  One of the most well known features of active matter is the tendencey of
self-propelled particles to undergo system-wide collective motion. With low
enough rotational noise or high enough global density, these systems
spontaneously break symmetry and transition to a state with nonzero net
momentum. The transition is currently understood as discontinuous, with phase
coexistence manifesting in terms of dense travelling bands propagating through
a dilute background. Here we show that the phase transition appears continuous
in experiments with a system of hundreds of polar vibrated disks, and compare
the fluctuations in this system to the traveling bands that are present in
larger systems. We argue that this difference is due to finite size effects,
which are often considered an artifact of simulations in studies with
equilibrium systems, but that are of fundamental importance in many systems of
self-propelled particles such as those composed of living individuals.
