Prove Matrix Representations of p & x Don't Satisfy [-ih/2pi]

  • Thread starter Thread starter bon
  • Start date Start date
  • Tags Tags
    Matrices Proof
Click For Summary

Homework Help Overview

The discussion revolves around the properties of matrix representations of the momentum operator \( p \) and the position operator \( x \) in quantum mechanics, specifically addressing the commutation relation \([p,x] = -ih/2\pi\). The original poster seeks to prove that no finite dimensional representations satisfy this relation and questions how the argument changes in the context of infinite dimensional matrices.

Discussion Character

  • Conceptual clarification, Assumption checking, Mathematical reasoning

Approaches and Questions Raised

  • Participants explore the implications of taking the trace of both sides of the commutation relation and question the validity of the argument in finite versus infinite dimensions. They discuss the trace of the identity matrix and its implications for dimensionality, as well as the definition of the trace operation in infinite dimensions.

Discussion Status

Participants are actively engaging with the problem, raising questions about the mathematical definitions involved, particularly regarding the trace operation in infinite dimensions. There is an exploration of the consequences of the trace being zero in finite dimensions and how this might differ when considering infinite dimensional representations.

Contextual Notes

There is an ongoing discussion about the assumptions related to the dimensionality of the matrices and the implications of the trace operation being well-defined in infinite dimensions. Participants are questioning whether the trace of the identity matrix remains meaningful in this context.

bon
Messages
547
Reaction score
0

Homework Statement



By taking the trace of both sides prove that there are no finite dimensional matrix representations of the momentum operator p and the position operator x which satisfy [p,x] = -ih/2pi

Why does this argument fail if the matrices are infinite dimensional?


Homework Equations





The Attempt at a Solution



No idea really..

So I am guessing it'll be something like

[p,x] = px-xp

Let px-xp= some matrix C

Take trace of both sides,

Tr(px-xp) = Tr (C)

0=Tr(C)..

But don't see if/how this answers the question/what would be different in infinite case?

Thanks!
 
Physics news on Phys.org
You also need to note that in this finite dim representation

[tex]C = - i\hbar I[/tex]

where [tex]I[/tex] is the identity matrix.
 
Oh i see. Thanks

so i have Tr(-ih/2pi I)=0 so assuming p and x are nxn, we have (-ih)^n Tr(I) = n(-ih)^n = 0 which implies n = 0?

But how do things change if it is infinite?
 
bon said:
Oh i see. Thanks

so i have Tr(-ih/2pi I)=0 so assuming p and x are nxn, we have (-ih)^n Tr(I) = n(-ih)^n = 0 which implies n = 0?

Is n=0 consistent with actually having a matrix representation?

But how do things change if it is infinite?

Is the trace operation well-defined for infinite-dimensional matrices?
 
umm well I am not sure..can't it just be the sum to infinity rather than to some finite limit?
 
bon said:
umm well I am not sure..can't it just be the sum to infinity rather than to some finite limit?

What about Tr I? Is that well-defined?
 
it's infinity? oh i see..we run into problems? is that all that needs to be said?

Thanks!
 
Well you have similar problems trying to define Tr(xp) and Tr(px), so it's hard to define the difference between those traces as well.
 

Similar threads

  • · Replies 5 ·
Replies
5
Views
14K
Replies
25
Views
8K
  • · Replies 1 ·
Replies
1
Views
3K
  • · Replies 1 ·
Replies
1
Views
3K
  • · Replies 2 ·
Replies
2
Views
4K
Replies
3
Views
8K
  • · Replies 2 ·
Replies
2
Views
2K
  • · Replies 2 ·
Replies
2
Views
4K
  • · Replies 38 ·
2
Replies
38
Views
12K
  • · Replies 1 ·
Replies
1
Views
4K