momentum
We can work out what this invariant is by first arguing that, since it is a scalar, it does not matter in which reference frame we calculate it, and then by transforming to a frame where the total momentum is zero.






P


2


=
−


(



E

rest


c


)


2


=
−
(
m
c

)

2


.


{\displaystyle \mathbf {P} ^{2}=-\left({\frac {E_{\text{rest}}}{c}}\right)^{2}=-(mc)^{2}.}


