Electron-positron collisions at SLAC produce a resonance that results in an entangled pair of mesons. When one meson decays at time , the identity of the other is “tagged” but not measured specifically. In the top panel, the tagged meson is a “”. This surviving meson decays later at , encapsulating a time-ordered event, which in this case corresponds to “” . To study time reversal, the BaBar collaboration compared the rates of decay in one set of events to the rates in the time-reversed pair. In the present case, these would be the “” events, shown in the bottom panel.