Expert: Daniel Mazur - 3/9/2006

Question
This question has come up in a discussion and we would like someone well versed in Special Relativity to give us insight in how the theory deals with this problem.
A transparent rocket ship with a light wave emitter at its center and light sensors equally spaced forward and aft. The sensors are connected to a computer that will indicate the arrival time of the growing light sphere at both.
We believe that computer will show simultaneous arrival whether the ship is moving or not.
Our question is how would an observer standing on the ground watching the ship pass overhead view this event?
Also, if the ground observer had two light sensors (equal distances from the emitter) connected to a computer, what would the ground computer record?

Answer
Hi,

honestly, my skills in special realtivity are rather basic and I am in a hurry with my own deadlines to look things up for you now. The answer to the first problem, however, should be a resolute yes. Within the rocket, everything is unmoving with respect to everything, only light is moving at c with respect to everything else.

The second and third problems are more involved, I daresay the simultaeneity of the two light-detection events is likely to be broken, when the source or observer is moving with respect to both detectors. The result will depend on, if the whole rocket is emerging, just passing or leaving, when the light pulse is emitted. I don't know the exact answer by heart and I would have to look it up in my undergrad textbook (FYI: Halliday-Resnick-Walker, "Fundamentals of Physics", any edition).

I will return to this problem in two weeks time, when my schedule is less tight.
Daniel