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Comment by vorg | Hacker News Reader

> There is no known physics theory that is true at every scale—there may never be

The article mentions how GR and QM have never been successfully combined together into a testable theory, how SR and QM produce infinities that need to be "renormalized" when combined together, and how there's no "direct" evidence that QED and QCD can be combined together.

The article didn't mention Thermodynamics that operates at scales between GR and QM, and how its reliance on a directed time dimension means it also doesn't combine with time-reversible GR, e.g. where has the information gone after a black hole evaporates via Hawking radiation, or what happens at the Cauchy horizon in a rotating black hole. QM is also time-reversible so I guess TD can't be combined with QM either.

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Not a physicist, but I do not think thermodynamics is a physical theory in the same sense that QED or GR are: it isn't about any specific physical system or interaction, but rather a general pattern when one looks at certain properties of bulk matter on (relatively) long timescales. Feynman had a good explanation of this, but for the life of me I can't remember the reference right now -- it's either in one of the lectures in the Character of Physical Law (try the one on the distinction between past and future) or one of the Feynman Lectures chapters on thermo.

IIRC thermodynamics per se does not have to "change" to adapt to systems where quantum effects play a role. How to link abstract thermodynamic concepts to the microscopic dynamics of a specific system in which quantum effects play a role would require a quantum theory, but for that we have quantum statistical mechanics (https://en.wikipedia.org/wiki/Quantum_statistical_mechanics) which is a fairly well-developed subject.

(This leaves out connections to relativity, special or general, about which I don't know much.)

My understanding is that the black hole's entropy gets encoded into a hologram on it's event horizon, and then is distributed with the Hawking radiation. Last I read, it sounded like it was still a somewhat open question, though.

Thermodynamics is time reversible in general. It's just not reversible once you postulate a small entropy starting position.

I am not aware of any incompatibility between thermodynamics and either QM or GR, except for stuff involving black holes.

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