  Vacuum bubbles nucleate at rest with a certain critical size and subsequently
expand. But what selects the rest frame of nucleation? This question has been
recently addressed in [1] in the context of Schwinger pair production in 1+1
dimensions, by using a model detector in order to probe the nucleated pairs.
The analysis in [1] showed that, for a constant external electric field, the
adiabatic "in" vacuum of charged particles is Lorentz invariant, and in this
case pairs tend to nucleate preferentially at rest with respect to the
detector. Here, we sharpen this picture by showing that the typical relative
velocity between the frame of nucleation and that of the detector is at most of
order \Delta v ~ S_E^{-1/3} << 1. Here, S_E >> 1 is the action of the instanton
describing pair creation. The bound \Delta v coincides with the minimum
uncertainty in the velocity of a non-relativistic charged particle embedded in
a constant electric field. A velocity of order \Delta v is reached after a time
interval of order \Delta t ~ S_E^{-1/3} r_0 << r_0 past the turning point in
the semiclassical trajectory, where r_0 is the size of the instanton. If the
interaction takes place in the vicinity of the turning point, the semiclassical
description of collision does not apply. Nonetheless, we find that even in this
case there is still a strong asymmetry in the momentum transferred from the
nucleated particles to the detector, in the direction of expansion after the
turning point. We conclude that the correlation between the rest frame of
nucleation and that of the detector is exceedingly sharp.
