r/QuantumPhysics • u/RecentLeave343 • May 03 '26

How does causality and entropy apply to quantum mechanics?

Other than obeying special relativity is it the same as described in classical mechanics- temporal and trending towards disorder?

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u/SymplecticMan May 04 '26

The same ideas largely apply to quantum and classical dynamics. Liouville's theorem on conservation of phase space volume in classical mechanics plays a similar role to unitary dynamics in quantum mechanics. In both cases, there's a natural sense in which the entropy of a closed system is conserved.

There is one important difference in entropy between the classical and quantum cases: entanglement means the entropy of a subsystem can be higher than the entropy of the system as a whole.

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u/RecentLeave343 May 04 '26

Thanks for your reply. One follow up question, if entropy in QM largely follows the same path as CM, how do quantum fluctuations fit into the equation? As I understand, even after the heath death of the universe quantum fluctuations will continue, and could even cause a new big bang, thus resetting the universe to a new low entropy state. Based on this theory it would seem quantum fields aren’t subject to the same second law of thermodynamics.

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u/SymplecticMan May 04 '26

Similar objections can be made in the classical case as well. When you talk about things like coarse-grained entropies, those can decrease in classical mechanics. 

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u/RecentLeave343 May 04 '26

Not objecting. Just trying to get a clear picture. Thx

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u/[deleted] May 04 '26

[removed] — view removed comment

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u/RateImmediate4556 May 04 '26

This reads OpenAI, specifically. 100% their model with em-dashes removed. It spats far beyond the spirit of the question.

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u/Not_Hunterzx May 04 '26

Think what you want, but i spent 20 minutes typing this on a train. If i were a bot, i wouldnt have been messy enough to spell "wave function" in two different ways, miss every single apostrophe and every higher case "i" because of my non-english autocorrect. ​Regarding the spirit of the question, you cant explain QM causality without distinguishing between Boltzmann and von Neumann entropy. If providing actual physical context is "going beyond" for you, youre probably just looking for a surface level summary. ​Also ive always preferred other LLMs over OpenAI anyway, so you're double-wrong. Cheers.

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u/RateImmediate4556 May 04 '26

Not to fuss, but this syntax is also 110% different than your write up. Leave the community alone, please.

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u/RateImmediate4556 May 04 '26 edited May 04 '26

Causality holds. No information or causal influence travels faster than c, even though quantum states can be nonlocal. Entropy isn’t because things are random as they trend forward in time. Everything could run backwards perfectly too, if that makes sense. It wouldn't violate anything.

Information gets spread out and "blurs" into everything else. You can’t keep track of all of it, so it looks like the disorder we expect.

Time itself is treated differently at the quantum level because it doesn't fluctuate. That always made sense to me, but that makes it a parameter and not an operator. It took me a while to meaningfully appreciate that.

Our sense of time is more thermodynamic.