The Four Fundamental Forces
Gravity: Gravity was the first force to be understood scientifically. Gravity is the naturally occurring force by which all things are brought together. Isaac Newton is given credit for discovering the Law of Universal Gravitation, which is discovered by watching an apple fall off of a tree, which states that a particle attracts every other particle in the universe using a force (gravity) that is directly related to the product of their masses as well as proportional to the square distance between them. Therefore, associate Newton with gravity, and gravity with the force that attracts things together. It is also why we do not float into space or plummet to the center of the Earth.
Weak Nuclear Force: The weak nuclear force is one of the four fundamental forces. It is responsible for radioactive decay, which plays a major role in nuclear fission. It is stronger than gravity, but is only effective at very short distances. Without the weak nuclear force, things such as the sun would not exist.
Strong Nuclear Force: The strong nuclear force is responsible for the binding of protons and neutrons, which form atomic nuclei. The strong nuclear force is the strongest of the four fundamental forces. However, the range is short, meaning the particles must be close before the effects of the strong nuclear force are felt. The main job of this force is to hold together subatomic particles.
Electromagnetic Force: Electromagnetic force is a physical interaction that occurs between electrically charged particles. It usually exhibits electromagnetic fields, and includes both electricity and magnetism.
EINSTEIN'S Theory of Realitivity
Einstein first formed the Theory of Relativity in 1905 by determining that the laws of physics are the same for all non-accelerating objects, and that the speed of light in a vacuum is independent of the motion of all objects. In other words, black holes! This theory was officially published in 1915. This theory means that space and time are interwoven into what is called the space-time continuum. This means that events that could appear to happen at one time to one person, could happen to a different person at a different time. Massive objects can distort the space-time, creating what is called a black hole, which is so intense that not even light can escape them.
Quantum mechanics largely involves probability. It is believed that one cannot know the outcome, only the probability of the outcome. It describes how the universe works at the subatomic level and explains all of the fundamental forces at a subatomic level aside from gravity. Quantum mechanics is the branch of mechanics that deals with the mathematical description of the motion and interaction of subatomic particles. Quantum mechanics is the science of what is very small.
Unification was Einstein's goal, a goal that he died trying to achieve. Unification is the law that describes everything in the universe, and could potentially be solved by the string theory. When unification was trying to be worked out, physicists were running into the problem of anomalies, which meant that the theory was not correct. After years of work, in 1984 there was a match on both sides of the equations reading "496", meaning that the anomalies were no more and proved unification correct. However, the overall theory has yet to be proven. All four forces could now potentially be encompassed, including the force of gravity. This theory is commonly referred to as "The Theory of Everything".
The String Theory
The String Theory states that there are 11 dimensions with parallel universes. The discovery of The String Theory was an accident and it is difficult because it cannot be tested. There is a current battle between physicists and string theorists. Several papers were sent and reviewed, but several were also declined and never published. The String Theory is a theoretical idea in which the particles of particle physics are replaced by one-dimensional objects that are referred to as strings. All objects of the universe are composed of the strings in addition to membranes of energy according to the theorists. Along with unification, the string theory is another attempt to unify the four fundamental forces into one unified theory.