It is not controversial among physicists that all particles are "energy". Loop theorists, sometimes in rivalry with string theory, claim that spacetime itself converts into the particles. Matter's being a special variant of energy was a consequence of Einstein's special theory of relativity, and thereupon Einstein formalized the mass-energy equivalence, E=mc. When sufficiently energetic photons collide, they can combine and generate matter—matter creation. All particles have antiparticles, and atoms of matter have antiatoms of antimatter, whose union annihilates the particles and matter while leaving energy.