A short laser pulse strikes the air, and a tiny lightning bolt flashes at the point of impact. It lives longer than you'd think: the bright glow is visible for half a millisecond, and a faint one for nearly twenty milliseconds. All the while, free electrons race around inside the plasma, and for the first time, scientists have precisely measured how many there are and how hot they run.

Electrons in such a spark zoom around at a temperature of about seventy thousand degrees. Their density is enormous: in one cubic centimeter, there are two hundred million billion of them. To get these numbers, physicists used two independent methods: one illuminates the electrons with a laser and catches the scattered light, the other watches how the emission lines of hydrogen atoms broaden. Both approaches yielded the same result.

Over time, the electrons disappear: they fly apart and merge with ions, restoring neutrality to the atoms. This process was successfully described by simple collision laws, like a crowd gradually dispersing from a square, sometimes pairing up. Such measurements will become a benchmark for computer models of laser discharges.