For a long time, people have wondered why we can't move at light speed. They imagine that the speed of light is an arbitrary speed limit, like the speed of sound. Therefore, they reason, we will eventually get to light speed. Or they ask the question 'What happens if I travel at the speed of light in a car and then turn on the headlights?'

The answers to these questions are somewhat subtle, and plagued Einstein from the time he was a young boy. Interestingly, the answer to the question of riding on a light beam leads to the Special Theory of Relativity. In Einstein's day, there were two major realms of Physics which had already been settled - Electromagnetism and Newton's Laws. Each gave detailed descriptions of different areas of Physics. Newton's Laws dealt with how objects made of matter, such as bowling balls, cars, and people moved; Electromagnetism explained how photons (little light particles) moved. However these two ideas were at odds with one another when they said what happened to light beams.

In Newton's Laws, the speed of objects add. For example, if you are in a car moving at 60 miles per hour and throw a ball ahead of you out the window at 20 miles per hour, someone outside of the car would see the speed of the ball as 60 + 20 = 80 miles per hour. However, in Electromagnetism, the speed of light is a constant. So imagine you were moving in a car at 60 miles per hour and shone a light beam ahead of the car. By the Laws of Electromagnetism, the speed of the light beam is not 60 miles per hour faster than it would be if the car were still - it is still the same speed in either case.

In 1905, Einstein was a patent clerk working in Switzerland thinking about this contradiction. Who was right about the nature of light? Newton, or Maxwell (who discovered the equations of Electromagnetism)? Most researchers at this time thought that it was Newton that was correct. That is, they thought the speed of light changed depending on the speed of the person emitting it. This wasn't easily observed because the speed of light is so incredibly fast. In fact, it's over 186,000 miles per second!

Einstein realized that he could make sense of the problem by making the speed of light a constant for all observers - meaning the person sitting still and the person in the car turning on the headlights. If they both measure how fast the light beam is moving, they both agree that it is some particular number - the same number for both of them.

Now let's try to think about what happens as we get closer to the speed of light. If we were moving in a car and emitted a light (say, a laser) in front of the car, we would say its speed is the same speed as though we were sitting still. In order to make this work, consider one little equation:

Speed = Distance/Time

If the speed of what we are measuring is light, then to make it the same values as what someone outside of the car would measure, we need to literally measure different distances and different times elapsed from a person sitting outside of the car.

This means that as we get closer to the speed of light, we measure distances as getting shorter and times as getting longer. Getting closer to the speed of light makes all distances we measure for objects besides ourselves as getting smaller and smaller. It also makes the amount of time we see passing for everyone else getting longer and longer. This happens until, at the speed of light itself, we would see the entire Universe along our direction of motion become a point, and time would stop! Unfortunately, as we move faster and faster, our mass would also increase. This would keep happening until at the speed of light we would literally have an infinite mass. This isn't possible, because as we get more and more massive it would take more and more energy to make us go a little bit faster. It would therefore take an infinite amount of energy to get us to light speed, which is impossible to have. And this radical conclusion, that we can't get to the speed of light no matter how hard we try or how large of a rocket ship we have, is the core of Einstein's Theory of Special Relativity.

A really great video about Einstein's Theory of Relativity is Brian Greene's "The Elegant Universe, which goes into more detail about Einstein's Theory, and how it eventually ties in possibly with Quantum Mechanics using String Theory.