Light speed and relativity ... will the light on the train shine or not?

In today's edition of "Dear Richard" I provided a tongue in cheek answer to this question:

If you are standing on the front of a train that is moving the speed of light, and your turn on a flashlight, will that light then be moving twice the speed of light?

As it turns out, many readers wanted to know the real answer to the question. Fortunately, as an amateur theoretical physicist, I can offer a reasonable explanation to the conundrum.

In physics, forces, motion and time are relative to the system in which they occur. Everything inside the system appears normal to any participant.

For example, on the system of the earth, we do not notice that we are rotating on the earth at nearly a thousand miles an hour, or that we are revolving around the sun at over 18 miles per second, or that our solar system in moving within our galaxy, the Milky Way at over 150 miles per second, or that the Milky Way is moving around the universe at over 180 miles per second.

But from our vantage point in the system, it appears that other objects are moving but that we are not. To an outside observer though, we are moving very far very past.

The same is true with the flashlight on the train. To participants in the system (on the train) the light will shine at the normal speed of light. To those outside the system, the light will not appear to shine.

Also, the relative time to those on the train will mover slower than the time noted by outside observers. As a matter of fact ... as the speed on light is approached ... time will nearly stand still as viewed from outside the system!

So, could a space traveler leave the earth for a ten year journey is a very fast spaceship and only age five years compared to the ten years of the loved ones left behind.

Surprisingly, the answer is yes!

But that's only theoretical, right? WRONG! Very accurate atomic clocks installed on spacecraft that orbit the earth for long periods at a time have to be annually re-adjusted for the relativity lag.

How cool is that?

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