An international team of astrophysicists has discovered that the basic laws of nature as understood today may be changing slightly as the universe ages, a surprising finding that could rewrite physics textbooks and challenge fundamental assumptions about the workings of the cosmos.

The researchers used the world's largest single telescope to study the behavior of metallic atoms in gas clouds as far away from Earth as 12 billion light years. The observations revealed patterns of light absorption that the team could not explain without assuming a change in a basic constant of nature involving the strength of the attraction between electrically charged particles.

If confirmed, the finding could mean that other constants regarded as immutable, like the speed of light, might also have changed over the history of the cosmos.

The work was conducted by scientists in the United States, Australia and Britain and was led by Dr. John K. Webb of the University of New South Wales in Sydney, Australia. It is to be published on Aug. 27 in the field's most prestigious journal, Physical Review Letters.

Scientists who have examined the paper have not been able to find any obvious flaws. But because the consequences for science would be so far-reaching and because the differences from the expected measurements are so subtle, many scientists are expressing skepticism that the discovery will stand the test of time, and say they will wait for independent evidence before deciding whether the finding is true.

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On the other hand, the finding would fit with some theorists' new views of the universe, particularly the prediction that previously unknown dimensions might exist in the fabric of space.

Even scientists on the project have been deliberately cautious in presenting their result. Describing the implications of what his team observed, Dr. Webb said, ''It's possible that there is a time evolution of the laws of physics.''

Dr. Webb added, ''If it's correct, it's the result of a lifetime.''

Dr. Rocky Kolb, an astrophysicist at the Fermi National Accelerator Laboratory who was not involved in the work, said the finding could not only force revisions in cosmology, the science of how the universe began and later evolved, but also add credence to an unproven theory of physics called string theory, which predicts that extra dimensions exist.

''The implication, if it is true, would just be so enormous that it's something people should look at and take seriously,'' Dr. Kolb said. ''This would upset the apple cart.''

The magnitude of the change apparently observed by the group is minute, amounting to just 1 part in 100,000 in a number called the fine structure constant over 12 billion years. That constant, also referred to as alpha, is defined in terms of more familiar quantities like the speed of light and the strength of electronic attractions within atoms.

But even that small change would rock physics and cosmology, said Dr. Sheldon Glashow of Boston University, who received a Nobel Prize in physics in 1979. The importance of such a discovery, Dr. Glashow said, would rank ''10 on a scale of 1 to 10.''

Considering the unexpected nature of the finding, both Dr. Glashow and Dr. Kolb said the chances were high that some more mundane explanation for the results would turn up.

Dr. John Bahcall, an astrophysicist at the Institute for Advanced Study in Princeton, N.J., said the complicated analysis that was required to infer the tiny changes from the observations could -- in principle, at least -- be obscuring possible errors.

''The effect does not scream out at you from the data,'' Dr. Bahcall said. ''You have to get down on all fours and claw through the details to see such a small effect.''

But others said that the team had been very careful and that any unknown source of error would have to be extremely subtle to be missed.

''If they were claiming anything less dramatic, probably most people would find their work very careful and believable,'' said Dr. Massimo Stiavelli, an astrophysicist at the Space Telescope Science Institute in Baltimore.

''Exceptional results deserve extraordinary proof,'' Dr. Stiavelli said, adding that he was reserving judgment until further evidence became available.

The work relied on observations of light from distant beacons called quasars, which shine with a brightness equivalent to billions of suns. The light is probably emitted by matter torn from young galaxies by the powerful gravity of a black hole.

Besides Dr. Webb, the team included three other scientists at the University of New South Wales, Michael T. Murphy, Dr. Victor V. Flambaum, and Dr. Vladimir A. Dzuba; and one physicist at Cambridge University in Britain, Dr. John D. Barrow. Three American astronomers who are experts on quasars were also members of the team: Dr. Christopher W. Churchill of Pennsylvania State University; Dr. Jason X. Prochaska of the Carnegie Observatories; and Dr. Arthur M. Wolfe of the University of California at San Diego.

The observations, made by the 30-foot-wide Keck Telescope on Mauna Kea, in Hawaii, looked in detail at the absorption of quasar light by gas clouds in deep space between Earth and the quasars. Metal atoms like zinc and aluminum are often present in trace amounts in the clouds.

The absorption of light by such atoms creates dark spikes at various wavelengths in the quasar's spectrum, with a pattern so well defined that it is often likened to a fingerprint. The value of those wavelengths is directly related to the value of the fine structure constant.

But the fingerprint seemed to change in time, Mr. Murphy said, indicating that the constant grows larger as one goes nearer to the present and was not really constant.

''What we have found is that, statistically, there is a difference between the fine structure constant a long time ago and here on earth,'' he said.

Far from being of interest only in understanding atomic behavior, said Dr. Barrow of Cambridge University, the effect would be important ''because it gives you such a feedback into fundamental physics.''

String theory, for example, could accommodate changes in quantities that accepted physics theory considers immutable. String theorists postulate that space contains tiny, unseen dimensions. Any change in the size of those dimensions -- much like the expansion of the universe in the space we are familiar with -- could change quantities like the fine structure constant, said Dr. Paul Steinhardt, a physicist at Princeton University.

Dr. Steinhardt said most theorists would have expected those changes to have occurred in the first seconds of the universe's life and be virtually unobservable by astronomers today. Still, he pointed out that several years ago, other astronomers unexpectedly found that the present universe is apparently filled with a mysterious kind of energy that counteracts gravity on large scales. Perhaps the two effects are somehow related, Dr. Steinhardt said.

Other scientists pointed out that geologic processes, like naturally occurring nuclear fission, have been used to determine that the fine structure constant has probably changed little over the past two billion years on Earth. But researchers on the new paper point out that their results reach back much farther in time, and that interpreting the geological results is also a complicated matter.

But a few physicists, like Dr. Jacob D. Bekenstein of Hebrew University in Israel, noted that some theories have long been predicting a change in some of nature's apparent constants. Dr. Bekenstein called the findings ''potentially revolutionary'' and said he was inclined to believe them.

''After much thinking about this issue,'' Dr. Bekenstein said, ''I think the quasar observations may have found the real variation.''

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