  Bell's theorem implies that any completion of quantum mechanics which uses
hidden variables (that is, preexisting values of all observables) must be
nonlocal in the Einstein sense. This customarily indicates that knowledge of
the hidden variables would permit superluminal communication. Such superluminal
signaling, akin to the existence of a preferred reference frame, is to be
expected. However, here we provide a protocol that allows an observer with
knowledge of the hidden variables to communicate with her own causal past,
without superluminal signaling. That is, such knowledge would contradict
causality, irrespectively of the validity of relativity theory. Among the ways
we propose for bypassing the paradox there is the possibility of hidden
variables that change their values even when the state does not, and that means
that signaling backwards in time is prohibited in Bohmian mechanics.
