Einstein's Train Example Seems to Exceed c...?

  • #1

Main Question or Discussion Point

Youtube vid of an Einstein's Train example:
From the perspective of the man on platform, the rear of the train seems to be approaching the front lightning's light at greater than c (train's speed + c). Is this correct? I thought there was a rule against that...
 

Answers and Replies

  • #2
Ibix
Science Advisor
Insights Author
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That's fine. What is forbidden is that you ever measure speeds greater than c relative to you.

Time dilation, length contraction, and the relativity of simultaneity conspire so that the guy at the back of the train measures the light coming towards him at c (you might want to Google for the relativistic velocity addition formula).

Nothing is moving faster than c relative to you. Nothing is moving faster than c relative to the guy on the train. No rules broken.
 
  • #3
Doc Al
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From the perspective of the man on platform, the rear of the train seems to be approaching the front lightning's light at greater than c (train's speed + c). Is this correct?
Yes, perfectly correct. The "closing speed" of the light and the rear of the train is greater that c.

I thought there was a rule against that...
No. Note that no one measures anything to be moving faster than light with respect to themselves.
 
  • #4
Well, alright then. Thank you both
 
  • #5
50
0
Well, alright then. Thank you both
I think not the rear of the train but front of the train approaches faster to the front strike and rear of the train "escapes" from the rear strike.
 

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