Inconstants of Nature
If the constitution of nature itself were changing in time, how would you know?
BY IGOR TEPER
Why should the future resemble the past? Well, for one thing, it always has. But that is itself an observation from the past. As the philosopher David Hume pointed out in the middle of the 18th century, we can’t use our experience in the past to argue that the future will resemble it, without descending into circular logic. What’s more, physicists remain unable to explain why certain fundamental constants of nature have the values that they do, or why those values should remain constant over time.
The question is a troubling one, especially for scientists. For one thing, the scientific method of hypothesis, test, and revision would falter if the fundamental nature of reality were constantly shifting. And scientists could no longer make predictions about the future or reconstructions of the past, or rely on past experiments with complete confidence. But science also has an ace up its sleeve: Unlike philosophy, it can try to measure whether the laws of nature and the constants that parameterize those laws are changing.
The fine-structure constant, ?, is among the most ubiquitous and important of the fundamental constants of nature. It governs how strongly light and matter interact. If it were even slightly different from its present-day value of about 1/137, the universe would look very different indeed—and would almost certainly be inhospitable to life. While the laws of physics permit ? to vary over time, few thought that it actually did. That is, until 1999, when scientists conducted an analysis of the light reaching us from very bright, very distant astrophysical objects called quasars.
This analysis took advantage of the fact that the atoms of every element preferentially absorb or emit certain colors of light in a manner that intimately depends on the value of ?. These absorptions and emissions can be seen as bright or dark lines when light is broken into a spectrum, as when a prism splits white light into a rainbow of color. As light from quasars passed through gas clouds on its way to us, certain atoms in the gas clouds imprinted dark absorption lines on the light’s spectrum, which were then compared to the same atomic absorption lines as produced and measured in a laboratory.
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