Tom's rocket ship is moving away from Kathy at 0.75c. He fires a laser beam in the backward direction, toward Kathy. According to Galilean relativity. How fast does the laser beam move relative to Kathy, assuming that Tom observes the beam to move away from him at speed c? How fast according to Einstein's relativity? Why?
According to Galilean relativity, you can add/subtract speeds to find the relative speed. So, in this case you would find that the speed is c - 0.75 c = 0.25 c.
According to special relativity the speed remains c. This is actually a basic postulate of special relativity, so it is wrong to search for explanations of this fact within special relativity itself. Special relativity has two postulates: All inertial frames are equivalent and ...
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