# Deriving Klein-Gordon from Heisenberg

1. Apr 7, 2009

### waht

1. The problem statement, all variables and given/known data

Sort of stuck deriving the Klein-Gordon equation from Heisenberg equation of motion

$$\dot{\varphi} = i [H, \varphi ]$$

2. Relevant equations

$$\dot{\varphi} = \frac{\partial\varphi}{\partial t}$$

$$H = \int d^3x \mathcal{H}$$

$$\Pi (x) = \dot{\varphi}(x)$$

$$\mathcal{H} = \Pi \dot{\varphi} - \mathcal{L}$$

3. The attempt at a solution

$$\dot{\varphi} = \int d^3x [ \dot{\varphi}^2 - \mathcal{L}, \varphi ]$$

if I expand that, then it becomes a real mess, not sure if I'm on the right track?

Last edited: Apr 7, 2009
2. May 24, 2011

### mingda

first, from the condition i*d(phi)/dt=[H,phi], you can arrive an equation, which makes [intgral(delta_phi)^2,phi]=0, then you differentiate the Heisenberg, equation, get d^2(phi)/dt^2=[H,[H,phi]], and substitute back. Then you get K-G equation immediately

3. May 24, 2011

### mingda

\dot{\varphi} = i [H, \varphi ], we obtain

\int d^3x [ \delta{\varphi}^2, \varphi ]=0 Eq(1)

differentiate Heisenberg Eq. again, we have

\dot\dot{\varphi} = i [H, \dot{\varphi} ]=- [H, [H, \dot{\varphi} ] ]

substitute \Pi (x) = \dot{\varphi}(x) back,

using the specific form of H and use Eq.1, you'll get the answer of KG equation.