Numerical Evidence for Firewalls

We study thermal configurations in the BFSS matrix model produced by numerical simulations. We do this by adding a probe brane to a typical configuration and studying the fermionic degrees of freedom connecting the probe to the configuration. Depending on the parameters of the probe there is a region where these fermion modes are gapless. We argue that it is natural to excise the gapless region from the geometry and that the black hole horizon is located exactly at the edge of this zone. The physics inside the gapless region is maximally non-local and effectively 1+1 dimensional. When a probe, considered as an observer, crosses into the gapless zone there is a break down of effective field theory where the off-diagonal fermions are integrated out. We argue that this breakdown of effective field theory is evidence for firewalls on black hole horizons.