What is Grand Unification Theory?

In the mid-1970's physicists were excited with the recent success of Steven Weinberg, Abdus Salam and Sheldon Glashow in creating a unification theory for the electromagnetic and weak forces. By applying what is called 'group theory' , physicists such as Glashow, Georgi and others proposed that you could use the symmetries of 'SU(5)' to unite the weak and electromagnetic forces with the strong nuclear force which is mediated by gluons. This became known as 'Grand Unification Theory' or 'GUT', and quickly evolved into many variants including 'super-symmetric GUTs (SUSY- GUTs)', 'super gravity theory' and 'dimensionally-extended SUSY GUTs', before being replaced by string theory in the early 1980's.

It produced a lot of excitement in the late-70s and early-80's because it seemed as though it could provide an explanation for the strong, weak and electromagnetic forces, and do so in a common mathematical language. It's major prediction was that at the enormous energy of 1000 billion billion volts (10^15 GeV) the strong nuclear force would become similar (or unified) with the electromagnetic and weak forces. Applying these ideas to cosmology also led to the creation of Inflationary Cosmology.

Today, the so-called Standard Model of nuclear physics unifies physics (except for gravity) and uses some of the basic ideas of GUT to do so. Physicists worked very hard to confirm several basic ideas in GUT theory such as 'spontaneous symmetry breaking' by looking for the Higgs Boson. In 2012 this elusive particle was discovered at the Large Hadron Collider some 50 years after it was predicted. This wass a revolutionary discovery because it demonstrated that the entire concept of spontaneous symmetry breaking seemed to be valid. It was the keystone idea in the unification of the electromagnetic and weak forces for which Abdus Salam, Steven Weinberg and George Glashow received the Nobel Prize in the mid-1970s. SSB was also the workhorse concept behind much of the mathematical work into GUTs.

GUT research in the booming 1970s also uncovered a new 'Supersymmetry' in nature, which continues to be searched for. The unpleasant thing about the current Standard Model is that it has several dozen adjustable constants that have to be experimentally fine-tuned to reproduce our physical world including such numbers as the constant of gravity, speed of light, fine structure constant, and the constants that determine how strongly the leptons and quarks interact. Physicists think that this is way too much, and so the search is on for a better theory that has far fewer ad hoc constants. There is also the problem that the Standard Model doesn't include gravity.

The hope that gravity could some how be incorporated into GUTs pursued in the 1970s was ultimately never realized because of the advent of String Theory which provided a newer way to look at gravity as a 'quantum field'. Yet most popular versions of string theory include supersymmetry, hense they are called superstring theories.

Supersymmetry has grown to become a lynchpin concept behind many ideas for unifying all of the four foruces including gravity, however after five years of searching for signs of it at the CERN Large Hadron Collider, not so much as a trace of it has been detected. It seems as though the Standard Model is all there is, but in which the strong force and the 'electroweak' forces may possibly not be unified further.

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