The measurement of a is based on the way atoms absorb and emit light. An isolated atom can emit light in particular wavelengths, as shown in the upper part of the drawing. The frequencies of these lines are characteristic of each chemical element. In certain elements, one of these lines can be split. An example is the bright yellow pair of lines in sodium, responsible for the yellow color of sodium street lamps. The amount of this splitting is proportional to the fine-structure constant.

Formation of emission and absorption lines (drawing courtesy of Jose Wudka, University of California at Riverside)

A quasar and its host galaxy (Image courtesy of the Space Telescope Science Institute, operated by the Association of Universities for Research in Astronomy, Inc., from NASA contract NAS5-26555)

In addition to such emitted light, spectral lines are also observed in absorbed light. (See the lower part of the drawing.) When a continuous spectrum of light passes through a cool region of atoms, the atoms absorb out of the continuous spectrum the very same frequencies that these atoms can emit. Astronomers match such dark absorption lines to bright emission lines observed in the laboratory to identify the species of atoms doing the absorbing. (See drawing.)

Imagine that out in space there is a cool cloud of a gas characterized by the split spectral lines mentioned above. Passing a continuous spectrum of light through this gas produces two nearby absorption lines. Just as with the emission lines, the spacing between the absorption lines is proportional to the fine-structure constant.

Astronomers have identified quasars, extremely bright and distant objects, whose light passes through such cool gas clouds on its way to Earth. The separation in the absorption lines is slightly different from the separation measured in a laboratory on Earth, to about a part in 100,000. The gas clouds are extremely distant, so the interpretation of the results is that the value of the fine-structure constant has changed during the time required for the light to reach Earth.