  It is well known that a receding mirror in Minkowski spacetime can model the
formation of a black hole, producing Hawking-like radiation at late times. We
ask what an observer would need to do to discern whether the radiation is
fermionic or bosonic. Specialising to massless fields in 1+1 dimensions, we
find that an Unruh-DeWitt detector accomplishes this: the late time transition
rate of a detector coupled linearly to the scalar density of a spinor field is
proportional to the Helmholtz free energy density of a fermionic thermal bath,
hence showing a clear sign of Fermi-Dirac statistics, with no counterpart in
the response of a detector coupled linearly to a scalar field or its
derivative. By contrast, an observer examining just the stress-energy tensor
sees no difference between a fermion and a boson, neither at late times nor
early.
