An example of E=mc2

Picture from CERN 2-metre hydrogen bubble chamber exposed to a beam of negative kaons () with energy 4.2 GeV. This piece corresponds to about 70 cm in the bubble chamber.

One beam particle interacts with a proton producing four outgoing tracks, two positive and two negative. The short dark stopping track is immediately identifiable as a proton. In the absence of measurements a reasonable interpretation for this event is:

The production of extra particles in the final state shows that some of the kinetic energy of the beam has been converted into new particles ( and ), an example of E = mc2.

Possible exercise on charge conservation

There are no clear knock-on electrons in this picture from which to deduce the direction of the magnetic field. Starting from the short dark track immediately identifiable as a proton, use charge conservation to show that the final state has charge 0 and that the beam particle is negative.

For a more spectacular example of E=mc2 ... click here.