Fischler–Susskind holographic bound

In theoretical physics, the Fischler–Susskind holographic bound is a conjectured bound on the maximum amount of entropy that a region of spacetime could have. It was proposed by Willy Fischler and Leonard Susskind in 1998 to overcome numerous issues faced by the spatial entropy bound. While the spatial bound uses the area of a spherical surface as a bound on the maximum allowed entropy contained on spatial slices enclosed by the sphere, the Fischler–Susskind bound instead uses this area as a bound on the entropy passing through the past-directed ingoing lightcones emanating from that surface. This proposal however proved to be inadequate in some physically reasonable spacetimes, which led to it being refined in the form of the covariant entropy bound the next year.