  A recent proof, couched in the symbolic language of modal logic, shows that a
well-defined formulation of the question posed in the title is answered
affirmatively. In a paper with the same title Unruh has tried in various ways
to translate the symbolic proof into normal prose, and has claimed that the
proof must fail in some way or another. A correct translation is given here,
and it is explained why the difficulties encountered by Unruh do not actually
arise.
