[Physics] Meaning of the Angular Momentum in Quantum Mechanics

angular momentumhydrogenquantum mechanics

There was a discussion about the angular momentum in QM in an online course that I am attending now. The discussion and the answers did not satisfy me so I wanted to ask it on Physics SE.

In the classical picture we think of hydrogen atom as an electron circling around a nucleus with plus charge, which has obvious flaws such as the instability of the atom because of the EM radiation.

In QM we have the operator $\mathbf{L = p \times r}$ and we know that the the electron does not radiate EM-Waves because it is not actually circling around the nucleus. It is sometimes here and there. However the wave function of the electron may have an eigenvalue of $\mathbf{L}^2$ ie. it can have a definite angular momentum. Since we know that the angular momentum is also conserved in QM ($[ \mathbf{L}^2,H ]=0$) I would assume (classically) that the point at which we measure the angular momentum should be the nucleus since only then the angular momentum can be conserved (correct me if I am wrong). However that would mean that the electron is actually circling around the nucleus.

How can it have an angular momentum without circling around the nucleus and its angular momentum being conserved? If it is circling, we are back to square one, why does it not radiate EM-Waves? Alongside these sub-questions that beg for an answer, my real question is this: How correct it is to associate the classical angular momentum with the angular momentum algebra in QM, which arise actually from a rotational symmetry. I don't know much about the experimental side of the story but if we are able to measure the angular momentum of an electron bound to the nucleus, then how does it fit in to all of this?

Best Answer

It is a misconception to say that

In QM [...] we know that the the electron does not radiate EM-Waves because it is not actually circling around the nucleus. It is sometimes here and there.

In QM the notion of "circling round the nucleus" does indeed fail to make sense, but this is not why electrons don't radiate EM waves. Instead, an atom in its ground state will not radiate because its state space simply does not contain any states with lower energy.

In essence, this is due to the uncertainty principle. If you have a hydrogen atom and you try to compress the 'electron orbit' any further, then the electron's position uncertainty will decrease, and this will drive a corresponding increase in the momentum uncertainty. This momentum uncertainty is proportional to $p^2$ and thus to the kinetic energy. This means that a decrease in the mean radius will decrease the potential energy but increase the minimum allowed kinetic energy, so there is a trade-off involved. For the ground state, this trade-off is optimal, and you cannot increase or decrease the orbit's spatial extent without increasing the energy.

On another track, you ask

How can it have an angular momentum without circling around the nucleus and its angular momentum being conserved?

The answer to that is that you need to stop thinking about the electron "circling" anything. The only relevant physical quantity is the electron's wavefunction. The electron is said to have orbital angular momentum if its wavefunction has significant changes in phase over paths which go around the nucleus. That's it.

Finally, in terms of

How correct it is to associate the classical angular momentum with the angular momentum algebra in QM, which arise actually from a rotational symmetry.

you are severely underestimating the extent to which angular momentum arises as the algebra associated with rotational symmetry within classical physics. Indeed, the algebra is exactly the same; you only replace commutators $i[·,·]$ with Poisson brackets $\{·,·\}$. Angular momentum is as related to rotational symmetry in classical physics as it is in quantum mechanics.

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