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# Re: Help w/ Relativity Problem?

A spaceship has a length of 200m in its own reference
frame. It is traveling at 0.95c relative to Earth.
Suppose that the tail of the spaceship emits a flash
of light. (a)In the reference frame of the spaceship,
how long does the light take to reach the nose?
(b)In the reference frame of the Earth, how long does
this take? Calculate the time directly from the motions of
the spaceship and the flash of light, and explain
why you cannot obtain the answer by applying the
time-dilation factor to the result from Part (a).
(from Ohanian's Principles of Physics)

I would rather have the
understanding of the problem and work from there.

The crazy thing is, although the book says "explain why you cannot obtain
the answer by applying the time-dilation factor to the result of Part
(a)", that is exactly what you have to do to get the answer they list!?
What's going on here? Am I missing some basic step or do I understand
even less than I previously thought!? Any insight or advice would be
greatly appreciated!

Jonathan Gillis

In any relativity problem, a lot of confusion can be alleviated by
looking at the sequence of events. Clearly identify each emission and
reception event and put them in time order and your problem should
then be trivial. That is also how you go about clearing up most
relativity paradoxes. You can't use time dilation because you aren't
comparing clock rates.

An excellent reference on how to do this sort of thing is Taylor and
Wheeler's Spacetime Physics. If you are going to teach introductory
level relativity, I would highly recommend picking up a copy.

Paul J. Camp "The Beauty of the Universe
Assistant Professor of Physics consists not only of unity
Coastal Carolina University in variety but also of
Conway, SC 29526 variety in unity.
pjcamp@csd1.coastal.edu --Umberto Eco
(803)349-2227 The Name of the Rose
fax: (803)349-2926