Copyright 1994
Thomas J. McFarlane
www.integralscience.org
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)
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.
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 leptonsThe 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 bottomThe 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 LeptonsCertainly 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.