Macroscopic processes are usually not completely reversible: you can’t unmix or unbake cake, and perfume released from a bottle won’t spontaneously recollect. However, these phenomena involve huge numbers of particles. On the level of individual particles in elementary physics, the direction of time doesn’t matter to the forces involved. The exception to this is the weak force, one of the four fundamental forces of nature. However, no experiment thus far had been able to demonstrate time asymmetry unambiguously until now.

New results from the BaBar detector at the Stanford Linear Accelerator Center (SLAC) have uncovered this asymmetry in time. Researchers measured transformations of entangled pairs of particles, including the rates at which these transformations occurred. Through analyzing over 10 years of data, they found clear time-reversal asymmetry with an error of only one part in 10⁴³, a clear discovery by any standard. These results are a strong confirmation of predictions of the Standard Model, filling in one of the final missing details of that theory. [Read more…]