“The light emitted from them takes billions of years to reach us from our point of view as observers in the Milky Way. During this time, the expansion of the Universe causes space to stretch, and the energy of the emitted photons to drop tremendously: a cosmological redshift. Yet despite this incredible journey, the photon itself experiences none of what we know as time: it simply is emitted and then instantaneously is absorbed, experiencing the entirety of its travels through space in literally no time. Given everything that we know, a photon never ages in any way at all.”
Whether you’re at rest or in motion, you can be confident that — from your point of view — the laws of physics will behave exactly the same no matter how quickly you’re moving. You can move slowly, quickly or not at all, up to the limits that the Universe imposes on you: the speed of light. But what if you’re actually a photon? What if you don’t move near the speed of light, but at the speed of light? As it turns out, the way any massless particle experiences time, distance, and the Universe in general is entirely counterintuitive, and there’s nothing in our common experience that matches up.
It’s a relatively interesting story if you want to think about it deeply, and yet once you arrive at the answer, it couldn’t be any simpler.