This Echo is READ ONLY !   NO Un-Authorized Messages Please!
 ~~~~~~~~~~~~~~~~~~~~~~~~   ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Public Information Department
Lawrence Berkeley National Laboratory

Contact:
Lynn Yarris, (510) 486-5375, lcyarris{at}lbl.gov

June 18, 2003

Berkeley Lab Physicist Challenges Speed of Gravity Claim

BERKELEY, CA -- Albert Einstein may have been right that gravity
travels at the same speed as light but, contrary to a claim made
earlier this year, the theory has not yet been proven. A scientist at
Lawrence Berkeley National Laboratory (Berkeley Lab) says the
announcement by two scientists, widely reported this past January,
about the speed of gravity was wrong. 

Stuart Samuel, a participating scientist with the Theory Group of
Berkeley Lab's Physics Division, in a paper published in Physical
Review Letters, has demonstrated that an "ill-advised" assumption
made in the earlier claim led to an unwarranted conclusion.

"Einstein may be correct about the speed of gravity but the
experiment in question neither confirms nor refutes this," says
Samuel. "In effect, the experiment was measuring effects associated
with the propagation of light, not the speed of gravity."

According to Einstein's General Theory of Relativity, light and
gravity travel at the same speed, about 186,000 miles (300,000
kilometers) per second. Most scientists believe this is true, but the
assumption was that it could only be proven through the detection of
gravity waves. Sergei Kopeikin, a University of Missouri physicist,
and Edward Fomalont, an astronomer at the National Radio Astronomy
Observatory (NRAO), believed there was an alternative. 

On September 8, 2002, the planet Jupiter passed almost directly in
front of the radio waves coming from a quasar, a star-like object in
the center of a galaxy billions of light-years away. When this
happened, Jupiter's gravity bent the quasar's radio waves, causing a
slight delay in their arrival on Earth. Kopeikin believed the length
of time that the radio waves would be delayed would depend upon the
speed at which gravity propagates from Jupiter. 

To measure the delay, Fomalont set up an interferometry system using
the NRAO's Very Long Baseline Array, a group of ten 25-meter radio
telescopes distributed across the continental United States, Hawaii,
and the Virgin Islands, plus the 100-meter Effelsberg radio telescope
in Germany. Kopeikin then took the data and calculated
velocity-dependent effects. His calculations appeared to show that
the speed at which gravity was being propagated from Jupiter matched
the speed of light to within 20 percent. The scientists announced
their findings in January at the annual meeting of the American
Astronomical Society. 

Samuel argues that Kopeikin erred when he based his calculations on
Jupiter's position at the time the quasar's radio waves reached Earth
rather than the position of Jupiter when the radio waves passed by
that planet. 

"The original idea behind the experiment was to use the effects of
Jupiter's motion on quasar-signal time-delays to measure the
propagation of gravity," he says. "If gravity acts instantly, then
the gravitational force would be determined by the position of
Jupiter at the time when the quasar's signal passed by the planet.
If, on the other hand, the speed of gravity were finite, then the
strength of gravity would be determined by the position of Jupiter at
a slightly earlier time so as to allow for the propagation of
gravitational effects." 

Samuel was able to simplify the calculations of the
velocity-dependent effects by shifting from a reference frame in
which Jupiter is moving, as was used by Kopeikin, to a reference
frame in which Jupiter is stationary and Earth is moving. When he did
this, Samuel found a formula that differed from the one used by
Kopeikin to analyze the data. Under this new formula, the
velocity-dependent effects were considerably smaller. Even though
Fomalont was able to measure a time delay of about 5 trillionths of a
second, this was not nearly sensitive enough to measure the actual
gravitational influence of Jupiter. 

"With the correct formula, the effects of the motion of Jupiter on
the quasar-signal time-delay are at least 100 times and perhaps even
a thousand times smaller than could have been measured by the array
of radio telescopes that Fomalont used," Samuel says. "There's a
reasonable chance that such measurements might one day be used to
define the speed of gravity, but they just aren't doable with our
current technology." 

Berkeley Lab is a U.S. Department of Energy national laboratory
located in Berkeley, California. It conducts unclassified scientific
research and is managed by the University of California. Visit our
Website at www.lbl.gov . 

Additional information

For a copy of the Samuel's paper: "On the speed of gravity and the
v/c corrections to the Shapiro time delay", see
      http://arxiv.org/abs/astro-ph/0304006

 - END OF FILE -
==========

@Message posted automagically by IMTHINGS POST 1.30
---