A ‘boom’ of light that appears when a particle exceeds the speed of light set by a medium could, in other contexts, signal a kind of quantum instability that could trigger what’s known as vacuum decay.

If ever spotted in the emptiness of space, according to theoretical physicist Eugeny Babichev of the University of Paris-Saclay, the eerie blue glow of Cherenkov radiation could be interpreted as a manifestation of negative-energy ghost perturbations.

Why does it matter? Because our current theory of gravity is incomplete, and such a signal would offer rare insight into how spacetime behaves in regimes where existing theories break down, and potentially narrow the search for better models.