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Imagine a spaceship traveling at the speed of light firing a photon forward to a target 1 at a distance of one light-year. For a stationary observer, the photon will hit the target after 1 year. The event of hitting the target must happen for an observer in the spaceship as well. However, since he is moving at the speed of light, time doesn’t flow for him compared to that of a stationary observer which means he will see the photon hitting the target after an infinite amount of time has passed which means he never sees it!

I know there is something wrong with what I said above but I can’t figure out what it is. Please help me.

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  • $\begingroup$ The mistake is to suppose that a n observer can travel at the speed of light. $\endgroup$
    – WillO
    Commented Jun 20, 2021 at 13:27
  • $\begingroup$ Yes. You cannot have a scenario with either a massive object or an inertial reference frame moving at c. $\endgroup$
    – Dale
    Commented Jun 20, 2021 at 13:34
  • $\begingroup$ When velocities are close to the velocity of light one has to use special relativity and Lorentz transformation to calculate the kinematics. The velocity of light cannot be reached by a particle/object with mass, as your spaceship, see hyperphysics.phy-astr.gsu.edu/hbase/Relativ/ltrans.html#c2 $\endgroup$
    – anna v
    Commented Jun 20, 2021 at 18:53
  • $\begingroup$ Okay. Forget spaceship. Can I take an inertial frame or reference moving at the speed of light? If not, why? $\endgroup$
    – Muhammed Roshan
    Commented Jun 21, 2021 at 1:49

1 Answer 1

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Not only can space ships not travel at the speed of light, there are no inertial frames traveling at the speed of light. The question is ill-posed.

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  • $\begingroup$ Can I take an inertial frame of reference that is moving at the speed of light? $\endgroup$
    – Muhammed Roshan
    Commented Jun 21, 2021 at 1:50
  • $\begingroup$ @MuhammedRoshan Only if you can find a Lorentz transformation that gets you there. $\endgroup$
    – JEB
    Commented Jun 21, 2021 at 15:02

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