Starting from the question of what the equations in school physics textbooks really mean, the author traces the search for a single theory that would describe every force in the universe. Newton's law of gravity and Maxwell's four equations of electromagnetism are presented as early unifications, followed by Einstein's General Relativity (1915), which replaced instantaneous Newtonian gravity with curved space and predicted gravitational waves travelling at the speed of light, finally detected by LIGO in February 2016. Quantum mechanics then revealed a probabilistic atomic world with two further forces, the strong and weak nuclear forces, and Glashow, Salam and Weinberg shared the 1979 Nobel Prize for unifying these with electromagnetism in a Standard Model where forces are carried by gauge bosons such as the gluon, the W and Z bosons and the photon, with the graviton postulated for gravity.
Gravity, however, still resists a quantum description, and the need for a Theory of Everything is illustrated by black holes, where both General Relativity and quantum mechanics give nonsensical results. String Theory, in which fundamental particles are tiny vibrating one-dimensional strings of energy in an eleven-dimensional universe whose extra seven dimensions are curled up too small to notice, is described as the most studied candidate alongside Loop Quantum Gravity. The article concludes that strings are too small to be detected even by the Large Hadron Collider, so the theory can neither be confirmed nor disproved yet, leaving open the hope of one day describing the whole universe in a single equation.
