  The appealing theoretical measure of irreversibility in a stochastic process,
as the ratio of the probabilities of a trajectory and its time reversal, cannot
be accessed directly in experiment since the probability of a single trajectory
is zero. We regularize this definition by considering, instead, the limiting
ratio of probabilities for trajectories to remain in the tubular neighborhood
of a smooth path and its time reversal. The resulting pathwise medium entropy
production agrees with the formal expression from stochastic thermodynamics,
and can be obtained from measurable tube probabilities. Estimating the latter
from numerically sampled trajectories for Langevin dynamics yields excellent
agreement with theory. By combining our measurement of pathwise entropy
production with a Markov Chain Monte Carlo algorithm, we infer the
entropy-production distribution for a transition path ensemble directly from
short recorded trajectories. Our work enables the measurement of
irreversibility along individual paths, and path ensembles, in a model-free
manner.
