A second possible break in the laws of physics has been noted

“WHO ORDERED that?” This was the reaction, famous in particle-physics circles, of Isidor Isaac Rabi to the discovery of the muon. Rabi, a Nobel laureate who helped America develop the atom bomb, was reflecting physicists’ general surprise that muons, which are, to all intents and purposes, just heavy and unstable versions of electrons, actually exist. To an orderly physicist’s mind they somehow seemed superfluous to Nature’s requirements.Establishing the muon’s nature was, though, an important part of the creation of what is known as the Standard Model of particle physics. This, along with Einstein’s general theory of relativity (actually a theory of gravity), is one of the two foundation stones on which modern physics is built. Yet the Standard Model is known to be incomplete for several reasons, one of which is precisely the fact that it does not yet embrace gravity. So it seems fitting that an answer to Rabi’s question, and with it a path to an explanation of physics beyond the Standard Model, may now have been opened by a measurement made on muons.