  The idea that a quantum magnet could act like a liquid crystal, breaking
spin-rotation symmetry without breaking time-reversal symmetry, holds an
abiding fascination. However, the very fact that spin nematic states do not
break time-reversal symmetry renders them "invisible" to the most common probes
of magnetism - they do not exhibit magnetic Bragg peaks, a static splitting of
lines in NMR spectra, or oscillations in muSR. Nonetheless, as a consequence of
breaking spin-rotation symmetry, spin-nematic states do possess a
characteristic spectrum of dispersing excitations which could be observed in
experiment. With this in mind, we develop a symmetry-based description of
long-wavelength excitations in a spin-nematic state, based on an SU(3)
generalisation of the quantum non-linear sigma model. We use this field theory
to make explicit predictions for inelastic neutron scattering, and argue that
the wave-like excitations it predicts could be used to identify the symmetries
broken by the otherwise unseen spin-nematic order.
