Momentum Transfer in Scattering

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1. Aug 9, 2016

Muh. Fauzi M.

1. The problem statement, all variables and given/known data
This is just a simple proof of substitution, but after one day struggle, I still can't get where that minus sign appear.

2. Relevant equations
Here is the equation:
$$(p_1 - p_3)^2=-(\mathbf p_1 - \mathbf p_3)^2$$
where $p_1=(E/c,\mathbf p_1)$ and $p_3=(E/c, \mathbf p_3)$ is four-momentum of incident and scattered electron respectively, with stationary target.

3. The attempt at a solution
Here is my doodling:
$$(p_1 - p_3)^2=(E/c+\mathbf p_1-E/c-\mathbf p_3)^2=(\mathbf p_1 - \mathbf p_3)^2.$$
Which step that I miss?

2. Aug 10, 2016

Muh. Fauzi M.

ah, i've got it. solved by myself.