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First speed of gravity measurement
revealed
20:30 07 January 03 NewScientist.com news service
The speed of gravity has been measured for the first
time. The landmark experiment shows that it travels at the
speed of light, meaning that Einstein's general theory of
relativity has passed another test with flying colours.
Ed Fomalont of the National Radio Astronomy
Observatory in Charlottesville, Virginia, and Sergei Kopeikin
of the University of Missouri in Columbia made the
measurement, with the help of the planet Jupiter.
"We
became the first two people to know the speed of gravity, one
of the fundamental constants of nature," the scientists say,
in an article in New Scientist print edition. One important consequence of the result
is that it places constraints on theories of "brane worlds",
which suggest the Universe has more spatial dimensions than
the familiar three.
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.
You can read Fomalont and Kopeikin's account of
their unique experiment in an exclusive, full-length feature
in the next issue of New Scientist print edition, on sale from
9 January.
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.
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But how can you measure the speed of gravity? One
way would be to detect gravitational waves, little ripples in
space-time that propagate out from accelerating masses. But no
one has yet managed to do this.
The opportunity to do
this arose in September 2002, when Jupiter passed in front of
a quasar that emits bright radio waves. Fomalont and Kopeikin
combined observations from a series of radio telescopes across
the Earth to measure the apparent change in the quasar's
position as the gravitational field of Jupiter bent the
passing radio waves.
From that they worked out that
gravity does move at the same speed as light. Their actual
figure was 0.95 times light speed, but with a large error
margin of plus or minus 0.25.
Their result, announced
on Tuesday at a meeting of the American Astronomical Society
meeting in Seattle, should help narrow down the possible
number of extra dimensions and their sizes.
But experts
say the indirect evidence that gravity propagates at the speed
of light was already overwhelming. "It would be revolutionary
if gravity were measured not to propagate at the speed of
light - we were virtually certain that it must," says Lawrence
Krauss of Case Western Reserve University in Cleveland, Ohio.
Hazel Muir
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