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fxdung
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Is Quantum Statistical Mechanics being the application of Quantum Mechanics on the separate particles of bulk matter or the application of QM on whole agregate matter?
Non-relativistic QFT is one way, in my opinion the most clever way, to describe quantum many-body systems. NRQFT is comprehensive, i.e., it includes all of the "1st quantized formalism" but it allows for more, e.g., the description of many-body systems in terms of "quasi particles", where the "quasiparticles" are not particles in the usual sense but can be quantized collective excitations of a many-body system, e.g., the lattice vibrations (sound waves) in a solid. The mathematics is pretty similar to a QFT description of particles and that's why one calls such excitations, treated in this way mathematically, "quasiparticles". They also have names like "phonons" for the lattice vibrations.fxdung said:Then what is the difference between non-relativistic QFT (NRQFT) and quantum statistical mechanics?
The inverse of the coefficient of the Hamiltonian in the exponent of the assumed form of the density operator, e.g., in the canonical ensemble ##\rho=e^{-H/kT}##.fxdung said:But what is temperature of radiation,of QFT,of QM?
Only in the thermodynamic equilibrium. Statistical physics (either classical or quantum) studies also systems far from thermodynamic equilibrium, in which case the concept of temperature does not make sense.atyy said:The most important concepts in quantum statistical mechanics that are not present in pure quantum mechanics are the thermodynamic concepts like temperature, and the thermodynamic limit.
?Demystifier said:Only in the thermodynamic equilibrium. Statistical physics (either classical or quantum) studies also systems far from thermodynamic equilibrium, in which case the concept of temperature does not make sense.
In this case you have a local thermodynamic equilibrium. But sometimes you don't have even that.A. Neumaier said:The concept of temperature makes perfect sense in nonequilibrium thermodynamics, as a local temperature field.
Demystifier said:In this case you have a local thermodynamic equilibrium. But sometimes you don't have even that.
If the region is dense enough, local temperature makes sense. In particular, close to the big bang.atyy said:can we say there is always a temperature if we average over large enough regions?
Did you want to suggest that the early universe had such a density matrix?Demystifier said:Suppose, for instance, that the density matrix of a system is
$$\rho =\frac{A}{{\rm Tr}A}$$
where
$$A=e^{-\beta_1 H}+e^{-\beta_2 H}$$
##H## is the Hamiltonian and ##\beta_1 \neq \beta_2##. What is the temperature of that system? No temperature at all!
No.A. Neumaier said:Did you want to suggest that the early universe had such a density matrix?
True! Similarly, for small systems, most pure states are also not realized in Nature. Yet quantum mechanics, as a theory, is a theory of all such pure states, whether realized in Nature or not. Likewise, quantum statistical mechanics, as a theory, is a theory of all such mixed states, whether realized in Nature or not. That's the only point I wanted to make.A. Neumaier said:For large systems, most density matrices are not realized in Nature.
No. It is a theory of only such states that conform to assumptions found in reality, otherwise one could never obtain irreversibility from unitarity. Only concentrating on states representing reality rather than arbitrary states makes quantum statistical mechanics work.Demystifier said:quantum statistical mechanics, as a theory, is a theory of all such mixed states
Then I am using a wider definition of statistical mechanics than you do. For example, with my definition I can study the possibility of Boltzmann brains, the hypothetical and never observed phenomenon which, in some sense, violates "the principle of macroscopic irreversibility". For some recent research on Boltzmann brains see e.g.A. Neumaier said:No. It is a theory of only such states that conform to assumptions found in reality, otherwise one could never obtain irreversibility from unitarity. Only concentrating on states representing reality rather than arbitrary states makes quantum statistical mechanics work.
You may study them. But I prefer to study (like all mainstream work in statistical mechanics) what can be observed. This dramatically reduces the weirdness of QM.Demystifier said:hypothetical and never observed phenomenon
Fair enough!A. Neumaier said:You may study them. But I prefer to study (like all mainstream work in statistical mechanics) what can be observed. This dramatically reduces the weirdness of QM.
But we never know whether a symmetry is fundamental or merely emergent, valid approximately on large distances only.vanhees71 said:On the theory side the only restriction of possible states are superselection rules (mostly based on fundamental symmetries).
I restricted my interest just to what is observable. I don't think experimentalists will ever be able to prepare unobservable states...vanhees71 said:Who knows, what kinds of "weird" states the experimentalists are able to prepare in the future?
I believe they prepare them all the time, but they just don't know it because the states are ... well, unobservable.A. Neumaier said:I don't think experimentalists will ever be able to prepare unobservable states...
Demystifier said:Then I am using a wider definition of statistical mechanics than you do. For example, with my definition I can study the possibility of Boltzmann brains, the hypothetical and never observed phenomenon which, in some sense, violates "the principle of macroscopic irreversibility". For some recent research on Boltzmann brains see e.g.
http://lanl.arxiv.org/find/grp_physics/1/ti:+AND+Boltzmann+brain/0/1/0/all/0/1
Thanks, I think I fixed the problem now:ddd123 said:Link doesn't seem to be working...
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.vanhees71 said:What do you mean by "unobservable states"? It's just a matter of technical development what we are able to prepare or not, right?
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).Demystifier said:I believe they prepare them all the time, but they just don't know it because the states are ... well, unobservable.
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 said: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.
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 said: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.
Quantum statistical mechanics is a branch of physics that studies the behavior of systems made up of a large number of particles, using the principles of quantum mechanics. It aims to understand the macroscopic properties of a system based on the microscopic behavior of its individual particles.
Classical statistical mechanics follows the laws of classical mechanics, which describe the behavior of macroscopic objects. On the other hand, quantum statistical mechanics takes into account the wave-like nature of particles at the microscopic level, and uses the principles of quantum mechanics to describe their behavior.
The key concepts in quantum statistical mechanics include the wave function, which describes the probability of finding a particle in a particular state, and the Hamiltonian operator, which represents the total energy of a system. Other important concepts include the density matrix, partition function, and the Boltzmann distribution.
Quantum statistical mechanics has many practical applications, including the study of materials at the nanoscale, the behavior of gases and liquids, and the properties of magnetism and superconductivity. It is also used in fields such as quantum computing and quantum information theory.
Quantum statistical mechanics plays a crucial role in our understanding of the universe, as it helps us explain the behavior of matter and energy on a microscopic scale. It has also led to important discoveries, such as the existence of quantum entanglement and the uncertainty principle, which have greatly expanded our understanding of the fundamental laws of nature.