  In the description of \emph{relative} motion in accelerated systems and
gravitational fields, inertial and tidal accelerations must be taken into
account, respectively. These involve a critical speed that in the first
approximation can be simply illustrated in the case of motion in one dimension.
For one-dimensional motion, such first-order accelerations are multiplied by
$(1-V^2/V_c^2)$, where $V_c=c/\sqrt{2}$ is the critical speed. If the speed of
relative motion exceeds $V_c$, there is a sign reversal with consequences that
are contrary to Newtonian expectations.
