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Let's say I am on spaceship A and it is moving with speed of 0.6c. Also, there's other spaceship B which is moving with the speed of 0.4c opposite direction from me.

And after some time in our travel, I look to spaceship B and what I would see spaceship B is moving in what speed?

According to Einstein's velocity addition we are moving at 0.8c relative to each other, but if I see the spaceship B, the light from spaceship B is coming to me with speed of light and we are moving each other 0.8c, so apparently will I see spaceship B is moving 0.2c or same as our relative speed?

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  • $\begingroup$ This question is not related to astronomy. Physics might be better suitable. $\endgroup$
    – planetmaker
    May 18 at 17:45
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    $\begingroup$ "it is moving with speed of 0.6c" Relative to what? $\endgroup$
    – Philip Wood
    May 18 at 21:15

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When A measures the velocity of B, the result is $0.8c$. But the light speed has always, coming from B or not, the same velocity for A, and that is $c$.

How the B speed is measured? I can be by a doppler effect for example, comparing the spectra of a known element as H, and recording the blue or red shift. The frequency and wavelength of the incoming light changes, but not its product, the speed $c$.

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