Einstein created the Theory of Relativity which states that gravity comes from the curvature in the fabric of space time. 'Space time' is the fabric of the universe. Imagine it as a bed sheet with marbles on it, the heavier the marble the more impact it has on the sheet therefore creating gravity. The theory also states that the speed of light is the ultimate speed limit of the universe. General relativity is very good at explaining the large things in the universe like planets but doesn't do a good job of explaining small things like atoms and subatomic particles.
There are four fundamental forces that rule the universe: Gravity, Electromagnetism, Strong nuclear force, and Weak nuclear force.
Gravitational force is known to be the weakest force by far and is what keeps us on the earth. Electromagnetism is the force that exists between oppositely charged particles in an atom. Strong nuclear force is the force that keeps the nucleus of an atom together. Weak nuclear force is responsible for radioactive decay.
Quantum mechanics is the part of physics that deals with the small particles that general relativity leaves out. It uses the Strong Nuclear force to explain why the nucleus of an atom stays together, as well as electromagnetism for why electrons stay close to each other. However it does not agree with Eisenstein's theory of relativity, the math does not work.
The goal of unification is to explain everything in the known universe, big or small, under one equation or idea.
Here is a video on trying to unify the theory's
String theory is the theory that unites Quantum Mechanics with General relativity. It states that the universe is made up of tiny little 1 dimensional strings that exist inside of subatomic particles. It says that these strings wiggle randomly like violin strings and are responsible for how different elements get their unique properties and even explain why gravity exists.
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