Homework Help: Commutators: solving with jacobian

1. May 17, 2013

Rorshach

1. The problem statement, all variables and given/known data
Hell My problem is as follows:
Determine the following commutators: [px2,x],[pxx2],[px2,x2],[]. The calculation can be done in two ways, either by inserting a test function, and using the explicit expressions for the operators, or by utilizing Jacobi identity and using the fact that [x,px]=ihbarred . You should master both methods.

2. Relevant equations
[A,B]=AB-BA
[x,px]=ihbarred
[A,BC]=B[A,C]+[A,B]C
px=-ihbarred*d/dx
p2=-hbarred2*d2/dx2

3. The attempt at a solution
Here's the problem- I just cannot find good paper with proper explanation how to do it and examples with solutions. I am also not familiar with term and meaning of jacobian- in the textbook I am using there is no explanation of jacobian, or how to solve commutators, not even mentioning solving commutators with jacobians.

2. May 17, 2013

MisterX

The Jacobi identity is not the same thing as the Jacobian. They are both named after the same person, but they are not the same thing. Here is the Jacobi identity.

$[A, [B, C]] + [B, [C, A]] + [C, [A, B]] = 0$

It applies to the bracket operation for Lie algebras, so it applies to cross products and Poisson brackets as well as commutators.

3. May 17, 2013

MisterX

Also I have no idea why you should use the Jacobi identity with the commutators given to you. The unnamed identities $[A,BC] = [A,B]C + B[A,C]$ and $[AB,C] = A[B,C] + [A,C]B$ seem like a more appropriate things to use.

Last edited: May 17, 2013
4. May 17, 2013

Rorshach

I don't know that either, but that is the content of the problem, so I have to do it. I have no idea how to solve commutator if there is a second derivative (px2).

5. May 17, 2013

vela

Staff Emeritus
Use $\hat{p}_x^2 = \hat{p}_x\hat{p}_x$. You need to show some effort at working the problem out on your own.

6. May 17, 2013

Rorshach

ok, but what if there is a factor in front of the commutator? for example like [px2,x]ψ?
Is it supposed to be like this:

[px2,x]ψ=p[px,x]ψ+[px,x]ψpx?

7. May 17, 2013

vela

Staff Emeritus
I don't see a factor in front of the commutator. Could you elaborate?

8. May 18, 2013

Rorshach

One of the ways of solving commutators was to put a function after commutator, like this
[p,x]ψ - and then solve it. But unfortunately no matter if I put the function there or not- I can't get the correct value. I couldn't find anywhere how to solve commutators [p2,x], or [p2,x[/SUP]2[/SUP]] step by step. When I try do solve this on my own, I just keep getting confused by the order of operators in the equation.

9. May 18, 2013

vela

Staff Emeritus
Show what you tried.

10. May 18, 2013

Rorshach

ok, so I did the calcuation as follows:
[pxpx,x]=px[px,x]+x[px,x]px
px(pxx-xpx)+(pxx-xpx)*px
-ihbarred(∂/∂x)(-ihbarred(∂x/∂x)+ihbarredx(∂/∂x))+(-ihbarred(∂x/∂x)+ihbarredx(∂/∂x))*(-ihbarred(∂/∂x))
-ihbarred(∂/∂x)(-ihbarred+ihbarredx(∂/∂x))+(-ihbarred+ihbarredx(∂/∂x))*(-ihbarred(∂/∂x))
-ihbarred^2(∂/∂x)+xhbarred^2(∂/∂x)-hbarred^2(∂/∂x)+xhbarred^2(∂/∂x)(∂/∂x)

and that is pretty much the result that I obtain every single time I calculate this. Where am I making mistake?

11. May 18, 2013

vela

Staff Emeritus
Once you get to this point, use the fact that $[x,p_x]=i\hbar$.

12. May 18, 2013

Rorshach

But that is what I want to do with the jacobi identity, so the demands in the problem are satisfied. I tried to do this the way I can see step by step what is coming from where, that is why I insisted on step by step explanation.

13. May 18, 2013

vela

Staff Emeritus
Your mistake is when dealing with terms like $p_x x$. It doesn't simplify to 1, like you have. The operator $p_x$ acts on everything to the right of it, so not just $x$ but the test function $\psi$ as well, i.e.,
$$p_x x \psi = -i\hbar \frac{\partial}{\partial x} (x\psi) = \cdots$$

14. May 18, 2013

Rorshach

so in case of [px2,x] (if I want to do it step by step) I am supposed to put the test function after the operator p?

15. May 18, 2013

vela

Staff Emeritus
Yes. Why wouldn't it be at the end?

16. May 19, 2013

Rorshach

It just confuses me so I wasn't sure if it was supposed to be before or after the p operator. I tried with using ψ function, unfortunately it didn't help. first bracket zeroes:
-ihbarred(∂/∂x)(-ihbarred(∂x/∂x)ψ+ixhbarred(∂ψ/∂x))
but the second one in no way gives me the value -2ihbarredpx, which is supposed to be an answer for this problem:
(-ihbarred(∂x/∂x)+ixhbarred(∂/∂x))(-ihbarred(∂ψ/∂x))
where am I making mistake?

17. May 19, 2013

vela

Staff Emeritus
Using $[AB,C] = A[B,C] + [A,C]B$, you have $[p_x^2,x] = p_x[p_x,x] + [p_x,x]p_x$. Now it's basic substitution:
$$[p_x^2,x]\psi = (p_x[p_x,x] + [p_x,x]p_x)\,\psi.$$ You can't just move $\psi$ past $p_x$.

How'd you get 0? Also, you're missing a term. You should have
$$p_x [p_x, x] \psi = p_x (p_x x - x p_x)\psi = -i\hbar\frac{\partial}{\partial x} \left[-i\hbar\frac{\partial}{\partial x} (x\psi) - x \left(-i\hbar\frac{\partial}{\partial x}\right) \psi\right].$$ Now use the product rule to expand the first term in the brackets.

18. May 19, 2013

Rorshach

You were right, I confused the order of calculations, hence the zeroing of the first bracket. My result of the first bracket came out to be -2(hbarred)^2(∂ψ/∂x). The second one comes to -(hbarred)^2(∂ψ/∂x)+x(hbarred)^2(∂/∂x)(∂ψ/∂x). unless I miss something similar to what I did in the first bracket.

19. May 19, 2013

Rorshach

and also- if the Jacobi identity is [A,[B,C]]+[B,[C,A]]+[C,[A,B]]=0, then for [p2x,x] it would be [px,[px,x]]+[px,[x,px]]+[x,[px,px]]=0, right? But how do I do this if there is commutator like [px2,x2]?

20. May 19, 2013

Rorshach

I tried calculation with Jacobi identity according to the scheme I wrote above, but the commutators just zero themselves, so it seems kind of pointless. What am I doing wrong?

21. May 20, 2013

vela

Staff Emeritus
Like MisterX, I don't see how the Jacobi identity is very helpful here.

22. May 20, 2013

Rorshach

Do You mean it is not possible to solve those commutators with jacobi identity? And also, could You please tell me what am I doing wrong with second bracket? The result of the second bracket that I obtained is equal to -(hbarred)^2(∂ψ/∂x)+x(hbarred)^2(∂/∂x)(∂ψ/∂x)

23. May 22, 2013

Rorshach

24. May 22, 2013

vela

Staff Emeritus
It's kind of hard to offer help when all you do is post your incorrect answer and ask what went wrong. How are we supposed to tell if we can't see the steps you took?

25. May 22, 2013

Rorshach

I' sorry, here are my steps:
from the first bracket $px(pxx-xpx)ψ$ I obtained the following result: $-2\hbar2\frac{∂ψ}{∂x}$. In the second bracket I made following steps:
$$(pxx-xpx)[tex]pxψ (-i/hbar\frac{∂x}{∂x}+xi/hbar\frac{∂}{∂x})(-i/hbar\frac{∂ψ}{∂x}) -/hbar2\frac{∂ψ}{∂x}+x/hbar2r\frac{∂ψ}{∂x}r\frac{∂}{∂x}$$

It seems that LaTeX does not want to play, no idea why. Tried to describe more clearly and visibly, if You know why it is like that please tell me