[JD557]

Unrelated to the word "daemon", but related to the article, I was a bit surprised by this assertion:

> Eventually, though, the theory of quantum mechanics showed why it wouldn't work.

I was familiar with the information theory arguments (the same presented in Wikipedia[1]). Is that why they mean here by "quantum mechanics" or is there another counterargument to Maxwell's daemon?

1: https://en.m.wikipedia.org/wiki/Maxwell's_demon#Criticism_an...

[n4r9]

I'm guessing that the daemon's ability to allow only fast molecules through the gate depends on knowing their position and velocity simultaneously?

[roywiggins]

It seems to come from measuring the particles at all. One result is that the demon has to store information about the particles, and erasing that information to free up memory increases the entropy of the gas/demon system.

https://en.m.wikipedia.org/wiki/Maxwell%27s_demon#Criticism_...

[BlueTemplar]

Yes, exactly, and this is what allows quantification.

http://www.av8n.com/physics/thermo/entropy-more.html#sec-pha...

With 'entropy' being an obsolete term for (lack of) information, and

> “classical thermodynamics” is a contradiction in terms.

[Vecr]

On the "information" part, I'm not sure that's correct, have you seen the papers that dispute the Bayesian versions of thermodynamics?

I think practically though, even before you hit anything "quantum", the requirement that you physically interact with the system is what dooms you.

[BlueTemplar]

No, got a link ?

Searching for them did bring me to an interesting discussion :

https://www.lesswrong.com/posts/YSFRazdoWXKHgNakz/link-the-b... (2015)

and then to :

https://bayes.wustl.edu/etj/articles/second.law.pdf (1998)

Which confirms my suspicions, but also sheds lights on how old the confusion is !

There are a bunch of assumptions that are easy to make (because they almost always are true), but very hard to get rid of when they aren't :

- that entropy is objective/ontological rather than subjective/epistemic

- that entropy is equivalent to disorder

- that temperature can always be defined

- that entropy is extensive

(- I think there was at least another one, but I had to do something else in-between and I don't remember now)

- oh yeah, maybe it was that there's a difference between a distribution and a macrostate ? (not sure about it myself)

Now, I don't know what the Bayesian framework can bring to the table here (not being sufficiently familiar with it down to the nuts and bolts of calculations), but if it can prevent us (and future students) from making these mistakes over and over and over again, it would be real progress.

[Vecr]

https://arxiv.org/abs/1508.02421v1

https://arxiv.org/abs/1508.02421v3 (updated, 2017)

They claim to fix the criticisms, see the section "The Bayesian arrow of time."

Who knows how well they did though.

As far as I can tell they're still making impossible assumptions, because certain Bayesian problems can't be calculated under a certain amount of energy, and some can't be calculated at all while embedded in spacetime (excepting time travel, and sometimes even then).

I think it's necessary to increase (on expectation) the entropy in a closed system when measuring, unless you take measuring to be magic and not a physical process.

[BlueTemplar]

Thanks, I need to look into those whenever I can find the time, however it also sounds like they are trying to fit General Relativity in there ?

I can't say for sure if this is doomed in the first place because GR infamously is not compatible with quantum mechanics (and trying to match them by force is only going to produce confusion and nonsense), or potentially revolutionary in actually managing to reconcile them...

[eru]

But the daemon doesn't need to know them all that precisely.

[n4r9]

I think you'd have to be pretty precise to know if it's heading towards a hole that's only just large enough for it.

[eru]

Why would you make the hole so small?

The main requirement for the hole is that it's small enough that (with high enough probability) only at most one or a handful of molecules will make its way through.

And that size is completely independent of the size of your molecules, and only depends on how many there are per unit volume. There's a lot of 'empty space' between molecules in a gas.

[n4r9]

Good question. I was going off the linked article which states:

> In the middle of the divider was a tiny gate, just large enough to admit one molecule of gas.

Still, that's quite a small hole relatively speaking. So you'd have to be fairly precise about both position and velocity. Potentially more than is allowed by Plank's constant. I dunno though, this isn't one of the counterarguments in the Wikipedia page, so probably you're right.

[eru]

Air molecules are large and massive, so their de Broglie wavelength is actually much smaller than their physical size (and that's much smaller than the hole needs to be to let in one at a time at ambient pressure and temperature), and you don't need to know their speeds all that well, eg if you just want to 'pump' all the air from one chamber into another.

So all in all, a classic description would work reasonably well. (Remember that quantum uncertainty is related more to de Boglie wavelength than physical size.)

[Vecr]

It probably (if the calculations are right) is unable to actually do much of anything useful (because it's too complex to avoid being extremely correlated with the rest of the universe ("embedded")), and even if it could it wouldn't be better than a standard ASI in most real-world situations.

That's assuming you aren't trying to claw back more energy than you lose, I'm pretty sure that's not possible to reliably do without crazy hypothetical physics.