The vacuum is not as empty as it seems. If an observer moves with acceleration, the vacuum around them heats up: this was predicted by William Unruh back in the 1970s. The effect is akin to black hole radiation, but in flat space. The trouble is that detecting it is nearly impossible: a temperature of one kelvin requires an acceleration of 10²⁰ m/s², as if a fly accelerated to the speed of light in an instant.
In a new study, physicists propose a way to dodge this difficulty. Instead of chasing extreme accelerations, they place a rotating atom in an optical cavity — a tiny trap for light with very high quality (finesse Q). The cavity reshapes the density of states of the electromagnetic field and turns the barely noticeable corrections from acceleration into a distinct shift in the atom's energy levels — the very Lamb shift.
By varying the rotation speed and the cavity tuning, the shift can be amplified, suppressed, or even nullified. The most astonishing part: at an acceleration of just 0.5 m/s², like a person leisurely walking down a corridor, the shift reaches 10 Hz. This is twenty orders of magnitude below the threshold previously thought necessary for direct detection of the Unruh effect. Thus, a laboratory table becomes a window into the physics of curved spacetime.
Half a meter per second squared is the typical acceleration of a pedestrian. At such motion, according to calculations, an atom already 'feels' the warmth of the vacuum, previously associated only with black holes.
In Robert Ibatullin's novel 'The Rose and the Worm', the acceleration of a spaceship heats the vacuum to temperatures that change the fate of civilization — the Unruh effect is not named, but it is unmistakably recognizable.