Energy and Mass in Relativity Theory presents about 30 pedagogical papers published by the author over the last 20 years. They deal with concepts central to relativity theory: energy E, rest energy E₀, momentum p, mass m, velocity v of particles of matter, including massless photons for which v = c. Other related subjects are also discussed.According to Einstein's equation E₀= mc2, a massive particle at rest contains rest energy which is partly liberated in the nuclear reactions in the stars and the Sun, as well as in nuclear reactors and bombs on the Earth. The...