Introduction to General Relativity
Before Albert Einstein space and time were seen as separate from each other. This makes sense as there seems to be no connection between matter and the endless ticking of a clock but Einstein would prove otherwise. In the early 20th century, Einstein published his book on general relativity and it changed the way that modern physicists viewed gravity as well as the universe. There was already a widely used theory of gravity proposed by Isaac Newton. It had been used to predict everything from apples falling to the ground and the orbits of planets. However, the problem with the previous theory of Newtonian gravity was that there was nothing producing the force of gravity. It was said that gravity just came from the center of every object but it could not be determined what exactly caused the phenomenon. Einstein wanted a theory that explained why there is gravity, not just explain how it works. Along with this problem scientists had been seeing light from distant stars being shifted on the light spectrum and no one could explain why. This “red shift” caused all of the light from far away to appear more red than it should. Einstein would attempt to solve this problems but also bring about new ones with his new theory of relativity.

Einstein's Theory
Einstein succeeded when he took the two separate dimensions of space and time and combined then into spacetime. To explain it he thought of a two dimensional plane called spacetime. Planets and everything in the universe would be essientally moving across lines on this plane. What Einstein proposed was that everything that was on this plane would interact with it and make it sag and dip. A planet would make a very large dip because it has such a large mass. This dip, Einstein explained, was what caused gravity. Objects would follow the lines of spacetime and when they came too close to a large object like a planet, would be drawn into it because of the dip that the planet made in spacetime. This explained where gravity came from and why it was stronger with more massive objects. The more mass that an object had, the more it would affect the curvature of spacetime. Since he proposed that everything moves across spacetime, everything is affected by the gravity produced by it. This even includes light and time which was a strange concept for most physicists because it was always thought that these were constant. Einstein explained the red shift by saying that as light traveled across spacetime it interacted the curves made by other objects. This made the frequency of the light stretch out which would make it appear to shift towards a more red look.

A depiction of what a massive object does to spacetime.
A depiction of what a massive object does to spacetime.

Consequences of General Relativity
Time was more difficult to understand. Einstein theorized that gravity would affect the rate at which time would pass. This was because at different distances from massive objects, the gravity is different. As spacetime is warped by the object, time is warped because it is a part of this spacetime. As a consequence, time ticks slower the closer that a person gets to a massive object because spacetime is warped. It is hard to think of time as a physical thing but that is why it ticks slower when closer to massive objects. Another consquence of Einstein’s theory is a point in space so dense that it sucks everything into it, including light. These are known now as black holes because you cannot see them so they would appear black. They would occur when a star died and collasped on itself, leaving all of its mass in one point. This point would curve spacetime so much that nothing could escape its gravity, not even light. Einstein’s theory predicts that these could exist but they have not be observed in nature as of now.

Experimental Evidence
Many people were skeptical of Einstein’s theory but many aspects of it have been proven experimentally. Time dilation was proven by having two identical clocks and placing one on Earth and the other orbiting the Earth. Eventually the clocks showed different times and even though it was an incredibly small amount, a few nanoseconds, it proved Einstein correct. The red shift was proven in laboratories by using incredibly sensitive light readers that measure the amount that the frequency shifted over a certain distance.



Significance
This new way of thinking proposed by Albert Einstein changed the face of modern physics. Physicists saw the universe in a new light now and moved that much closer to understanding how it worked. The theory of general relativity described many phenomenon that were previously a mystery and paved the way for new discoveries in the future. Above all it is one of the most important theories because it is believed to apply to the whole universe. This gave physics the ground work for the fundamental make up of the universe and improved the understanding of concepts that were thought to be understood completely. General relativity was truly a revolutionary idea that would open the doors to the wonders that the universe still had to offer.