Posted on 09/04/2002 5:23:02 AM PDT by ShadowAce
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.
Which just might account for some seemingly missing mass.
(See? I told everyone it wasn't all due to we neutrinos)
Well, I'm no physicist, but this don't pass the smell test.
Speed (unit-of-distance over time) is not Force (ergs) is not Velocity (speed+direction).
Does gravity respect particle theory?
Perhaps the article was translated (poorly) from another language. Perhaps it's too early for this sort of thing... where's my coffee cup?
Well, seeing as how it is from the University of Missouri-Columbia, I would kinda doubt that.
You ever work around these guys?
It's a different language.
:)
No it doesn't. It depends on the mass of Jupiter. The larger the mass of the intervening object the greater the bending of the light.
You're confusing the field (gravity, analogous to electromagnetism) with waves (gravitational waves, analogous to light), which are undulations in the field. Gravitational attraction depends upon the field strength; changes in the local field (such as would be caused by the motion of a heavy mass such as Jupiter) are transmitted by waves, which propagate at some velocity.
The language of the article is confusing. It doesn't make sense to talk about the "speed of gravity"; what they mean is the speed of gravitational waves.
First, we have excellent indirect evidence for the existence of gravitational waves. Measurements of the timing of the famous (and Nobel Prize-worthy) Hulse-Taylor binary pulsar show that energy is being radiated at exactly the rate predicted by general relativity through the mechanism of gravitational radiation:
(I believe the y-axis is in seconds.)
As for LIGO, it has always been understood that it was probably too insensitive to see any but the largest astronomical signals, which should only happen very rarely. The real purpose of LIGO has always been to develop the technique, to find ways of eliminating external noise. LIGO is the first instrument of its kind. A detector that can measure the ambient gravitational wave spectrum is feasible, but nobody wants to spend that kind of money on an experiment until the technique is mature. It's a lot tougher than they thought it would be, but give it time.
Jupiter will pass very close to the primary quasar.
Okay so I got the wrong impression of LIGO... I recall a few years back we had a colloquium speaker on the topic and he didn't sound overly-impressed about the project as a whole, considering the expense... I just remember his harping on about how sensitive it was to dumptrucks and such...
No one has ever tested this prediction, even though the assumption that gravity travels in waves or gravitons with a finite speed underpins much of theoretical physics. The difficulty is that if light and gravity travel at the same speed, how can you hope to see evidence of gravity's speed?The answer, says Sergei Kopeikin of the University of Missouri, Columbia, is by watching a distant quasar as the planet Jupiter moves in front of it and its gravity bends radio waves from the quasar. This event is due to happen over the weekend of September 7 and 8. "When I first gave a talk about the idea everybody got excited and said we have to do this," says Kopeikin.
Subtly warped
Together with Ed Formalont of the National Radio Astronomy Observatory, Kopeikin will get data on Jupiter's eclipse of the quasar from the NRAO's Very Long Baseline Array, a series of ten 25-metre radio telescopes strung between the Virgin Islands and Hawaii. The data will show the changes to the quasar's radio image to an accuracy of more than a millionth of one degree.
Astronomers usually assume that so-called "lensing events" convert a point-like source like the quasar into a ring, as the radio waves are bent round the edges of the lensing object - in this case Jupiter. But this assumes that the gravitational fields are static, while the light or radio waves move.
So in 2001 Kopeikin decided to develop a more realistic model. In this, as Jupiter moves, the changing gravitational field interacts with the radio waves coming from the quasar. The calculations show that if gravity has a finite speed, the ring-like image seen at Earth will be subtly warped compared to the shape expected if the gravitational changes propagate instantaneously.
It was a neat piece of work, but Kopeikin never expected to be able to test his idea so soon. "These lensing events only happen about once per decade," he says. The results from the experiment should be known within two months.
Disclaimer: Opinions posted on Free Republic are those of the individual posters and do not necessarily represent the opinion of Free Republic or its management. All materials posted herein are protected by copyright law and the exemption for fair use of copyrighted works.