The so called Standard Model of physics is a masterpiece of mathematical reasoning.  It is the most successful theory of nature ever devised by humans.  Most of its predictions have been verified by three decades of experimentation.  Before the Standard Model, there were numerous theories proposed to explain physical reality. Almost as soon as a new theory was devised, an experiment would discover a new particle or a new type of interaction, sending the theorists back to the drawing board. But the Standard Model has reigned supreme me since its creation in the early 70s.The most popular alternative to the SM is the supersymmetry theory, in which each fundamental particle has an as yet, undiscovered super partner. And then there is the so-called superstring theory.

It uses 17 particles to explain all matter and forces in the universe. These are, 6 quarks, 6 leptons, (electron, muon, tauon, and their respective neutrinos) the Z boson, the W boson, the gluon, the photon, and the Higgs particle. All of these particles have been verified beyond any doubt to exist except the Higgs boson, however there is some strong suggestive evidence for it.

Quarks come in three generations, or energy levels.  These are whimsically named: Up & down quarks, Charm & strange, Top & Bottom. Only the first generation of quarks is used to build the particles of atomic nuclei. In other words, protons and neutrons are made up only of Up and Down quarks. The other two generations of quarks combine to create strange massive particles with very short lives. Leptons also come in three generations, the electron along with the electron neutrino form the first generation. The Muon and Tauon with their respective neutrinos form the next two generations. The muon and tauon are like massive versions of the electron and are very short lived particles, created in certain nuclear reactions.

Only four forces are needed to explain everything in physics. These are the electromagnetic force, the strong nuclear force, the weak nuclear force, and gravity.  Gravity is by far the weakest, being many orders of magnitude weaker than the other forces. Think about it, a tiny bar magnet can pick up a nail, even though the gravitational field of the entire Earth is pulling it down.

Of course, the Standard Model (SD from now on) is not perfect. Like previous theories, it will eventually be superceded by a new theory. That does not necessarily mean it’s wrong.  When a new theory in physics supercedes an older theory, it does not invalidate the older theory. It incorporates the older theory as a special case. For instance, Einstein’s relativity theory incorporates Newtonian mechanics as a special case. When one is dealing with speeds much less than light speed, or gravitational fields much less than that of a neutron star, Newton’s theory is accurate to several decimal places.

There are, of course, some problems with the SM. No one has ever been able to reconcile the SM with general relativity. These two theories, explain the nature of physical reality quite well, the SM dealing with everything except gravity, and general relativity dealing only with gravity.  Another particle, postulated by quantum theory, but never discovered, is the graviton.  The graviton is postulated as the particle responsible for the warping of the space-time continuum, in general relativity.  There is some tantalizing evidence for the existence of the graviton, mostly coming from studies of the slowdown rates of binary systems containing a neutron star. The rate of energy lost due to gravitational spin down, is equal to (within the tolerances of measurement errors) what theory predicts from gravitational radiation, i.e. gravitons. But this is far from conclusive.  If the graviton is discovered, it would add another particle to the SM.

There are still certain phenomena that the SM cannot explain. Some of these phenomena are not only inexplicable in the SM but cannot even be accommodated by it. For instance, the universe seems to contain a tremendous concentration of energy, even in empty space.

1. This cosmological energy, depending on whether it is positive or negative, should have either quickly collapsed the universe long ago or expanded it to far greater than its present size.
2. The recently discovered acceleration of the expansion of the universe cannot be accommodated by the SM.
3. The inflation theory, (a modification of the big bang, developed in the early 70s) postulates an extremely rapid expansion of the universe in its first fraction of a second, faster than light, in fact. It is the most successful cosmological theory ever devised. The field or fields responsible for inflation cannot be accommodated by the SM.
4. The universe is made of matter, with only small, fleeting bits of anti-matter created here and there in nuclear reactions. The SM does not allow for this.
5. The so-called dark matter which dominates the universe cannot be accommodated by the SM.
6. In the SM, the Higgs field interacts with particles, bestowing on them the property of mass. The Higgs interaction cannot be adequately explained by the SM.
7. The mathematics of quantum theory make the Higgs boson’s mass huge. This in turn, makes the mass of all particles far larger than they are known to be. This is a serious contradiction.
8. Gravity, because it is mathematically structured so differently from the other three forces, cannot be accommodated by the SM.
9. The values of the masses of quarks and leptons cannot be explained by the SM. Why should they have the particular masses that they have? A more comprehensive theory should explain this.
10. There are three generations of particles in the SM.  Our everyday world is made up of only the first generation.  The particles of the other two generations, are created in nuclear reactions and have lives measured in minute fractions of a second.  The SM describes all three generations with no contradictions or inconsistencies, but cannot explain why more than one should exist.  Why are there heavier versions of the electron (Muon & tauon) each with their won corresponding neutrino?  Why are there heavier versions of the two quarks that make up protons and electrons?  The SM explains the nature of these particles quite well, with no contradictions or inconsistencies, but it can’t explain why they exist.  Why should they exist?

Of course physicists have been trying to unify the four forces with one elegant set of equations. Einstein worked most of his life on this and failed. There has been some success unifying the electromagnetic force with the weak nuclear force. The real holy grail of physics is to unite Gravity, as explained in general relativity, with quantum theory, as elucidated and elaborated by the SM. One popular theory explains the weakness of gravity by stating that there are parallel universes and gravitons can leak from our universe into another, thus weakening the force of gravity. In fact the standard inflationary big bang theory, has given way to much speculation and mathematical tinkering with the idea of an infinite number of parallel universes, always being created, in big bang like fashion, and running their courses and dying. Each one can have totally different laws of physics, with some being close to ours in their properties. With an infinite number, some would be exactly like ours. Is there another me, out there somewhere, sitting at a computer typing the same thing that I am now, except that he is listening to Clementi while I am listening to Mozart? Maybe the Mets won the World Series in a parallel universe.  Maybe the Beatles kept Pete Best.