Vast amounts of entropy are produced in black hole formation, and the amount
of entropy stored in supermassive black holes at the centers of galaxies is now
much greater than the entropy free in the rest of the universe. Either mergers
involved in forming supermassive black holes are rare,or the holes must be very
efficient at capturing nearly all the entropy generated in the process.
We argue that this information can be used to constrain supermassive black
hole production, and may eventually provide a check on numerical results for
mergers involving black holes