Here we are dropping all factors of c. This is based on the freedom to find a set of units where c=1.






The 4-vectors are 4-compoment objects that transform under the Lorentz transformation as a vector.


Boost along the x-directions.



Is an invariant under boosts.


You can define a 4-vector momentum


       is the usual three momentum

       E is the energy


is an invariant.


Let us evaluate this in a general frame



What is this in the rest frame of the particle?

and we identify the energy for a particle at rest as the mass energy



reaction 1+2=3+4


Energy and momentum conservation



Suppose a particle decays into n particles



Suppose you identify the three particle in the final state so that you know their rest masses.


For two particles