I’m not really sure why you say volume is changing here.
Think about putting a packet of gas next to a vacuum and allowing it to expand. In this case it’s even easier to see that the requirements of your thought experiment hold—you know the exact state of the vacuum, because it has no microstates. Yet the total system entropy will still increase as the molecules of gas expand to fill the vacuum. Even if you have perfect information about the gas at the beginning (zero entropy), at the end of the experiment you will not. You will have some uncertainty. This is because the phase space itself has expanded.
If we’re having some sort of disconnect I’d be happy to talk more,
I think we are. I suggest becoming familiar with R Landauer and C H Bennet’s work. I’d be happy to discuss this further if we are on the same page.
Think about putting a packet of gas next to a vacuum and allowing it to expand. In this case it’s even easier to see that the requirements of your thought experiment hold
Oh, I see, you’re thinking of particle exchange, like if one dumped the water into the tea. This case is not what I intended—by thermal contact I just mean exchange of energy.
With identical particles, the case with particle exchange gets complicated. There might even be some interesting physics there.
The thermodynamics of energy exchange and mass exchange are actually similar. You still get the increase in entropy, even if you are just exchanging energy.
One the one hand, this is a good point that points out a weakness in my argument—if states are continuous rather than discrete, one can increase or decrease entropy even with deterministic time-evolution by spreading out or squeezing probability mass.
But I don’t know how far outside the microcanonical this analogy you’re making holds. Exchanging energy definitely works like exchanging particles when all you know is the total energy, but there’s no entropy increase when both are in a single microstate, or when both have the same Boltzmann distribution (hm, or is there?).
Think about putting a packet of gas next to a vacuum and allowing it to expand. In this case it’s even easier to see that the requirements of your thought experiment hold—you know the exact state of the vacuum, because it has no microstates. Yet the total system entropy will still increase as the molecules of gas expand to fill the vacuum. Even if you have perfect information about the gas at the beginning (zero entropy), at the end of the experiment you will not. You will have some uncertainty. This is because the phase space itself has expanded.
I think we are. I suggest becoming familiar with R Landauer and C H Bennet’s work. I’d be happy to discuss this further if we are on the same page.
Oh, I see, you’re thinking of particle exchange, like if one dumped the water into the tea. This case is not what I intended—by thermal contact I just mean exchange of energy.
With identical particles, the case with particle exchange gets complicated. There might even be some interesting physics there.
The thermodynamics of energy exchange and mass exchange are actually similar. You still get the increase in entropy, even if you are just exchanging energy.
One the one hand, this is a good point that points out a weakness in my argument—if states are continuous rather than discrete, one can increase or decrease entropy even with deterministic time-evolution by spreading out or squeezing probability mass.
But I don’t know how far outside the microcanonical this analogy you’re making holds. Exchanging energy definitely works like exchanging particles when all you know is the total energy, but there’s no entropy increase when both are in a single microstate, or when both have the same Boltzmann distribution (hm, or is there?).
I’ll think about it too.