The Four Fundamental Forces:
Gravity - the force that attracts a body toward the center of the earth, or toward any other physical body having mass.
Electromagnetic - the force that consists of electrical and magnetic waves that act between charged particles through attraction or repulsion.
Weak Nuclear - the force which is responsible for radioactive decay, which plays an essential role in nuclear fission.
Strong Nuclear - the force that acts upon the protons and neutrons in order to hold together the nucleus of an atom.
Theory of General Relativity
Proposed by Einstein in 1915, the Theory of Relativity states that objects in space all abide to the same laws of gravitation. He stated that objects attract each other through their indentation in space time. The theory can be explained by envisioning a trampoline; the indentation an object creates in the trampoline pulls other objects towards it. Thus building upon Newtons Law of Gravitation which stated that all particles have an attraction to other particles.
While Einsteins Theory of Relativity explained the observable world, it couldn't explain the Quantum world. He sought an answer of unification til his death on April 18th, 1955 at the age of 76.
Quantum Mechanics deal with the mathematical equations that explain the rules that govern the subatomic particles which make up our world.
In the Quantum World, Gravity has no real influence; the Strong Nuclear, Weak Nuclear, and Electromagnetic forces influence the way the Quantum world operates. These incorporate the concepts of quantization of energy, wave-particle duality, the uncertainty principle, and the correspondence principle.
Need more explanation? Watch this video!
Unification is the attempt at a master formula that unites all the four fundamental forces together. As of now, the forces of quantum mechanics and general relativity butt heads. Yet, theoretical scientists have found a theory, called "String Theory", that could explain the way the world as we know it operates.
The String Theory
In recent years, string theory has morphed into a promising theory to unite General Relativity and Quantum Mechanics. The String Theory states that everything we know, even the tiniest of atoms, are made up of incredibly tiny vibrating strings. So small, that its possible that we will never actually be able to see one. These vibrating strings, vibrate at different frequencies to create the world as we know it, but there's even more to it. The string theory also goes on to say that there are up to 11 dimensions, 7 more than we know already exist. Yet, the String Theory is able to mostly unite the Four Fundamental Forces.