Physics - Demons, Engines and the Second Law in Scientific American of November 1987

This document contains comments about the article Demons, Engines and the Second Law by Charles H. Bennett In Scientific American of November 1987.
Since 1987 physicists have been trying to resolve the conundrum of Maxwell's demon: a creature that seems to violate the second law of thermodynamics. An answer comes from the theory of computing.
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page 88

One manifestation of the second law of thermodynamics is that such devices as refregirators, which create inequalities of temperature, require energy in order to operate.
This sentence is clear for a professional, but not clear for a layman. Specific what is: the second law of thermodynamics. See also: Reflection 2 - Proper Science It is better to write: The only way to cool water is by using a refregirator, which requires energy.
To invalidate this claim, please try to cool water without using energy.
Conversely, an existing inequality of temperature can be exploited to do useful work - for example by a steam engine, which exploits the temperature difference between its hot boiler and its cold condenser.
See also: Reflection 2 - Proper Science It is better to write: The only way to heat water is by using a heater, which requires energy. To invalidate this claim, please try to heat water without using energy.
Now let us suppose that such a vessel is divided into two poortions, A and B, by a division in which there is a small hole, and that a being, who can see the individual molecules, opens and closes this hole, so as to allow only the swifter molecules to pass from A to B, and only the slower ones to pass from B to A.
The issue is does this work, to be done by a being, involves any energy? Ofcourse it does.
He will thus, without expenditure or work, raise the temperature of B and lower that of A, in contradiction to the second law of thermodynamics.
Please build on.
Since Maxwell's day numerous versions of the demon have been proposed. One of the simplest creates a pressure difference (rather then a temperature difference) by allowing all molecules, fast or slow, to pass from B to A but preventing them from passing from A to B.
They are easy to propose, but do they work?
A one-way valve for air molecules could substitute for the compressor, effortesly collecting air from the surroundings into the high pressure tank
That would be wonderfull. But how does it work. To create compressed air requires energy.
One might think such an arrangement would violate the law of conservation of energy (otherwise known as the first law of thermodynamics), but it would not.
The only thing that is important is: does it work?
The energy for cutting concrete could be taken from heat in the air collected by the oneway valve; the air's temperature would drop as it passed through the machinery.
This could be a problem in the winter.
To analyze the demon's actions closely, then, one must understand some of the subtleties of the second law.
It can never be the second law, it are the subtleties of the underlying physical processes.
The second law was originally expressed on the restriction on the possible transformations of heat and work, but it is now seen as being a statement about the increase of disorder in the universe.
Proper science implies to clearly define the concepts used. See: Reflection 2 - Proper Science .
We humans cannot say anything about the total universe, because we cannot clearly define what it is.
According to the second law, the entropy, or disorder, of the universe as a whole cannot be made to decrease.
How do you measure the entropy of the universe? (As a whole?) If that is not possible than please don't use it as a physical argument.

page 90

The reason the molecules spread out to fill both chambers is mathematical rather than physical, if such a dinstiction can be made.
Understanding physical processes involves physical arguments, is based on physical details, and not on numbers, mathematical operations, chance or equations.

page 92

Even though a simple mechanical demon cannot work, perhaps an intelligent one can.
What we are discussing are simple processes. For example a waterfall and the generating of electricity. Falling water is simple. To return the water, and to close the loop, as a physical process is a very complex water. THere is no reason to believe that an intelligent demon can perform this task.
Indeed, some time after Maxwell had described the demon, many investigators came to believe intelligence was the critical property that enabled the demon to operate.
More detail is required about the definition of intelligence.
However even more important is to describe what the demon is supposed to do.
In the next step the engine determines which half of the apparatus the molecule has trapped in.
This is difficult to perform in reality and costs energy.
The engine's memory device has three possible states: a blank state to signify that no measurement has been made, an L to signify that the molecule has been observed in the left half of the apparatus and the R to signify that the molecule has been observed in the right half.
My first thought is that any measurement always cost energy at the expense of the environment.

page 93

One of the surprising consequences of the second law, however, is that it is impossible to see anything in a vessel at uniform temperature by the vessel's own glow.
That effect is not caused by the second law. The second law is no explanation for any physical effect. The explanation is part of the details of the effect. By experiment we know that in a dark room we can see nothing. By experiment we know that in a dark room we can see these objects when we use red light. See:
If one looks into a kiln in which pots are being fired, for example, one will see a uniform orange glow almost devoid of contrast, even though the pots in the kiln may have very different colors, brightness and surface textures.
The most important issue is if objects have the same temperature or not.

page 95

In order to see the objects in a furnace, then, one must shine light in from an external source, such as a flashlight that has a filament hotter than the furnace temperature.
In general to see any object, one must shine light towards that object. The reflected signal, against a dark background, makes the object vissible. That means an extra external source is required, to make the objects vissible.
Brillouin and others, armed with an understanding of the photon gas, argued that Maxwell's demon cannot observe the molecules it sorts without some kind of light source.
That inturn means that the demon needs some sort of eyes.
Therefore, they said, the demon cannot violate the second law.
What this means that sorting something requires energy, as part of a measuring process.
Such arguments, although they are not completely rigorous, seemed to substantiate Szilard's belief that acquiring a given amount of information entails producing a corresponding amount of entropy.
This type of text is to abstract to explain something.
Szilards Engine, modeled after a machine described in 1929 by Leo Szilard, seems to convert heat from its surroundings into work, contrary to the second law.
That means no energy is require.
But what is the source of this heat? See also
Moving the piston requires no work, since it compresses empty space.
To all it compressesion is a little doubtfull. The more important issue is to undo this requires a lot of work (energy)
(The one-molecule gas expands against the pistol.) Energy lost by the molecule as it works against the piston is replaced by heat from the environment.
This involves energy. This is an irreversible operation.
When the piston has returned to its original position(7), the memory is erased (8) and the cycle can begin again

page 96

Landauer identified several other operations that are thermodynamically irreversible.
Consider one physical bit which can have the value of 0 or 1.
In cycle 1 this bit has a value 1. In cycle 2 the value 0. In cycle 3 the value 0. And in cycle 4 the value 1.
From a physical point of view, in each cycle, a physical value has to be stored in that bit. It does not matter which value, but the previous value is erased. Each cycle as such is an irreversible operation.
What all these operations have in common if that they disgard information about the computers past state.
That is correct. In each operation, or cycle the previous physical state is erased.
To use the concept information, is a typical human interpretation.
In Landauer's phrase, such operations are logical irreversible.
They are physical irreversible. All operations are physical operations.
To prove this point I have designed a reversible measurement device, which measures and records the position of the molecule without undergoing any thermodynamically irreversible steps.
Please demonstrate this measurement device.
In general any measurement involves energy.

We have then, found the reason the demon cannot violate the second law: in order to observe a molecule, it must first forget the results of previous observations.
Please explain what physical means: to forget.
The physical fact is that a demon to do any work, needs energy.
Physical understanding requires that each physical action requires energy.
To only way to disprove this claim is to perform something physical without energy or effort.
If the demon had a very large memory, of course, it could simply remember the results of all its measurements.
This requires an infinite amount of energy.
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