Figure 1: Electron-positron collisions at SLAC produce a $\mathrm{{\rm Y}}(4s)$ resonance that results in an entangled pair of $B$ mesons. When one meson decays at time $t_1$ , the identity of the other is “tagged” but not measured specifically. In the top panel, the tagged meson is a “${\overline{B}}^0$”. This surviving meson decays later at $t_2$ , encapsulating a time-ordered event, which in this case corresponds to “${\overline{B}}^0$” $\to B_{-}$ . 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 “$B_{-}$” $\to {\overline{B}}^0$ events, shown in the bottom panel.