ABSTRACT
In the Einstein box calculation, the conservation of momentum is explicitly assumed. In discussing the ultimate speed experiment, the readers took it for granted that a calorimetric measurement would, through energy conservation, give us an exact knowledge of the kinetic energy brought in by electrons traveling at speeds close to the speed of light. All the kinetic energy of collision would then be available for particle creation. To produce colliding beams of particles traveling in opposite directions is technically a great deal harder than to have one beam striking a stationary target, but the payoff can be great. In optical spectroscopy the Doppler effect manifests itself as a small though quite measurable broadening of spectral lines, resulting from the random thermal motions of the emitting atoms. In high-energy particle physics, however, it will frequently happen that photons are emitted from unstable particles traveling with speeds.
