FROM remote observatories on the Tibetan plateau to a cave in a Shanghai suburb, Chinese researchers were this week poised for an ambitious once-in-a-century experiment. The plan is to investigate a controversial notion that few in the west have the means or motivation to look at: the possibility that gravity suddenly drops by a minute amount during a total solar eclipse.
As New Scientist went to press, geophysicists from the Chinese Academy of Sciences were preparing an array of sensitive instruments at six sites across China ready to take gravity readings on Wednesday, 22 July, as the eclipse passed over the south of the country. The experimenters hope the results, which will be analysed in the coming months, will confirm once and for all that anomalous fluctuations recorded during past eclipses are a real phenomenon, rather than an experimental artefact.
“It would be revolutionary if it turned out to be true,” says of Delft University of Technology in the Netherlands. “I’m not really convinced the anomaly exists,” he says. Most scientists outside China take a similar view.
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The first indication that gravity may fluctuate during an eclipse came in 1954, when French economist and physicist Maurice Allais noticed a swinging pendulum behaving erratically when an eclipse passed over Paris.
A pendulum’s back-and-forth swing typically undergoes a steady shift under the combined influence of gravity and the rotation of the Earth. At the start of the eclipse, however, Allais observed a marked shift in the pendulum’s swing, suggesting a sudden change in gravitational pull.
Gravity has been measured during some 20 subsequent total solar eclipses, in China, Zambia, Australia and elsewhere. A 1997 experiment by the Chinese Academy of Sciences in north-eastern China provided some of the best evidence of a possible anomaly, according to Duif. But most of these tests featured only one or two pendulums or gravimeters – precise scales that can measure minute changes in gravity – set up close to each other. The results remain inconclusive. For the 1999 eclipse over Europe, NASA organised a multi-site experiment, but the researcher heading the project left soon after the eclipse and the findings were never made public.
If the anomaly exists, it would challenge our ideas about how gravity works. Neither Newtonian physics nor general relativity can explain it.
A number of mundane explanations have been suggested. “There could be different reasons: atmospheric changes in temperature or air pressure, people suddenly moving or not moving during an eclipse, or other sudden changes,” says of the Tata Institute of Fundamental Research in Mumbai, India.
However, in 2004 Duif posted a paper on the Arxiv physics preprint server which concluded that none of these external influences could account for the magnitude and timing of observed anomalies ().
In the run-up to the 22 July eclipse, a team led by Tang Keyun of the Chinese Academy of Sciences prepared eight gravimeters and two pendulums, spread across six monitoring sites (see Map). The researchers hope that the long distance between the sites – roughly 3000 kilometres between the most easterly and most westerly stations – and the number and diversity of instruments will eliminate the chance of instrument error or local atmospheric disturbances.
“If our equipment operates correctly, I believe we have a chance to say the anomaly is true beyond all doubt,” says Tang. The opportunity won’t come again soon. Lasting over 5 minutes, the event is the longest total solar eclipse predicted for this century. What’s more, it occurs at a time when the sun is high in the sky, which Tang says should mean that any potential gravitational anomaly should appear most strongly.
The fact that the eclipse crosses China is also significant. “Nowhere else would you get the funding,” says Duif. “It’s seen too much as fringe science. No US geophysical organisation, for example, would want to connect their name with such an experiment.”
To make the most of their opportunity and boost their credibility, the Chinese researchers are doing all they can to eliminate external factors from disturbing their results. Each observation site is a China Seismological Bureau station either inside a cave or at a remote location where human disturbance is minimal. The researchers will also monitor changes in air temperature and pressure as well as seismological disturbances that could affect their results.
If Tang is successful, it wouldn’t be the first time gravitational anomalies observed during an eclipse have had an impact on theoretical physics. In 1919 the bending of starlight by the gravitational field of the sun during an eclipse confirmed Einstein’s theory of general relativity. Duif, however, points out one major difference this time. “You only have the observations; you don’t have a good theory to explain them.”
“It wouldn’t be the first time anomalies during an eclipse have had an impact on theoretical physics”
Tang told New Scientist that he has been working for years on a theory to explain the apparent anomalies. He hopes to publish his ideas soon. “I think we’re about to restudy Newton’s laws and general relativity to see if they are exactly correct or only partially correct,” he says.