If a photon is emitted from a light source travelling with some velocity, then surely the photon emitted in the direction of travel would have a velocity equal to the speed of light plus the velocity of the light source. So travelling faster then the speed of light?

  • 3
    $\begingroup$ No, because of the relativistic addition of velocities. $\endgroup$
    – Andrew
    Aug 6, 2022 at 15:46
  • $\begingroup$ The whole point of the theory of relativity is to answer this question. So the answer is in that theory and it is not simple. If you have questions about relativity, then you should ask those. $\endgroup$
    – Themis
    Aug 6, 2022 at 16:16
  • $\begingroup$ Possible duplicates: physics.stackexchange.com/q/79331/2451 and links therein. $\endgroup$
    – Qmechanic
    Aug 6, 2022 at 16:29
  • $\begingroup$ Imagine it this way. Photons are carried away at the speed of light, rather than being emitted from the source. $\endgroup$ Aug 6, 2022 at 16:43

1 Answer 1


You're assuming that velocities are additive. This result, while commonly accepted with everyday's intuition, is incorrect. The higher velocities are, the worse the mistake becomes if you assume additivity.

You need special relativity to find the correct way to combine velocities.


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