Fractons cannot move. That is their defining property — fractionalized excitations whose mobility is restricted to lines (lineons) or planes (planeons), or forbidden entirely. This immobility makes them theoretically fascinating and experimentally invisible: you cannot scatter them, transport them, or drag them through a detector.
Tseng, Hart, and Nandkishore show that the immobility itself produces a spectroscopic fingerprint. Using pump-probe techniques on the X-cube fracton model, they demonstrate that the signal carries three qualitatively distinct features: three-dimensional anyonic braiding statistics, multi-anyon bound state formation, and lineon-specific mobility restrictions. Each feature alters the spectral response in ways that no conventional spin liquid can replicate.
The bound states are the key distinguishing element. In ordinary topological phases, fractionalized excitations braid and the spectral signal oscillates at frequencies set by the braiding phase. In fracton phases, some of these excitations form bound states that change the long-time asymptotic behavior of the signal — the decay envelope shifts because bound-state lifetimes differ from free-anyon lifetimes. This is a qualitative, not quantitative, difference.
The structural point: fractons are experimentally detectable precisely because they cannot move. Their immobility constrains the spectral response so severely that it becomes a unique signature — like identifying a species by the tracks it cannot leave. Mobility restrictions, usually obstacles to detection, become the detection mechanism itself.