International Team of Scientists Attempts to Measure Speed of Gravity

COLUMBIA, Mo. – Ever since Albert Einstein proposed the general theory
of relativity in 1916, physicists worldwide have tested the theory’s
underlying principles. While some principles – such as the speed of
light is a constant – have been proven, others have not. Now, through a
combination of modern technology, the alignment of a unique group of
celestial bodies on Sept. 8, and an experiment conceived by a University
of Missouri-Columbia physicist, one more of those principles might soon
be proven.

“According to Einstein’s theory, the speed of gravity is assumed to be
equal to the speed of light,” said Sergei Kopeikin, MU associate
professor of physics and astronomy. “While there is indirect evidence
this is true, the speed has never been measured directly, and that’s
what we’re attempting to do in an experiment that will not be possible
again for another decade.”

The experiment will involve precisely measuring the angular distances
between several quasars, celestial objects in distant galaxies that
resemble stars. On Sept. 8, Jupiter will pass very close to the primary
quasar. When it does, its gravity will cause the quasar’s position in
the sky to shift by a distance that depends on the speed of gravity.
Kopeikin and Ed Fomalont, a radio astronomer with the National Science
Foundation’s National Radio Astronomy Observatory (NRAO), will use an
observational technique they developed to compare the position of the
primary quasar to the position of other quasars unaffected by Jupiter.
Using their data, they hope to confirm the accuracy of Einstein’s theory
further.

Measurements will be made using the NRAO’s Very Long Baseline Array
(VLBA), a series of 10, 25-meter radio telescopes located from the
Virgin Islands to Hawaii, and the 100-meter radio telescope in
Effelsberg, Germany, which is operated by the Max Planck Institute for
Radio Astronomy. “Results from recent VLBA test observations indicate we
can reach the accuracy necessary to determine the speed of gravity if
the experiment goes well,” Fomalont said.

“Japanese and NASA scientists also will conduct the experiment
independently using other telescopes around the world, so we’ll be able
to compare our findings,” Kopeikin said. “We believe the general theory
of relativity is correct and that the speed of gravity is equal to the
speed of light.”

“The techniques we’ve employed for this experiment can also be used to
more precisely determine the position of other objects in space,”
Fomalont said. “With more exact positioning of satellites, we could
improve telecommunications. Unmanned space navigation could also be
improved, allowing us to explore the solar system more deliberately.”

The scientists said final results from the experiment should be
available in mid-November.