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[Phys-l] Sample relativity problem giving me fits...



Hi all,

I am new to the forum, but hoping someone out there can help. I teach at a small, relatively isolated, college and haven't had any luck working this one with my colleagues.

Assume there is a light emitter at x = 0 and a light absorber at x = 150,000,000 m. Observer O' is moving along the x-axis at -0.8c and watches some light get emitted and then absorbed. Here are the two questions I had thought to ask...
a) What is the spatial separation between emitter and absorber according to O'?
b) How long does the emit-absorb process take according to O'?

For part a) I believe we have simple length contraction, so we get 9x10^7 m.

For part b) I believe I can get this two ways. One I can find the travel time for light towards the absorber which is moving away from the light. If I do this I get 1.5 seconds. I also think I should be able to time-dilate the time observed by O. The time O sees is 0.5 seconds, which time dilates to 0.833 seconds, not 1.5 seconds.

I hope what I have said is clear. If anyone can point out what I am doing wrong I would be very appreciative.

Jeff

--
Jeff Loats, Ph.D.
Assistant Professor of Physics
Fort Lewis College
650 Berndt Hall
970.247.7553
loats_j@fortlewis.edu
http://faculty.fortlewis.edu/loats_j/