The Four Fundamental Forces: General Relativity, Electromagnetism, and Strong and Weak Nuclear Force.
In 1687, Sir Isaac Newton noticed an apple fall out of a tree. This got him to thinking: "What could have caused this apple to fall?" After years of trying to answer this question, he finally came up with the Universal Law of Gravitation. Gravity became the first understood force. It was later found that gravity was actually one of the weakest forces.
Albert Einstein was one of the most influential physicists of the 20th century. He marks his finding of general relativity as his greatest work. The last few years of his life were spent pondering over the unification of general relativity and quantum mechanics.
General relativity, the explanation of the motion of large things, became the new way to look at gravity. It explains that what we perceive as the force of gravity is actually caused by the curvature of space and time. GR describes the universe as predictable and stable.
Electromagnetism (a combination of electricity and magnetism) was discovered by James Clerk Maxwell in 1873. Electricity is a form of energy resulting from charged particles. Magnetism is the phenomena of the force provided by magnets, the ability to attract and repel other objects.
Strong Nuclear Force is the force which keeps bonds together, specifically the bonds between protons and neutrons, or subatomic particles.
Weak Nuclear Force is responsible for radioactive decay; the process that occurs when an unstable atomic nucleus loses energy by emitting radiation. It is the second weakest force, after gravity.
Quantum Mechanics explains the motion of very small objects (atoms, electrons, etc). It describes the universe as a game of chance, unstable, and unpredictable. Each outcome has a probability of happening, but in a different parallel universe.
Unification was one of the last things Albert Einstein worked on before he passed away. He was looking for a way to unify both general relativity and quantum mechanics. Since you can't have 2 theories to explain how things move in the universe, there needed to be a theory to explain the movement of big and small things.
String Theory is an attempt to unify all four fundamental forces of the universe into one theory. The theory goes that all matter and energy are made up of small vibrating rings, called strings. Each string equals one fundamental particle, and there are different vibration patterns which signify the different fundamental particles. This theory allows for there to be one explanation of the movement of big and small objects in the universe.