Abstract:
We study the perturbative unitarity of noncommutative scalar
field theories. Field theories with space-time
noncommutativity do not have a unitary S-matrix. Field
theories with only space noncommutativity are perturbatively
unitary. This can be understood from string theory, since
space noncommutative field theories describe a low energy
limit of string theory in a background magnetic field. On
the other hand, there is no regime in which space-time
noncommutative field theory is an appropriate description of
string theory. Whenever space-time noncommutative field
theory becomes relevant massive open string states cannot be
neglected.