- #1
eudo
- 29
- 8
Maxwell field commutation relations
I'm looking at Aitchison and Hey's QFT book. I see in Chapter 7, (pp. 191-192), they write down the canonical momentum for the Maxwell field [itex]A^\mu(x)[/itex]:
[tex]
\pi^0=\partial_\mu A^\mu \\
\pi^i=-\dot{A}^i+\partial^i A^0
[/tex]
and then write down the commutation relations
[tex]
[\hat{A}_\mu(\boldsymbol{x},t),\hat{\pi}_\nu(\boldsymbol{y},t)]=ig_{\mu\nu}\delta^3(\boldsymbol{x}-\boldsymbol{y})
[/tex]
and state that if you assume the commutation relations
[tex]
[\hat{A}_\mu(\boldsymbol{x},t),\hat{A}_\nu(\boldsymbol{y},t)]=[\hat{\pi}_\mu(\boldsymbol{x},t),\hat{\pi}_\nu(\boldsymbol{y},t)]=0
[/tex]
we see that the spatial derivatives of the [itex]\hat{A}[/itex]'s commute with the [itex]\hat{A}[/itex]'s, and with each other, at equal times.
They state it as if it's obvious, so maybe I'm missing something, but I don't see why the spatial derivatives of the [itex]\hat{A}[/itex]'s commute with the [itex]\hat{A}[/itex]'s, and with each other.
I'm looking at Aitchison and Hey's QFT book. I see in Chapter 7, (pp. 191-192), they write down the canonical momentum for the Maxwell field [itex]A^\mu(x)[/itex]:
[tex]
\pi^0=\partial_\mu A^\mu \\
\pi^i=-\dot{A}^i+\partial^i A^0
[/tex]
and then write down the commutation relations
[tex]
[\hat{A}_\mu(\boldsymbol{x},t),\hat{\pi}_\nu(\boldsymbol{y},t)]=ig_{\mu\nu}\delta^3(\boldsymbol{x}-\boldsymbol{y})
[/tex]
and state that if you assume the commutation relations
[tex]
[\hat{A}_\mu(\boldsymbol{x},t),\hat{A}_\nu(\boldsymbol{y},t)]=[\hat{\pi}_\mu(\boldsymbol{x},t),\hat{\pi}_\nu(\boldsymbol{y},t)]=0
[/tex]
we see that the spatial derivatives of the [itex]\hat{A}[/itex]'s commute with the [itex]\hat{A}[/itex]'s, and with each other, at equal times.
They state it as if it's obvious, so maybe I'm missing something, but I don't see why the spatial derivatives of the [itex]\hat{A}[/itex]'s commute with the [itex]\hat{A}[/itex]'s, and with each other.