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The theory of relativity is built around the proposition that light travels at light speed. While this proposition seems self evident it is really quite counterintuitive. If we assume something to be traveling at A speed we generally mean that it is traveling at that speed relative to something else. If we are racing down the highway at 60 miles an hour we mean that we are going 60 miles an hour with respect to the ground. If we talk about the speed of a bullet we mean the speed of the bullet relative to the muzzle. It is of very little use to know that your car is moving 3600 mph relative to a distant star but that is as equally true as our more conventional speed of 60 mph. In this case the can be measured to have two different speeds by two different people sitting on the road side and looking down from the heavens because they are moving at different speeds relative to the car. But when we said that light always travels at light speed we did not say with reference to what. The reason for that omission is that light travels at the same speed relative to everything.

I doubt that I am the only person to wonder as a child what would happen if i turned on a flashlight on a ship traveling at light speed. Would the light just collect at the back of the flashlight growing into a dense ball? Or would it just fly away from me going twice the speed of light getting a boost from the ships own speed like a bullet fired from an aeroplane? Einstein was a like soul and he tried to immagine as a youth what a light beam might look like when you caught up with it. when he whas older he found out that the equations of electromagnetism didn't allow for a light beam to sit still. The equations seemed to have a certain speed built into them, the speed of light. With this in mind he came to realize that no matter how fast you went you could never catch up with a light beam. Light always traveled by you at exactly the same speed. If you were in a space ship that traveled 99 percent the speed of light and you turned on your flashlight you would see the light beam go at exactly 299792458 m/s and so would someone outside the spaceship watching you go by. For that matter people on annother space ship going 99 percent the speed of light the opposite direction would see the light beam passing them at just the speed of light though the relative motion of the ships would seem logically to be more than the speed of light.

Such a radical statement deserves some sort of explanation. Einstein reasoned that since the light beam travels the same distance according to all observers and since speed is defined as distance traveled per unit time then the amount of time measured by the observers as the light beam travels a certain distance is different for observers moving at different speeds. According to einstein time moves slower for observers traveling close to the speed of light. To an observer outside the ship the light beam from a flashlight could move very slowly away from it since the flashlight is following it at close to the speed of light. However to those inside the ship they see things happening much much faster than the person outside and so see the beam of light zoom off at exactly the speed of light. The equation that tells us exactly how fast time travels for an observer moving at a speed v is given by T=t*sqrt(1-(v2/C2)) where T is the time measured by the moving observer and t is the time measured by an observer at rest and c is the speed of light.

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