So if I understood well, Normal ordering just comes due to the conmutation relation of a and a⁺? right? Is just a simple and clever simplification.(adsbygoogle = window.adsbygoogle || []).push({});

Wick Theorem is analogue to normal ordering because it is related to the a and a⁺ again (so related to normal ordering, indeed).

However I do not know why and exactly where the Time order operator comes. Is this like onion layers? Normal ordering -> time order operator -> wick's theorem ?

And can someone explain me this equation? Please.

$$ <0|T\big\{\phi^{(0)}(x)\phi^{(0)}(y)\big\}|0>=G^{(0)}_F(x-y)$$

This really vanishes just because of a(p)|0>=0?

$$<0|:\phi(x)\phi(y):|0>=0$$

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# About Wick's Theorem, Time Order Operator, Normal Ordering and Green's Function

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