General Relativity- November 25th, 1915, renowned scientist Albert Einstein published his ideas on general relativity. His theory suggested that the earth, as well as other planets, orbited around the sun following the curves of the fabric of space time. The fabric of space time is said to be very geometric. Dense large objects, such as the sun, created distortions in the fabric of space time causing the curves that planets would orbit in. Einstein also created a set of equations to describe his new theory of gravity.
Quantum Mechanics- Describes the microscopic world of chance (this opposes Einstein's perception of the world). Niels Bohr proclaimed that probability ruled the microscopic world, however, Einstein believed that the universe was geometric and predictable. There are several different outcomes depending on which parallel universe an event occurs. One of the largest contributions quantum mechanics made was unlocking the secrets of the atom. There are three different forces involved with quantum mechanics; electromagnetism, strong nuclear, and weak nuclear. However, these three forces do not combine nicely with gravity.
The Four Fundamental Forces-
The most familiar of the forces is also the first to be discovered, gravity. Issac Newton unified nature with gravity over 300 years ago. His equations laid the foundation down for scientists in the future to calculate the path to the moon. It was later understood that the fabric of space time creates gravity. However, Einstein's studies on light contradicted with Newton's idea of instantaneous gravity. It was recently discovered that gravity moves in waves and can only travel as fast as light. Gravity is the force linked to general relativity; the other three forces are related to quantum mechanics.
Electromagnetism is much stronger than gravity. It was discovered in the 1920's that gravity is actually pretty weak on earth. James Maxwell unified electricity and magnetism and created four equations to describe the force just like Newton did. Electromagnetism made the invention of the telegraph possible.
Strong nuclear and weak nuclear are the two newer forces discovered among the four. The strong nuclear force keeps the nucleus together; in other words it keeps protons and neutrons together in the center of an atom. Weak nuclear force is responsible for radiation. Combining electromagnetism and weak nuclear (electroweak) would produce a very hot temperature; this extremely hot temperature would be similar to the heat of the big bang. Electroweak combined with strong nuclear would create an ultimate super force, however, combining the three forces involved with quantum mechanics with gravity would be much more difficult to put together.
Unification- The ultimate goal of unification is to combine all of the laws of the universe into one law. Einstein spent the last two decades of his life (even to his death bed) trying to figure out the master equation. The goal for scientists today is to unify everything through string theory. However, it is easier said than done; there were too many anomalies between general relativity and quantum mechanics (x cannot equal both eight and negative seven).
String Theory- The ultimate theory, a theory of everything. There have been many debates on whether string theory is a theory of physics or just a philosophy that can't be proved. However, Michael Green solved the equations and on both sides of the board he got 496, therefore, string theory was free of anomalies. The theory states that the world is made up of tiny vibrating strings of energy. Just as the strings of a cello can create a wide variety of sounds using the same string, so can the strings of energy. String theory unifies all of the particles that make up the different forces, such as a graviton for gravity. String theory does come with a cost, parallel universes, eleven different dimensions to be exact. There are also five different perspectives of string theory (though they all deal with extra dimensions); some believe that the strings are loops of energy while others think the strings are open ended. Later the "M" theory stated that the five different flavors of string theory are actually just different views on the same thing.