Title: First-Passage Duality

Abstract: We show that the distribution of times for a diffusing particle to first hit an absorber is \emph{independent} of the direction of an external flow field (with the caveat that for flow away from the absorber, the hitting time is conditioned on the particle actually reaching the absorber). Thus in one dimension, the average time for a particle to travel to an absorber a distance $\ell$ away is $\ell/|v|$, \emph{independent} of the sign of $v$. This duality extends to all moments of the hitting time. In two dimensions, the distribution of first-passage times to an absorbing circle in the radial velocity field $v(r)=Q/(2\pi r)$ again exhibits duality. Our approach also gives a new perspective on how varying the radial velocity is equivalent to changing the spatial dimension, as well as the transition between transience and strong transience in diffusion.