Does a photon experience time as we perceive it?

Question

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

Answer 1

A photon _does not have_ a perspective.

Answers to questions like this rely on the introduction of a “co-moving inertial reference frame”, which is a mathematical transformation to a frame of reference where the observer is stationary. It’s a core principle of relativity that we can predict the result of observations made from any inertial reference frame using measurements made in any other inertial reference frame. That’s how we can make statements about observations from another observer’s “perspective”: by mathematically transforming our measurements into the co-moving inertial reference frame of the other observer.

But the other core principal of relativity is that the speed of light is constant in _all_ inertial reference frames. That means there is _no_ frame of reference where light is stationary, and _no_ principal we can use to make predictions about observations “from the photon’s perspective”.