The time evolution operator $\hat{U}(t) = e^{-i\hat{H}t/\hbar}$ was introduced in Chapter 8 as the solution to the Schrodinger equation for time-independent Hamiltonians. Without looking back, can you show that $\hat{U}(t)$ is unitary? (Hint: use $\hat{H}^\dagger = \hat{H}$.) Can you verify that $\h