Another index notation question hopefully pretty easy

  • #1
How do I make sense of the index notation expression

[tex]
r_j r_i p_j
[/tex]

?

What I really *want* it to be is

[tex]
\vec{r} (\vec{r} \cdot \vec{p})
[/tex]

And it turns into this, as long as I can commute the [itex]r_i[/tex] and [itex]r_j[/itex]...right?

(By the way, here [itex]\vec{r}[/itex] is just the position vector and [itex]\vec{p}[/itex] is the momentum vector.)
 

Answers and Replies

  • #2
HallsofIvy
Science Advisor
Homework Helper
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Well, those are simply coordinates, i.e. numbers, so they are commutative, aren't they?
 
  • #3
192
0
Yes, you can commute them just fine. I know how you feel, after doing a lot of quantum operator calculations it feels like such a breeze to have the commutative property!
 
  • #4
Thanks guys! Yes, this is actually something that's come up in trying to solve a quantum problem. And yeah...I've done enough of these to where I pretty much don't believe anything commutes anymore :(
 

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