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da_willem
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I heard nonrelativistic QM is time-reversible. How does this follow from the Schrodinger equation?
da_willem said:but if QM is time reversible
Tom Mattson said:It's not time reversible.
Let T=time reversal operator, and start from Schrodinger's equaiton:
Hψ(x,t)=(i*hbar)(∂ψ(x,t)/∂t)
Now let Tψ(x,t)=ψ(x,-t), and you get:
Hψ(x,-t)=(-i*hbar)(∂ψ(x,t)/∂t)
Note the negative sign, which means that the Schrodinger equation is not invariant under this transformation. In order to recover quantum mechanics under time reversal, you have to do a transformation to the wavefunctions as well, namely:
Tψ(x,t)=ψ*(x,-t).
Fredrik said:Seratend, what you did here is a nice exercise, but I don't think that this calculation can be used to argue that the Schrödinger equation is time reversible. You just multiplied both sides of an equation with an operator from the left, and showed that the equation still holds. That by itself can't prove anything of course.
However, you used both the antilinearity property of T (Ti=-iT) and the commutation relation [H,T]=0 to get the necessary result. If you had used one of those properties but not the other, you would have obtained a contradiction. That means that what your calculation really showed is that [H,T]=0 if and only if T is antilinear.
I wouldn't say that the result implies that the Schrödinger equation is "time reversible" or "invariant under a time reversal transformation". To me that would mean that if ψ is a solution, then so is Tψ, but that's not the case:
[tex]i\hbar\frac{\partial}{\partial t}T\psi(\vec x,t)=i\hbar\frac{\partial}{\partial t}\psi(\vec x,-t)=-i\hbar\frac{\partial}{\partial t}\psi(\vec x,t)=-H\psi(\vec x,t)=H(-\psi(\vec x,t))[/tex]
This is clearly not always equal to
[tex]HT\psi(\vec x,t)=H\psi(\vec x,-t)[/tex]
so Tψ is not always a solution to the Schrödinger equation.
Fredrik said:If you had used one of those properties but not the other, you would have obtained a contradiction. That means that what your calculation really showed is that [H,T]=0 if and only if T is antilinear.
Fredrik said:I wouldn't say that the result implies that the Schrödinger equation is "time reversible" or "invariant under a time reversal transformation". To me that would mean that if ψ is a solution, then so is Tψ, but that's not the case:
da_willem said:I think I'm looking for an argument that shows that
[tex] H(T \Psi)=-i \frac{\partial T \Psi}{ \partial t} [/tex]
is equivalent to the original Schrodinger equation.
The Schrodinger equation is a mathematical formula that describes how the quantum state of a physical system changes with time. It is a fundamental equation in quantum mechanics and is used to predict the behavior of particles on a microscopic level.
Time reversibility in the context of the Schrodinger equation refers to the ability to reverse the direction of time and still obtain the same results. This means that if the initial conditions of a system are known, the Schrodinger equation can be used to predict the future behavior of the system, as well as the past behavior if time is reversed.
The concept of time reversibility is closely related to the second law of thermodynamics, which states that entropy (a measure of disorder) always increases with time. The Schrodinger equation is time reversible, but the laws of thermodynamics are not, as they only work in one direction of time. This is because they take into account the irreversible process of energy dissipation.
No, the Schrodinger equation is only applicable to systems at the microscopic level, such as atoms and subatomic particles. It cannot accurately predict the behavior of larger, macroscopic systems due to the influence of external factors such as friction and other forms of energy dissipation.
In classical mechanics, time irreversibility is a fundamental concept that is built into the laws of motion and thermodynamics. In quantum mechanics, on the other hand, the Schrodinger equation is time reversible, but the observable behavior of particles is not necessarily time reversible due to the probabilistic nature of quantum mechanics.