- #1
hideelo
- 91
- 15
I'm taking thermal physics this semester and I have two questions that have been floating around my head,
1. Time's arrow and second law:
If I have system (box of gas) and at t_i I know all the positions and momenta of all the particles, let's assume it's not in a state with maximum entropy. If I wanted to know the state of the system at some other time t_j I could say that given that the particles in my system have momenum, given that they are in a box with walls and the particles inevitably collide with those walls, I know that that the system must evolve into some other state and the particles will in general have different momenta and positions. I could now use statistics and try and predict what that other state might be. Turns out, that in general I should expect that at any other time t_j I should expect the entropy to go up simply based on the fact that a state with more entropy has a greater multiplicity, and is therefore more likely. However no where so far have I placed any constraints on if t_i < t_j or t_i > t_j . So why can't I argue that given any system, I could expect that in the past it should have had a higher entropy? In other words it seems to me that I should assume that however and whenever I find a system, it is most probably in the state of lowest entropy it ever has or will be.
Now, this must be false because that gives predictions about nature that strongly depend on when I choose to measure the system, and worse this argument goes against the second law. So why is this false?
2. Entropy and symmetry
If I have some crystalline solid, its symmetry group will be S_k for some k depending on the exact structure of the crystal. I know that as a liquid it has higher entropy and as a gas even higher entropy. However in addition to the increase in entropy, the symmetry of this system also just increased, it's symmetry group is now S^2 which has continuous symmetry under arbitrary rotation. Is increase in entropy somehow related to increase in symmetry? Is there some generalization under which this holds?
TIA
1. Time's arrow and second law:
If I have system (box of gas) and at t_i I know all the positions and momenta of all the particles, let's assume it's not in a state with maximum entropy. If I wanted to know the state of the system at some other time t_j I could say that given that the particles in my system have momenum, given that they are in a box with walls and the particles inevitably collide with those walls, I know that that the system must evolve into some other state and the particles will in general have different momenta and positions. I could now use statistics and try and predict what that other state might be. Turns out, that in general I should expect that at any other time t_j I should expect the entropy to go up simply based on the fact that a state with more entropy has a greater multiplicity, and is therefore more likely. However no where so far have I placed any constraints on if t_i < t_j or t_i > t_j . So why can't I argue that given any system, I could expect that in the past it should have had a higher entropy? In other words it seems to me that I should assume that however and whenever I find a system, it is most probably in the state of lowest entropy it ever has or will be.
Now, this must be false because that gives predictions about nature that strongly depend on when I choose to measure the system, and worse this argument goes against the second law. So why is this false?
2. Entropy and symmetry
If I have some crystalline solid, its symmetry group will be S_k for some k depending on the exact structure of the crystal. I know that as a liquid it has higher entropy and as a gas even higher entropy. However in addition to the increase in entropy, the symmetry of this system also just increased, it's symmetry group is now S^2 which has continuous symmetry under arbitrary rotation. Is increase in entropy somehow related to increase in symmetry? Is there some generalization under which this holds?
TIA