The Concept of Mass in the Einstein Year

Various facets of the concept of mass are discussed. The masses of elementary particles and the search for higgs. The masses of hadrons. The pedagogical virus of relativistic mass. 1 From “Principia” to Large Hadron Collider (LHC) The term “mass’ was introduced into mechanics by Newton in 1687 in his “Principia” [1]. He defined it as the amount of matter. The generally accepted definition of matter does not exist even today. Some authors of physics text-books do not consider photons – particles of light – as particles of matter, because they are massless. For the same reason they do not consider as matter the electromagnetic field. It is not quite clear whether they consider as matter almost massless neutrinos, which usually move with velocity close to that of light. Of course it is impossible to collect a handful of neutrinos similarly to a handful of coins. But in many other respects both photons and neutrinos behave like classical particles, while the electromagnetic field is the basis of our understanding of the structure of atoms. On the other hand, the so-called weak bosons W, W−, Z are often not considered as particles of matter because they are too heavy and too short-lived. Even more unusual are such particles as gluons and quarks. Unlike atoms, nucleons, and leptons, they do not exist in a free state: they are permanently confined inside nucleons and other hadrons. There is no doubt that the problem of mass is one of the key problems of modern physics. Though there is no common opinion even among the Presented at the 12th Lomonosov conference on Elementary Particle Physics, Moscow State University, August 25-31.