Greisen-Zatsepin-Kuzmin limit

Etymology
From the surnames of the three physicists who proposed this on long distance travel of energetic particles. From. Named after American physicist, and Soviet (now Russian) physicists , and. The Soviet team and the American each discovered the limit separately in the 1960s, being independently computed in 1966.

Proper noun

 * 1)  A theoretical upper limit on the energy of cosmic ray protons travelling from other galaxies through the intergalactic medium to our galaxy: it is 5×1019 eV, or about 8 joules (the energy of a proton travelling at ≈ 99.99999999999999999998% the speed of light).