The helium atom is like a tiny solar system: electrons orbit the nucleus, but their orbits are governed by strict rules. Scientists have learned to observe these motions with incredible precision. In a new experiment, they measured the frequency of a transition between two states of the helium-4 atom, as if capturing the moment a planet jumps from one orbit to another.
To do this, helium-4 atoms were cooled to a state called a Bose-Einstein condensate: millions of atoms merge into a single cloud where all move in unison. The cloud was held in a laser trap tuned so as not to distort the measured transition. The frequency was determined with an uncertainty of just 48 hertz: that's like noticing that a clock's pendulum swung a billionth of a second early. Comparison with a clock at a remote station via an optical fiber link provided calibration.
The precise value of the difference in nuclear sizes of helium-3 and helium-4 helps test quantum electrodynamics — the theory describing the interaction of light and matter. Previously, there was a discrepancy between theory and experiment for the ionization energies of excited helium states. The new result shows that this discrepancy is absent in the isotope shift, meaning the puzzle lies in other aspects of the theory. Thus, by measuring tiny shifts in atomic spectra, physicists test the most fundamental laws of nature.
An uncertainty of 48 hertz at a transition frequency of about 1.7 petahertz is equivalent to measuring the distance from Earth to the Moon to the thickness of a human hair.
In Greg Egan's story "The Halo," precise measurements of helium atomic spectra are used to test subtle effects of quantum field theory.