Physicist Who Conceived Experiment Led Effort

COLUMBIA, Mo. — For the first time, scientists have measured
the speed of gravity, one of the fundamental constants of
Albert Einstein’s 1916 general theory of relativity. Led by
Sergei Kopeikin, a physicist at the University of Missouri-
Columbia, a team of scientists took advantage of a rare
cosmic alignment on Sept. 8 to test Einstein’s assumption
that gravity moves at the speed of light.

“Newton thought that gravity’s force was instantaneous,”
Kopeikin said. “Einstein assumed that it moved at the speed
of light, but until now, no one had measured it.”

Kopeikin worked with Ed Fomalont, a radio astronomer with
the National Science Foundation’s National Radio Astronomy
Observatory (NRAO) in Charlottesville, Va. On Sept. 8, the
scientists measured the shift of a quasar, a celestial
object that resembles a star. Jupiter’s gravitational
force caused the quasar to shift as Jupiter passed by it
closely.

“We have determined that gravity’s propagation speed is
equal to the speed of light within an accuracy of 20
percent,” Fomalont said.

The scientists presented their findings to the American
Astronomical Society’s meeting in Seattle, Wash. The
landmark measurement is important to physicists working
on field theories that attempt to combine particle
physics with Einstein’s general theory of relativity and
electromagnetic theory.

To conduct the experiment, the scientists used the
National Science Foundation’s Very Long Baseline Array
(VLBA), a continent-wide, radio-telescope system, and a
100-meter radio telescope in Effelsberg, Germany, to make
an extremely precise observation when Jupiter passed in
front of the quasar. The observation recorded a slight
“bending” of the radio waves coming from the quasar
because of the gravitational effect of Jupiter. The
bending resulted in a small change in the quasar’s
apparent position in the sky in addition to the deflection
of light calculated by Einstein in 1915.

“Because Jupiter is moving around the sun, the precise
amount of the bending depends slightly on the speed at
which gravity propagates from Jupiter,” Kopeikin said.
“Since the effect is very small, Einstein neglected it
in his calculations.”

In 1999, Kopeikin extended Einstein’s theory for light
propagation to include the gravitational effects of a
moving body on light and radio waves. Prior to this
study, no one had tried to measure the speed of gravity
because most physicists had assumed that the only way
to do so was to detect gravitational waves, Kopeikin
said. The MU scientist realized that if Jupiter moved
closely in front of a star or radio source, he could
test his theory.

The VBLA system is a result of a general radio technique
known as Very Long Baseline Interferometry (VLBI).

“I believe this experiment sheds new light on fundamentals
of general relativity and represents the first of many
more studies and observations of gravitation available
presently with existing VLBI technique,” Kopeikin said.
“We have a lot more to learn about this intriguing cosmic
force and its relationship to the other forces in nature.”