Where Did the Idea Start?
Gravity was an idea developed by Isaac Newton when he observed an apple fall from a tree. As this apple fell he began to wonder if the force that makes the apple fall from the tree is the same force that makes the earth revolve around the sun. This would later develop into Newton's three laws of motion.
Einstein's Idea of General Relativity
The key idea of Einstein's theory of general relativity is that gravity is not an ordinary force, but rather a property of space-time geometry. Space-time is the concepts of time and three-dimensional space regarded as being fused in a four-dimensional continuum.
In Newton's theory, gravity makes particles that leave a straight path. However, in Einstein's theory of general relativity gravity is a distortion of space-time. Because space-time is distorted, particles accelerate as if they were under the influence of what Newton called the gravitational force.
Gravity was the first of the four fundamental forces to be discovered. Gravity is the reason why earth orbits around the sun, and why things fall, such as apples from trees. The idea of space-time being distorted is where gravity comes from.
For example, think of a large fabric sheet stretched out really tight. If you were to drop a bowling ball on the sheet, it would make a distortion in the fabric's surface. Now if you were to drop a tennis ball, it makes a much smaller distortion. If the bowling ball were the sun, and the tennis ball were the earth and it spun, space-time can be visioned in this way, and the dent the ball makes in the fabric is gravity.
Electromagnetism is the interaction of electric currents or fields and magnetic fields. This force plays a major role in determining the internal properties of most objects. Matter takes its form as a result of inter-molecular forces between individual atoms and molecules, it is a manifestation of the electromagnetic force. Electrons are bound by the electromagnetic force to atomic nuclei, their orbital shapes and their influence on nearby atoms with their electrons.
Strong Nuclear Force
The strong nuclear force deals with binding protons and neutrons into atomic nuclei. It is the strongest force of the four, however, it also has the shortest range, meaning that particles must be extremely close before its effects are felt. If this force is separated, the energy exerted is extremely strong. This energy is what you see in atomic bombs.
Weak Nuclear Force
The weak nuclear force is what turns neutrons into protons. The weak interaction is responsible for radioactive decay, which plays an essential role in nuclear fission.
The Four Forces and the Big Bang
It has been theorized that when the Big Bang occurred, the weak, strong, and electromagnetic forces were bound together as one force, excluding gravity.
Quantum mechanics was developed in the 1920's, it was able to describe the microscopic realm with great success. This part of modern physics would shatter the previous ways of viewing the universe.
Quantum mechanics shows us how everything works at the microscopic level except gravity. It explains that everything is measured by chance, and that we cannot know for certain the outcome of any experiment.
The problem With Quantum Mechanics
Quantum mechanics is a fundamental equation and form of modern physics that helps us view the universe, like gravity. However, the equation for gravity and quantum mechanics do not mix. If we were to look at a black hole's center, we would need to use both equations to help understand it, which is the problem of not being able to use both equations in one universal equation.
What is Unification?
Unification is the theory of formulating a law that puts the universe into a single equation, thus binding gravity and quantum mechanics. After Einstein had developed his theory of general relativity, he began non-stop research trying to find a way to make a singular equation that can solve the universe. When Einstein passed away in 1955, he was unable to finish his studies about unification, and no scientists wanted to pick up on his research.
What Is String Theory?
String theory is a new belief that everything in the universe is made up of the smallest object known as strings. These strings vibrate and make up all the constituents of nature, but very controversial. If these strings are real, they could help scientists with unifying everything in the universe, and they could help us understand a black hole. String theory however is still in it's infancy, and not testable.
String Theory and Multiverses
There a few versions of sting theory, all of which require extra dimensions of spacetime for their math. One theory says spacetime is 26-dimensional, while others say there are 10 and 11 dimensions. Each of these dimensions is occurring simultaneously, each with different versions of our reality. Although there is no proof of this and it is hard to predict what these dimensions look like.