Unification

The Problem : Too Many Fundamental Particles

By the early 1980's, a plethora of over 50 "fundamental" fermions & bosons were observed with no direction for their unification. An impasse of sorts was reached in high energy theory. In other words, there were too many "fundamental" entities and their was a challenge to theory to synthesize and integrate these entities into simpler underlying structures.

The Solution : Unification

One of the most important achievements in theoretical physics was the unification of electricity and magnetism by James Clerk Maxwell in the late 1860s and early 1870s.

More recently, electromagnetism and weak interactions were unified into the electroweak model by Steven Weinberg and Abdus Salam in the 1970s.

Einstein's theory of general relativity (GR) unifies energy and geometry into a single entity.

Unification is very fundamental and important in physics as it reduces the number of basic assumptions required in theory and also gives us a common way of understanding apparently unrelated phenomena.

The following disparate concepts need to be unified

1. Unify bosons and fermions into a single "higher" entity.
2. Unify electroweak with the strong force into a grand unified theory (GUT)
3. Unify gravity with quantum mechanics into quantum gravity (GR is in conflict with quantum mechanics).
4. Unify quantum gravity with the other forces.

Supersymmetry

Fermions and bosons are combined into a superfield using the concept of supersymmetry, which implies that fermions and bosons come in pairs with equal masses; hence we expect all observed particles to have (as yet unobserved) superpartners, i.e.

Fermions	Bosons
electron	selectron
photino	photon
quark	squark
gluino	gluon

GUT (Electroweak + Strong)

All attempts to unify based on local quantum fields predict that the proton will decay, and experiments rule this out. One positive outcome is that the best choice for group G which contains U(1) x SU(2) x SU(3) gauge fields is E₆ (an exceptional Lie group).

Quantum Gravity

All attempts to obtain a local quantum field theory for gravity have quantum fluctuations in the geometry only at scales around 10^-35 m. Due to large quantum fluctuations in energy, virtual pairs of black holes increasingly dominate & the very concept of a point in spacetime is no longer valid-----spacetime melts into a foam due to quantum randomness of geometry.

A positive outcome from these attempts to unify physics was that it become clear that one must consider supergravity (graviton + gravitino) and spacetime manifolds with dimensions greater than four.

Last updated: 06 March, 2000