Einstein's Train: Does Speed Exceed c?

[Chris Frisella]

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...

 

[Ibix]

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.

 

[Doc Al]

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.

 

[Chris Frisella]

Well, alright then. Thank you both

 

[Dimani4]

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.