The cited article https://www.sciencealert.com/faster-than-light-speed-in-jets-that-produce-gamma-ray-bursts https://www.sciencealert.com/faster-than-light-spee

The cited article https://www.sciencealert.com/faster-than-light-speed-in-jets-that-produce-gamma-ray-bursts https://www.sciencealert.com/faster-than-light-speed-in-jets... talks about particles moving faster than the phase velocity of light in a medium. This is well known phenomenon also called luminal boom; see https://en.m.wikipedia.org/wiki/Cherenkov_radiation https://en.m.wikipedia.org/wiki/Cherenkov_radiation