  Signal causality, the prohibition of superluminal information transmission,
is the fundamental property shared by quantum measurement theory and
relativity, and it is the key to understanding the connection between nonlocal
measurement effects and elementary interactions. To prevent those effects from
transmitting information between the generating and observing process, they
must be induced by the kinds of entangling interactions that constitute
measurements, as implied in the Projection Postulate. They must also be
nondeterministic as reflected in the Born Probability Rule. The nondeterminism
of entanglement-generating processes explains why the relevant types of
information cannot be instantiated in elementary systems, and why the
sequencing of nonlocal effects is, in principle, unobservable. This perspective
suggests a simple hypothesis about nonlocal transfers of amplitude during
entangling interactions, which yields straightforward experimental
consequences.
