Feature

Constant Changes

If a constant of nature can vary, then so might laws of physics

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10:31am, October 2, 2001
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Sometimes it's the tiniest differences that change everything. This summer, astrophysicists reported tantalizing evidence of just such a discrepancy.

Using one of the world's largest telescopes, a team of Australian, British, and U.S. astrophysicists observed clouds of gas in space backlit by beams of radiation from ancient, superpowerful quasars.

By doing so, they have found evidence that one of the constants of nature, which are never ever supposed to vary, was smaller billions of years ago than it is today. The quantity that was measured, known as alpha, wasn't smaller by much–less than 1 part in 100,000–but the finding has sent tremors through physics and astronomy.

"Atoms, the whole periodic table, and the way it exists are dependent on the value" of alpha, notes Barry N. Taylor of the National Institute of Standards and Technology (NIST) in Gaithersburg, Md. "If alpha didn't have the value it has, Earth as we know it wouldn't exist," he adds.

Other striking interpretations of the new data are also possible. For instance, a changeable alpha may indicate that extra dimensions of space exist beyond the three familiar to us.

On the other hand, a much less exotic explanation may lurk behind the new measurement of alpha–it may simply be an error. "It's a pretty amazing result, so you have to treat it with extreme skepticism," admits the team's leader John K. Webb of the University of New South Wales in Sydney, Australia. Although no flaw has been found so far in the study, researchers are rushing to measure alpha's ancient value by other approaches that wouldn't be prone to the same potential sources of error.

Electromagnetic force

Alpha is known formally as the fine-structure constant. On its Web site, NIST defines this constant as "the strength of the electromagnetic force that governs how electrically charged elementary particles (e.g., electrons, muons) and light (photons) interact."

Since the universe was born some 15 billion years ago, it has ceaselessly expanded and changed. Nonetheless, a few characteristics of the cosmos appear to have remained immutable across all of space and time. These fundamental constants of nature include alpha, the gravitational constant, and the speed of light in a vacuum.

The constants have been viewed as fixtures of reality. They are part of the foundation of physics, embedded deeply in both the classical science and quantum mechanics, as well as in relativity and the so-called standard model of particle physics.

If the measured variation in alpha turns out to be real, then one of the most basic assumptions of science–that the laws of physics are the same everywhere and at all times–will prove untrue, notes Michael S. Turner of the University of Chicago.

"Constants are invented by man to help him describe the natural world that he sees. You have to keep that in mind," points out Taylor, a physicist who since the 1960s has been a leader in assessing the values of constants.

Physicists periodically recheck those values partly because advances in instrumentation enable them to measure the quantities with greater precision. New, improved measurements, in turn, lead to refinements in all calculations that depend on the constants' values.

Also, since the constants and their values are integral components of the leading theories of physics, searching the universe for discrepancies in those values is a way to grope for cracks in current physical understanding. "There's a whole industry of people thinking about the variation of constants," Taylor notes.

One way that that industry looks for such inconstancy is to examine what colors of light are absorbed by giant gas clouds floating far out in space. Many such clouds dot the universe, and some conveniently lie between Earth and brilliant quasars.

From high-precision laboratory experiments, scientists know that some atoms, primarily metals, absorb two or more wavelengths of radiation separated by a telltale spacing. So, when patterns of dark absorption lines with just those spacings show up in the spectra of quasars, astronomers conclude that an intervening gas cloud contains particular types of atoms.

Since the late 1960s, observers have been checking to see whether the spacings between the absorption lines in quasar spectra differ slightly from those observed in laboratory experiments. According to theory, alpha is one of the factors that affects the size of the spacings. So, if alpha during the earlier phases of the universe was slightly off from today's value, that difference might show up in the spectra of quasar light traversing gas clouds on its way to Earth.

That's where the new spectral measurements that Webb and his colleagues harvested come in. In the Aug. 27 Physical Review Letters, they present data suggesting that the spacings between absorption lines for several types of atoms 8 to 12 billion years ago were different than they are today.

In their analysis of spectra from 49 different gas clouds, the researchers find consistent evidence that alpha was smaller in the early universe. The difference from today's value of 0.007297352533 is minuscule, affecting only the seventh decimal place and beyond.

More evidence for the discrepancy appears to be on its way. Webb says that a preliminary analysis of an additional set of observations twice as extensive as the one described in the Aug. 27 report also indicates that alpha was once a wee bit smaller than today.

The team is able to discern such a tiny discrepancy thanks to a combination of factors including better instruments and a focus on elements that have many absorption lines, says Christopher W. Churchill of Pennsylvania State University in State College, an astronomer and a coauthor of the new report.

The first hints that alpha was smaller in the distant past than it is now came in 1999. However, those findings were less robust than are the latest ones because they relied on data from fewer clouds and fewer types of atoms in those clouds, Churchill notes.

The new finding is "one of those results that's extremely important . . . if it's true," comments David N. Spergel of Princeton University. Adds Taylor, "To the best of my knowledge, there's been no definitive observation of a time variation in a constant. This case may be the strongest that we've seen."

A slight variation

The implications of even a slight variation in alpha are many. The grand scale of some of these may explain why the John Templeton Foundation of Radnor, Penn., a spiritual organization with a goal of using new scientific discoveries to broaden theology, is in part supporting the research by Webb's team. On its Web site (http://www.templeton.org), the foundation states that it takes particular interest in evidence that some unseen hand is molding the universe over time.

One of the most profound implications for science would be that the presumption of immutability for the laws of physics may be wrong. Although the universe is full of evidence of the constancy of these laws, the new finding suggests that "maybe there's a tiny violation of that," says Turner, a cosmologist.

On the other hand, it may not prove easy to distinguish a revision of the laws from merely a better understanding of parameters found in those laws.

As an example, Turner points out the accepted theoretical claim that elementary particles known as the W boson and the Z boson had no mass when the universe first exploded into being. Modern accelerator experiments have shown, however, that both are very massive today. Even so, physicists have not concluded that the laws of physics have changed. Instead, they envision that as the universe evolved according to the steady laws of physics, the inherent possibility for W and Z bosons to become massive was realized. Something similar may be behind the apparent discrepancy between ancient and modern values of alpha.

Taylor explains that the laws that describe the forces between charged particles, such as atomic nuclei and electrons, wouldn't necessarily change even if the values of alpha or other parameters do in fact vary. However, if alpha has enlarged in the past 12 billion years, the strength of that atom-binding force would have grown slightly. "I would take the view that, until we know better, the law might be universal but the values of these parameters may be time-dependent. It's hard to say," he adds.

String theory

In the 1990s, the burgeoning branch of physics known as string theory fueled efforts to find variation in fundamental constants. In string theory, the fundamental particles of the universe are not points as they are in today's dominant theories. Instead they are vibrating, elongated stringlike entities. And, there are not just four dimensions–three spatial ones and time–but as many as 11 (SN: 2/19/00, p. 122: http://www.sciencenews.org/20000219/bob1.asp).

According to some string models, the values of certain constants are dependent on the scales of those extra dimensions. Although today those extra dimensions would remain tightly curled up and hidden on subatomic scales, some of them may have been more spread out shortly after the universe was born.

Although the string-theory models suggest that alpha variation would have taken place much earlier in the universe than 12 billion years ago, Webb's team may be seeing a "tail" remaining from a larger, earlier variation, Turner speculates. In any case, an ancient, slightly diminished alpha might be a sign that extra dimensions exist, and it might provide a coveted window onto their properties, says John D. Barrow of the University of Cambridge in England, a theorist on the varying-alpha team.

Complicating the interpretation of a once-smaller alpha, the quantity's magnitude relies on the values of other fundamental constants. Those are the size of the electron's charge, the speed of light, and Planck's constant, which defines the scales at which quantum phenomena operate.

A once-smaller alpha, therefore, could indicate that radiation may have zipped around slightly faster in the early universe than it does today, Webb says. As a result, interactions might have been possible between more widely separated regions in the early universe than in today's. That, in turn, might have affected the uniformity of the universe and some of its other properties, Webb adds.

Alpha's growth

Alpha's possible growth since the distant past also raises the question of whether it is still in flux today. This June in Seattle at a meeting about controlling frequencies of atomic clocks and other devices, Sébastien Bize of the Paris Observatory and his colleagues reported fresh evidence that alpha is holding steady. Using the world's most precise clocks, known as atomic fountain clocks (SN: 8/7/99, p. 92), the scientists have made a preliminary estimate that the annual variation in alpha, if any, must be less than 1 part in 100 trillion.

Although the measurements in the clock experiments indicate that alpha's value now is fixed, they don't cast doubt on the new astrophysical findings, Bize says. The minuscule disparity that turned up in the quasar spectra covers 12 billion years, which would correspond to an average annual shift of 1 part in 1,000 trillion. That's smaller than the clock laboratories can currently detect. What's more, it may be that the rate of change in alpha itself varies.

That's exactly what Barrow and a couple of other theorists have proposed in a theoretical model they posted July 26 on the Internet physics preprint archive (http://xxx.lanl.gov/abs/astro-ph/0107512). They suggest that alpha stopped being a variable about 3 or 4 billion years ago when the expansion of the already-ballooning universe presumably began accelerating (SN: 2/12/00, p. 106: http://www.sciencenews.org/20000212/bob1.asp).

Still, in this model, which agrees with the astrophysical data but has no obvious ties to string theory, the gravity of massive objects such as stars may also tweak alpha's value. Bize says the Paris experimenters plan to probe this possible gravitational influence in future tests. To do so, they'll compare frequency measurements of fountain atomic clocks when Earth's orbit carries it relatively near or far from the sun's gravity.

Taking an alternative approach to the problem, Webb and his coworkers have launched a program of radio telescope observations. The researchers will employ equipment quite unlike the optical instruments, such as the 10-meter Keck I telescope in Hawaii that they previously used to make spectral observations of gas clouds. Radio astronomy also homes in on different substances in the clouds, including carbon monoxide and other organic molecules, Webb explains.

Other, more radically dissimilar means may also provide a cross-check on the ancient value of alpha. In one, scientists plan to analyze subtle fluctuations in the cosmos discerned by satellite-based telescopes. Those telescopes generate sky maps of relic radiation, known as the cosmic microwave background (CMB), that dates from not long after the birth of the universe (SN: 6/23/01, p. 394: An Illuminating Journey).

Almost a year ago, a team of Portuguese and British scientists proposed that the microwave pattern favored an ancient alpha that was smaller than today's. However, says Spergel, those microwave data have since been reanalyzed, weakening that interpretation.

Measurements to come may tilt the scales one way or another. For example, the recently launched Microwave Anisotropy Probe, or MAP, satellite can measure the CMB's faint glow with 100 times the precision that has been available. MAP's successor, dubbed Planck, is slated to launch around 2007 and should detect variations in the CMB with even greater precision.

"When the [CMB] data gets better, we'll see either [the radiation pattern predicted by] the standard model or we'll see a signature for something new, like this time-varying alpha," Spergel says.

If the slight shift in alpha measured by Webb and his colleagues holds under further scrutiny, then scientists may have to forgo their long-held ideal that the constants of nature are perpetually unchanging.


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