  In ordinary quantum field theory, one can define the algebra of observables
in a given region in spacetime, but in the presence of gravity, it is expected
that this notion ceases to be well-defined. A substitute that appears to make
sense in the presence of gravity and that also is more operationally meaningful
is to consider the algebra of observables along the timelike worldline of an
observer. It is known that such an algebra can be defined in quantum field
theory, and the timelike tube theorem of quantum field theory suggests that
such an algebra is a good substitute for what in the absence of gravity is the
algebra of a region. The static patch in de Sitter space is a concrete example
in which it is useful to think in these terms and to explicitly incorporate an
observer in the description.
