  To investigate how quantum effects might modify special relativity, we will
study a Lorentz transformation between classical and quantum reference frames
and express it in terms of the four-dimensional (4D) momentum of the quantum
reference frame. The transition from the classical expression of the Lorentz
transformation to a quantum-mechanical one requires us to symmetrize the
expression and replace all its dynamical variables with the corresponding
operators, from which we can obtain the same conclusion as that from quantum
field theory (given by Weinberg's formula): owing to the Heisenberg's
uncertainty relation, a particle (as a quantum reference frame) can propagate
over a spacelike interval.
