  The concept of negative temperature (T < 0) is unique to quantum physics and
describes systems that are hotter than any positive temperature system. For
decades negative temperatures have been shown in a number of spin systems, but
experiments only recently demonstrated atomic ensembles with negative
temperatures in their motional degrees of freedom. An observed behavior of such
negative temperature ensembles is that despite highly attractive forces between
an arbitrary number of particles, there is a self-stabilization against
collapse. Negative temperatures are only possible in quantum systems because
there exists upper bounds on the energy of particles -- a property not found in
classical physics. Here we consider whether event horizons set up similar upper
limits within black holes, giving rise to negative temperature systems just
within event horizons. Combining black hole thermodynamics with experimentally
observed negative temperature effects could imply a quantum-based outward
pressure in black holes.
