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If 2 cars are traveling at the speed of light and the one in the back turns on its headlights, would the car in front be able to see them? Why or why not?

It is not possible for objects with weight to travel the speed of light. So, let's ask a different question. If 2 cars are traveling nearly the speed of light and continue to get closer and closer to the speed of light, what happens when the car in the back turns on its headlights?

 If you look at the question from the point of view of someone in the front car, not much happens.  The distance between the cars does not change, so the light always takes the same time to catch the car in front. For example, if they are 300,000 km apart then the light will always take one second to catch up. 

 It's more interesting when one considers someone on the ground watching the cars go by.  In this case, two things happen.  First, from this person's point of view the distance between the cars shrinks the faster they go.  Second, the light takes longer to go from the rear car to the front car the faster they go. So as the cars approach the speed of light, the cars would become very, very close together and the light would take forever to get from one car to the next! For example if they are going  99% the speed of light,  the observer measures them to be 43,320 km apart, but the light takes 7.09 sec to travel the distance.


Submitted by J.(Texas, USA)
(September 29, 1997)



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