The Four Fundamental Forces
Gravity: Gravity is the theory that brought the Heavens and Earth together by holding them down together by the same force. Issac Newton derived the theory, famously after watching an apple fall from a tree. It is a relatively weak force that acts between objects of mass. It warps and curves through time, called general relativity. It's what makes the earth revolve around the Sun, what makes the Moon revolve around the Earth, and what ties us down to our beloved planet.
Electromagnetism: Electromagnetism states that electricity and magnets have a special relationship. Maxwell unified electricity and magnetism through a single equation called electromagnetism. Electromagnetic force is billions of times stronger than gravity. It keeps us from falling to the center of the Earth by the negative repelling forces of the ground atoms and our feet.
Strong Nuclear Force: Strong nuclear force is the force that binds the protons and neutrons together to form the atomic nuclei. It is the strongest out of the four forces. It holds the particles together at the atomic level. It is only effective at very short distances.
Weak Nuclear Force: Weak nuclear force is responsible for radioactive decay. It is stronger than gravity, but only effective at very short distances.
Einstein's Theory of General Relativity
At 26, Einstein figured out a new theory about space and time. It simplified physics by treating space and time as one thing, spacetime. He figured out that mass causes spacetime to curve. Since objects of mass curve spacetime, objects of mass curve toward the most massive objects. That is what gravity is, warped spacetime. He claimed that the universe was once a very dense, small ball that is continuing to expand today. Physicists have made up the theory of dark energy to counterbalance the expansion of the universe, hence the black hole.
Quantum Mechanics is a theory so radical that it shattered the previous theories of Gravity and Electromagnetism. Albert Einstein thought that the universe is predictable. However, Niels Bohr claimed that the Universe is complex and by chance. Quantum mechanics is based largely on probability. It states that we cannot know the outcome, just the probability. It explains the universe on the subatomic level using mathematical equations to describe their motion.
Albert Einstein longed for a single equation that explained the entire universe into it. Unfortunately, he died before he could achieve it. Physicists were able to combine electromagnetic, strong nuclear, and weak forces into one equation, but not gravity. An equation for unification was theorized. However, there were anomalies, meaning that it wasn't true. In 1984, the matching numbers of "496" on both sides of the equation indicated that the anomalies were gone and the unification equation was true. All four forces could be combined into a single equation. This is known as "The Theory of Everything".
String Theory paved the way for the Theory of Everything. It is the building block of nature. String Theory stitches quantum theory and general theory together. It states that there are 11 dimensions with parallel universes. The discovery of it was an accident and it is difficult to test. According to String Theory, particles are replaced by one-dimensional strings that make up our universe.