What is Quantum Statistical Mechanics?

[vanhees71]

I believe they prepare them all the time, but they just don't know it because the states are ... well, unobservable.

Hm, I still don't know, what you mean by "unobservable states". If a state is (in principal) unobservable, then it's not a state. So an "unobservable state" seems to be a constradictio in adjecto. The only thing, I'm aware of are superselection rules which forbid certain states, but that means that they simply do not exist (e.g., the charge superselection rules forbidding superpositions of states of different charge or the angular-momentum superselection rule forbidding superpositions of half-integer with integer angular momenta).

 

[vanhees71]

Whatever will be preparable any time in the future must be a state that comes from partial tracing of the state of a bigger system including its environment. Thus it is covered by my definition of the states that can appear in Nature. Whereas one cannot observe a state that doesn't occur in Nature, and one cannot prepare such a state.

Well, If I prepare a cup of coffee and leave it at rest a while on my desk, I don't trace anything but prepare a thermal-equilibrium state by just waiting long enough. That's an easy preparation without in any way tracing out anything.

 

[A. Neumaier]

Well, If I prepare a cup of coffee and leave it at rest a while on my desk, I don't trace anything but prepare a thermal-equilibrium state by just waiting long enough. That's an easy preparation without in any way tracing out anything.

To be able to say what you said you traced out the whole universe except for the coffee in the cup. For only that part is in equilibrium. If you wait longer, maybe the bigger system consisting of coffee, cup and desk will be in thermal equilibrium. But as long as you are in the room, the whole room will not be in equilibrium. Thus you need to trace out at least yourself. And the outside of the building your desk is in. And your computer if it is running...

 

[vanhees71]

Wow, I'm a very mighty being, being able to trace out the whole universe by cooking a cup of coffee ;-)), but when I come to thermal equilibrium with my environment, I'm dead. So I better don't trace myself out...