[Physics] How does an operator transform under time reversal

operatorsquantum mechanicstime-reversal-symmetry

We know that a time-reversal operator $T$ can be represented as
$$T=UK$$
where $U$ is some unitary operator and $K$ is the complex conjugation operator.

Then under time-reversal operation, a quantum state $|\psi\rangle$ will transform as the following:
$$|\psi^R\rangle=T|\psi\rangle=UK|\psi\rangle=U|\psi^*\rangle$$
If we require time-reversal symmetry to the system, then we need to have
$$\langle\psi^R|O^R|\phi^R\rangle=\langle\psi|O|\phi\rangle$$
where $|\psi\rangle$ and $|\phi\rangle$ are some arbitrary quantum states and $O$ is some operator. From the above equation, we would have
$$\langle\psi^*|U^{\dagger}O^RU|\phi^*\rangle=\langle\psi|O|\phi\rangle$$
So based on this equation, how do we obtain the result (given in the book "random matrices" by Mehta) that
$$O^R=UO^TU^{\dagger}$$
where $O^T$ means the transpose of $O$.

My second question is that, what if we do NOT assume time reversal symmetry?

Best Answer

Time reversal is not only complex conjugate, what it does is also to transpose the items on which it acts (vectors, matrices).

$$T\langle \phi|\hat{O}|\psi\rangle = \langle \psi T|\hat{O}|T \phi\rangle.$$

Notice the change of places of the functions in the right wing with respect to the left wing. Also, I used the fact that $\hat{O}$ is unchanged at time-reversal.

Now we do the following change which is allowed under the integral if the two functions vanish at infinity:

$$\langle UK\psi|\hat{O}|UK\phi\rangle = \langle \phi|U\hat{O}U^\dagger|\psi\rangle.$$

So, we got the time-reversed of $\hat{O}$.