  We mathematically construct focal lenses for positive-mass particles in
Minkowski space. Particles emanating from a point source, refocus with
synchronized proper time regardless of initial direction and velocity.
Minkowski space offers a useful representation of the passenger queue formed
during airplane boarding. The blocking relations between passengers coincide
with the past-future relation in spacetime, and the boarding time is determined
by the longest blocking chain. All blocking chains can be kept equally long if
slow passengers are placed according to a lens in Minkowski space. The two
media of the lens correspond to a grouping of fast and slow passengers, and the
lens specifies the optimal placement of fast and slow passengers in the queue
at the gate. Not only the boarding time, but also its variance is minimized.
