The Standard Model (particle physics grand unified theory) and Gravity
All quantum field theories are based ultimately upon simple extensions of Dirac's mathematical work in attempting to unify special relativity with quantum mechanics in the late 1920s. People such as Dr Sheldon Glashow and Dr Gerard t'Hooft developed the framework. A quantum field theory, the 'Standard Model' [gauge groups SU(3) x SU(2) x U(1)] is built on a unitary group, U(1), as well as two symmetry-unitary groups, SU(2) and SU(3).
U(1) describes electric charge (having a single vector field or gauge boson, the photon). Because bosons are spin 1, the force can be attractive or repulsive, depending on the signs of the charges. (To have a charge which is always positive or attractive, like gravity, would require a spin 2 boson which is why the postulated quantum gravity boson, the unobserved graviton, is supposed to have a spin of 2.)
SU(2) describes weak isospin interactions (having 3 vector fields or 3 gauge bosons: Z, W+, W-).
SU(3) describes the strong nuclear force, the 'colour charge' interactions (having 8 vector fields or 8 gauge bosons: gluons). Gauge bosons are force mediators, 'gauge' coming from the size scale analogy of railway line gauges, and 'boson' coming from Einstein's collaborator Bose, who worked out the statistical distribution describing a gas of light photons.
SU(2) allows left handed fields to form doublets, while left handed fields in SU(3) allows triplets of quarks (baryons like neutron and proton) and singletons (leptons like electron and muon) to form. The right handed fields are the same for SU(3) but only form a pair of two singletons (mesons) for SU(2).
To work, mass must be provided by an uncharged massive particle, the 'Higgs field boson'. SO(3) is another symmetry group which describes the conservation of angular momentum for 3 dimensional rotations. Is the Standard Model a worthless heap of trash, as it requires the existence of an unobserved Higgs field to give rise to mass? No, it is the best available way of dealing with all available physics data, and the Higgs field is implied as a type of ether. If you see an inconsistency between the use of special relativity in quantum field theory and the suggestion that it implies an ether, you need to refresh yourself on the physical interpretation of general relativity, which is a perfect fluid (ether/spacetime fabric) theory according to Einstein. General relativity requires an additional postulate to those of special relativity (which is really a flat earth theory, as it goes not allow for curved geodesics or gravity!), but gives rise to the same mathematical transformations as special relativity.
Peter Woit has some very interesting ideas for proceeding with the Standard Model in the sense of explaining the electroweak symmetry using geometric spinors and Clifford algebras (Quantum Field Theory and Representation Theory: A Sketch, http://arxiv.org/abs/hep-th/0206135). Personally, I have various reservations with the popularisation of the existing Standard Model. For one thing, the basic physics described by the maths is usually ignored by the modellers, who for example don't care if Maxwell made errors or fiddles in his 'electromagnetic unification' which is encapsulated as U(1). For another thing, these calculations are fine for building up a representation of the possible particles and fields, and the representation predicts - by gaps in the pictorial symmetry - missing particles that are later discovered. But never forget that these theories do not predict the masses of particles. The electroweak prediction of the relative force of the weak and electromagnetic force replies on you putting into the calculation experimentally determined values of the particle masses. So the correct prediction is not completely theoretical, it is unifying two things, but is using other data to do so, so it is really just a correct scaling procedure. Tony Smith has a prediction of the top quark mass, relying on Higgs field assumptions. (Unfortunately, he is suppressed by arXiv.org.)
Traditional extensions of the Standard Model involve, for example, SUSY, 'SUper-SYmmetry'. SUSY is supposed to be a helpful girl as she matches or pairs up fundamental particles with 'superpartners'. Each superpartner has a spin that is different from the original particle by half a unit. For SUSY to model reality, the particle and superpartner cannot have exactly the same mass, so SUSY is a broken symmetry theory. None of the superpartners have so far been detected, so they must be very massive! The advantage of SUSY is to unify the strengths of all fundamental forces at very short distances from particle core, i.e., when all forces including electromagnetism are 137 times that of the electromagnetism force as seen from a long distance. Saying 'short distances from the particle cores' is identical to saying 'at extremely high energies', because you need to accelerate them to extremely high energies to break through the polarised shield of quantum foam particles around the particle's core!
However, SUSY is very naughty girl, predicting unobserved dark matter, and is too close to arm-waving parallel universes, string theory, and 11 dimensional M-theory to be defended as science. Why can't the gravity mechanism be taken seriously? The hypocrisy of the 'branes' of string theory is bad. The string theorists, for example Dr Edward Witten in Physics Today (April 1996) and Dr Lisa Randall in her 2005 book Warped Passages claim that general relativity can be unified with the Standard Model using various aspects of string theory. I've discussed them in detail on previous posts. It is not correct. As Dr Roger Penrose points out in Road to Reality, and as Dr Peter Woit confirms, simply putting quantum gravity into the same package as the Standard Model by means of specifying ad hoc 11 dimensions and 'branes scenarios' with no experimental justification or testable predictions is a dead end. What is needed is the more classical mechanism that makes testable and tested predictions...