  A recent experimental study reports on measuring the temporal duration and
the spatial extent of failed attempts to cross an activation barrier (i.e.,
"loops") for a folding transition in a single molecule and for a Brownian
particle trapped within a bistable potential. Within the model of diffusive
dynamics, however, both of these quantities are, on the average, exactly zero
because of the recrossings of the barrier region boundary. That is, an observer
endowed with infinite spatial and temporal resolution would find that finite
loops do not exist (or, more precisely, form a set of measure zero). Here we
develop a description of the experiment that takes finite experimental
resolution into account and show how the experimental uncertainty of localizing
the point, in time and space, where the barrier is crossed leads to observable
distributions of loop times and sizes. Although these distributions generally
depend on the experimental resolution, this dependence, in certain cases, may
amount to a simple resolution-dependent factor and thus the experiments do
probe inherent properties of barrier crossing dynamics.
