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This question What is the relationship between how time is viewed in thermodynamics and how time is viewed in general relativity? is close to what I was wondering, but it didn't get into repetition specifically.

I'm not a physicist, my Scientific American level of understanding is that the universe is constantly changing, I think that is specifically the changing state of entropy, and that constant change is what creates the arrow of time.

Nothing is ever truly unchanging and so the constant change is how we perceive time.

But doesn't repetition fit in somehow?

If some state of the universe repeated, wouldn't that be a reserving of the arrow of time?

I suppose the answer might simply be no, and if I am confused by the repetition, that's just a problem with my perception.

But, anyway, do these theories of arrow of time and entropy say anything about repetition?

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  • $\begingroup$ Welcome to the site. Such prompt acceptance of an answer to such a major question is remarkable, but tends to discourage other answers, without necessarily prohibiting your approval (but not your "acceptance", in PSE's terminology) of them as well. Have you looked at the Wikipedia item on "Poincarre recurrence", mathematically proven since 1919 but very controversial in physics? $\endgroup$
    – Edouard
    Commented Feb 1, 2022 at 16:08
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    $\begingroup$ @Edouard, I had a sense that my question wouldn't attract more answers. If someone posts a better answer, I can switch my accepted answer. I'm also on the SE music theory forum and some of my answers there prompted questioners to change their accepted answer to mine :-) $\endgroup$
    – Michael Curtis
    Commented Feb 1, 2022 at 17:05
  • $\begingroup$ Although I'm an interpreter, & familiar with physics mainly through verbiage, your question & comment were remarkably helpful to me, as the related questions & their answers & commentary led me to the concept of "strain", with which I had been unfamiliar, and was useful in a debate (again, very heavily reliant on verbiage) on Quora. $\endgroup$
    – Edouard
    Commented Feb 3, 2022 at 21:15
  • $\begingroup$ Have a look at this video: youtube.com/watch?v=Sm7bXpNjGOM $\endgroup$
    – Yitian Chen
    Commented Nov 11 at 21:08

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The arrow of time and the second law of thermodynamics are the same thing. They are just different ways to look at it. We basically associate increases in entropy with forward time. If you were to see a system whose entropy was decreasing, you would perceive time to be flowing backwards (think of coffee and creamer suddenly un-mixing).

There are definitely systems that undergo cycles. Just think about heat engines which operate on cycles. However, just because the entropy of the heat engine is increasing and decreasing in a cycle does not mean that the entropy of the entire universe is doing this as well. Indeed, heat engines have to pump heat out of the system, and hence increase the entropy of its surroundings (the rest of the universe).

If we assume the universe is an isolated system then the entire state of the universe actually cannot be repeated. It is statistically impossible e.g. the second law of thermodynamics. Perhaps if the universe was not isolated then some external factor could in fact cause the universe to truly repeat.

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  • $\begingroup$ As you're not capitalizing "universe" (whose capitalization generally appears to represent a "local universe" or LU, in a multiverse which is usually inflationary), I'm assuming you're not considering we're in a multiverse. The local universes in the multiversal models I've seen appear generally to be causally-separated from each other, but their causal separations generally appear to vary, with some interactions between them during their beginning, and occasionally because of catastrophic circumstances (usually collisions) at other times. Am I reading you right, or incorrect, about that? $\endgroup$
    – Edouard
    Commented Feb 1, 2022 at 16:04
  • $\begingroup$ @Edouard, in this answer... "We assume the universe is an isolated system" and "if the universe was not isolated" ...he laid out either possibility. For a general summary didn't that make it clear? $\endgroup$
    – Michael Curtis
    Commented Feb 1, 2022 at 17:09
  • $\begingroup$ In a broad reading of the statement, it might; however, as BioPhysicist conditioned the last paragraph with "we assume", he appears to be referring to physicists generally, whereas, in fact, some of them (notably, Leonard Susskind) have had varying opinions about the plausibility of Poincare recurrence (potentially, an extremely general form of repetition), while others (such as Lubos Motl) have tended to favor it "in principle", and still others (notably, Laura Mersini-Houghton) seem to have felt it to have a spatially limited applicability, in a core region of a multiverse. $\endgroup$
    – Edouard
    Commented Feb 1, 2022 at 17:26
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    $\begingroup$ @Edouard I was not considering anything dealing with multiverses or differences in capitalization. I think you are reading more into the answer in an area that was not the purpose to be read into. I have modified the last paragraph slightly to help $\endgroup$
    – BioPhysicist
    Commented Feb 1, 2022 at 17:37

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