One spaceship leaves planet X and heads toward planet Y,which is 70 ly away,as measured in the rest frame of the two planets.100 ly later (again,in the two planets' rest frame),a second spaceship leaves planet Y and heads for planet X.Calculate the space time interval between these two events.
A)71 ly
B)57 ly
C)80 ly
D)89 ly

Question

Quizplus · Accepted Answer

The space time interval is the same in all inertial frames, so we can use the rest frame of the two planets. In this frame, the first spaceship travels 70 ly and the second spaceship travels 100 ly, so the space time interval is 70 + 100 = 170 ly.

One Spaceship Leaves Planet X and Heads Toward Planet Y,which