# 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.

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

JAN
17
2009
 Very cool Richard. You found the fountain of youth there. All my theoretical Physics questions are going now to YOU!!  : ) 10:14pm • #1
 Amazingly cool. t just makes you stop and think, "How much do we really not know ........." 10:15pm • #2
 Ok, but if you shoot a bullet from the front of the train, it will decelerate more quickly than one fired from a standing position... right?!? 10:22pm • #3
 Gary... I have always believed that you can move forward in time, but not backward ... it's really fun to think about it! Steve... There is so much more to discover ... it excites me to think about it! Lane... It depends on the system ... the bullet would behave normally inside the train .., whether shot front to back or back to front ... and it depends on the point of view of the observer. In simple Newtonian Physics a bullet fired from the front of the train would decelerate at a faster RATE but it would take longer to slow down. You have to identify all of the forces acting upon the Newtonian mass ... friction (drag,) gravity, rotation etc.... As you approach light speed the bullet will act exactly the same to the observer on the train as it would under anyy other circumstance. 10:37pm • #4
 Richard, Very interesting post, really interesting. Very cool question and a clever answer. Thank you. 10:38pm • #5
 Jon... It's wild stuff, but all true! Thanks for the comment. 11:23pm • #6
JAN
18
2009
 Way to deep for this mornings reading...I hope it's noon somewhere, I need a drink! 10:28am • #7
 God bless you Google, suddenly everyone is now a scientist. 12:54pm • #8
 Bill... I think that would help! JL... Actually, I had the Time-Life series of science books when I was a kid, and the full explanation of the train scenario was in them. However, at that time the "theory" was unproven. It has since been verified! 3:25pm • #9
JAN
19
2009
 Time-Life was awesome but I remember reading Tiger Beat.  That explains a lot. 8:22am • #10

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Richard Weisser

Richard Weisser Coweta Newnan Homes for Sale

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Author Bio: Richard Weisser is a Georgia real estate broker, licensed auctioneer, a candidate for GA House District 71 and has been writing about Georgia Real Estate since 1999.

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