On March 14, 1879, Albert Einstein was born in Ulm, Germany. When he was in grade school, Einstein was considered a terrible student by most of his teachers; they thought he was slow and badly behaved. In fact, he did so poor in school that he ended up working in a patent office as his first career. Now, however, Albert Einstein is widely regarded as one of the most intelligent people in the history of science. In addition to creating the famous equation E=mc^2, Einstein made several other groundbreaking discoveries, such as proving the speed of light (186,000 miles per second) and showing how gravity works (as opposed to just showing what gravity does). One of his most prominent works is his theory of relativity. Einstein’s theory of relativity can be divided into two parts: special relativity and general relativity. Einstein wrote about special relativity before general relativity, as general relativity was really created as a result of special relativity. The central idea of Einstein’s theory of special relativity is that the speed of light is the speed limit of the universe. In other words, nothing can exceed the speed of light.
To understand this, imagine two situations. First, imagine a boy riding on a skateboard at a speed of 185, 999 miles per hour (which is very near the speed of light) while holding a baseball. As the boy skates down the road, he throws the baseball. He then gets off his skateboard and, retrieving the ball, throws the baseball while stationary.
During the time of Einstein, Newton’s laws were thought of as the laws of the physical universe. According to his laws, velocity is calculated by dividing the distance that an object travels by the amount of time it takes for the object to travel the given distance (v=d/t). the distance that an object travels is divided by the amount of time that it took for the object to travel the given distance (v=d/t) to find the velocity of an object. In order to find the velocity of an object thrown in the same direction as a moving source, the velocity of the moving source is added to the velocity of the traveling object if it was thrown from a stationary source. Therefore, the velocity of the baseball while thrown from the boy riding a skateboard would be the sum of the boy’s traveling velocity and the ball’s velocity if the boy was stationary. This would exceed the velocity of the baseball if the boy was stationary. Now, if the boy is traveling very nearly at the speed of light, the baseball should exceed the speed of light. However, this is not the case. Now matter how hard the skateboarder hurls the ball, it will never exceed the speed of light. But why is this true? There are three factors that restrict the ball from ever exceeding the speed of light: time dilation, Lorentz contraction, and mass increase. Because these factors only become apparent at very high speeds—close to the speed of light—Newton never noticed or considered them.
Time dilation refers to the slowing of time in relation to the speed of a moving object. Imagine two friends, both about the same age, with identical watches ticking in perfect unison. One friend boards a rocket and flies around outer space close to the speed of light while the other friend stays on earth. The friend on earth waits for about a year before the space traveler returns. Because the space-traveling friend was traveling close to the speed of light, time slowed for her. Her watch ticked slower than her earth friend’s watch. Furthermore, because time slowed, the space-traveler would be younger when she returns than the earth friend.
Lorentz contraction refers to the contraction of an object moving at very high speeds, and it is relative to the person viewing the object. In other words, an object looks shorter when it is moving compared to when it is at rest depending on where the viewer is. For example, imagine two corn snakes that are of equal length. One snake is stretched out on a rock, resting in the sun. The other snake has mastered the ability to slither at incomprehensibly high speeds. As the moving corn snake slithers at 183,000 miles per hour, she would say that she does not look or feel any different in size. On the other hand, the resting snake, who was watching his friend, would say that the moving snake was shorter than she would be if she was resting. Both cases are right; because of Lorentz contraction, the moving snake would appear shorter when traveling than when resting. Since the moving snake is the one that is traveling, she would see herself as a normal length.
Mass increase refers to the increase in mass of an object as it moves at higher speeds. In order for an object to travel at a higher speed, it requires more energy. Mass can refer to many things, but in this case, think of inertial mass. Inertial mass is how much an object resists a change in velocity, so the more drastic a change in velocity, the greater the object resists the change. This increased resistance leads to a greater inertial mass, thus increasing the mass of an object.
Now, knowing all of this, think back to the baseball that was thrown as the boy was traveling at the speed of light. First of all, because of time dilation, time is slowing for the boy because he is traveling at 185, 999 miles per second. Therefore, time dilates even more for the ball that is hurled with as much force as possible. Because of Lorentz contraction, both the ball and the distance that it is traveling contracts. Because both the time and the distance traveled are reduced, the velocity does not exceed the speed of light. Finally, because of mass increase, the mass of the boy, his skateboard, and his ball would really be too great to do any of this. Because the ball is resisting such a great change in velocity, its inertial mass would increase to an almost infinite number.
Einstein’s theory of special relativity revolutionized physics, changing the way that scientists looked at the universe. His discoveries defied Newton’s laws, which were previously thought to be the laws of the universe. People who formerly believed that Newton’s laws governed the universe became unsure of the world around them. Einstein revealed one of the secrets of the universe, and it paved the road for the inventions and discoveries of countless future scientists.