Chinese scientists announced this week that they have conducted the most precise measurement yet of the speed of gravity. Their finding that gravity’s influence propagates at the speed of light supports both earlier, less precise measurements and the predictions of general relativity.
By conducting six observations of total and annular solar eclipses, as well as Earth tides, a team headed by Tang Keyun, a researcher with the Institute of Geology and Geophysics under the Chinese Academy of Sciences (CAS), found that the Newtonian Earth tide formula includes a factor related to the propagation of gravity.
“Earth tide” refers to a small change in the Earth’s surface caused by the gravity of the moon and sun.
Based on the data, the team, with the participation of the China Earthquake Administration and the University of the CAS, found that gravitational force released from the sun and gravitational force recorded at ground stations on Earth did not travel at the same speed, with the time difference exactly the same as the time it takes for light to travel from the sun to observation stations on Earth.
[…] By applying the new data to the propagation equation of gravity, the team 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 showing that gravity travels at the speed of light.
The first measurement of the speed of gravity was announced by Ed Fomalont and Sergei Kopeikin in 2003, but was considerably less precise and has since been disputed. From Hazel Muir’s report on Fomalont and Kopeikin’s experiment at New Scientist:
John Baez, a physicist from the University of California at Riverside, comments: “Einstein wins yet again.” He adds that any other result would have come as a shock.
Isaac Newton thought the influence of gravity was instantaneous, but Einstein assumed it travelled at the speed of light and built this into his 1915 general theory of relativity.
Light-speed gravity means that if the Sun suddenly disappeared from the centre of the Solar System, the Earth would remain in orbit for about 8.3 minutes – the time it takes light to travel from the Sun to the Earth. Then, suddenly feeling no gravity, Earth would shoot off into space in a straight line.
But the assumption of light-speed gravity has come under pressure from brane world theories, which suggest there are extra spatial dimensions rolled up very small. Gravity could take a short cut through these extra dimensions and so appear to travel faster than the speed of light – without violating the equations of general relativity.