  We study the time structure of vacuum jet evolution via a simple uncertainty
principle estimate in the kinematic range explored by current heavy ion
collisions at the LHC. We observe that a large fraction of the partonic
splittings occur at large times, of the order of several fm. We compare the
time distribution of vacuum splittings with the distribution of path lengths
traversed by jets in a heavy ion collision. We find that if no medium induced
modification of the jet dynamics were present, a very large fraction (larger
than 80% for inclusive jets) of the jet splittings would occur outside of the
medium. We confront this observation with current available data on jet
properties in heavy ion collisions and discuss its implications for the
dynamics of jet-medium interactions.
