

Physicists carry out Maxwell's "demon" thought experiment - waterhouse
http://www.scientificamerican.com/article.cfm?id=demonic-device-converts-inform

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btilly
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.

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zbyszek
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-...](http://blog.joerg.heber.name/2010/11/14/the-demon-is-out-of-the-
bottle/)

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.

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calibraxis
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...)

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rubidium
One nice way to think about is to imagine a very light but strong door, and
very massive particles. The energy it would take to move the door back and
forth would be trivial compared to the energy of the system.

You can even use a reversible process (e.g. the doors on a spring so the
energy put into it to close the door is regained by letting the spring's
energy reopen it) to make it use effectively zero energy.

Then all the "energy" in the system is in the demon's observation of the
system, and not in the actual mechanics of opening and closing the door.

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younata
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.

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IgorPartola
"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.

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ErrantX
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.

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alphaBetaGamma
s/energy/entropy/ and then proof read.

