  We study airplane boarding in the limit of large number of passengers using
geometric optics in a Lorentzian metric. The airplane boarding problem is
naturally embedded in a 1+1 dimensional space-time with a flat Lorentzian
metric. The duration of the boarding process can be calculated based on a
representation of the one-dimensional queue of passengers attempting to reach
their seats, into a two-dimensional space-time diagram. The ability of a
passenger to delay other passengers depends on their queue positions and row
designations. This is equivalent to the causal relationship between two events
in space-time, whereas two passengers are time-like separated if one is
blocking the other, and space-like if both can be seated simultaneously.
Geodesics in this geometry can be utilized to compute the asymptotic boarding
time, since space-time geometry is the many-particle (passengers) limit of
airplane boarding.
  Our approach naturally leads to a novel definition of an effective refractive
index. The introduction of an effective refractive index enables, for the first
time, an analytical calculation of the average boarding time for groups of
passengers with different aisle-clearing time distribution. In the past,
airline companies attempted to shorten the boarding times by trying boarding
policies that either allow slow or fast passengers to board first. Our
analytical calculations, backed by discrete-event simulations, support the
counter-intuitive result that the total boarding time is shorter with the slow
passengers boarding before the fast passengers. This is a universal result,
valid for any combination of the parameters that characterize the problem ---
percentage of slow passengers, ratio between aisle-clearing times between the
fast and the slow groups, and the density of passengers along the aisle. We
find an improvement of up to 28% compared with the Fast-First boarding policy.
