If ˆT(Δx)=e−iℏˆpΔx is the spatial translation operator, then there exists a function f from R to the ket space V such that ˆT(Δx)f(x)=f(x+Δx). Namely, the function that sends x to the position eigenstate |x⟩.
Similarly if ˆU(Δt)=e−iℏˆHΔt is the time evolution operator (for time-independent Hamiltonians), then there exists a function f from R to V such that ˆU(Δt)f(t)=f(t+Δt). Namely, the function that sends t to |ψ(t)\rangle, the state of a particle at time t.
And similarly if \hat{R}_z(\Delta\theta) = e^{-\frac{i}{\hbar}{\hat{L}_z\Delta \theta}} is the orbital rotation operator, then there exists a function f from \mathbb{R} to V such that \hat{R}_z(\Delta \theta)f(\theta) = f(\theta+\Delta\theta). Namely the function which sends \theta to |r,\theta,\phi\rangle, an eigenstate of the position operator \hat{\theta} in spherical coordinates.
But my question is, if \hat{R}_z(\Delta\theta) = e^{-\frac{i}{\hbar}\hat{J}_z\Delta \theta} is the intrinsic rotation operator, then does there exist a function f from \mathbb{R} to V such that \hat{R}_z(\Delta \theta)f(\theta) = f(\theta+\Delta\theta)? By intrinsic rotation operator I mean the rotation operation related to spin angular momentum.
I suspect the answer is no, because there is no operator corresponding to \theta, the parameter of the intrinsic rotation operator, since in quantum mechanics it doesn't really make sense to think of spin as a particle rotating about its own axis. But then again, there is no time operator in non-relativistic quantum mechanics, and yet the time evolution operator satisfies the property.
In any case, assuming that the answer to my question is no, I'd like a formal proof that there cannot exist such an f.
EDIT: I posted a follow-up question here.
No comments:
Post a Comment