Proof of E=mc2

Before Einstein, it was known that a beam of light pushes against matter, this is known as radiation pressure.

This means the light has momentum.

Consider a cylinder of mass M.A pulse of light with energy E is emitted from the left side. The cylinder recoils to the left with velocity v=EMc. If the mass of the cylinder is large, it doesn't move far before the light reaches the other side. So, the light must travel a distance L, requiring time t=Lc. In this time, the cylinder travels a distance x=vt=[EMc](Lc).

This theory was demonstrated by Cockcroft and Walton in 1932, experimentally.