The Universe is not the same forwards and backwards in time | by Ethan Siegel | Starts With A Bang! | Jul, 2025
Whether you run the clock forward or backward, most of us expect the laws of physics to be the same. A 2012 experiment showed otherwise.
No matter when, where, or what you are in the Universe, you experience time in only one direction: forward. In our everyday experiences, clocks never run backward; scrambled eggs never uncook and unscramble themselves; shattered glass never spontaneously reassembles into an intact vessel. But if you were to look at the laws of physics that govern the way the Universe works — from Newton’s laws of motion down to the quantum physics of subatomic particles — you’d find something peculiar and unexpected: nearly all of the rules that govern nature are exactly the same, irrespective of whether time runs forward or backward.
This corresponds to a certain symmetry of nature: T-symmetry, also known as time-reversal invariance. Our everyday experience indicates to us, quite strongly, that the laws of physics must violate this symmetry, but for decades, we couldn’t demonstrate a single violation of time-reversal invariance. From Newtonian physics to Maxwell’s electromagnetism to the strong nuclear force, every individual interaction ever observed appears to obey this time-reversal symmetry. It was only…
