If a photon travelled a billion light years to come to Earth, how do those billion years pass from the photon’s perspective?
In: Physics
The spacetime interval between the photon leaving its origin to arriving at earth is s^2 = ct^2 – x^2, which is equal to zero since x=ct (the photon travels at the speed of light). For any ordinary object traveling at a speed less than the speed of light, the proper time experienced by that object between any two events happening on its own trajectory (world line) is equal to the spacetime interval between the events. It is unclear whether this idea can be extended to photons, but if we assume it can, it means the proper time between the two events is zero. That is, from the photon’s perspective, it leaves and arrives at the exact same time.
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