olegt
Posts: 1405
Joined: Dec. 2006
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niwrad summarizes the discussion on the 2nd law of thermodynamics:
| Quote | It seem to me an important questions is: Maxvell’s demon does violate or does not violate SLoT? Just here not all commenters agree. In my opinion Maxvell’s demon can be considered in two main senses: (1) a machine, an artificial system (one-way filter); (2) a thermodynamic metaphor of intelligence.
(1) Maxvell’s demon as a machine. But there are many kinds of machines, and then we have again to distinguish.
(A) Maxvell’s demon as a mechanical-thermo machine. In this case I agree with Monastyrski #71 when says “the decrease in entropy caused by the intelligent demon is more than compensated for by an increase in the demon’s own entropy”. SLoT is not violated.
(B) Maxvell’s demon as a computer. If the Maxvell’s demon is a computer for which the Landauer’s principle is involved, according to givemeabreak #75, there is no increase of entropy because computation per se does not consume energy. SLoT is violated.
(2) Maxvell’s demon as intelligence. But what is intelligence in the first place? This is one of the above fundamental and difficult questions. Without knowing what intelligence is how can we to speak about Maxvell’s demon, which is one of its symbols? Intelligence can be considered in two main senses: (A) physical intelligence; (B) pure intelligence or metaphysical intelligence.
(A) If intelligence is a physical agent then energy is involved. SLoT is not violated.
(B) If intelligence is a metaphysical entity then no energy is involved. SLoT is violated. |
This is a sensible summary of his options. He is still wrong on 1B, and trivially so. Landauer's principle is a consequence of the 2nd law of thermodynamics. If something works in accordance with Landauer's principle, it isn't going to violate the 2nd law. The total entropy of the system gas + demon will be the same or increase.
Here is Charles H. Bennett, a computation theorist from IBM, explaining what Landauer's principle is about (emphasis mine):
| Quote | In his classic 1961 paper [2], Rolf Landauer attempted to apply thermodynamic reasoning to digital computers. Paralleling the fruitful distinction in statistical physics between macroscopic and microscopic degrees of freedom, he noted that some of a computer’s degrees of freedom are used to encode the logical state of the computation, and these ”information bearing” degrees of freedom (IBDF) are by design sufficiently robust that, within limits, the computer’s logical (i.e. digital) state evolves deterministically as a function of its initial value, regardless of small fluctuations or variations in the environment or in the computer’s other non-information bearing degrees of freedom (NIBDF). While a computer as a whole (including its power supply and other parts of its environment), may be viewed as a closed system obeying reversible laws of motion (Hamiltonian or, more properly for a quantum system, unitary dynamics), Landauer noted that the logical state often evolves irreversibly, with two or more distinct logical states having a single logical successor. Therefore, because Hamiltonian/unitary dynamics conserves (fine-grained) entropy, the entropy decrease of the IBDF during a logically irreversible operation must be compensated by an equal or greater entropy increase in the NIBDF and environment. This is Landauer’s principle.
arXiv:physics/0210005
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Bennett adds:
| Quote | Earman and Norton have pointed out with some justice that Landauer’s principle appears both unnecessary and insufficient as an exorcism Maxwell’s demon, because if the Demon is a thermodynamic system already governed by the Second Law, no further supposition about information and entropy is needed to save the Second Law. On the other hand, if the Demon is not assumed to obey the Second Law, no supposition about the entropy cost of information processing can save the Second Law from the Demon.
I would nevertheless argue that Landauer’s principle serves an important pedagogic purpose of helping students avoid a misconception that many people have fallen into during the 20’th century, including giants like von Neumann, Gabor, and Brillouin and even, perhaps, Szilard. This is the informal belief that there is an intrinsic cost of order kT for every elementary act of information processing, e.g. the acquisition of information by measurement, or the copying of information from one storage medium into another, or the execution of a logic operation by a computer, regardless of the act’s logical reversibility or irreversibility. In particular, the great success of the quantum theory of radiation in the early 20’th century led Gabor and Brillouin to seek an exorcism of the Demon based on a presumed cost of information acquisition, which in turn they attributed to the energy cost of a thermal photon, or in the case of Gabor’s high-compression Szilard engine [16], to the cost of recreating a static radiation field localized to one end of a long cylinder, into which the molecule would wander to trigger the power stroke. Landauer’s principle, while perhaps obvious in retrospect, makes it clear that information processing and acquisition have no intrinsic, irreducible thermodynamic cost, whereas the seemingly humble act of information destruction does have a cost, exactly sufficient to save the Second Law from the Demon.
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So options 1A, 1B, and 2A are in accord with the 2nd law of thermodynamics. The only construct violating it is 2B (pure, metaphysical intelligence). OK, I suppose.
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