Cosmic rays are, in fact, not rays, but high energy subatomic particles of cosmic origin that continually bombard Earth. The measurements scientists make of them, both on the ground and from probes in space, are the only direct measurements that are made of matter originating outside the solar system.

Among the cosmic rays are electrons, protons, and the complete nuclei of all the elements. Their energies range from below the rest mass of an electron, easily attainable in terrestrial accelerators, up to energies 10¹¹ times the rest mass of a proton. Matter with such energies is moving at speeds so close to the speed of light that there is an enormous relativistic time dilation, so that in its proper frame only 10^-11 of the time has elapsed that an observer on Earth would have measured. An early verification of German-born American physicist Albert Einstein's special theory of...