  Single quantum system, such as Unruh-DeWitt detector, can be used to
determine absolute acceleration by local measurements on a quantum field. To
show this, we consider two kinematically indistinguishable scenarios: an
inertial observer, Bob, measuring the field of an uniformly accelerated cavity,
and his non-inertial twin Rob accelerating and making measurements in a
stationary cavity. We find that these scenarios can be distinguished in the
non-relativistic regime only by measurements on highly excited massive fields,
allowing one to detect non-inertialness of the reference frame.
