The Standard Model of particle physics — a theory of all forces and particles except gravity and a theory that has survived all tests over the past thirty years — says it is possible to convert matter entirely into energy. Old-fashioned nuclear physics allows some matter to be converted into energy, but because nuclear physics requires the number of heavy particles like neutrons and protons, and light particles like electrons, to be separately conserved in nuclear reactions, only a small fraction (less than 1%) of the mass of the uranium or plutonium in an atomic bomb can be converted into energy. The Standard Model says that there is a way to convert all the mass of ordinary matter into energy; for example, it is in principle possible to convert the proton and electron making up a hydrogen atom entirely into energy. Particle physicists have long known about this possibility, but have considered it forever irrelevant to human technology because the energy required to convert matter into pure energy via this process is at the very limit of our most powerful accelerators (a trillion electron volts, or one TeV).
I am very much afraid that the particle physicists are wrong about this Standard Model pure energy conversion process being forever irrelevant to human affairs. I have recently come to believe that the consistency of quantum field theory requires that it should be possible to convert up to 100 kilograms of ordinary matter into pure energy via this process using a device that could fit inside the trunk of a car, a device that could be manufactured in a small factory. Such a device would solve all our energy problems — we would not need fossil fuels — but 100 kilograms of energy is the energy released by a 1,000-megaton nuclear bomb. If such a bomb can be manufactured in a small factory, then terrorists everywhere will eventually have such weapons. I fear for the human race if this comes to pass. I very hope I am wrong about the technological feasibility of such a bomb.