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
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
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
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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