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Comment by tim333 | Hacker News Reader

The reasoning sounds a bit iffy as in:

“Finally, we can understand why a cup of coffee equilibrates in a room,” said Tony Short, a quantum physicist at Bristol. “Entanglement builds up between the state of the coffee cup and the state of the room.”

I think you can understand coffee cooling quite well without any quantum stuff - the atoms in the coffee are moving faster than those in the room. There will be a tendency when one impacts with an atom of the air in the room for that to speed up and the coffee atom to be slowed.

Actual quantum entanglement is a strange and interesting thing. It's a shame people tag the term on things it is not really relevant to try to sound impressive for the most part.

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I don't know why tim333 is being downvoted. The particular quote does sound iffy - Boltzmann did explain the cooling of coffee via purely classical processes.

The basic idea - the particle system moves throughout phase space. The vast majority of phase space consists of areas where thermal equilibrium is reached. If you compute the time it would take for the system to return to a non-equilibrium state, it's way larger than the age of the universe.

(Note: I'm not an amateur, though I did leave the field a few years back.)

I think the author pretty clearly explained the connection:

    the arrow of time does not seem to follow from the underlying
    laws of physics, which work the same going forward in time as in
    reverse. By those laws, it seemed that if someone knew the paths
    of all the particles in the universe and flipped them around,
    energy would accumulate rather than disperse: Tepid coffee would
    spontaneously heat up

What he's saying is a bit unclear: we know the how, which is even a kind of plausible explanation as to why. What the author says we couldn't explain before is the converse: why does the opposite never happen? So I interpret that quote more like "Finally, we can understand why a cup of coffee only equilibriates..."

Statistical mechanics doesn't say why a coffee cup never heats up spontaneously - rather, it says that the probability of that happening is incredibly small. You end up with probabilities like 10^-100000. This doesn't require quantum at all, and is effectively the same.

Two objects always attract under gravity, never repel under gravity. Why is this less mysterious than thermodynamic equilibrium, which is some sense just an opposite of gravity (repulsive instead of attractive)?

I think of the arrow of time like this. We started in an extremely exceptional state of very low entropy. The state space is very high dimensional. Therefore the entropy is much higher, pretty much everywhere we go. In this space we do a random walk.

Now, the mystery that remains to be solved is: Why did we start with such an exceptional (low entropy) state?

One possible answer is based on complexity: we were more likely to start in a low entropy state because it had a description with low Kolmogorov complexity, or something like that. (K-complexity induces a probability distribution on the state space that is very different from the uniform distribution.) If that reasoning works, it seems to reduce the question "why did we start in a low entropy state?" to the question "why do we often observe hypotheses with low complexity to be true?", which is pretty much the problem of induction. I'd love to know the answer to that.

Another possible answer is anthropic: you see an ordered universe because otherwise it wouldn't create an ordered mind like you. Unfortunately, the anthropic answer seems to predict that we're in a tiny ordered bubble that will be swallowed by chaos any moment now, so the complexity-based answer looks more promising to me.

>Another possible answer is anthropic: you see an ordered universe because otherwise it wouldn't create an ordered mind like you.

Well, DUH.

This, so called, "anthropic" answer is still a naive non-answer. It's as if those that use the anthropic priniple in this sense have a very shallow understanding of Epistemology and/or Philosophy of Science.

It's obvious that the question concerns at least one level deeper than what the "anthropic principle" attempts to answer. Now just "how come" (possibillity of it being observed by an ordered entity) but "why" (actual mechanism causing this).

Because you can't distinguish maximal entropy from minimal entropy in a closed frame of reference.

I think the issue is that the quantum stuff is more real than the macroscopic systems.

Sure we can describe coffee cooling without quantum stuff, but the quantum stuff is the only part that is essential. Everything we experience aside from gravity must emerge from the quantum stuff. So the question wouldn't be "do we need the quantum stuff," it would be "do we understand why our macroscopic experience must emerge this way from the quantum stuff?"

I'm certainly no expert but I think the issue is that the rules of quantum mechanics work just as well in either direction of time.

The physical implications of entanglement might be strange but on a mathematical level it's all just vector addition so it's not really that strange.

The strangeness is the fact that they add and that the sum is as physically meaningful as the components. In a nutshell, so to speak.

Gleason's theorem speaks to that. Mathematically, any "well-behaved" density function can be expressed as linear combination of quantum states

your claim is that the quantum physicist cited is making the analogy just to sound impressive?

Actually I was thinking more of the quantum consciousness self help folks. The physicist cited may well have something.

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