  To study potential limitations of controllability of physical systems I have
earlier proposed physically universal cellular automata and Hamiltonians. These
are translation invariant interactions for which any control operation on a
finite target region can be implemented by the autonomous time evolution if the
complement of the target region is 'programmed' to an appropriate initial
state. This provides a model of control where the cut between a system and its
controller can be consistently shifted, in analogy to the Heisenberg cut
defining the boundary between a quantum system and its measurement device.
However, in the known physically universal CAs the implementation of
microscopic transformations requires to write the 'program' into microscopic
degrees of freedom, while human actions take place on the macroscopic level. I
therefore ask whether there exist physically universal interactions for which
any desired operation on a target region can be performed by only controlling
the macroscopic state of its surrounding. A very simple argument shows that
this is impossible with respect to the notion of 'macroscopic' proposed here:
control devices whose position is only specified up to 'macroscopic precision'
cannot operate at a precise location in space. This suggests that reasonable
notions of `universal controllability' need to be tailored to the manipulation
of relative coordinates, but it is not obvious how to do this. The statement
that any microscopic transformation can be implemented in principle, whenever
it is true in any sense, it does not seem to be true in its most obvious sense.
