The UnifierTheory of Everything

The Unifier carries four keys and has fitted two of them together. The other two do not fit yet.

The first line on this page is the Standard Model's symmetry group: SU(3) for the strong force, SU(2) × U(1) for the electroweak force. That second piece is the one success of unification so far. In the 1960s Sheldon Glashow, Abdus Salam and Steven Weinberg showed that electromagnetism and the weak force are two faces of one interaction. The second line is the bookkeeping: the electron's charge is the SU(2) coupling times the sine of the weak mixing angle, and the W and Z masses are tied together through that same angle. The theory predicted a neutral weak current, seen at CERN's Gargamelle chamber in 1973, and the W and Z themselves, found at CERN in 1983.

Grand unified theories try to fold the strong force in as well. Their signature prediction is that protons decay, very slowly. None has ever been seen to. The Particle Data Group's 2024 listings put the partial lifetime for the favoured decay, into a positron and a neutral pion, above 2.4 × 10³⁴ years: more than a million billion billion times the age of the universe.

Gravity is the hard part. The third line, Einstein's field equation, describes it superbly on every scale that has been tested. But it treats spacetime as smooth and classical, and attempts to quantize it the way the other forces were quantized break down at very high energies. String theory and loop quantum gravity are candidates. Neither has an experimental confirmation.

So there is no accepted theory of everything. There is one confirmed unification, one proposed, and one missing.

The Unifier is not finished. Three equations on the page, and the one that joins them is still blank.