which changes as the step sweeps into denser parts of the BEC, as would be expected from the BCR mechanism. (The time dependence of this growth also appears roughly compatible with that observed in the experiment.) Second, a spacetime plot of the magnitude of the deviation of the GP wave function from the background flow \[Fig. \[fig:stimulated\_HR\](b)\] shows that the standing wave first arises at the WH, and then propagates to the BH \[as illustrated in Fig. \[fig:stimulated\_HR\](a)\]. And third, the standing wave has zero frequency in the WH frame. This is evident by inspection of Fig. \[fig:stimulated\_HR\](b), which shows that the lines of constant phase are parallel to the WH horizon worldline.

Figures \[fig:stimulated\_HR\](a) and (b) also reveal that the WH horizon has a smaller velocity than the BH horizon. The BCR is therefore Doppler shifted to a nonzero frequency in the BH frame (which is the rest frame of the potential step), as can also be seen in the figure. Although the relative velocity of the horizons is small, this Doppler shift is larger than might be expected, because the BCR has a very large wave vector. With its nonzero frequency in the BH frame, the BCR classically 