Category: time reversal violation

No, The Laws Of Physics Are Not The Same Forwards And Backwards In Time

“It took the creation of over 400 million ϒ(4s) particles to detect time-reversal violation directly, and this was accomplished by the BaBar collaboration back in 2012. The test for the reversal of initial and final entangled states is, to date, the only direct test ever performed to see if T-symmetry is conserved or violated in a direct fashion. Just as anticipated, the weak interactions violate this T-symmetry, proving that the laws of physics are not identical whether time runs forwards or backwards.

In particle physics, the gold standard for experimental significance is a threshold of 5-sigma. Yet BaBar physicists achieved a significance of 14-sigma: a remarkable accomplishment. The reason you’ve likely never heard about it? It was overshadowed by slightly bigger particle physics news occurring in the same year: the discovery of the Higgs boson. But this result maybe Nobel-worthy, too. The laws of nature are not the same forwards and backwards in time. After seven years, it’s time the world felt the impact of this discovery.”

Imagine you took a ball and threw it off of a tower, watching its trajectory as it flew through the air and eventually hit the ground. If you were to take that same ball and throw it with the right speed and angle from the ground, it would fly through the air and wind up exactly at the point it was launched from in the first example. It would, in fact, follow the exact same trajectory as if you video recorded the first throw and ran the video backwards in time. This is called time-reversal invariance, and it’s valid for Newton’s laws of motion. But it isn’t valid for all laws of physics! 

We once thought they were, and there’s only ever been one experiment to measure its violation directly: BaBar, in 2012. Here’s what we know and how we know it!