Maxwell's Demon and the Uncertainty Principle

[iVenky]

Maxwell's demon measures the position and velocity of the particle. How can it do that when it violates the uncertainty principle? Does that mean uncertainty principle is unavoidable otherwise we will violate II law of thermodynamics as in the case of Maxwell's demon?

 

[Vanadium 50]

Maxwell's demon ... How can it do that

It isn't real.

 

[iVenky]

I agree but is it possible to explain it with uncertainty principle?

 

[Nugatory]

Maxwell's demon measures the position and velocity of the particle. How can it do that when it violates the uncertainty principle?

The logic of the Maxwell's Demon argument starts "If there were such a thing, then..." and considers the consequences of that assumption. That logic is valid whether or not such a thing can actually be created.

(It woud also be a good exercise to calculate the actual limits the uncertainty principle places on the effectiveness of a hypothetical Maxwell's Demon. The demon doesn't have to sort every single atom that comes by, just enough to produce a detectable effect).

 

[iVenky]

The logic of the Maxwell's Demon argument starts "If there were such a thing, then..." and considers the consequences of that assumption. That logic is valid whether or not such a thing can actually be created.

(It woud also be a good exercise to calculate the actual limits the uncertainty principle places on the effectiveness of a hypothetical Maxwell's Demon. The demon doesn't have to sort every single atom that comes by, just enough to produce a detectable effect).

So you are saying even within the limits of uncertainty, the demon could produce a detectable effect hypothetically?

 

[Nugatory]

So you are saying even within the limits of uncertainty, the demon could produce a detectable effect hypothetically?

I'm saying that you should try calculating an upper bound for yourself

 

[Demystifier]

Maxwell's demon measures the position and velocity of the particle. How can it do that when it violates the uncertainty principle? Does that mean uncertainty principle is unavoidable otherwise we will violate II law of thermodynamics as in the case of Maxwell's demon?

The Maxwell's demon (being real or not) is supposed to measure all initial data that, according to a given theory, completely determine the future evolution of the system. If the given theory is classical mechanics, then those initial data are positions and velocities of all particles. But if the given theory is quantum mechanics, then the initial data is the initial state in the Hilbert space, the future evolution of which is determined by the Schrödinger equation. The latter corresponds to a quantum Maxwell's demon, which is compatible with the uncertainty principle.