Don’t worry, no fundamental laws of physics were broken, and Einstein’s theory of relativity remains as sound as ever. The simple explanation is that while the electrons were travelling at tremendous speeds, the light was relatively slower in the medium. This allowed researchers from the Tata Institute of Fundamental Research (TIFR), the Rutherford Appleton Laboratory in UK and Centre for Intense Laser Studies and Applications (CELIA) in France to study a phenomenon known as Cherenkov radiation.
The setup was relatively simple, and involved a glass target in a tabletop vacuum chamber. A targeted high power laser, focused to within a micrometre radius, irradiated the glass target. The electrons in the glass target were almost immediately kicked out at speeds approaching that of the speed of light in free space. The intensity was enough to create millions of amperes of current pulses. The electron pulses were extremely short, as the light pulses were also extremely short, in the range of tens of femtoseconds. Since light slowed down within the glass, the electrons moved faster than light within the medium. Because of this, the electrons emitted the Cherenkov radiation. Measuring the radiation, allowed the researchers to find out the number of FTL electrons, as well as for how long they last within the medium.
The Cherenkov radiation lasted for extremely short durations. So short, that it was beyond the measuring abilities of current electronic circuits. The team used an ultrafast shutter triggered by the same laser to measure the duration of the radiation. The shutter remained open for two picoseconds, and allowed for measuring the radiation with a temporal resolution that was a thousand times better than conventional approaches. The observations showed that the electrons actually lasted longer than theoretical predictions. In fact, they lasted longer than the time it takes to cross the medium. The electrons actually lasted for about 2,000 times the duration of the laser pulse. The researchers from CELIA are investigating aspects of the unexpected behavior.
The high speed electrons can be useful in generating some of the most exotic states of matter known. Researchers can get to peek into conditions similar to the hearts of stars and planets, in a lab environment. The research can advance development of x-ray, ion and electron sources for medical and industrial applications. The research can also potentially help in developing laser driven fusion.