The first law of thermodynamics, better known as Conservation of Energy, says that you can't create energy from nothing: it prohibits perpetual motion machines of the first type, which run and run indefinitely without consuming fuel or any other resource. According to our modern view of physics, energy is conserved in each *individual *interaction of particles. By mathematical induction, we see that no matter how large an assemblage of particles may be, it cannot produce energy from nothing - not without violating what we presently believe to be the laws of physics.

This is why the US Patent Office will summarily reject your amazingly clever proposal for an assemblage of wheels and gears that cause one spring to wind up another as the first runs down, and so continue to do work forever, according to your calculations. There's a *fully general* proof that at least one wheel must violate (our standard model of) the laws of physics for this to happen. So unless you can explain how *one* wheel violates the laws of physics, the *assembly* of wheels can't do it either.

A similar argument applies to a "reactionless drive", a propulsion system that violates Conservation of Momentum. In standard physics, momentum is conserved for all individual particles and their interactions; by mathematical induction, momentum is conserved for physical systems whatever their size. If you can visualize two particles knocking into each other and always coming out with the same total momentum that they started with, then you can see how scaling it up from particles to a gigantic complicated collection of gears won't change anything. Even if there's a trillion quadrillion atoms involved, 0 + 0 + ... + 0 = 0.

But Conservation of Energy, as such, cannot prohibit converting heat into work. You can, in fact, build a sealed box that converts ice cubes and stored electricity into warm water. It isn't even difficult. Energy cannot be created or destroyed: The net change in energy, from transforming (ice cubes + electricity) to (warm water), must be 0. So it couldn't violate Conservation of Energy, as such, if you did it the other way around...

Perpetual motion machines of the second type, which convert warm water into electrical current and ice cubes, are prohibited by the *Second* Law of Thermodynamics.

The Second Law is a bit harder to understand, as it is essentially Bayesian in nature.

Yes, really.

The essential *physical* law underlying the Second Law of Thermodynamics is a theorem which can be proven within the standard model of physics: *In the development over time of any closed system, phase space volume is conserved.*

Let's say you're holding a ball high above the ground. We can describe this state of affairs as a point in a multidimensional space, at least one of whose dimensions is "height of ball above the ground". Then, when you drop the ball, it moves, and so does the dimensionless point in phase space that describes the entire system that includes you and the ball. "Phase space", in physics-speak, means that there are dimensions for the momentum of the particles, not just their position - i.e., a system of 2 particles would have 12 dimensions, 3 dimensions for each particle's position, and 3 dimensions for each particle's momentum.

If you had a multidimensional space, each of whose dimensions described the position of a gear in a huge assemblage of gears, then as you turned the gears a single point would swoop and dart around in a rather high-dimensional phase space. Which is to say, just as you can view a great big complex machine as a single point in a very-high-dimensional space, so too, you can view the laws of physics describing the behavior of this machine over time, as describing the trajectory of its point through the phase space.

The Second Law of Thermodynamics is a consequence of a theorem which can be proven in the standard model of physics: If you take a volume of phase space, and develop it forward in time using standard physics, the total volume of the phase space is conserved.

For example:

Let there be two systems, X and Y: where X has 8 possible states, Y has 4 possible states, and the joint system (X,Y) has 32 possible states.

The development of the joint system over time can be described as a rule that maps initial points onto future points. For example, the system could start out in X_{7}Y_{2}, then develop (under some set of physical laws) into the state X_{3}Y_{3 }a minute later. Which is to say: if X started in 7, and Y started in 2, and we watched it for 1 minute, we would see X go to 3 and Y go to 3. Such are the laws of physics.

Next, let's carve out a subspace S of the joint system state. S will be the subspace bounded by X being in state 1 and Y being in states 1-4. So the total volume of S is 4 states.

And let's suppose that, under the laws of physics governing (X,Y) the states initially in S behave as follows:

X_{1}Y_{1}-> X_{2}Y_{1}

X_{1}Y_{2}-> X_{4}Y_{1}

X_{1}Y_{3}-> X_{6}Y_{1}

X_{1}Y_{4}-> X_{8}Y_{1}

That, in a nutshell, is how a refrigerator works.

The X subsystem began in a narrow region of state space - the single state 1, in fact - and Y began distributed over a wider region of space, states 1-4. By interacting with each other, Y went into a narrow region, and X ended up in a wide region; *but the total phase space volume was conserved.* 4 initial states mapped to 4 end states.

Clearly, so long as total phase space volume is conserved by physics over time, you can't squeeze Y harder than X expands, or vice versa - for every subsystem you squeeze into a narrower region of state space, some other subsystem has to expand into a wider region of state space.

Now let's say that we're *uncertain* about the joint system (X,Y), and our *uncertainty* is described by an equiprobable distribution over S. That is, we're pretty sure X is in state 1, but Y is equally likely to be in any of states 1-4. If we shut our eyes for a minute and then open them again, we will expect to see Y in state 1, but X might be in any of states 2-8. Actually, X can only be in *some* of states 2-8, but it would be too costly to think out exactly which states these might be, so we'll just say 2-8.

If you consider the Shannon entropy of our uncertainty about X and Y as individual systems, X began with 0 bits of entropy because it had a single definite state, and Y began with 2 bits of entropy because it was equally likely to be in any of 4 possible states. (There's no mutual information between X and Y.) A bit of physics occurred, and lo, the entropy of Y went to 0, but the entropy of X went to log_{2}(7) = 2.8 bits. So entropy was transferred from one system to another, and decreased *within *the Y subsystem; but due to the cost of bookkeeping, we didn't bother to track some information, and hence (from our perspective) the overall entropy increased.

If there was a physical process that mapped past states onto future states like this:

X2,Y1 -> X2,Y1

X2,Y2 -> X2,Y1

X2,Y3 -> X2,Y1

X2,Y4 -> X2,Y1

Then you could have a physical process that would actually *decrease entropy,* because no matter where you started out, you would end up at the same place. The laws of physics, developing over time, would compress the phase space.

But there is a theorem, Liouville's Theorem, which can be proven true of our laws of physics, which says that this never happens: phase space is conserved.

The Second Law of Thermodynamics is a corollary of Liouville's Theorem: no matter how clever your configuration of wheels and gears, you'll never be able to decrease entropy in one subsystem without increasing it somewhere else. When the phase space of one subsystem narrows, the phase space of another subsystem must widen, and the joint space keeps the same volume.

Except that what was initially a *compact* phase space, may develop squiggles and wiggles and convolutions; so that to draw a simple boundary around the whole mess, you must draw a much larger boundary than before - this is what gives the appearance of entropy increasing. (And in quantum systems, where different universes go different ways, entropy actually does increase in any local universe. But omit this complication for now.)

The Second Law of Thermodynamics is actually probabilistic in nature - if you ask about the probability of hot water spontaneously entering the "cold water and electricity" state, the probability does exist, it's just very small. This doesn't mean Liouville's Theorem is violated with small probability; a theorem's a theorem, after all. It means that if you're in a great big phase space volume at the start, but you *don't know where,* you may assess a tiny little *probability* of ending up in some particular phase space volume. *So far as you know,* with infinitesimal probability, this particular glass of hot water may be the kind that spontaneously transforms itself to electrical current and ice cubes. (Neglecting, as usual, quantum effects.)

So the Second Law really *is* inherently Bayesian. When it comes to any real thermodynamic system, it's a strictly lawful statement of your *beliefs about* the system, but only a probabilistic statement about the system itself.

"Hold on," you say. "That's not what I learned in physics class," you say. "In the lectures *I* heard, thermodynamics is about, you know, *temperatures.* Uncertainty is a subjective state of mind! The temperature of a glass of water is an objective property of the water! What does heat have to do with probability?"

Oh ye of little trust.

In one direction, the connection between heat and probability is relatively straightforward: If the only fact you know about a glass of water is its temperature, then you are much more uncertain about a hot glass of water than a cold glass of water.

Heat is the zipping around of lots of tiny molecules; the hotter they are, the faster they can go. Not all the molecules in hot water are travelling at the same speed - the "temperature" isn't a uniform speed of all the molecules, it's an average speed of the molecules, which in turn corresponds to a predictable statistical distribution of speeds - anyway, the point is that, the hotter the water, the faster the water molecules *could be* going, and hence, the more uncertain you are about the velocity (not just speed) of any *individual* molecule. When you multiply together your uncertainties about all the individual molecules, you will be *exponentially* more uncertain about the whole glass of water.

We take the logarithm of this exponential volume of uncertainty, and call that the entropy. So it all works out, you see.

The connection in the other direction is less obvious. Suppose there was a glass of water, about which, initially, you knew only that its temperature was 72 degrees. Then, suddenly, Saint Laplace reveals to you the exact locations and velocities of all the atoms in the water. You now know perfectly the state of the water, so, by the information-theoretic definition of entropy, its entropy is zero. Does that make its *thermodynamic* entropy zero? Is the water colder, because we know more about it?

Ignoring quantumness for the moment, the answer is: Yes! Yes it is!

Maxwell once asked: Why can't we take a uniformly hot gas, and partition it into two volumes A and B, and let only fast-moving molecules pass from B to A, while only slow-moving molecules are allowed to pass from A to B? If you could build a gate like this, soon you would have hot gas on the A side, and cold gas on the B side. That would be a cheap way to refrigerate food, right?

The agent who inspects each gas molecule, and decides whether to let it through, is known as "Maxwell's Demon". And the reason you can't build an efficient refrigerator this way, is that Maxwell's Demon generates entropy in the process of inspecting the gas molecules and deciding which ones to let through.

But suppose you already *knew* where all the gas molecules were?

Then you actually *could* run Maxwell's Demon and extract useful work.

So (again ignoring quantum effects for the moment), if you *know* the states of all the molecules in a glass of hot water, it is cold in a genuinely thermodynamic sense: you can take electricity out of it and leave behind an ice cube.

This doesn't violate Liouville's Theorem, because if Y is the water, and *you* are Maxwell's Demon (denoted M), the physical process behaves as:

M1,Y1 -> M1,Y1

M2,Y2 -> M2,Y1

M3,Y3 -> M3,Y1

M4,Y4 -> M4,Y1

Because Maxwell's demon *knows* the exact state of Y, this is mutual information between M and Y. The mutual information decreases the joint entropy of (M,Y): H(M,Y) = H(M) + H(Y) - I(M;Y). M has 2 bits of entropy, Y has two bits of entropy, and their mutual information is 2 bits, so (M,Y) has a total of 2 + 2 - 2 = 2 bits of entropy. The physical process just transforms the "coldness" (negentropy) of the mutual information to make the actual water cold - afterward, M has 2 bits of entropy, Y has 0 bits of entropy, and the mutual information is 0. Nothing wrong with that!

And don't tell me that knowledge is "subjective". Knowledge has to be represented in a brain, and that makes it as physical as anything else. For M to physically represent an accurate picture of the state of Y, M's physical state must correlate with the state of Y. You can take thermodynamic advantage of that - it's called a Szilard engine.

Or as E.T. Jaynes put it, "The old adage 'knowledge is power' is a very cogent truth, both in human relations and in thermodynamics."

And conversely, *one subsystem cannot increase in mutual information with another subsystem, without (a) interacting with it and (b) doing thermodynamic work.*

Otherwise you could build a Maxwell's Demon and violate the Second Law of Thermodynamics - which in turn would violate Liouville's Theorem - which is prohibited in the standard model of physics.

Which is to say: **To form accurate beliefs about something, you really do have to observe it.** It's a very physical, very real process: any rational mind does "work" in the thermodynamic sense, not just the sense of mental effort.

(It is sometimes said that it is erasing bits in order to prepare for the next observation that takes the thermodynamic work - but that distinction is just a matter of words and perspective; the math is unambiguous.)

(Discovering logical "truths" is a complication which I will not, for now, consider - at least in part because I am still thinking through the exact formalism myself. In thermodynamics, knowledge of logical truths does not count as negentropy; as would be expected, since a reversible computer can compute logical truths at arbitrarily low cost. All this that I have said is true of the logically omniscient: any lesser mind will necessarily be less efficient.)

"Forming accurate beliefs requires a corresponding amount of evidence" is a very cogent truth both in human relations and in thermodynamics: if blind faith actually worked as a method of investigation, you could turn warm water into electricity and ice cubes. Just build a Maxwell's Demon that has blind faith in molecule velocities.

Engines of cognition are not so different from heat engines, though they manipulate entropy in a more subtle form than burning gasoline. For example, to the extent that an engine of cognition is not perfectly efficient, it must radiate waste heat, just like a car engine or refrigerator.

"Cold rationality" is true in a sense that Hollywood scriptwriters never dreamed (and false in the sense that they did dream).

So unless you can tell me which *specific step* in your argument violates the laws of physics by giving you true knowledge of the unseen, don't expect me to believe that a big, elaborate clever argument can do it either.

Another solid essay.

To form accurate beliefs about something, you really do have to observe it.How do we model the fact that I know the Universe was in a specific low-entropy state (spacetime was flat) shortly after the Big Bang? It's a small region in the phase space, but I don't have enough bits of observations to directly pick that region out of all the points in phase space.

Without discounting the predictive power of the second law, my confidence in our understanding of its physical basis has been seriously reduced. This after viewing a recent series of talks held at MIT, Meeting the Entropy Challenge.

Of particular interest were the discussions about the lack of an established definition of entropy.

the more uncertain you are about the velocity (not just speed)Isn't speed the same as velocity?

What is phase space? Is it the same as state space? You didn't define it.

Does that make its thermodynamic entropy zero? Is the water colder, because we know moreabout it?

Ignoring quantumness for the moment, the answer is: Yes! Yes it is!

I guess that if you stick your finger into the water it will still get burned, am I wrong?

And conversely, one subsystem cannot increase in mutual information with another subsystem,without (a) interacting with it and (b) doing thermodynamic work.

It is not entirely clear to me how you arrived at this conclusion.

Another pertinent example might be this: a metal shaft could spin so fast that its atoms' velocity distribution could be the same as that of the (hotter!) gaseous form of the same metal. Yet the spinning of the shaft does not evaporate the metal.

Why? Because, to a typical observer of the shaft, its degrees of freedom are significantly more constrained. So, since the observer knows more about the shaft (that its atoms are in a solid lattice that moves in relative unison), that

makesthe shaft colder -- and it allows you to extract more mechanical work from the shaft than if it were a hot gas with the same average particle velocities!"Isn't speed the same as velocity?"

Nope, speed is a scalar, while velocity is a vector.

Eliezer, I was going to point out that you never defined "phase space", but Roland beat me to it. It's a small hole in an otherwise excellent post.

Roland,

Eliezer is using the physics jargon meanings of speed and velocity: speed is a magnitude, a raw number; velocity is magnitude and direction together. A car might be travelling at a speed of 65 mph; if you include a direction, e.g., 65 mph east, then you've got its velocity.

Phase space from Wikipedia.

Added definition of phase space.

I guess that if you stick your finger into the water it will still get burned, am I wrong?Only if you're silly enough to stick in your finger at the wrong moment. Stick in your finger at exactly the right moment, and your finger will get colder while the water gets hotter - because you've timed it so that all the molecules next to your finger happen to be moving very slowly.

Of course you will usually have to wait so long for the right moment that all the protons evaporate before you see a chance to stick your finger in. And of course the trick relies on your knowing the exact behavior of the water, so that it has no entropy regardless of its temperature.

I guess that if you stick your finger into the water it will still get burned, am I wrong?

Only if you're silly enough to stick in your finger at the wrong moment.Ok, I thik we have a problem of definition here. You said the water got colder in a thermodynamic definition. But you agree that if I take a thermometer and insert it into the water and leave it there for a while it will still indicate 'hot water'. Right?

What I don't understand is your thermodynamic definition of colder. And I'm no physicist. Btw, I understand that with the information about the velocities you have power to make the water colder, but that doesn't mean that it actually will get colder(at least not right now).

Velocity is the same as speed, where speed or velocity is a scalar which has magnitude only and size. A state of change of speed or velocity however is acceleration and is a vector, which has both magnitude (size) and direction, but don’t confuse angular momentum with direction, look into angular momentum. You can also have constant acceleration that is not to be confused with constant velocity which is in no way acceleration. There is also a rate of change of acceleration and a constant for the rate of change of acceleration...

Good post, lays it down very nicely. Quick question:

Why is it that you can’t turn warm water into ice cubes and electricity, but reversible computing can use an arbitrarily small amount of energy? My guess is that computing (logic?) must be fundamentally different from

workin this sense. Logic is, in a sense, ‘already there,’ whereas work requires energy."But suppose you already knew where all the gas molecules were?"

I assume by this you mean I have an exact knowledge of position and momentum. Why should I suppose a scenario that is contrary to what I know of the uncertainty principle? Can't I reject it for the same reasons you reject the possibility of waking up with a purple tentacle.

I suggest a lot of caution in thinking about how entropy appears in thermodynamics and information theory. All of statistical mechanics is based on the concept of energy, which has no analogue in information theory. Some people would suggest that for this reason the two quantities should not be called by the same term.

the "temperature" isn't a uniform speed of all the molecules, it's an average speed of the molecules, which in turn corresponds to a predictable statistical distribution of speedsI assume you know this, but some readers may not: te... (read more)

Eliezer Yudkowsky (I'll remove the underscores from now on): I enjoyed reading this, but I don't quite understand all the references to the impossibility of turning warm water into electricity and ice cubes: You don't need extra information or violation of the laws of physics to do this. You could run a heat engine off the temperature difference between the warm water and the environment, and have that work drive a refrigerator. It's just that you couldn't exploit this phenomenon to make a perpetual motion machine.

I probably missed an implicit (or explicit!) qualification of all that, and if so, reader new to thermodynamics can just take this as a clarification.

Will Pearson, you can indeed summarily reject the possibility, and that's why I kept saying, "ignoring quantum". For quantumness, you would need a total description of the quantum state of the water, and this you can never obtain by any physical means. Though this is true even under classical mechanics: The third law of thermodynamics, on the impossibility of obtaining absolute zero, implies the impossibility of obtaining infinite-precision knowledge.

Silas, 'twas explicit: I said "sealed box".

I don't mind people ignoring elements of science when they are not important. e.g. ignoring general relativity when calculating ball trajectories.

But molecules and atoms are very much in the quantum realm. So it seemed to me to be like saying, ignoring special relativity, when things are travelling faster than the speed of light then this analogy holds, from this we can conclude blah. To me it seems unlikely to hold any insights.

I don't see why I should accept any conclusion drawn from the premises if I do not hold with the premises. But this brings up an ... (read more)

Maxwell's demon is ruled out by information theory. That's not

quitethe same thing as saying that it's Bayesian."Is the water colder, because we know more about it? ...Yes! Yes it is!"

You're kidding, right? Knowing something about a system doesn't change the system (neglecting quantum, of course). The statistical way to define entropy (as you mentioned) is the log of the number of microstates. The fact that you know all the trajectories/positions couldn't matter less to the glass of water, the only thing that matters is (using your jargon) the phase space volume it occupies.

Reshape the space for a second. Call it 6-D, with each particle a point, inste... (read more)

The fact that you know all the trajectories/positions couldn't matter less to the glass of water, the only thing that matters is (using your jargon) the phase space volume it occupies.What, precisely, do you think it means for a statistically viewed system to "occupy a volume of phase space"? When you talk about the "number of microstates", what exactly do you think you are counting?

I think we may have to Taboo the word "cold" in this posting. As I understand it, an object is colder than another if it has a lower temperature than another (in other words, the average kinetic energy of its molecules is lower than those of another object). Therefore, knowing the exact position and velocity of all the (classical) molecules in an ideal gas doesn't make the gas "colder" until you actually DO use Maxwell's Demon on it. Saying that an object that you know enough about to use Maxwell's Demon on is "colder" than another conflicts with my understanding of the word. It's not actually colder, it's only "potentially colder" (by analogy to potential energy), if that makes any sense.

Yes, we're arguing about words, but that's because we're getting confused. :(

two systems in thermal contact trade energy to maximize the net entropy of the ensemble.Actually the assumption is that two systems in thermal contact come to some equilibrium state.

Let this equilibrium state

maximize something, call it S, and use calculus.Energy is conserved.

Therefore the energy change in on system equals minus the energy change in the other, and the change in S wrt the energy change in each system has to be equal in both systems at the maximum of total S.

Call that change wrt energy the (inverse) temperature. Two systems in thermal conta... (read more)

You lost me there.

1) If Alice and Bob observe the system in your first example, and Alice decides to keep track precisely of X's possile states while Bob just says "2-8", the entropy of X+Y is 2 bits for Alice and 2.8 for Bob. Isn't entropy a property of the system, not the observer? (

Thisis the problem with "subjectivity": of course knowledge is physical, it's just that it depends on the observer and the observed system instead of just the system.)2) If Alice knows all the molecules' positions and velocities, a thermometer will still ... (read more)

Isn't entropy a property of the system, not the observer?Nope. It's a property of the observer, but one that behaves in such a lawful and inescapable way that it seems to you like a property of the system.

Your ignorance of next week's winning lottery numbers is a property of you, not just a property of the lottery balls, but good luck on ignoring your ignorance.

Someone elsewhere said:

Almost all the time, I stick with this idea: Temperature of a gas is the mean kinetic energy of its molecules.Aren't there vibrational degrees of freedom that also contribute to kinetic energy, and isn't that why different materials have different specific heats? I.e., what matters is kinetic energy per degree of freedom, not kinetic energy per molecule? So you actually do have to think about a molecule (not just measure its kinetic energy per se) to determine what its temperature is (which direction heat will flow in, compared to another material), even if you know the total amount of heat - putting the same amount of heat into a kilo of water or a kilo of iron will yield different "temperatures".

But the more important point: Suppose you've got an iron flywheel that's spinning very ra... (read more)

But the more important point: Suppose you've got an iron flywheel that's spinning very rapidly. That's definitely kinetic energy, so the average kinetic energy per molecule is high. Is it heat? That particular kinetic energy, of a spinning flywheel, doesn't look to you like heat, because you know how to extract most of it as useful work, and leave behind something colder (that is, with less mean kinetic energy per degree of freedom).Systems in thermal contact (by radiation of nothing else) come to the same temperature. That makes it pretty objective if one of the systems is a thermometer, whether it's heat or not.

To form accurate beliefs about something, youreallydo have to observe it.Does this not confuse

accurate beliefwithknowledge? Leaving aside doubts about whetherjustified accurate beliefis sufficient for knowledge (e.g., the Gettier problem), there is certainly more to knowledge than just accurate belief, and while I accept your statement for knowledge, it does not seem true for mereaccurate belief.I suppose the issue hinges on -- and perhaps this is your point -- whether

accuratemeansprobability of being correctorwhether it turns out to have been... (read more)Joseph Knecht:

The problem with your argument is that justification is cheap, while accuracy is expensive. The canonical examples of "unjustified" beliefs involve mis-calibration, but calibration is easy to correct just by making one's beliefs vaguer and less precise. Taken to the extreme, a maximum-entropy probability distribution is perfectly calibrated, but it adds zero bits of mutual information with the environment.

anonymous:

I don't see how your response addresses my concern that saying

accurate belief requires observationimplies unacceptable consequences for the man on the street, such as that his correct belief that the Giants would win on Sunday is nevertheless not an accurate belief.Eliezer, you seem awfully close to Shalizi's paradox. Could you address it?

Unless I'm missing something, Shalizi usually makes more sense than this.

1) Measurements use work (or at least erasure in preparation for the next measurement uses work). They do not simply magically reduce our uncertainty without thermodynamic cost. Even if you measure and never erase, the measuring system must be in a prepared state, cannot be used again, and still produces entropy if you are not operating at absolute zero / infinite precision, which you can't do (third law of thermodynamics).

2) Because we are not logically omniscient, we lose information we already have as the result of not being willing to expend the computational cost of following every atom. Liouville's Theorem preserves a volume of probability but it can get awfully squiggly, so if you preserve a simple boundary around your uncertainty, it gets larger.

3) Quantum universes branch both ways and create new uncertainty in their branched agents.

Done.

If you have the time I would be interested in seeing a mathematical description of a system that increases its mutual information with the environment, with the total entropy of the system+environment increasing.

I enjoyed this article. As several commenters have suggested, it seems not just counterintuitive but actually non-physical to say that the warm water has become colder just because I know more about it. The subjective nature of entropy that this seems to imply is absurd. As has been pointed out, a stationary thermometer in the water will show the same reading after my visit from Saint LaPlace as it did before.

I think the problem is resolved if we consider our system boundary to include not just the water, but the observer as well. The entropy of the water ... (read more)

Shalizi usually makes more sense than thisa sign to give it more consideration.

Your response seems to be that Shalizi assumes an ideal observer, while you assume an observer-in-the-system. That's fine, as far as it goes, but often you assume an ideal observer, and statistical mechanics is able to function with some kind of ideal observer. If you can build a model with an ideal observer, you should!

In particular, when you say that knowledge of particles makes something colder, makes it possible to extract work, you've gone back to the ideal observer.

More ta... (read more)

In particular, when you say that knowledge of particles makes something colder, makes it possible to extract work, you've gone back to the ideal observer.I think emphatically not! To extract work, you've got to be inside the system, extracting it.

If you take the perspective of a logically omniscient perfect observer outside the system, the notion of "entropy" is pretty much meaningless, as is "probability" - you never have to use statistical thermodynamics to model anything, you just use the deterministic precise wave equation.

I love this blog. Best blog ever.

Douglas Knight: "In particular, when you say that knowledge of particles makes something colder, makes it possible to extract work, you've gone back to the ideal observer."

Eliezer: "I think emphatically not! To extract work, you've got to be inside the system, extracting it."

I think what Douglas may be implying is that unless you are perfectly insulated from what you are getting knowledge of (e.g. and ideal observer), the act of getting knowledge of something will heat it up. As you are doing work and increasing the entropy in the surro... (read more)

Ridiculously good post, ridiculously good comments, in my opinion.

Indeed is one hell of a post, i am from computer science background, had to read the post 5 to six times and most of the comments at least twice, its worth it.

If someone is still following the post, i would like to know, can randomness of the particles be measured? or is it calculated according to probability? i remember vaguely from my college reading that entropy is random energy, so, for a perfect transfer X-> Y, how is the final state determined (because of the randomness).Arent accurate beliefs functions of randomness?

So after doing the Maxwell's Demon thing, you say that mutual information decreases, the entropy of Y decreases, so we are left with the same amount of total entropy:

M1,Y1 -> M1,Y1

M2,Y2 -> M2,Y1

M3,Y3 -> M3,Y1

M4,Y4 -> M4,Y1

However, I don't see why the mutual information would be lost; would the Demon know where he "put" the molecule, thus making the transition look more like:

M1,Y1 -> M1,Y1

M2,Y2 -> M1,Y1

M3,Y3 -> M1,Y1

M4,Y4 -> M1,Y1

This would of course shrink the phase space, violate the second law, etc. I just do not see how M would stay the same when Y changed (i.e. lose the mutual information).

While the notion of "entropy" seems to make a lot more sense when considered as observer-dependent, what continues to confuse me about this is what happens when you have time-reversed observers. If phase space volume is simply conserved, then the same principles apply to time-reversed observers, i.e., they also see entropy increasing. But this would imply that any time-reversed observer would have to draw boundaries very differently from us, and it's not at all clear how simply negating the 't' coordinate causes you to draw your boundaries in such a way that you know more about two gases when mixed rather than when unmixed. I feel I must be making some fundamental mistake here but I can't tell what.

So this is how warming and cooling spells work in the Rational Potterverse?

Article linked from Reddit, which I haven't read: Demonic device converts information to energy (Scientific American).

So in the following transformation:

X1Y1 -> X2Y1 X1Y2 -> X4Y1 X1Y3 -> X6Y1 X1Y4 -> X8Y1

You say that while true entropy has not increased (it stays at 2 bits), apparent entropy has, due to the observer not keeping track of X and just lumping its possible states into X2-X8. If this is the case, why doesn't observed entropy

decreaseas well, since phase space is preserved with the following?X2Y1 -> X1Y1 X4Y1 -> X1Y2 X6Y1 -> X1Y3 X8Y1 -> X1Y4

"it prohibits perpetual motion machines of the first type, which run and run indefinitely without consuming fuel or any other resource"

That's only right if you're able to extract work from it and it still runs undiminished.

Otherwise it's only a perpetual motion machine of the second type.

Liouville's theorem alone does not suffice to obtain the Second Law. You might want to look up the objections to Boltzmann's derivation of H-theorem made by Zermelo (wait long enough and the system will return to a state arbitrarily close to the original state, due to Poincare's recurrence theorem) and Loschmidt (reverse the speeds of all particles and the entropy will decrease to its original value). Boltzmann killed himself in a bout of depression because he could not find a satisfactory answer to these objections. More than a century later, we still don... (read more)

Do we have moderators who kill obvious spam like this?

Could the second law of thermodynamics also be understood as "the function between successive states as described by the laws of physics is bijective"?

Does this mean, then, that it is not merely

difficult, butmathematically impossiblefor any matter to ever reach 0 Kelvin? This would seem to violate Liouville's Theorem as stated here.When I worldbuild with magic, this is somehow automatically intuitive - so I always end up assuming (if not necessarily specifying explicitly) a 'magic field' or smth that does the thermodynamic work and that the bits of entropy are shuffled over to. Kind of like how looking something up on the internet is 'magic' from an outside observer's POV if people only have access nodes inside their heads and cannot actually show them to observers, or like how extracting power from the electricity grid into devices is 'magic' under... (read more)

I'm confused - what does "cold" and "hot" mean in this context? What predictions that I make on the water before knowing the trajectories of all the molecules should change, once that information is revealed to me, to resemble the predictions I would make if I believed the water was cold in the traditional meaning of the word?