How do we know that the fundamental forces become unified as the energy of particle interactions increases?
1 Answer
Two of the fundamental forces have already been proved to unify at high energies.
Explanation:
The standard model of particle physics defines the interactions of fundamental forces as being due to particles called bosons.
The electromagnetic force is propagated by the photon, which is a boson.
The weak nuclear force has two bosons. The W is a charged particle which is responsible for changing a quark from once flavour to another. This is what causes beta decay where a neutron is turned into proton, en electron and an electron antineutrino. The other boson is the Z which is involved when neutrinos are scattered off other particles.
Both forces are described mathematically by symmetry groups. When the W and Z were discovered it was possible to prove that electromagnetism and the weak force can be unified into the electroweak theory. It worked by creating a unifying symmetry group which shows that the photon and the Z boson are indistinguishable at high energies.
The next step is to discover a Grand Unified Theory (GUT) which unifies the colour force which binds quarks together with the electroweak force. The strong nuclear force is actually a residual effect of the colour force.
There are several candidate theories for a GUT. The problem is that the unification energy if so high that we can't conceive of how to build a particle accelerator to reach it. Candidate GUTs require additional particles which we haven't yet discovered.
It is likely that a GUT will be found but not soon.
Gravity poses a real problem. There is no quantum theory of gravity. A Theory of Everything which unifies all four forces requires new physics. String theory made be the answer but it has a long way to go.
