In physics, four fundamental forces are known thus far:
1) Gravity is by far the weakest force, but is the force that has the greatest large-scale impact on the universe. Unlike the other forces, gravity works universally on all matter and energy, and is (so far as we know) universally attractive. Any matter or energy anywhere and at any time in the universe attracts all other matter and energy in the universe, as long as it is inside its light cone. This is explained in detail in General Relativity, which describes gravity in terms of spacetime. One active area of research involves merging the theories of general relativity and quantum mechanics into a more general theory of quantum gravity. It is widely believed that in a theory of quantum gravity, gravity would be mediated by a particle which is known as the graviton.
An interesting theory, negative gravity (also called dark energy), arose while trying to explain the recent discovery that the expansion of the universe is actually accelerating.
2) Electromagnetism is the combination of electrostatic and magnetic forces. It is the force between charged particles, such as the force between two electrons, or the force between two current carrying wires. The quantum theory of electromagnetism is known as quantum electrodynamics (QED). In QED, virtual photons transfer this force.
3) The weak nuclear force mediates beta decay. The weak force is transferred by W and Z bosons. Neutrinos interact with other matter only through the weak nuclear force and gravity, and hence can pentrate large amounts of matter without being scattered. Electromagnetism and the weak force can be seen as two aspects of the same underlying force, the electroweak force.
4) The strong nuclear force is the force holding together the protons and neutrons inside the atomic nucleus. The strong force is transferred by gluons and it acts on particles that carry "color charge", i.e. quarks and gluons.
Most particle physicists perceive the existence of different forces each with separate theories to describe them to be inelegant and believe that all of the forces can be described in a general theory of everything. In the late 1960s and early 1970s, a successful theory which forms part of the standard model was proposed to unify electromagnetism and the weak force into a single electroweak force. There is also active work on various forms of grand unified theories which attempt to unite the strong and electroweak forces. Many of these theories predict proton decay which has not been observed.
Much more speculative are theories that attempt to reconcile quantum field theory with General Relativity, in order to find a successful theory for quantum gravity, and then to combine this into a general theory of everything. Unlike grand unified theories, most propsed theories of everything do not yet give experimentally testable predictions.
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It is currently believed that all interactions can be explained in terms of these four forces. For instance, friction is a result of the electromagnetic force.
However, an exotic fifth force has been proposed by some physicists from time to time, mostly to explain discrepancies between predicted and measured values of the gravitational constant. As of 2003, all of the experiments which seem to indicate a fifth force have been explainable in terms of experimental errors.
Also of note is that all four of these forces are conservative forces which is to say that the effect of the force on an object moving from one point to another is independent of the path of the object.