  Antiferromagnets (AFMs) are promising platforms for the transmission of
quantum information via magnons (the quanta of spin waves), offering advantages
over ferromagnets with regard to dissipation, speed of response, and immunity
to external fields. Recently, it was shown that in the insulating van der Waals
(vdW) semiconductor, CrSBr, strong spin-exciton coupling enables readout of
magnon density and propagation using photons of visible light. This exciting
observation came with a puzzle: photogenerated magnons were observed to
propagate 10$^3$ times faster than the velocity inferred from neutron
scattering, leading to a conjecture that spin wavepackets are carried along by
coupling to much faster elastic modes. Here we show, through a combination of
theory and experiment, that the propagation mechanism is, instead, coupling
within the magnetic degrees of freedom through long range dipole-dipole
coupling. This mechanism is an inevitable consequence of Maxwell's equations,
and as such, will dominate the propagation of spin at long wavelengths in the
entire class of vdW magnets currently under intense investigation. Moreover,
identifying the mechanism of spin propagation provides a set of optimization
rules, as well as caveats, that are essential for any future applications of
these promising systems.
