Fundamental Particles and Forces

The Four Fundamental Forces and Their Unification

Date  Gravity     Electricity  Magnetism   Weak     Strong
----  -------     -----------  ---------  -------   -------
1687   Newton      unknown      unknown   unknown   unknown

                   \________  _________/
                            \/
1873                    Classical         unknown   unknown
                     Electrodynamics

1915  Einstein                            unknown   unknown


1948                  QED: Quantum       no theory  no theory
                     Electrodynamics

                  \______________  ______________/
                                 \/
1967                      Electroweak theory        no theory


1972                                                    QCD


                  \___________________  ____________________/
                                      \/
?                        Grand Unification Theory (GUT)

     \___________________________  _________________________/
                                 \/
?                     Theory of Everything (TOE)

Gravity

When Newton discovered his law of gravitation, gravity became the first force to be understood. Indeed, it was the first force even to be recognized. Because charges combine to form electrically neutral atoms, the electric force only has indirect effects in our usual realm of experience, and so it is easy to overlook. But all matter has positive mass, and so gravity is cumulative and we feel it regularly. It governs the motion of falling bodies and the planets orbiting in the heavens. Newton's theory was a unification of terrestrial forces with extraterrestrial forces, a recognition that the same force governs both the laws of heaven and the laws of earth.

Electricity and Magnetism

In the hundred years preceding Maxwell's theory, electricity and magnetism were discovered and studied as separate forces. Early on it was known that there was a connection between the two, but it wasn't until Maxwell that these two forces were understood as two aspects of the same electromagnetic force. From Maxwell's equations all the known equations of electricity and magnetism can be derived. But Maxwell's unification also predicted the existence of certain laws relating electricity and magnetism that were unknown at the time. In particular, Maxwell derived the wave equation for electromagnetic radiation, and it was soon confirmed that light was electromagnetic radiation. Radio was a direct spin-off of electromagnetic unification.

Quantum Electrodynamics

Twenty years after quantum mechanics was discovered, Feynman and others developed a consistent quantum theory of the electromagnetic field, called quantum electrodynamics, or QED. This theory has been verified to an high degree of precision, and successfully explains many subtle quantum effects. With QED, nearly all the (non-gravitational) phenomena at our energy scale can be understood.

The Strong and Weak Nuclear Forces

When it was recognized in the 1930's that the nucleus of the atom consisted of protons and neutrons, physicists hypothesized a strong force in the nucleus that acts to bind protons and neutrons together in spite of the great electromagnetic repulsion between the positively charged protons. Moreover, with cloud chambers and particle accelerators, physicists discovered dozens of new particles and unexplained interactions between them. As a consequence, the weak nuclear force was also hypothesized as a mediator of certain interactions. Finally the weak force was unified with the electromagnetic force, yielding the electroweak theory, and the strong force was explained by quantum chromodynamics, or QCD. Together, these two theories comprise what is now called the Standard Model. Like QED before it, this theory is verified to a high degree of precision and no known (non-gravitational) phenomena fall outside its domain.

Theorists have proposed grand unification theories that unify the electroweak theory with QCD, but there are no consequences of such theories that can be experimentally verified without particle accelerators that can produce much higher energies than those in existence today. The same situation holds for proposed "theories of everything" that include gravity.

The Standard Model

By our present understanding, there are four fundamental forces (gravitational, EM, weak, strong). Quantum field theory associates to each force particles (vector bosons, or gauge bosons) that serve to mediate the force. For example, charged particles act on each other through the exchange of (virtual) photons, the particle that mediates the electromagnetic force.


      Gravity      EM          Weak       Strong
      -------      --          ----       ------
      graviton     photon      W+,W-,Z    8 gluons
      massless     massless    massive    massless
      spin 2       spin 1      spin 1     spin 1

      Acts on      Acts on     Acts on    Acts on
      everything   charged     quarks &   quarks
                   particles   leptons

The fundamental particles of matter fall into two groups, leptons and quarks. Each group has six particles, which fall into two groups, as follows.

                      Leptons                     Quarks
                 --------------------       ----------------

Charge             -1          0           +2/3       -1/3
                ---------  -----------     ------    --------
Generation 1:   electron   e-neutrino        up        down

Generation 2:      mu      mu-neutrino     charmed    strange

Generation 3:     tau      tau-neutrino      top      bottom

The universe is composed primarily of Generation 1 particles. The other particles arise only in high-energy interactions and quickly decay into Generation 1 particles. All these fundamental particles have spin 1/2. Any particle, fundamental or composite, with integral spin is called a boson, and any particle with half-integral spin is called a fermion. All the quarks and leptons are fermions. All the mediating particles are bosons. But fermions can combine to form both bosons and fermions. In addition to the particles shown, there are the corresponding antiparticles, which have the same mass and spin but opposite charge, color, and flavor.

These fundamental particles combine to form composite particles which fall into various classes. Quarks, which have color-charge, combine to form color-neutral composites called hadrons. Hadrons are of two types: mesons, which consist of two quarks, and baryons, which consist of three quarks. For example, protons (composed of two up quarks and a down quark, uud) and neutrons (two down quarks and an up quark, udd) are baryons. These two baryons are also called nucleons, since they are the particles that comprise the atomic nucleus. Atoms, of course, are composed of a nucleus together with electrons. And molecules are composed of atoms. The once-indivisible atoms are today seen to have a rich structure, as shown below.

                 Mesons
Quarks=>Hadrons_/         Other Baryons
                \Baryons_/
                         \Neucleons=>Nuclei
                                            \Atoms=>Molecules
          __Electrons_______________________/ 
Leptons__/
         \__Other Leptons

Certainly our present understanding of matter is incomplete. It is not understood why there are only six quarks and six leptons. We do not understand why matter should break up into these two classes. Nor do we fully understand why particles have the masses they do. And perhaps the biggest mystery of all is gravity. We still have a poor understanding of how gravity can be understood in terms of quantum mechanics. Most likely a revolutionary new perspective is needed that will fundamentally change the way we understand the fundamental forces and particles of nature.