Testing the Fine Structure Constant: The More Things Change, the More They Don’t

Via the arxiv Blog, a review article has been posted by the Haensch group with the title“Testing the Stability of the Fine Structure Constant in the Laboratory.” The fine structure constant, usually referred to by the symbol α is a ratio of fundamental constants– the electron charge squared divded by Planck’s Contant times the speed of light (e2/hc)– and usually assumed to be constant. Some beyond-the-Standard-Model theories of physics, though, include effects that could cause this ratio to change over time.

For this reason, people have been looking to see if the fine structure constant is really constant for quite some time. These measurements involve things like the natural nuclear reactor at Oklo and observations of spectral lines in distant galaxies, and their interpretation is highly controversial (despite what some people write).

The new paper is a very nice review of the state of laboratory measurements of the stability of the fine-structure constant. If the constant were changing, this would change the frequencies of light emitted by different atoms, so you can test whether the constant is really constant by (for example) measuring the frequencies of light absorbed by two different elements, waiting a while, and then measuring the same two frequencies a second time. If the frequencies are the same, then α is a constant; if they’re different, then α might be changing.

The possible change is really small– current experiments limit it to less than 0.00000000000000001 per year– but new laser-based experimental techniques make it possible to measure frequencies at the level needed. That number is actually drawn from a laser-based measurement comparing transition frequencies in mercury and aluminum ions. The same techniques might also prove useful as a reference in measuring the frequencies of lines seen in distant galaxies, and thus help settle the question of what those measurements really mean.

The linked article is intended for a conference proceeding, and contains a very nice explanation of the various techniques used to make these ultra-precise measurements. There are also references aplenty if you want more detail about the state of play of the fine-structure-constant business.