xepma said:
I'm not convinced this is enough. You would need to see if it holds for all ppssible states.
Using the (-+++) metric and \dot{\phi}=i[H,\phi]=iH\phi-i\phi H:
i\int d^3x e^{-ikx} \bar \partial_0 \phi(x)=<br />
i[i\int d^3x e^{-ikx}H\phi(x)-i\int d^3x e^{-ikx}\phi(x)H-i\int d^3x e^{-ikx} E_k\phi(x)]
Using that \phi(x)=\int d^3 \tilde q [a(q,t)e^{iqx}+a^\dagger(q,t)e^{-iqx}] this becomes:
<br />
=-H[\frac{a(k,t)}{2E_k}+\frac{a^\dagger(-k,t)}{2E_k}e^{2iE_kt}]<br />
<br />
+[\frac{a(k,t)}{2E_k}+\frac{a^\dagger(-k,t)}{2E_k}e^{2iE_kt}]H<br />
<br />
+E_k[\frac{a(k,t)}{2E_k}+\frac{a^\dagger(-k,t)}{2E_k}e^{2iE_kt}]<br />
Now consider the operation of this on a state |M> with energy M. The 2nd terms (the creation operators) in each of the lines cancel because the Hamiltonian in the first line pulls out an energy of -(M+E
k), the second line pulls out an energy of M, and the third line is just E
k: these add to give zero. So far so good, so examine the 1st terms (the destruction operators). Consider two cases, where |M> contains the particle |k>, and not. If M does not, then all the destruction operators will produce zero acting on |M>. If |M> contains |k>, then the Hamiltonian brings out a -[M-E
k] in the 1st line, a +M in the second line, and an E
k in the 3rd line. This adds to 2E
k, and multiplying this by the term \frac{a(k,t)}{2E_k}, this leaves just the destruction operator.
Anyways, there are some subtleties that I'm bothered by, but I'm convinced that a(k,t)= i\int d^3x e^{-ikx} \bar \partial_0 \phi(x) is still true in an interacting theory. What's remarkable is if you plug this expression in for a(k,t) to calculate the commutator of the a's, and use the canonical commutation relations, then you get the standard equal time commutation relations for a(k,t):
[a(k,t),a^\dagger(q,t)]=\delta^3(k-q)(2\pi)^32E_k
All that's required is that \Pi=\frac{\partial \mathcal L}{\partial \dot{\phi}}=\dot{\phi}, so that you can identify the time derivative terms in a(k,t)= i\int d^3x e^{-ikx} \bar \partial_0 \phi(x) as the canonical momentum, so the commutation relation is easy to take.
So basically, we're pretty much required to have the Lagrangian be no more than 2nd order in time derivatives so that the canonical momentum is just the time derivative of the field, or there is no equal time commutation relations for the creation operators.
So basically the relation a(k,t)= i\int d^3x e^{-ikx} \bar \partial_0 \phi(x) is actually more fundamental than the Lagrangian!
Anyways, I was reading Srednicki again, and he showed that a^\dagger(k,t) can actually create multiparticle states when acting on the vacuum! This is equation (5.23). However, as t goes to +- infinity, you don't have this happening.
This is interesting stuff, but I heard that getting into more detail on this stuff takes constructive quantum field theory.