The four forces found in nature
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There are four known forces: physicists dream of the day when they can combine all of these into one force with four forms, all describable with one set of equations, just as James Clerk Maxwell unified electricity and magnetism, but that day is not here yet. The solution may even come as a surprise as it did when Maxwell published his famous equations. The bonus was an explanation of the behavior of light, and a calculated value for the speed of light in a vacuum.
Gravitation
Gravity holds galaxies and stars together. This force operates on any two objects with mass, and is easily detected on our scale of observation. It is by far the weakest of the forces but the only one which affects us directly.
The standard model of gravity is Einstein's general relativity theory. The problem with this theory is that it yields us an unquantized force, while all of the others come in small units. Unless gravity can be quantized, it is hard to see how it can fit into any grand unified theory of everything. Until we detect and measure gravity waves, we will have trouble fitting gravity in to a big scheme of things.
The electromagnetic force
Electromagnetic forces exist between charged particles, and are apparent on our scale of observation. We tend to think of electricity and magnetism as separate, but they are in fact linked in a complex way. A moving magnet may give rise to an electrical current and a moving electric charge possesses a magnetic field.
The strong interaction (strong nuclear force)
A strong force holds the nucleus together, binding protons and neutrons together, in spite of the electromagnetic forces that try to push the protons apart. Interestingly, it has no effect beyond an individual nucleus, so it must only act at extremely short range, about 10 to the minus 15 meters.
The strong interaction operates between quarks, binding them together into more complex particles like the proton and the neutron. These particles are jointly known as the hadrons, and with the exception of the proton, they are all unstable in the isolated state.The force between quarks is small when they are close together, but increases as they move further apart. Quarks contain something a bit like electrical charge, referred to as 'color', but while you can get separate electrons and protons, it does not seem to be possible to isolate particles of simple color, and the groups that can be achieved are held together by the strong force.
The strong force only acts on particles which have color. A proton, with one red, one blue and one green quark, has no overall color, and is unaffected by the strong force. When protons and neutrons are close together in a nucleus, the colored quarks in neighboring particles can act on each other, which is how the nucleus is held in one piece.
The weak interaction (weak nuclear force)
This is most easily described as the force that controls nuclear decay. A neutron by itself will decay in a little under 15 minutes, on average, releasing an electron and an anti neutrino, and becoming a proton. This puzzling behavior is most easily explained by assuming a force, about one ten thousandth the strength within the nucleus of the strong force, which is transferred between particles.The weak force is transmitted by two charged particles, the W particle
(with a charge of + or -), and by the uncharged Z particle. These have a mass of about 100 times that of a proton, which is possible, because each particle has a degree of uncertainty about it, as described in the Heisenberg uncertainty principle.Over a long period of time, the uncertainty about a particle's position and mass is limited, but in very short time spans, the uncertainty can be very great, and almost anything can happen. In a sense, these W and Z particles can briefly take on their heavy mass, so long as they disappear again before the universe has time to notice what is going on.
One interesting theory suggests that the electromagnetic force and the weak force are really just aspects of each other, but that in the sort of environment we inhabit, the two are broken down into separate components. At higher energies and temperatures, say the theorists, the two combine into an electroweak force.