  When it comes to performing thought experiments with black holes,
Einstein-Bohr like discussions have to be re-opened. For instance one can ask
what happens to the quantum state of a black hole when the wave function of a
single ingoing particle is replaced by an other one that is orthogonal to the
first, while keeping the total energy and momentum unaffected. Observers at
$t\rightarrow\infty$ will not notice any difference, or so it seems in certain
calculational schemes.
  If one argues that this cannot be correct for the complete theory because a
black hole should behave in accordance with conventional quantum mechanics,
implying a unitary evolution, one is forced to believe that local quantum field
theory near the black hole horizon is very different from what had hitherto
been accepted. This would give us very valuable information concerning physics
in the Planck length region, notably a mathematical structure very close to
that of super string theory, but it does lead to conceptual difficulties.
  An approach that is somewhat related to this is to suspect a breakdown of
General Relativity for quantum mechanical systems. It is to some extent
unavoidable that Hilbert space is not invariant under general coordinate
transformations because such transformations add and remove some states.
Finally the cosmological constant problem also suggests that flat space-time
has some special significance in a quantum theory. We suggest that a new
causality principle could lead to further clues on how to handle this problem.
