Hmm... I see that the speed of light is different. I can't solve the problem the
Well, you said that the speed of light is always the same, namely, 
c, no
matter the velocity of the source (here the train moving at 3 4
c) or of the
recipient (here the person on the platform). So, the answer must be c. Right?
Right you are. So, here you don't just add the speeds to get 1 3 4
c.
I knew that that was wrong anyway because 1 3 4
impossibility. It is then that I reviewed carefully in my mind what you had
said and got the correct answer.
But, Mom. Wait a minute. There is something pretty weird going on here.
Well, the light travels further in a given time relative to the ground than it
does relative to the train. So, if the light is moving at the same speed relative
to both, then we have a paradox: it is covering at the same time, and at the
same speed, two different distances.
You mean I am right? Then what is the way out of this paradox?
Well, the answer to that question goes to the heart of what is involved in the
study of relativity theory. For example, one of Einstein's famous results is
that time runs slower on a moving train than it does from the perspective of
the platform. So, the time traveled by the light we have been discussing is
different on the train and relative to the platform. This is where you made a
mistake in what you said.
Different? How can that be? I have never noticed any difference in my watch
when we have traveled on trains. It doesn't run slow.
Right you are. But that is because we are not traveling at speeds close to the
speed of light. It is in this case that the difference becomes noticeable.
Hold on now. Is that why on Star Trek, one twin can go away on a space
ship traveling close to the speed of light and return to earth without having
aged as much as the twin who stayed at home?
further. A full explanation would require some careful thought experiments,
