Consider three galaxies, Alpha, Beta and Gamma. An astronomer in Beta sees each of the other two galaxies moving away from him in opposite directions at 0.70c. At what speed would an
Observer in Alpha see the galaxy Gamma moving?
A) 0.94c
B) 0.98c
C) 0.82c
D) 0.70c
E) 1.4c

Question

Quizplus · Accepted Answer

The speed of galaxy Gamma relative to Alpha can be calculated using the relativistic velocity addition formula: $v = \frac{v_1 + v_2}{1 + \frac{v_1 \cdot v_2}{c^2}}$, where $v_1$ and $v_2$ are the velocities of the two objects (in this case, 0.70c each) and $c$ is the speed of light. Plugging in the values, we get $v = \frac{0.70c + 0.70c}{1 + \frac{(0.70c)(0.70c)}{c^2}} = \frac{1.4c}{1 + 0.49} = \frac{1.4c}{1.49} \approx 0.94c$.

Consider Three Galaxies, Alpha, Beta and Gamma