Lets suppose a spaceship travels with v = 0.9c relative to the Earth. The time inside the spaceship would pass slower than on Earth. Would the astronauts measure a different speed (that means, a different one that the observer on Earth does) in relation to the same reference frame (Earth)?

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    $\begingroup$ Speed of what? What are these astronauts measuring? $\endgroup$ – user10851 Jan 12 '13 at 1:32
  • $\begingroup$ @Chris White: I think OP is essentially asking Is the velocity of $A$ relative to $B$ the opposite of the velocity of $B$ relative to $A$? $\endgroup$ – Qmechanic Jul 7 '13 at 9:21

The time inside spaceship would pass slower than Earth and distance measured by astronauts would be less than distance measured by Earth scientists. As $speed=distance/time$, they would observe different speed of something other than light. Speed of light would be observed same by all.


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