When light comes from a source, there are numerous photons associated with the energy packets. Now, what is the velocity of a single photon with respect to another photon coming form the same source i.e. the relative velocity between two photons?
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$\begingroup$ From the perspective of an observer with mass, the relative velocity would be 0. As the speed of light is constant for all inertial observers. However, from the perspective of the light, the photons would 'see' the other photons moving also at the speed of light, due to relativistic effects. $\endgroup$– Joshua PasaCommented Jul 13, 2021 at 19:15
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$\begingroup$ Essentially a duplicate of physics.stackexchange.com/q/72654/2451 , physics.stackexchange.com/q/16018/2451 and links therein. $\endgroup$– Qmechanic ♦Commented Jul 13, 2021 at 19:26
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Technically, no reference frame really makes sense at exactly the speed of light, because the definitions of time and velocity and many other quantities just break down, but for the sake of trying to answer your question, just know the relative velocity has to be between 0 and the speed of light, and the same goes for between any other two frames.
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$\begingroup$ Nope. Massless particles will travel at c locally wrt any observer $\endgroup$– KP99Commented Jul 13, 2021 at 19:34
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1$\begingroup$ You're correct that we can't really define relative speed for photons since there's no reference frame in which a photon is at rest. That said, an external observer not moving at $c$ will observe two photons going in opposite directions increasing in distance at the rate of $2c$, despite the fact that no object can ever move faster than $c$. $\endgroup$ Commented Jul 13, 2021 at 19:34