Calculating the Commutator of H and r in 3D - What is the Correct Solution?

  • Thread starter Thread starter rcross5
  • Start date Start date
  • Tags Tags
    3d Commutator
Click For Summary
SUMMARY

The commutator of the Hamiltonian operator \(\hat{H}\) and the position vector \(\vec{r}\) in three dimensions is calculated as \([\hat{H}, \vec{r}] = -\frac{\hbar^2}{m} \nabla\). A common mistake identified in the discussion is the incorrect interpretation of the Laplacian as the gradient of a gradient, rather than the divergence of a gradient. The solution can be simplified by breaking it down into its Cartesian components: \([\hat{H}, x]\), \([\hat{H}, y]\), and \([\hat{H}, z]\), and then combining the results. This method leverages the symmetry of the equations in Cartesian coordinates.

PREREQUISITES
  • Understanding of quantum mechanics and operator algebra
  • Familiarity with vector calculus concepts, particularly divergence and gradient
  • Knowledge of Hamiltonian mechanics
  • Basic proficiency in multivariable calculus
NEXT STEPS
  • Study the properties of commutators in quantum mechanics
  • Learn about the divergence and gradient operations in vector calculus
  • Explore Hamiltonian mechanics and its applications in quantum systems
  • Investigate the role of the Laplacian operator in quantum mechanics
USEFUL FOR

Students and professionals in physics, particularly those studying quantum mechanics, as well as anyone interested in advanced vector calculus and operator theory.

rcross5
Messages
2
Reaction score
0

Homework Statement


[tex] [\hat{H},\vec{r}]= ?[/tex]

The Attempt at a Solution


onfb7.png


The answer is given, and I KNOW that factor of 6 shouldn't be there. The answer should be

[tex]-\frac{\hbar^2}{m} \nabla[/tex]

Anyway I've always been lurking these forums and I enjoy the discussions here, but this factor is really really bugging me and I was hoping you guys might be able to catch my probably simple mistake! Thanks :)
 
Physics news on Phys.org
You made a few mistakes in your solution. For example, the Laplacian is not the gradient of a gradient; it's the divergence of a gradient. Also, the gradient of a vector has no well-defined meaning in multivariable calculus, so you have to be very careful when dealing with them.

The easiest way to do this problem is to realize that [H,r] can be split into three parts: [H,x], [H,y], and [H,z]. Calculate each part separately, and combine the result into a vector. Note that after you get [H,x], you can just replace all the x's with y's to get [H,y], since all your equations are symmetrical in the Cartesian coordinates.
 
I guess I'm a bit rusty on my vector calculus! That's what I get for trying to be clever ;)
I'm too tired right now but if I have time I want to see if it's possible to directly solve it through like that. For now though, I've solved it by separating the components. Thanks!
 

Similar threads

  • · Replies 1 ·
Replies
1
Views
1K
  • · Replies 3 ·
Replies
3
Views
10K
  • · Replies 5 ·
Replies
5
Views
2K
  • · Replies 2 ·
Replies
2
Views
8K
  • · Replies 1 ·
Replies
1
Views
4K
Replies
5
Views
2K
Replies
3
Views
8K
Replies
1
Views
7K
  • · Replies 19 ·
Replies
19
Views
4K
  • · Replies 1 ·
Replies
1
Views
3K