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China Focus: Earth tides reveal first evidence for speed of gravity
[January 14, 2013]

China Focus: Earth tides reveal first evidence for speed of gravity

BEIJING, Jan 14, 2013 (Xinhua via COMTEX) -- Scientists have long been divided over the speed of gravity. Newton adherents believe it is instantly transmitted, while those siding with Einstein disavow the possibility that it could travel faster than the speed of light.

As physical scientists continue to look for a valid experimental or observational method to determine the true speed of gravity, Chinese scientists have found inspiration in Earth tides.

Earth tides refer to small changes in the Earth's solid surface caused by the gravity of the moon and the sun. These changes will, in turn, affect the gravity reading at a certain point on the Earth's surface.

After conducting six observations of Earth tides influenced by lunar and solar tidal forces during total and annular solar eclipses, a team headed by Tang Keyun, a research professor with the Institute of Geology and Geophysics under the Chinese Academy of Sciences (CAS), accidentally discovered time lags between the observational curves and the theoretical curve derived from the Newtonian formula. The differences ranged from eight to ten minutes, which seem to be related to the speed of gravity.

After ruling out various possibilities that could explain this phenomenon, Tang realized that the positions of the sun and the moon in the currently used formula were all apparent positions, or positions seen by observers -- not the true positions the spheres have already moved to in the amount of time light travels from the apparent positions to the Earth for us to see.

In other words, the practical Newtonian formula of Earth tide is no longer the classical one that deemed the speed of gravity infinite. Rather, it suggests that gravity and light are both released from the apparent positions of the sun and spend the same amount of time traveling to an observation point on Earth.

Meanwhile, the curves based on observational data, while differing from the classic Newtonian formula, roughly match the practical curve, proving that the hypothesis that gravity travels at the speed of light may be correct.

"In order to be more precise, I have derived an equation of the speed of gravity by comparing the observational curves and the practical curve," Tang said.

The team, consisting of researchers from the CAS Institute of Geology and Geophysics, the China Earthquake Administration and the University of CAS, then chose two Earth tide observation stations in Xinjiang Uygur Autonomous Region and Tibet Autonomous Region, two inland regions that are far away from all four oceans, and thus, have few ocean tide disturbances.

After using gravimeters to record data and correcting them, the team applied the data to the propagation equation of gravity and found that the speed of gravity is about 0.93 to 1.05 times the speed of light, with a relative error of about 5 percent, providing the first set of strong evidence that gravity travels at the speed of light.

Their findings, described by CAS Academician Teng Jiwen as a "breakthrough for Chinese scientists in the realm of gravitational research," are expected to offer new guidance for future gravity research and improve the accuracy of devices in astronautics, GPS and other fields.

The findings have been published online in a detailed English article by German science and technology publishing group Springer, and the printed version will be carried in an upcoming issue of Chinese Science Bulletin, according to the CAS Institute of Geology and Geophysics.

While revealing that his team will apply more accurate data to further reduce the relative error of the results, Tang called on various science and technology institutes to make full use of the country's western inland regions.

"Far away from all four oceans, Xinjiang and Tibet are the perfect locations for gravitational observations. Stations built there might become the center for international gravity-related research," Tang added.

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