system
Given the following post, respond as if you're commenting on Reddit:
human
Photons are emitted and absorbed instantaneously from their perspective. What's happening to the photons at the edge of our expanding universe?

Perhaps I'm running into a cognitive block dealing with the infinity that seems to arise from the expansion of our universe and the photons at the edge that would never seem to be absorbed by anything. My understanding is that from a photons perspective time doesn't exist. Their entire existence lasts an instant. But in a seemingly ever expanding universe, what's happening with the photons at the edge with nothing to absorb them?
gpt
Two things: first, it's not completely correct to say that time passes instantly for a photon. It's tempting to say that when you think about the limit of what it would look like for an object traveling closer and closer to the speed of light with respect to the rest of the stuff in the universe, but in fact it doesn't make sense to consider the perspective of a photon. In relativity photons do not have rest frames, and so they cannot have a perspective.

Second, it is a common misconception that when scientist talk about an expanding universe they're referring to a universe of finite size that's expanding outward in space from some central point. In fact, that's not how it works at all. The universe, as far as we know, is infinite in size (at least, we have no reason to believe it isn't infinite in size). Nevertheless, it's expanding. What is meant by the expansion of the universe is not that the total amount of space in it is getting larger but rather that the distances between objects on large scales like galaxies and clusters of galaxies are constantly increasing. In a sense, "new space" is being created in between everything. The point is that as far as we know there is no "edge" to the expanding universe.