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Origin of 'Daemon' in Computing

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236 points wizerno | 5 comments | 20 Oct 24 00:35 UTC | HN request time: 0.001s | source
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JD557 ◴[20 Oct 24 08:35 UTC] No.41893914[source]▶
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...

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n4r9 ◴[20 Oct 24 09:40 UTC] No.41894230[source]▶
I'm guessing that the daemon's ability to allow only fast molecules through the gate depends on knowing their position and velocity simultaneously?
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1. eru ◴[20 Oct 24 14:11 UTC] No.41895498[source]▶
But the daemon doesn't need to know them all that precisely.
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2. n4r9 ◴[20 Oct 24 18:02 UTC] No.41897222[source]▶
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.
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3. eru ◴[21 Oct 24 00:17 UTC] No.41899612[source]▶
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.

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4. n4r9 ◴[21 Oct 24 07:24 UTC] No.41901549{3}[source]▶
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.

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5. eru ◴[22 Oct 24 01:27 UTC] No.41910295{4}[source]▶
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.)