⏳ The laws of physics don't care which way time flows. So why does it only go forward?
Physicist Sean Carroll (Johns Hopkins) has spent decades on this question. His answer: the early universe started in an extraordinarily low-entropy state, and entropy has been climbing ever since. That gradient IS the arrow of time. Nobody knows why it started so low.
His newest idea (published May 2026): what if the universe is exactly periodic? Start with the Schrodinger equation instead of classical gravity, and you get cycles where entropy decreases toward a crunch, bounces, and produces a new Big Bang. Observers on the other side of the crunch would define "past" as the direction toward the crunch, so their arrow of time points opposite to ours.
In our future direction, no reversal. Just heat death. Stars burn out, black holes evaporate, the universe reaches maximum entropy. No observers, no entropy gradient, no arrow of time. It doesn't reverse. It just stops being meaningful.
Read more:
arxiv.org/abs/2605.30405
preposterousuniverse.com/blog/…
Toward a Phenomenologically Acceptable Quantum Cyclic Universe
We put forward a quantum model of cosmology that is exactly periodic but avoids the Boltzmann Brain problem.arXiv.org
Pusher of Pixels
in reply to jcrabapple • • •"Why does time not flow backwards?"
"nwonkun"
Ray Kelm
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