  Black hole event horizons, causally separating the external universe from
compact regions of spacetime, are one of the most exotic predictions of General
Relativity (GR). Until recently, their compact size has prevented efforts to
study them directly. Here we show that recent millimeter and infrared
observations of Sagittarius A* (Sgr A*), the supermassive black hole at the
center of the Milky Way, all but requires the existence of a horizon.
Specifically, we show that these observations limit the luminosity of any
putative visible compact emitting region to below 0.4% of Sgr A*'s accretion
luminosity. Equivalently, this requires the efficiency of converting the
gravitational binding energy liberated during accretion into radiation and
kinetic outflows to be greater than 99.6%, considerably larger than those
implicated in Sgr A*, and therefore inconsistent with the existence of such a
visible region. Finally, since we are able to frame this argument entirely in
terms of observable quantities, our results apply to all geometric theories of
gravity that admit stationary solutions, including the commonly discussed f(R)
class of theories.
