Boost generator transforms as vector under rotations

Click For Summary

Discussion Overview

The discussion revolves around the transformation properties of boost generators in the context of group theory and quantum field theory (QFT). Participants explore the implications of specific commutation relations and the definition of vector operators in quantum mechanics.

Discussion Character

  • Technical explanation, Conceptual clarification

Main Points Raised

  • One participant notes that the commutation relation [X_i,Y_j]=iε_{ijk}Y_k suggests that the vector of boost generators, denoted as \vec{Y}, transforms as a vector under rotations.
  • Another participant inquires about the definition of a vector operator in quantum mechanics.
  • A third participant describes a 3-vector as a set of quantities that transform correctly under rotations and provides the form of unitary rotation operators in Hilbert space.
  • This participant explains that a vector operator Y transforms according to the relation U(R)Y U†(R)=R Y, and mentions the expansion in a Taylor series for infinitesimal rotations leading to the commutation relations.
  • One participant expresses gratitude, indicating that the explanation provided clarity on the topic.

Areas of Agreement / Disagreement

The discussion appears to be largely exploratory, with participants seeking clarification on the transformation properties of boost generators. There is no explicit disagreement noted, but the initial participant's understanding was unclear, which was addressed by others.

Contextual Notes

Some assumptions about the definitions of vector operators and the nature of rotations in quantum mechanics may not be fully articulated, and the discussion relies on specific mathematical expressions that may require further context for complete understanding.

LAHLH
Messages
405
Reaction score
2
Hi,

I've read quite a few times now in group theory and QFT books that [tex][X_i,Y_j]=i\epsilon_{ijk}Y_k[/tex] can be regarded as saying that [tex]\vec{Y}[/tex], the vector of boost generators transforms as a vector under rotations (where X are SO(3) generators).

I don't really understand why this implies this fact, perhaps some could enlighten me.

Thanks
 
Physics news on Phys.org
What's the definition in quantum mechanics for a vector operator ?
 
Well I'm guessing this is it, but I don't really understand why.
 
A 3-vector is a set of three quantities that transform correctly under rotations. In a Hilbert space the unitary rotation operators are

[tex]U(R)=1+i\mathbf{a}\mathbf{J}[/tex]

where J is the total angular momentum and R is a clockwise rotation of angle |a| around the a/|a| direction. A vector operator Y transforms like

[tex]U(R)\mathbf{Y}U^\dagger(R)=R\mathbf{Y}[/tex]

If you expand this in first order taylor series (i.e. if you consider infinitesimal rotations), you'll find the commutation relations you mentioned.
 
Thanks a lot, that has cleared it up for me.
 

Similar threads

  • · Replies 1 ·
Replies
1
Views
1K
  • · Replies 31 ·
2
Replies
31
Views
6K
Replies
3
Views
2K
  • · Replies 5 ·
Replies
5
Views
2K
  • · Replies 9 ·
Replies
9
Views
944
  • · Replies 1 ·
Replies
1
Views
3K
  • · Replies 3 ·
Replies
3
Views
1K
  • · Replies 1 ·
Replies
1
Views
2K
Replies
13
Views
9K
  • · Replies 8 ·
Replies
8
Views
6K