**Ask a monkey a physics question thread**

[wufwugy]

Even though this isn't your domain, it's still good information. Thanks.

[OngBonga]

Anyway, I came here to talk about the heat death of the universe, something I've always been uncomfortable with. The latest episode of PBS Spacetime has some pretty cool ideas. The natural end state of the universe is pure radiation, ie light. That's a massless, timeless universe, and is indistinguishable from the beginning of the universe when everything was compacted into a singularity. This might not seem obvious at first, after all the beginning of the universe is infinitely small and the end is infinitely big. But without time, there is no spacetime. Photons do not experience time, because they travel at the speed of light. So a universe with only photons is a universe without time, and is therefore a universe without spacetime. The universe of photons, for all intents and purposes, is a singularity.

Head bending stuff, but it kinda makes sense, and more than that it's much more comforting (whatever that even means in this context) to believe the universe works in cycles.