  In this work we scrutinize the deterministic nature of globally hyperbolic
space-times from the point of view of an observer. We show that a space-time
point $q \in M$ that lies to the future of an observer at $p \in M$, receives
signals that are invisible (to be made precise) to the observer at $p$. Part of
the initial data on a Cauchy surface, required to predict what happens at $q$,
is also invisible to the observer at $p$. Therefore it is not possible for any
observer to predict a future event with certainty. The uncertainty increases as
one attempts to predict further future. An observer at $p$ can access the
entire data to determine what happens at $q$, if and only if $q \in J^-(p)$.
Classical uncertainty in prediction is not an intrinsic feature of the events
in space-time. It adds up with the usual quantum mechanical uncertainty to
limit our ability to predict the future. We also suggest a thought experiment
to elucidate the subject.
