There are 4 forces of nature, including (from the weakest to the strongest forces)
The "Holy Grail" of Physics among many theoretical physicist (Einstein being the most well known) was to "unify" all the 4 forces (just like Maxwell and his colleagues did for the Electric and Magnetic forces)
A series of theoretical and experimental developments in the 60's and 70's led to the development of a combined theory that covers 3 of the 4 forces of nature (Strong, electromagnetic and weak--leaving out only gravitation)-- which is now called "The Standard Model". This goes partially into the desire of Einstein, that of a "Unified theory of Forces". Many of us Physicists are working on models which will hopefully also incorporate gravity (e.g., "Supergravity theories", etc.)--so that Einstein's dream can become a reality. The Standard Model, which so far has been able to be conistant with all observed data, does, however, have too many un-explained parameters whose values have no logical basis--it is hope that a final combined theory, which the Standard model may be a small subset (just like Newtonian gravity is a small subset of the Special Relativity, and is usable only when the velocity is much less than the speed of light)--will in fact tell us that these values are logical and mean something. Wish us luck.
The Standard Model contains Matter and Force carriers --Matter consists of 3 families each of Leptons and Quarks.
Leptons do not have strong interactions--and [??? thus
are like Lepers that do not participate in the normal society]) .
The 3 family of Leptons --each contains 4 members --2 particles and 2 anti-particles--are
The 3 family of quarks are
In addition, there are particles (called Gauge Bosons) which carries the force between the matter particles --
Here's a brief description on one example on how the Gauge Bosons
"mediate" the various forces --
What happens is the constituents (e.g., a quark) of a matter particle (say, a
proton) emits a virtual gluon, which then is absorbed by the constituents (e.g.,
another quark) of another matter particle (say,
another proton)--in so doing, the 2 proton will experience an attractive
force, which will be sufficiently strong to overcome the repulsion from
the electromagnetic force (2 positive charge particles are mutually repelled)--
so that both proton can stay happily bound together in a Helium nucleus
(along with 1 or 2 neutrons). Note that since gluons have mass, this strong
force is of limited range
...the forces mediated by a massless particle like the photon, which mediated electromagnetic force--drops as inversely proportional to the distance squared;
...the forces mediated by a massive particle like the Gluon (or W and Z boson) falls much faster than the distance squared!!), and thus the Strong force is only "strong" (at least stronger than the EM forces) within the Atomic nucleus.
WHAT REMAIN TO BE FOUND IN THE STANDARD MODEL --While all the quarks and leptons of the 3 family as well as the gauge bosons have been discovered, (3 of them--top and bottom quarks, and the tau neutrino--discovered at Fermilab) there still are several items that have not yet been seen--even in the Standard Model.
The most important one is the Higgs Particle--H; it is a particle that has coupling strength proportional to the mass of the particle, and in fact is responsible for the mass of the particle. There is good indirect evidence that the mass of the H (Higgs particle) is not much more than 100 GeV or so, so it is possible that H is already being produced at the Fermilab Tevatron collider. However, the signature of the H is not very evident, and is thus difficult to distinguish from other physics processes. For each mode of decay of the H, there are other processes, often at a much higher level, which will confuse the issue. Thus, it is likely that even for the most promissing modes, a large number of Higgs event is required for us to definitely say that the Higgs exists.
Physicists are speculating on what lies beyond the standard model