Angular momentum commutes with Hamiltonian

In summary, the Hamiltonian and the angular momentum operator commute in systems with a central potential, such as the free particle, harmonic oscillator, and hydrogen atom. This means that energy is conserved under rotations and angular momentum is conserved under time translations. However, in systems with non-central potentials, this may not be the case. Understanding the physical meaning of this is important in understanding the behavior of these systems.
  • #1
Feynmanfan
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How can I prove that the Hamiltonian commutes with the angular momentum operator?

In spherical coordinates it is straightforward but I'd like to understand the physical meaning of it.

Thanks.
 
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  • #2
Feynmanfan said:
How can I prove that the Hamiltonian commutes with the angular momentum operator?
In spherical coordinates it is straightforward but I'd like to understand the physical meaning of it.
Thanks.
What hamiltonian ? The hamiltonian of the free particle ? Of the harmonic oscillator ? Of the hydrogen atom ?
All these do indeed commute with the angular momentum operator (Lx,Ly,Lz). But if you'd have a non-central potential, this would not be the case.
The physical meaning is this:
the hamiltonian is the generator of time translations (huh ? :-) Yes, that's the content of Schroedinger's equation: the time derivative of the state (wavefunction) is the hamiltonian applied to the wavefunction.
The angular momentum operator is the generator of space rotations.
If both commute, that means that it doesn't matter if you first rotate (a small bit) and then "translate in time" (advance a bit in time), or whether you do it in the opposite order (first advance a bit in time, and then rotate).
So that means that "energy" (the hamiltonian) is "conserved under rotations" (essentially that your problem is spherically symmetrical) ;
or:
that "angular momentum" (the angular momentum operator) is "conserved under time translations", meaning: angular momentum is constant during the motion: it is a constant of motion.
cheers,
Patrick.
 

1. What is angular momentum?

Angular momentum is a physical quantity that represents the rotational motion of a system. It is defined as the product of the moment of inertia and the angular velocity of the system.

2. What does it mean for angular momentum to commute with Hamiltonian?

When two quantities commute, it means that they can be measured simultaneously without affecting each other. In the context of physics, this means that the values of these two quantities can be known with certainty at the same time.

3. Why is it important that angular momentum commutes with Hamiltonian?

The fact that angular momentum commutes with Hamiltonian is a fundamental property of quantum mechanics. It allows for the conservation of angular momentum, which is crucial in understanding the behavior of particles and systems at the atomic and subatomic level.

4. How is the commutation of angular momentum and Hamiltonian related to Heisenberg's uncertainty principle?

The commutation of angular momentum and Hamiltonian is related to Heisenberg's uncertainty principle through the mathematical relationship between the two. The uncertainty principle states that the more precisely the angular momentum of a particle is known, the less precisely its Hamiltonian (energy) can be known, and vice versa.

5. Can the commutation of angular momentum and Hamiltonian be observed in real-life systems?

Yes, the commutation of angular momentum and Hamiltonian has been observed in various physical systems, such as atoms, molecules, and subatomic particles. This property has been confirmed through experiments and is essential in understanding the behavior of these systems.

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