Talk:Speed of light

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Suggested rewrite 2nd para[change source]

The theory of relativity is based on the fact that the measured speed of light is always the same no matter if the source of the light and the person doing the measuring are moving relative to each other. This is sometimes expressed as "the speed of light is independent of the reference frame."

This behavior is different from our common ideas about motion as shown by this example:

George is standing on the ground next to some train tracks. There is a train rushing by at 30 mph. George throws a baseball at 90 mph in the direction the train is moving. Tom, a passenger on the train, has a device( e.g. radar gun) to measure throwing speeds. Because he is on the train, Tom is already moving at 30 mph in the direction of the throw, so Tom measures the speed of the ball as only 60 mph.

In other words, the speed of the baseball, as measured by Tom on the train, depends on the speed of the train.

In the example above, the train was moving at 1/3 the speed of the ball, and the speed of the ball as measured on the train was 2/3 of the throwing speed as measured on the ground.

Now, repeat the experiment with light instead of a baseball, i.e. George has a flashlight, instead of throwing a baseball. George and Tom both have identical devices to measure the speed of light(instead of the radar gun in the baseball example).

George is standing on the ground next to some train tracks. There is a train rushing by at 1/3 the speed of light. George flashes a light beam in the direction the train is moving. George measures the speed of light as 186,000 miles per second. Tom, a passenger on the train, measures the speed of the light beam. What speed does Tom measure?

Intuitively you may think that the speed of the light from the flashlight as measured on the train should be 2/3 the speed measured on the ground, just like the speed of the baseball was 2/3. But in fact the speed measured on the train is the full value, 186,000 mile per second not 124,000 miles per second.

It sounds impossible, but that's what one measures.

A consequence of this fact is that nothing can go faster than the speed of light. Another consequence is that for objects with mass, no matter how much energy is used to increase the speed of an object, it will get closer and closer but it will never reach the speed of light.