  We calculate how the energy and the opening angle of jets in ${\cal N}=4$ SYM
theory evolve as they propagate through the strongly coupled plasma of that
theory. We define the rate of energy loss $dE_{\rm jet}/dx$ and the jet opening
angle in a straightforward fashion directly in the gauge theory before
calculating both holographically, in the dual gravitational description. In
this way, we rederive the previously known result for $dE_{\rm jet}/dx$ without
the need to introduce a finite slab of plasma. We obtain a striking
relationship between the initial opening angle of the jet, which is to say the
opening angle that it would have had if it had found itself in vacuum instead
of in plasma, and the thermalization distance of the jet. Via this
relationship, we show that ${\cal N}=4$ SYM jets with any initial energy that
have the same initial opening angle and the same trajectory through the plasma
experience the same fractional energy loss. We also provide an expansion that
describes how the opening angle of the ${\cal N}=4$ SYM jets increases slowly
as they lose energy, over the fraction of their lifetime when their fractional
energy loss is not yet large. We close by looking ahead toward potential
qualitative lessons from our results for QCD jets produced in heavy collisions
and propagating through quark-gluon plasma.
