The 100th Anniversary of the General Theory of Relativity

If you didn’t already know, this month marks the 100th anniversary of the General Theory of Relativity and the four lectures Albert Einstein gave regarding this theory. The General Theory of Relativity is one of the most revolutionary theories in all of science and is certainly something we should all be celebrating. In honour of this monumental achievement, let’s take a few moments to discuss how Einstein’s greatest work, the General Theory of Relativity, came to be.

The General Theory of Relativity is the creation of Albert Einstein and came to him from a series of thought experiments. Einstein had run away from school in Germany because he hated the emphasis on memorized and rote learning rather than imagination and creativity, and instead enrolled in a Swiss school that focused on visualizing concepts. During this time, his first thought experiment occurred as he wondered what light would look like if you travelled so fast that you were moving at the speed of light as well. If moving next to and at the same speed of light, he wrote, “I should observe such a beam of light as an electromagnetic field at rest”, or in simpler terms, the light wave would not look like it was moving. However, this thought did not agree with Maxwell’s equations and caused Einstein something he called “psychic tension”.

The next thought experiment came to Einstein while he was working in a Swiss patent office, where he would examine applications for devices that synchronize clocks, generally involving the use of electromagnetic waves. Einstein visualized what it would look like if you sent a signal from one clock to the other and observed the signal while travelling at the speed of light in the same direction of the signal versus what it would look like if you were travelling at the speed of light in the opposite direction of the signal. Confused? So am I. Einstein simplified this idea with another thought experiment. Imagine there are two strikes of lightning, a certain distance away from each other. Now suppose there are two people – one is standing on the midpoint between where the two bolts of lightning hit and the other is riding a train passing by this point. The lightning strikes simultaneously and the two people are asked what they see. The person in the middle will say that the two strikes occurred at the same time because the light from both bolts reaches him at the same time. However, the person riding the train is moving and will be slightly closer to one of the bolts. They will say the lightning bolt in front struck first. How can this be so?  Didn’t the lightning bolt strike at the same time? This led to the idea that time is relative, and later to the Special Theory of Relativity, from which Einstein showed that space and time are not independent but instead form a “space-time” fabric.

Einstein’s third thought experiment involved a person free-falling in a chamber with no windows and where you can’t see the floor. The person would not know they are falling until they hit the ground. The person would feel weightless, and if an object were placed next to them, the object would fall at the same speed and would look as if was floating relative to the falling person. Einstein would later write that this was the happiest thought of his life.

Einstein then wondered what would happen if there were a person and an object — take a spaceship for example — in outer space where there is no gravity. Now if the person steps on the spaceship while attached to a hook, they will feel as if there is gravity. Einstein wrote, “The man in the chamber will come to the conclusion that he and the chamber are in a gravitational field”, even though they are not. From this, Einstein postulated that the effects of gravity and acceleration “are both produced by one and the same structure”.

In the General Theory of Relativity, the fabric of “space-time”, from the Special Theory, is not just a container of objects but actually had two-way dynamics. This means that not only do the objects affect the fabric of “space-time”, but the fabric of “space-time” affects the objects as well. This was visualized with another thought experiment. Imagine a bowling ball on a trampoline. The ball curves this fabric. Now roll some billiard balls on the trampoline. These balls will move towards the bowling ball because of the curves in the fabric. Einstein was able to imagine this in the four-dimensional “space-time” fabric.

A hundred years ago, Einstein gave his four lectures on the General Theory of Relativity to the Prussian Academy of Sciences. In his final lecture on November 25, 1915, he gave the equations that describe the gravitational-inertial field. One of these equations is Rμν – ½ Rgμν = 8 π G Tμν, where the left side describes how objects affect the fabric of “space-time” while the right side describes how the warped fabric of “space-time” affects the moving objects.

With another thought experiment, Einstein noted that if gravity and acceleration are one and the same, then gravity should bend a light beam. Try imagining being in a room that is accelerating upwards. There is a pinhole in the wall of the room and a laser is being shone through. When it hits the wall, it will appear lower than where it came in because the room is moving upward.  If we tracked the trajectory of the laser, it would appear curved. Therefore, when light crosses a gravitational field, it should bend. This idea was proven true in May 1919 during an eclipse, during which a group of scientists led by Arthur Eddington measured how the light from a star was bent as it passed through the gravitational field near the sun.

The last of Einstein’s thought experiments related to the General Theory of Relativity is really the simplest. Einstein explained how he came up with his theory to his son using an analogy of a blind beetle. A blind beetle crawling on a curved branch will not know it is curved and Einstein said that he “was lucky enough to see what the blind beetle couldn’t”. Yes, the General Theory of Relativity is one of the most important concepts in science and has stood the test of time although many have tried to disprove it. However, it is not Einstein’s theories and equations that are the most important pieces of knowledge that we gain from him. If there is one thing that we should learn from Einstein, one of the greatest minds in all of history, it is that ideas and new concepts come from creativity and imagination, not from rote memory, and, as he once said, “Imagination is more important than knowledge”.  In celebrating the General Theory of Relativity, we are celebrating the power of creativity and imagination.