Register to reply 
Maxwell's Demon revisited 
Share this thread: 
#1
Feb2412, 01:56 AM

P: 98

Hi Folks,
(Skip to next paragraph if you already know what M's D is.) Maxwell's demon was a counterargument to the second law of thermodynamics (and hence the first) involving two chambers of air connected by a trap door which this demon would open and close to let fast molecules go one way and slow ones the other. One chamber gets hotter and can perpetually drive a heat engine. The catch apparently lies in the assumption that the demon himself can be neglected but in reality one has to consider the entropy associated with his decision. But this argument makes no reference to the size of the air molecules. What if they were whopping great cannonballs? Can one still claim that the energetic equivalent of whatever bits he has to erase in his head is comparable with the power station you could drive that way? How do we even make the comparison between energy and entropy here? Now that I think about it, I don't really understand why it would have been a problem in the first place. If I milk kinetic energy out of either chamber, all the cannonballs are gonna slow down eventually. But they did teach me in school that breaking the second law was tantamount to breaking the first. Confused, Adrian. 


#2
Feb2412, 02:12 AM

P: 98

Another problem with the "erasing bits in his head" argument is that it doesn't mention the temperatures involved, or how fussy he is. The number of bits he has to erase is independent of whether he sets his threshold at 100^C or 1000^C. But surely the entropy reduction is different in the two cases.



#3
Feb2412, 07:59 AM

Sci Advisor
P: 5,508

You have to erase the bits to ensure the process is cyclic your end state is the same as the initial state.



#4
Feb2412, 10:38 AM

P: 98

Maxwell's Demon revisited
I know I have to erase the bits, but I don't see how that problem can compare with an entropy reduction that seems to be in totally different units.



#5
Feb2412, 12:40 PM

P: 789

Also, entropy is "missing information" and may be different for different people. If person A knows only the temperature, pressure, volume, etc of a gas, then A will calculate some entropy for that gas, but if person B knows the position and velocity (speaking classically, now) of every particle in that gas, then they will say there is no entropy. Person A will write the second law of thermodynamics as dU=TdSPdV+µdN (µ is chemical potential, you have to include that when possibly changing the number of particles). Person B will write the second law as [itex]dU=\sum m_iv_i^2/2[/itex] where the index i runs over all particles. It won't be the second law, really, it will be a mechanical equation. I guess I don't understand how the energy expended by the demon can compare to the energy of one "hot" cannonball being allowed to pass through the door. The demon can use photons to determine the position and energy of a cannonball, but the energy used is negligible compared to the cannonball. Same goes for the energy needed for the demon to record the information. Can we say the energy required to open and shut the door is negligible? So this means that energy is flowing from one side to the other. The entropy of the system may be increasing, but the temperature of demon's process is way lower than the temperature of the cannonballs. 


#6
Feb2412, 03:20 PM

Sci Advisor
P: 5,508

Alternatively, there is the information required to specify the microstate itself this has been worked out in terms of algorithms, and the most straightforward definition is the Kolmogorov complexity. The two measures are somewhat related (for example, the amount of information required to transmit instructions for constructing a power plant is related to the information required to actually construct the power plant), but Shannon's result is better understood and is the basis for several data compression algorithms. Your example of a gas of cannonballs a very interesting one (especially if gravity is not important) defining the temperature of such a gas is not so obvious: see, for example, the hardsphere phase transition. The transition between fluid and crystal occurs at a particular volume fraction not the conventional sense of temperature. In terms of thermodynamics, the entropy of a gas of cannonballs will also depend on the volume fraction. 


#7
Feb2412, 06:17 PM

P: 1,407

The demon has to measure the speed of the cannonball by absorption of photons as you suggest, these are now high energy photons coming from a source of 2.4 x 10^22 Kelvin, which surely will increase the demon's temperature greatly is he is tiny enough to not have an energy expenditure of appreciable value when opening or closing the door. So the question is then how many cannonballs can the demon measure and let through the door before his own temperature rises to that of the cannonballs. I don't think size of "molecules" matters for Maxwell's demon. 


#8
Feb2412, 08:12 PM

P: 789

Notice that the demon knows more than the temperature, pressure, and volume. The demon knows the position and momentum of the cannonballs in his vicinity. (They are big enough to be treated classically). I don't think the size of the molecules matters either  which is why we should be able to prove that the second law is not violated for a human demon and a gas of cannonballs undergoing perfectly elastic collisions. 


#9
Feb2412, 09:33 PM

P: 98

Oh dear. I seem to have put the cat among the pigeons. Can we escalate this one? E.g. to Stephen Hawking or somebody like that ;)
I think 256bits must be on to something with the temperature of the cannonballs thing. Real cannonballs would probably heat up as they bounced around, taking energy out of the translational modes. We'd better include rotation as well. If we said there were no other modes but translation, then I think we'd be talking about a quantum system which might be a totally different ball game (scuse the pun) and if we approximated the collisions to perfectly elastic I think we'd just be agreeing to wait a very long time but not talking about perpetual motion or equilibrium. So let's take the case where the energy spreads out over all modes that a real cannonball has and see if we still have a problem. I think we do though because I can reduce the rate at which the demon gets sunbathed just by shrinking him and then I only need a very small fridge to keep him cool. I'm expecting to milk significant energy out of this contraption and I reckon I can spare that much for the fridge. Using Rap's numbers as an example, can anybody figure out how much energy we can harvest if we naively ignore the bits in the demon's head? I'm so clueless about thermodynamics that it still looks like zero to me, which means I don't understand why breaking the 2nd law means breaking the 1st or what was so demonic about this trapdoor in the first place. 


#10
Feb2412, 11:30 PM

P: 789

We should deal with the simplest situation possible that keeps the problem alive. Cannonballs colliding inelastically is a needless complication, it means the system is out of equilibrium  the cannonballs have an internal temperature and a "cannonball gas temperature" and they are vastly different. Equilibrium occurs when the two temperatures are the same  i.e. the cannonballs are practically motionless and have absorbed all the kinetic energy they used to have and that energy is now heat energy. When talking about entropy, you have to include the "internal entropies" of the cannonballs and the "cannonball gas entropy", etc. etc. Huge complication for the demon to keep track of. Do we really want to worry about Maxwell's demon in a nonequilibrium situation when the problem does not demand nonequilibrium? No.
We can forget about angular momentum too. Elastic collisions will impart no rotational energy to any cannonball during a collision, either from an offcenter hit or from a rotating cannonball transferring rotational energy to another cannonball. Just as the internal temperature of a cannonball is irrelevant, so is its rotational energy. Elastically colliding cannonballs are like a simple monatomic ideal gas, and Maxwells demon should give the same results in both cases. The problem is kept alive  what happens if you go to a "cannonball gas"? The bits that the demon will have to record will simply be the positions and momenta of the cannonballs, he won't have to worry about the internal entropy bits of the cannonballs, the rotation of the cannonballs, etc. Much simpler, and I don't think we will require quantum mechanics to resolve the issue. I'm not very good at Maxwell's demon, there must be something missing. Is it that the demon is required to be at the same temperature as the gas he is manipulating? If that is the case, then if you multiply the information entropy of those recorded bits times kT where T=10^22 or whatever, you get a very big energy, on the order of a cannonball's kinetic energy, and that plus the photon energy he used to measure the cannonballs may be more than the energy "generated" by the demon. I don't know. Also, violating the second law does not mean you violate the first. The second law is being violated all the time by small fluctuations, the first law, classically never. Its only in the "thermodynamic limit" of an infinite number of particles that the second law holds exactly. 


#11
Feb2512, 02:10 AM

P: 98

The elastic case is clearly simpler, but I'm worried that it might be too soft on me, i.e., make it too easy for me to make this case that the "bitsinthehead" argument is dodgy. Perhaps if we get a full blown paradox in the elastic case, the solution is that perfect elasticity isn't achievable in practice.
Not in theory either if the balls have nonzero diameter. That follows from relativity  there has to be a wave in the ball to tell the back face to change direction, and that wave can't go at infinite speed, so it's gonna bounce around and dissipate as heat. Put it this way: it would be more satisfying to me to make my case fly in the inelastic case than just to get away with it in the idealised case. As for rotation, you might be right that rotating modes won't get stimulated if the balls are *oily*, but if you're prepared to overlook the relativity argument, then we could also speak of perfect elasticity in the case that their surfaces have an enormous coefficient of static friction and therefore won't grind against each other. In that case, I think balls glancing off each other would end up rotating. I don't mind having oily balls though. I'm not entirely convinced that the temperature of the demon is relevant. I reckon 1 bit of information is just that. Is it really applicable to take equations like dU = TdS ... which originally came from huge statistical systems like gases and apply them to single bits. I'd like to hear a more mechanical argument that a bit really really has to use TdS of energy when you erase it. I also have my doubts about this 10^22 Kelvin. That would seem to imply that if I use 1kg balls of something inelastic like plasticine starting at absolute zero, then let all the KE turn to heat just by watching them all go splat, then they should reach this wild temperature. But in reality they only seem to heat up a few tenths of a degree or so. Perhaps the bug in the calculation is that pdV wasn't mentioned. 


#12
Feb2512, 06:00 AM

PF Gold
P: 3,080

Listen to Rap, he's making good sense. The first case to understand is the purely elastic case, because we should be able to account for that situation. Also, we do not want to make the Demon be the same T as the cannonballs, indeed it will be crucial that the Demon either not have a T associated with him at all, or else have a much lower T associated with him than the cannonballs have. Finally, the energy per "cannonball" is indeed very much like their temperature here, so if we have a counterintuitive case to understand, it is the case where the "Demon" is generating a huge T difference across the "door" by using what would seem to be a relatively small number of decisions.
First let me say there must not be anything "dodgy" about the "erasing bits" argument, even in the case of cannonballs and high T that argument must hit the nail on the head even in that situation, so this is what we need to understand. I would say the first thing we should do is distinguish the first and second laws the first merely being conservation of energy, the second counting entropy. What seems "paradoxical" here is that if the Demon can take a gas that is everywhere the same (very large) T, and make some decisions about a door, he can generate a huge T difference, which could then run a heat engine and do work, in a way that looks completely cyclic so sounds like it could be repeated over and over. This does not yield any problems with the first law, because energy is still being conserved if the heat engine does work, the cannonballs lose energy, and since nothing replaces that energy, there is no connection here to a perpetual motion machine. Perpetual motion machines are not power generators, they are simply machines that don't run down on their own, and that brings in the second law. So the real issue here is with the second law clearly a huge T difference has lower entropy than the original state of a huge T that is the same on both sides. So how did the Demon achieve that with just a few decisions? If we have cannonballs, we have the same number of decisions, but a much larger T difference, so isn't that a contradiction or paradox? No, it's fine. We just need to understand the connection between T and entropy. The first thing to do is get rid of the ridiculous units here we multiply T by k and divide S by k, so now S is S/k and means the natural log of the number of configurations N that are in the class associated with S, and T is kT and should be interepreted as the energy scale of changes that affect S/k. Specifically, an energy change of kT corresponds to augmenting S/k by one, which also corresponds to "one binary decision" about opening or closing the door. So you immediately see the key issue here if each cannonball has a huge energy, then one decision about the door automatically has a big effect on the energy difference, but not a huge effect on the entropy decrease! One decision by the Demon is always 1 unit of entropy lost in the gas, via the T difference it creates, regardless of the energy per cannonball. At any T, one decision corresponds to kT of energy added to the higherT gas, which always lowers the entropy by 1, even if T is huge. So in other words, we do not magnify the entropy consequences of the Demon's decisions simply by magnifying the energy of the cannonballs that is not the connection between entropy and energy. If there is already a huge energy per cannonball, than the entropy is "less impressed" by big energy shifts involving opening and closing the door. Either way, one bit in the head of the Demon is one kT of energy that is getting added to the highT, highS gas and removed from the lowT, lowS gas, and that always results in a net decrease in S/k of 1. But of course it also results in a net increase in the S/k in the Demon, so there is no issue, and you need to ignore (or "erase") that entropy increase in the Demon to get the "paradox." So the bottom line is, the "paradox" stems from not understanding the fact that when the energy scale is magnified, the entropy consequences are not because at higher energy, each "decision" corresponds to more energy, but at higher T, more energy is needed to have the same impact on the entropy. The bottom line is, in thermodynamics, the Demon does not have "intention" or "purpose" that motivates his actions he is just another "dumb gas" that will only do what he does if it results in a net increase in entropy in the total Demon+gas system. He does what he does because it is a more likely configuration of the whole system, that's "intention" in themodynamics. And there is no issue with extracting work from the gas if you have access to an entropygenerating mechanism (like a brain), you can use it to drop the entropy somewhere else, and extract work in a way that conserves energy overall. This just means that brains are even more important to have in situations where there is access to high temperature, because each decision by the brain has a relatively larger energy consequence, which is also why you need good brains with their "fingers on the button" if you see what I mean! 


#13
Feb2512, 08:33 AM

P: 98

Thanks Ken, you seem to have got this pointing in the right direction, but I have one last misconception.
I already mentioned the question of how fussy the demon is. Where in the probability distribution of temperature per molecule does he set his threshold for opening the door? Can we really say that the entropy decrease of the gas per decision is independent of the threshold, and/or can we say that the entropy value of the bits in his head are independent of the threshold? I'd be tempted to say nO and yES respectively, which would resuscitate the problem. I still haven't really understood this energyentropy connection. Let's just banish k by an appropriate choice of units, as Einstein banished c and QM people banish h and so on. Are you saying that one bit is just one bit of negentropy = ln(2), irrespective of any temperature? I think so. I think you'd only care about T if you cared about energy, which you already said was irrelevant because I can't make a power station out of this anyway. But the threshold does affect the number of phonons being moved over the divide in each decision. That's the point right? We're talking about the number of ways we can distribute a known number of phonons over a known (but inconstant) number of modes. If we can sort a lot of phonons in one go then we've made a big difference to the number of ways the whole system can be arranged. So the earlier nO seems supported. How about the yES? 


#14
Feb2512, 08:52 AM

P: 789

The lowered entropy (different temperatures) of the two gases can now be used to run an engine for a little while, until the temperatures are equal again (but lower than before). Then the demon can lower the entropy again, getting different temperatures again, which can then run the engine again for a little while (not delivering as much energy as before, because of lower temperatures). This could go on and on, but you could never get that engine to produce more energy than the amount of kinetic energy in the original gases. The thing about the cannonball gas is only that there is a lot of energy there to be converted (the cannonballgas temperature is so huge and there's a mole of cannonballs). Is this right? 


#15
Feb2512, 09:37 AM

P: 98

Here's a completely different point: Bennet's take is that the Demon has to erase a bit when he forgets what he observed, which means setting a bit (that might have had any value) to a distinct value, which means lowering entropy and thus using energy. That's supposed to help. But now that Ken has unravelled energy and entropy it seems more like a hindrance.
The demon doesn't have to erase any bits. He stores the bit by letting the molecule through! Oops: wasn't there a change of sign somewhere? I think so. I say the demon picks up negentropy from the chamber he observes and dumps it in the one behind the door. He doesn't need much RAM and his entropy is steady. Simple. Bennet's take is considerably more convoluted and I think he sneaks in a change of sign by using the very law that's under attack  the 2nd. Surely the demon was always maintaining a steady state  nobody ever asked him to store any internal state. It's multiply circular to argue that the demon might be in danger of acquiring a long memory when he never needed to, then to say that he has to take precautions against that, and that the precautions would seem to reduce entropy (although the danger would have reduced entropy as well) and then to blithely assume that the second law dictates some energy requirement when the second law is the very thing under attack by the paradox we are trying to solve. Whatever thermodynamic arguments are raised for claiming that the demon has to use energy, increase entropy or whatever, it should be possible to ground them in mechanistic, Newtonian style physics. Otherwise, there's a lot of danger of circularity. 


#16
Feb2512, 11:09 AM

PF Gold
P: 3,080




#17
Feb2512, 11:19 AM

PF Gold
P: 3,080




#18
Feb2512, 11:32 AM

P: 98




Register to reply 
Related Discussions  
Maxwell's Demon  Science & Math Textbooks  11  
Maxwell's Demon  Chemistry  3  
Maxwell's Demon  General Physics  6 