoing experiments searching for evidence of symmetry violation that could explain this. However, the problem is fairly well-constrained by the Sakharov Conditions for a baryon generating process. These three conditions must be satisfied for unequal production of matter and antimatter: 1) baryon number violation, 2) C and CP violation, 3) interactions outside of thermodynamic equilibrium.

These conditions give an excellent starting place for experiments, and there are many experiments searching for new sources of CP violation and baryon number violation.

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