Posted on 09/05/2002 9:08:22 AM PDT by RightWhale
Scientists attempt to measure speed of gravity
UNIVERSITY OF MISSOURI NEWS RELEASE
Posted: September 4, 2002
Ever since Albert Einstein proposed the general theory of relativity in 1916, physicists worldwide have tested the theory's underlying principles. Whil some principles - such as the speed of light is a constant - have been proven, others have enot. 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.
You could have stopped after two words.
Although Bill might disagree.
er - what was the topic of the thread again?
Shalom.
Physicist has some good things to add on this thread...
They better use my name.
I once thought so too, but Physicist explained to me that light's apparent mass (from its energy content) has real gravity. Yes, a photon has a zero rest mass, but it's never at rest.
Letter to the National Lampoon back in the 70's:
Dear Sirs:
If I'm so smart, how come I'm dead?
Sinerely,
Albert Einstein
Well like I said above I'm no gen-rel geek but I do believe one should, rather, discuss the speed of gravitational waves. LIGO's site (Laser Interferometer Gravitational-Wave Observatory) has a discussion about it--
"Gravitational waves are ripples in the fabric of space and time produced by violent events in the distant universe, for example by the collision of two black holes or by the cores of supernova explosions. Gravitational waves are emitted by accelerating masses much as electromagnetic waves are produced by accelerating charges. These ripples in the space-time fabric travel to Earth, bringing with them information about their violent origins and about the nature of gravity."
Well that was actually kind of lame. Lemme see if I can find anything better-- here's something written in '99--
"According to Newton's theory of gravitation, the binary period of two point masses (e.g., two stars) moving in a bound orbit is strictly a constant quantity. However, Einstein's general theory of relativity predicts that two stars revolving around each other in a bound orbit suffer accelerations, and, as a result, gravitational radiation is generated. Gravitational waves carry energy and momentum at the expense of the orbital decay of two stars, thereby causing the stars to gradually spiral towards each other and giving rise to shorter and shorter periods. This anticipated decrease in the orbital period of a binary pulsar was first observed in PSR 1913+16 by Taylor and Weisberg. The observation supported the idea of gravitational radiation first propounded in 1916 by Einstein... [who] showed that the first order contribution to the gravitational radiation must be quadrupolar in a particular coordinate system..."
The "quadrupole moment tensor of the energy density of the source", which is gotten to after you fix and gauge and pick a coordinate system and solve for Einstein's (linearized) equations Dal(h'_mu*nu) = -16PiGT^mu*nu (h' = trace-reversed perturbation, G = Einstein tensor, and I don't know what the hell T is... Tensor shorthand for something...) and fool around with a Fourier transform and take out the spatial part, is given on page 10 and has too many super/subscripts for me to put it here easily...
Photons only have percieved mass due to their speed. At rest, they would weigh nothing. Light has no mass, right?!
Likewise, gravitons have more mass than photons because gravitons travel CONSIDERABLY faster than photons.
Ergo, gravity is considerably faster than light.
You heard it here, first.
Oh, don't kid yourself. RightWhale had already considered that calculation, he was just too lazy to do the complex contravailing-factor mathematics involved.
Whaddya mean by that, you ask?
WELL, as is well known, Hillary! is a powerful locus of "dark energy", and therefore exerts a none-too-slight and all-too-perceptible repulsion amongst all right-thinking Life Forms -- thus contravailing the massive gravitational pull of the accumulated filth on Old Crusty.
Ergo, one has to divide the mass of right-thinking Life Forms present in the Empire State Building by the total mass thereof, etc., etc... the math ends up being mindbendingly annoying!! (which is another caused effect which has a powerful statistical correlation with Hillary! in all repeatable studies)
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