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Particles and interactions

Particles and interactions

Changes spotted in fundamental constant

02 Sep 2010 Hamish Johnston
The southern sky over the VLT

Billions of years ago the strength of the electromagnetic interaction was different at opposite ends of universe. That’s the surprising conclusion of a group of physicists in Australia, who have studied light from ancient quasars. The researchers found that the fine-structure constant, known as α, has changed in both space and time since the Big Bang.

The discovery – dubbed by one physicist not involved in the work as the “physics news of the year” – is further evidence that α may not be constant after all. If correct, the conclusion would violate a fundamental tenet of Einstein’s general theory of relativity. The nature of the asymmetry in α – dubbed the “Australian dipole” – could also point scientists towards a single unified theory of physics and shed further light on the nature of the universe.

A constant that varies?

The fine-structure constant, about 1/137, is a measure of the strength of the electromagnetic interaction and quantifies how electrons bind within atoms and molecules. It is a dimensionless number, which makes it even more fundamental than other constants such as the strength of gravity, the speed of light or the charge on the electron.

Despite being dubbed a constant, there are, however, good theoretical reasons why α might vary with space or time. A changing α could, for example, help solve the biggest mystery of physics – how to formulate a single unified theory that describes the four fundamental forces: gravity, electromagnetism, and the strong and weak nuclear forces. The leading contender for a unified theory, for example, requires extra spatial dimensions beyond our familiar three – and the existence of extra dimensions could be inferred from changes in α.

In 1998 John Webb, Victor Flambaum and colleagues at the University of New South Wales began looking for evidence of variations in α by studying light coming from distant quasars. Radiation from these extremely bright objects has travelled for billions of years before reaching Earth and will have passed through ancient clouds of gas along the way. Some of the light is absorbed at specific wavelengths that reveal the chemical composition of the cloud. Within the absorption spectrum is the eponymous “fine structure” from which the value of α can be extracted.

The team has so far studied hundreds of quasars in the northern sky and concluded that billions of years ago α was about one part in 100,000 smaller than it is today. This, however, remains a controversial result that is not accepted by all physicists.

Surprise in the southern sky

Now, Webb and colleagues have analysed 153 additional quasars in the southern sky using the Very Large Telescope (VLT) in Chile and have made an even more startling discovery. They found that in the southern sky, α was about one part in 100,000 larger 10 billion years ago than it is today. The value in the northern sky was still smaller, as found before.

This asymmetry in the two hemispheres – dubbed the “Australian dipole” by the researchers – has a statistical significance of about four sigma. This means that there is only a one in 15,000 chance that it is a random event.

This spatial variation in α is further evidence that the electromagnetic interaction violates Einstein’s equivalence principle – one of the cornerstones of relativity that says that α must be the same wherever and whenever it is measured. Such a violation is good news for those seeking unification because many leading theories also go against the equivalence principle.

Big breakthrough?

Wim Ubachs, a spectroscopist at the Free University of Amsterdam in the Netherlands, described the finding as “the news of the year in physics”, adding that the result both backs up previous findings and gives “a new twist to the problem”.

The fine structure and other fundamental constants determine the masses and binding energies of elementary particles – including dark matter. If these constants vary, the relative abundances of normal matter, dark matter and dark energy could be different in different parts of the universe. This could be seen as an additional anisotropy in the cosmic microwave background or as an asymmetry in the rate of expansion of the universe.

Perhaps the most intriguing aspect of the finding is with regards to the anthropic principle, which points out that we owe our very existence to the fact that the fundamental constants have values that allow matter and energy to form stars, planets and ultimately our own bodies. If α varies throughout space and time, it is possible that we owe our existence to a special place and time in the universe.

A paper describing the results has been submitted to Physical Review Letters.

In a separate preprint, Flambaum and UNSW colleague Julian Berengut argue that the Australian dipole is consistent with other measurements of the variation of α. In 2008, for example, studies with an atomic clock at the National Institute of Standards and Technology in the US suggested that α is constant to within about one part in 1017 in the course of a year. During that time, Earth moved a certain distance along the dipole, and Flambaum and Berengut calculate that this should have changed α by about one part in 1018 – well within the NIST limit.

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