Physicists carry out Maxwell's "demon" thought experiment

http://www.scientificamerican.com/article.cfm?id=demonic-device-converts-inform

by waterhouse • 16 years ago

43 10 16 years ago

younata• 16 years ago

I wonder how much energy was used to manipulate the stuff around the bead, and how much energy would be used to just directly move it up.

btilly• 16 years ago

This is cool. But it should be noted that it has already been proven that there is a minimum amount of entropy associated with a single irreversible bit flip, and as a result Maxwell's demon cannot actually violate the second law of thermodynamics.

zbyszek• 16 years ago

That's the trouble with exciting phrases like "This seems to violate the second law of thermodynamics". It is known that Maxwell's demon does not violate anything, and the article does explain this later. But that's not quite the point.

I think this is a better explanation of the experiment: http://blog.joerg.heber.name/2010/11/14/the-demon-is-out-of-...

It's direct empirical support for the idea that "there is energy in information", and it demonstrates how even supposedly tried-and-trusted science is still subject to empirical validation.

calibraxis• 16 years ago

This is the part I never understood about Maxwell's Demon:

> If the demon itself doesn’t use up any energy (which can be done), entropy would decrease, right?

Would anyone care to explain? Energy isn't spent moving the door around? (Particularly since the demon has to make sure the fast-moving molecules don't go right back through it... though I suppose with a sufficiently large box for fast-moving molecules, this may not happen...)

rubidium2• 16 years ago

You are referring to Landauer’s principle with the "minimum amount of entropy associated with a single irreversible bit flip". While it is proven (or "trivial" if you believe some scientists on the subject), its applicability to all possible computation systems is not clear.

Recent theory and experiment have re-emerged to study this field both in the classical and quantum regimes. See http://arxiv.org/PS_cache/physics/pdf/0210/0210005v2.pdf for a nice review and "Breakdown of the Landauer bound for information erasure in the quantum regime" (http://pre.aps.org/abstract/PRE/v64/i5/e056117) for a specific case.

ErrantX• 16 years ago

I may be missing the point here... but this seems like a simple feedback system, just with a slightly more complex energy exchange (i.e. the experimenters observation and action) which hides the fact and allows it to be presented as a "paradox".

In actual fact this is just bad reporting, really. The experiment is about showing energy exists in information, which it seems to do successfully.

IgorPartola• 16 years ago

"The bead is driven as a mini-rotor, with a information-to-energy conversion efficiency of 28%."

What exactly are the units of information in this case? The statement doesn't make much sense as I think the OP just took the quote out of context.

A simpler way to visualize what's going on: imagine your trained dog picks up a ball and tosses it up in your hands and you catch it. Now the ball is 1.5 meters higher off the ground than it was before. E = m * g * h = 1 kg (it's a heavy ball, all right?) * 9.8 m/s^2 * 1.5 m = 14.7 J. That's 14.7 J you got for free. But the dog expanded more energy than that to pick up the ball, overcome gravity and air resistance and toss it up. The total system (= you + ball + dog) lost energy and increased entropy.

alphaBetaGamma• 16 years ago

s/energy/entropy/ and then proof read.

Physicists carry out Maxwell's "demon" thought experiment

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