Suppose a photon is observed to have some three velocity v in an inertial frame O, another inertial frame O' is moving away from O with another three velocity u. Is there a general way to get the three velocity of the photon in the O' frame? It seems intuitive that the O' frame must observe the photon travelling at different directions than in the O frame, but since the first term of the four velocity dt/d(tau) is undefined four the photon, I can't simply apply the lorentz transformation matrix. Can anyone give me a hint?

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