Talk:Landauer's Principle

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[edit] The argument on one of the links is plainly wrong

I had a read of "Logic and Entropy Critique by Orly Shenker (2000)"

I Am Not A Physicist, but it seems to me that it is flat wrong. The author states:

Bennett's construct is a measuring device that determines which of two chambers contains a particle, and stores the information in a memory (the key with notch and peg). The device is a clever combination of gears, in which the only possible source of dissipation is friction. Bennett rightly concludes that this device shows that measurement cannot be associated with any minimum amount of dissipation. This conclusion is based – as it should - on the details of the counter example, regardless of any general or abstract argument that might be offered regarding the entropy of measurement.

It seems plainly wrong that "the only possible source of dissipation is friction". For the machine to move from the "ready" state into another state, the gears have to move. Even assuming a frictionless environment, if the gears have any mass at all, then this means that kinetic energy must be imparted to them. When the peg drops into a notch, this kinetic energy must be dissipated.

One can reduce the amount of energy involved by making the gears lighter, or by moving them more slowly. But then environmental heat becomes a problem. Even in a hard Vacuum, at any temperature above absolute zero the machine is going to be buffeted with photons at infra-red frequencies, which carry energy and momentum. If the gears are too light, it will take so little energy to accellerate them that these photons wil scramble the works. If it moves to slowly, it will take so long to move from A to B that once again enough photons will hit it to screw it up.

You can reduce this effect by reducing the temerature of the environmrnt - but that's the entire point.

So ... should this link be dealt with? Am I wrong? Pmurray bigpond.com 22:59, 22 February 2006 (UTC)

[edit] kT ln 2 is not necessarily Joules

A pet peeve of mine is when people give a dimensioned expression such as "kT ln 2" (which is already dimensioned in energy units) and then suffix it with some arbitrary energy unit such as Joules. This is technically incorrect since the expression "kT ln 2 Joule" would have dimensions of energy squared, since it multiplies the correct value kT ln 2 (already dimensioned in energy) by the arbitrary amount "1 joule" (also dimensioned in energy). Properly speaking, Boltzmann's constant k itself is not a number, but is a *physical quantity* that is dimensioned in units of energy over temperature, and it is *not* associated with any particular unit such as joules. You could express the very same physical constant using eV or ergs or any other unit. The expression "kT ln 2" only gives the number of Joules if you (say) measure k in J/K and T in K, and then drop all units from the result before adding Joules back in with your suffix, but none of these choices is dictated by the expression itself. So, I changed "joules" to "amount".

[edit] von Neumann-Landauer / Landauer bound

This expression for the minimum energy dissipation from a logically irreversible binary operation was first suggested by John von Neumann, but it was first rigorously justified (and with important limits to its applicability stated) by Landauer. For this reason, it is sometimes referred to as being simply the Landauer bound or limit, but if we wish to be more generous to its originator, we may prefer to refer to it as the von Neumann-Landauer (VNL) bound instead.

With all due respect to von Neumann for the idea, Wikipedia isn't the place to make up new terminology. Unless demonstrably established terminology, we shouldn't prefer to refer here to it as VNL, regardless of our generosity.