If two starships fly away from each other with speeds 0.6c each, at what rate the distance between?

Question

If two starships fly away from each other with speeds 0.6c (that is 60% of speed of light) each, at what rate the distance between the ships increases?

Answer 1

As measured by someone watching the two ships, each traveling at 0.6c, they separate at 1.2c.

As measured by someone on either of the ships, 0.9c as stated above.

Answer 2

It seems to me like you are talking about a 3rd spaceship or observer that is in between the 2 spaceships?

If each spaceship is traveling away from the spaceship in the middle at 0.6c then an astronaut on the spaceship in the middle would say that their relative velocities are 1.2c.

Relativity says that nothing can travel faster than c with respect to any reference frame. Neither spaceship is traveling faster than c with respect to the reference frame of the middle spaceship. It is perfectly ok for their relative speeds to be greater than c in this reference frame.

But, an astronaut in either of the spaceships that are moving away from the middle spaceship would say that the other is moving slower than the speed of light relative to themselves.

Answer 3

haha, got a test on this subject on friday... let's see..

the relative speed of one spaceship to the other is

u' = (ux-V)/(1-(V*ux)/(c^2))
with
ux = 0,6c
V = -0,6c
c = 1c
you should get
u' = (0.6c + 0.6c)/(1-(0,6c*-0,6c)/(c^2))
= 1.2c/(1-(0.36c)/c) = 1.2c / (1+.32) = 1.2/1.32c
is aprox. 0.9090909c