- #1
Xenosum
- 20
- 2
Homework Statement
Consider the Lagrangian, L, given by
[tex] L = \partial_{\mu}\phi^{*}(x)\partial^{\mu}\phi(x) - m^2\phi^{*}(x)\phi(x) . [/tex]
The conjugate momenta to [itex] \phi(x) [/itex] and [itex] \phi^{*}(x) [/itex] are denoted, respectively, by [itex] \pi(x) [/itex] and [itex] \pi^{*}(x) [/itex]. Thus,
[tex] \pi(x) = \frac{\partial L}{\partial(\partial_{0}\phi(x))} = \partial_0\phi^{*}(x) [/tex]
[tex] \pi^{*}(x) = \frac{\partial L}{\partial(\partial_{0}\phi^{*}(x))} = \partial_0\phi(x) .[/tex]
Upon quantizing the system, [itex] \phi(x) [/itex] and [itex] \phi^{*}(x) [/itex] are promoted to operators which satisfy the equal-time commutation relations:
[tex] [ \phi(x) , \pi(y) ] = i\delta^{(3)}(\vec{x} - \vec{y}) [/tex]
[tex] [ \phi^{*}(x) , \pi^{*}(y) ] = i\delta^{(3)}(\vec{x} - \vec{y}) [/tex]
(all others zero). In the Heisenberg regime, the time evolution of the operator [itex] \phi(x) [/itex], [itex] i \partial_0 \phi(x) [/itex], is given by
[tex] i \partial_0 \phi(x) = \left[ \phi(x) , H(y) \right]. [/tex]
The Hamiltonian may be derived from the Lagrangian, and we find that
[tex] i\frac{\partial \phi(x)}{\partial t} = \int d^{3}y \left( \left[ \phi(x) , \pi(y)\pi^{*}(y) \right] + \left[ \phi(x) , \nabla\phi^{*}(y) \cdot \nabla\phi(y) \right] + m^2 \left[ \phi(x) , \phi^{*}(y)\phi(y) \right] \right). [/tex]
Now here's my question. When we evaluate the commutators both my professor and a solution manual to Peskin and Schroeder claim that only the first commutator survives, because [itex] \phi(x) [/itex] commutes with everything except for the its conjugate momentum (by the canonical commutation relations). I don't see why. The canonical commutation relations only give us a relationship between [itex] \phi(x) [/itex] and [itex] \pi(y) [/itex], not e.g. [itex] \phi(x) [/itex] and [itex] \phi(y) [/itex]. The point is pressed by the fact that one can only show that the commutator [itex] \left[ \phi(x) , \phi(y) \right] [/itex] vanishes for space-like separation between the points x and y (this is the condition which preserves causality).
I guess it would be resolved if the commutator were instead [itex] \left[ \phi(x) , H(x) \right] [/itex], but this doesn't seem to be how it's done.
Thanks for any help!