  Pair production in a constant electric field is closely analogous to bubble
nucleation in a false vacuum. The classical trajectories of the pairs are
Lorentz invariant, but this invariance should be broken by the nucleation
process. Garriga et al. used a model detector, consisting of other particles
interacting with the pairs, to investigate how pair production is seen by
different Lorentzian observers. They found that particles (antiparticles) of
the pair are predominantly observed moving in the direction of (opposite to)
the electric field and concluded that observers see pairs nucleating
preferentially in the detector's rest frame. Here, we apply this approach to
the case where two detectors moving relative to one another are used to observe
the particle and antiparticle of the same pair. We find that each detector will
still observe nucleation to occur in its rest frame, regardless of the motion
of the other detector. However, if the relative velocity of the two detectors
is sufficiently high, the particle and antiparticle of the pair can be observed
moving towards one another with arbitrarily large momenta, contrary to the
usual expectation.
