In the mid-1800s, James Clerk Maxwell united electricity and magnetism in the electromagnetic field. He showed that electromagnetic waves travel at a constant speed, equal to that of the speed of light. This led him to the conclusion that light travels at the same speed of electromagnetic waves because light is an electromagnetic wave. (These waves react with chemicals in the retina to create vision.) Electromagnetic waves, including light, never stop and never slow down.

Albert Einstein was troubled by a hypothetical problem complicated by this theory. If you could go the speed of light, from your point of view light would appear stationary. Newton's law of motion agrees with this, but on the other side of the argument is Maxwell's theory which says this situation is impossible. How can light appear stationary? Can a person really hold light in their hand? So which theory was right, Newton's theory or Maxwell's? Up to that time, both had agreed spectacularly with experimental measurements and were equally credible.

Einstein settled this disagreement when, at 26 years old, he sent an article to the Annals of Physics that revised our view of space and time. His new theory was deemed special relativity because it showed that observers in relative motion perceive space and time differently. For example, if Bob takes off in his spaceship and goes from point a to point b traveling at a constant speed, something odd happens. Fred, who is stationary, will claim that the trip took more time than Bob measured. We are assuming that both of their stop watches are working properly and neither made any error whatsoever in their timing. This implies something else. If Fred knew exactly the speed Bob was going and measured how long it took the spaceship to pass him by, he could calculate the length of the spacecraft. He will find that it is shorter than if he had measured it when it was stationary. The thing is, Fred not only measures more time and a shorter distance, he also perceives that more time has passed and that Bob's spaceship is shorter. This effect is negligible at speeds that you are used to every day. The effect only becomes noticeable at speeds a significant fraction of the speed of light.

The effect on space is known as the Lorentz contraction, where an object is shorter in the direction of its motion. When the passage of time changes, it is known as time dilation.