New to this tour in physics? Start at THE BIG BANG.
The gravitational force.
Gravity is a natural phenomenon by which all physical bodies attract each other. It is most commonly experienced as the agent that gives weight to objects with mass and causes them to fall to the ground when dropped. Einsteins general relativity explains gravity.
The electromagnetic force,
This is the interaction responsible for almost all the phenomena encountered in daily life, with the exception of gravity. Electrons are bound by electromagnetic wave mechanics into orbitals around atomic nuclei to form atoms, which are the building blocks of molecules. Visible light is electromagnetism, as well as radio waves and x-rays. Photons is the carrier of the force from one atom to another.
The strong force.
At atomic scale, it is about 100 times stronger than electromagnetism, which in turn is orders of magnitude stronger than the weak force interaction and gravitation. In the context of binding protons and neutrons together to form atoms, the strong interaction is called the nuclear force (or residual strong force).The binding energy related to the residual strong force is used in nuclear power and nuclear weapons.
The weak force.
This force is termed weak because its field strength over a given distance is typically several orders of magnitude less than that of the strong nuclear force and electromagnetism. The weak interaction is responsible for both the radioactive decay and nuclear fusion of subatomic particles. It is best described as a non-contact force field having a finite range, albeit very short. In 1968 Sheldon Glashow, Abdus Salam and Steven Weinberg unified the electromagnetic force and the weak interaction by showing them to be two aspects of a single force, now termed the electro-weak force.
The GUT, Grand Unification Theory, theorizes today, that electromagnetism, strong and weak force could be considered as one force. Einsteins general relativity, explaining gravity, cannot merge with these forces. If you remember from the The Grand Unification, gravity separated from the other forces at the end of the Planck Epoch.
Next lecture THE ELECTROWEAK EPOCH.
These diagrams illustrates how the three forces merge at very high energies, and how the fourth, gravity, stand alone.