By Justin Mullins Controversy over whether the fundamental constants of nature change with time has reignited. A new study is casting doubt on an earlier claim that a key constant varied as the Universe evolved. The study looks at alpha, the fine-structure constant. Alpha is crucial to the debate because it dictates the strength of the interaction between an electron and a photon, and governs a host of physical processes, from how the Sun burns to the "inflation" of the Universe immediately after the big bang. A changing alpha has implications for the constancy of the speed of light, and would revolutionise traditional physics. So it was headline news in 2001 when astronomer John Webb of the University of New South Wales in Sydney, Australia, reported that alpha's value - based on observations of how gas clouds absorbed light from quasars - was different 12 billion years ago from what it is today (New Scientist, print edition, 18 August 2001). If Webb is correct, alpha may still be changing by as much as 1 part in 1014 per year, assuming a linear rate of change. Now Theodor Hänsch at the Max Planck Institute for Quantum Optics in Garching, Germany, and his colleagues have ruled out any such change to within 1 part in 1015. Energy level From 1999 to 2003, they compared the effect of alpha on the emission of photons from caesium and hydrogen atoms. The photons are emitted when electrons in these atoms move from one energy level to another. Any variation in alpha over time should show up in such a comparison. "We found no evidence of any change," Hänsch says (Physical Review Letters, vol 92, p 230802). This result negates at least one theory that assumes a linear rate of change in alpha over 12 billion years. Most theorists believe a linear change is unlikely. Joao Magueijo of Imperial College London says that any variation in alpha would have been dramatic when the Universe was expanding rapidly, but its rate of change would now have slowed to only 1 part in 1018. Hänsch admits that changes smaller than they can measure may be occurring, and the precision required to measure this kind of change will be available within the next few years. Meanwhile, Webb's original observations have also been questioned by French and Indian astronomers who have been unable to see any change in alpha in their survey of quasars (Physical Review Letters, vol 92, p 121302). "That is much more worrying," Magueijo says. Webb is now analysing a new set of quasar data and hopes to settle the question later in 2004.

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