Nuclear Spin Points out Arrow of Time

Compared to many weird-sounding concepts in physics, the idea that time has a preferred dirction seems downright obvious. After all, a broken glass won’t reassemble into one piece. But the origin of the arrow of time isn’t at all obvious to physicists. This is because the physical laws that describe microscopic systems are reversible: rewind the clock and two colliding particles will go back from where they came. Where then does irreversibility come from? Is there some undiscovered source of irreversibility at the microscopic scale? Or does it emerge when crossing some microscopic-macroscopic boundary? A new contribution to this already active dialogue [1–3] comes from Roberto Serra at the Federal University of ABC, Brazil, and colleagues [4]. They have, for the first time, experimentally measured the entropy production in a microscopic, quantum system: a nuclear spin (Fig. 1). A positive entropy production is a proxy for the arrow of time, and having measured it, the authors open the door to studying time’s arrow on the quantum scale.