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Towards Grand Unification

or

More than 3 or 2 interactions...

Philosophy and physicists dreams of unification.

Physicists have a secret dream: To be able to explain all phenomena in the Universe with the minimum number of elements: Reduce the number of particles (see the supercords theory) and the number of interactions to one. This quest, is it utopian or realisable?

• The Universe, does it restrict itself to a fixed number of simple laws, hidden from our eyes,  but accessible with our more and more sophisticated technology?
• Or is the Universe infinitely complex, essentially unknowable, and giving up nothing but scraps of partial understanding? We would then be like a spectator, knowing the world only by the chinese shadows projected on the wall (the platonic myth of the cavern, do you know it?)

• The multiplicity of natural materials (metals, stones, liquids...) has been explained by the atomic theory and all matter has found its foundations in the periodic table of elements.
• Atoms and the numerous particles discovered with the aid of the colliders have been disected, classified and reduced to a small number in the Standard Model.

The same evolution has occured concerning the forces which govern our Universe:

• Newton explained in 1687 that the astonomical force of attraction and terrestrial gravity are nothing but one and the same interaction called gravitational interaction.
• After Oersted, Maxwell expleined in 1873 that magnetism, electricity and light are nothing but different manifestations of  the same single interaction called the electromagnetic interaction.

THE GRAND PROJET FOR THE UNIFICATION OF THE  INTERACTIONS

This evolution towards the unification of forces may actually come to be understood following two different approaches:

the synthetic approach: the interactions observable by physicists would be nothing but different aspects of a single and same original force (the Superforce!)
 
• the historical approach: All of the interactions being unified at the birth of the Universe (the famous big bang), but having differentiated themselves since then.

The electroweak interaction
 

Our Standard Model finally describes only 4 interactions?
False. In reality, and since 1967, the weak and electromagnetic interactions have been unified by the theory of the electroweak interaction.

In 1979, two American physicists (Sheldon Lee Glashow and Steven Weinberg) and a pakistani physicist (Abdus Salam) received the Nobel Physics Prize for their theoretical work which in 1967 unified the weak and electromagnetic interactions.

The interactions are modelled mathematically in the form of a field of forces having a "gauge symmetry".
 

The important thing to understand here is that our Nobel Physics prize winners demonstrated that the gauge field from the weak interaction was structuraly identical to the electromagnetic field. Now the symmetry which invoked the weak electric theory imposed 4 mediator bosons of null mass: the photon, the Z^o and the two W.
This is in contradiction to the facts. In 1983, Carlo Rubbia at CERN had discovered the bosons Z^o and W predicted by the theory, but these had a mass (anticipated!), and even an enormous mass in the order of 90 times that of the proton!

How from here do we reconcile this mass of the Z^o and W with the electroweak theory?

Physicists are cunning, and they simply invented a new force field called the Higgs field (the name of the Scottish physicist Peter Higgs) which manifests itself by the intermediary of a mysterious boson evidently called the Higgs boson. This field only exists with the intermediary bosons Z^o, W^- and W⁺ to give them a mass, but which does not pair with the photon, preserving therefore its null mass. Once the Higgs field manifests itself, the two interactions, electromagnetic and weak, distinguish themselves from one another. We say that there is a break of symmetry.
From the moment that the Higgs mechanism is introduced into the equations, the electroweak theory functions perfectly well. It has permitted to be predicted, with precision, the mass of intermediary bosons well before their discovery by Rubbia.

Only weak point: the Higgs boson has never yet been detected and its existance is indespensible to validate the electroweak theory. We hope for its discovery thanks to the LHC hadron supercollider of CERN, operational in 2005.
The discovery of the Higgs boson will require enormous energies in the order of 100 GeV (GigaelectronVolts). Only the future LHC will be capable of recreating in its womb an environment of such energy density. Now it is at this energetic scale that physicists could maybe observe the appearance of the Higgs boson, because these 100 GeV correspond with its estimated mass (don't forget E = mc² !).
 

The electronuclear interaction
 

That leaves us with just the gravitational, strong and electroweak interactions.

The electronuclear interaction is also named the Grand Unification Theory or GUT. It unifies the strong and electroweak interactions.

This theory was proposed for the first time by Sheldon Glashow (him again) and Howard Georgi in 1973. Other versions have been proposed since: the one the most in vogue at present bears the subdued name of SU(5). This interaction is only possible at levels of enormous energy (more than 10¹⁶ GeV for the connoisseurs!), conditions which only ever existed at the very start of the big bang.
 

The SU(5) theory and leptoquarks

Because this theory groups together 5 particules (the electron, the neutrino and the d antiquarks of each colour) in a fundamental quintuplet. The other particles would be grouped in a decuplet.
The symmetry of the GUT would permit the invariance of nature by the permutation of a lepton (electron, neutrino...) with a quark: To put it plainly, leptons and quarks of the quintuplet would be transformable, one to another, and these transitions could be possible by the intermediary of new bosons called leptoquarks. These leptoquarks would then be bosons carrying a colour charge and a fractional electric charge.
This theory would permit explanation of the troubling fact that the value of the negative electric charge (Q= -1) of an electron corresponds to the same positive value (Q= +1) of the proton.
 

This theory predicts an appalling event: the proton, symbol of stability of matter, should have a limited lifetime! This lifetime would be 10³¹yrs; knowing that the Universe was born around 10¹⁰ years ago, there is still some spare time, phew! Enormous swimming-pool proton disintegration detectors have therefore been constructed: A proton emits two photons g and a positron e⁺ when it disintegrates; now the positron emits a blue luminous cone in water (Cerenkov effect for the connoisseurs) which photo-multipliers can detect. Alas, for the moment, no positive results have been announced, which rends this GUT theory much less solid than the electroweak theory.
 

The GUT theory could then unify all interactions except gravitation.
The theory which includes gravitation in one superforce exists: it is called the "theory of Everyhing". Lest look at this physicists dream...