Focus: A Constant that Isn’t Constant

Phys. Rev. Focus 8, 9
Spectra from distant quasars imply that a fundamental physical constant may have been increasing slightly over the past six billion years.
Figure caption
Perspective on the past. Quasar spectra recorded at the Keck Observatory in Hawaii imply that a fundamental physical constant may have been increasing slightly over the past six billion years.

Analysis of the light from distant quasars has shown that billions of years ago the laws of physics may have been slightly different. A research team has found evidence that the fine structure constant–which measures the strength with which subatomic particles interact with one another and with light–was different at earlier times in the history of the Universe. The new work, which will appear in the 27 August print issue of PRL, confirms earlier results by the same group, which suggested that six billion years ago the value was about one part in 105 smaller. They now have added more data and have ruled out more potential sources of systematic error. If confirmed by other experiments, the finding would profoundly change our understanding of the evolution of the Universe, but not all experts are convinced.

Since the 1930s physicists have discussed whether the constants that appear in the equations for the fundamental laws of physics–such as the speed of light in vacuum and the electron charge–are actually constant. If they have changed over time, nature may have worked in different ways at different times, even if the equations themselves have remained fixed. Modern theories that attempt to unify gravity with the other fundamental forces leave room for such a time-dependence. But it’s not easy to look for the effect. If the speed of light were slowly decreasing, for example, we might never know it, because our measuring apparatus might be shrinking at the same time.

John Webb, of the University of New South Wales in Australia, and his colleagues, focused on the fine structure constant, which goes by the Greek letter alpha, because it has no units and is independent of any measurement system. Its current value of roughly 1/137 could not have been very different in the past, as that would have spelled trouble for our very existence. A variation in alpha by more than a factor of ten would imply that carbon atoms could not be stable, and organic life could not have arisen.

The team looked at dark, narrow lines in the spectra of quasars, the highly active, bright cores of galaxies at the farthest reaches of our universe. Gas clouds between Earth and the quasars absorb some of this light and produce the spectral lines. The difference between the wavelengths absorbed by any two elements depends sensitively on the value of the fine structure constant. The researchers accurately measured the absorption wavelengths of several metal atoms in clouds seen at different cosmic times using the HIRES precision spectrograph at the 10-meter Keck telescope in Hawaii. In their 1999 paper [1] the team studied only iron and magnesium, but they have now also looked at absorption lines from silicon and other metals, further increasing their sensitivity and reducing their error bars. They also analyzed 13 potential sources of systematic error and found that none could explain their main result: Alpha is increasing.

“It would be extremely interesting if it were true, but some caution is due,” says Lennox Cowie of the University of Hawaii. Although the method is “very sensitive,” Cowie says there are still some potential sources of error that need to be addressed. For example, the various atomic species studied might have different relative velocities, which could mimic the trend caused by a variation in alpha. But team member Chris Churchill of Pennsylvania State University remains confident that he and his colleagues are really on to something. He mentions new, still unpublished data that reduce the error bars even more, although he looks forward to seeing other researchers repeat the measurements at other telescopes.

–Rob van den Berg

Rob van den Berg is a physicist and freelance science writer in Oegstgeest, the Netherlands.


  1. J.K. Webb et al.Phys. Rev. Lett. 82, 884 (1999)

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