  In recent reports, suggestions have been put forward to the effect that
parity and time-reversal (PT) symmetry in quantum mechanics is incompatible
with causality. It is shown here, in contrast, that PT-symmetric quantum
mechanics is fully consistent with standard quantum mechanics. This follows
from the surprising fact that the much-discussed metric operator on Hilbert
space is not physically observable. In particular, for closed quantum systems
in finite dimensions there is no statistical test that one can perform on the
outcomes of measurements to determine whether the Hamiltonian is Hermitian in
the conventional sense, or PT-symmetric---the two theories are
indistinguishable. Nontrivial physical effects arising as a consequence of PT
symmetry are expected to be observed, nevertheless, for open quantum systems
with balanced gain and loss.
