# Klein Gordon Field

1. Apr 30, 2014

### grimx

Hi everyone! Im' a new member and I'm studying Quantum Field Theory.

"The interpretation of the real scalar field is that it creates a particle (boson) with momentum p at the point x."

and :

$\phi$$\left(x\right)$ $\left|0\right\rangle$ = $\int \frac{d^3p}{(2\pi)^3(2\varpi_p)}$ $e^{-ipx} |p\rangle$ (1)

but I didn't understand this equality... i know that:

$\phi (x) = \int \frac{d^3p}{(2\pi)^3(2\varpi_p)} (a_p e^{ipx} + a^+_p e^{-ipx})$ (2)

So... where it goes the term $a_p e^{ipx}$ in the expression (1) ???

Can someone kindly show me all the steps?
I know it's a stupid question, but I can not understand.

thank you very much!!!!!

2. Apr 30, 2014

### WannabeNewton

The vacuum is annihilated by $a_p$ by definition.

3. Apr 30, 2014

### grimx

But in theory... $a_p$ it should not destroy the particle created by $a^+_p$??

What am I doing wrong?

Thank you.

4. Apr 30, 2014

### WannabeNewton

We don't have $a_p a^{\dagger}_p$ in the free KG field. We have $a_p$ attached to the negative frequency modes and $a^{\dagger}_p$ attached to the positive frequency modes so they act independently of one another.

As such $\phi(x)|0\rangle$ simply creates a particle at $x$.

5. Apr 30, 2014

Thanks!! :)