We start out with Einsteins famous formula



Here E stands for energy, p for momentum, m for mass and c for the speed of light. Momentum is a vector, and if you do not know what a vector is, follow Help 3: Vectors.

If you do a bit of algebra you can express the momentum as function of energy and mass:

In nature, momentum is conserved. This means: What goes in must come out, which in our particle collisions means that p_before=p_after. For a Z boson decaying to an electron-positron pair this would mean p_Z = p_e^- + p_e⁺.

Quiz-question: In the everyday-life version of momenta p=mv we encounter a problem with massless objects, their momenta would be 0. What happens with the momentum in case of a massless particle in the special relativity version of momentum?

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