• Question: If a spaceship travels at the speed of light past a stationary observer, and fires a photon in the opposite direction to the ships direction of travel, do the vectors add so the photon is stationary relative to the observer or does it travel at the speed of light in the opposite direction relative to the observer?

    Asked by prodegiaque to Ravi on 21 Jun 2013.
    • Photo: Ravi Kopparapu

      Ravi Kopparapu answered on 21 Jun 2013:

      It travels with the speed of light relative to the observer. Remember the ‘speed’ of light is a scalar (no direction). And light speed is unchanged in any reference frame.