A Jupiter Scientific report on the recent attempted measurement of the speed of gravity including background information, news reporting and a suggested resolution to the criticism concerning the experiment.
A Measurement of the Speed of Gravity
Perhaps, Perhaps Not
The Speed of Gravity: A News Story
On September 8, 2002, a celestial event took place that happens only once a decade on average. Jupiter passed near a quasar (called QSO J0842+1835) arriving at 3.7 arc minutes of angular distance from the line-of-sight of the faraway object. Meanwhile, observations of the quasar's signal were made across the globe using a network of radio telescopes. Two astrophysicists, Ed Fomalont of the National Radio Astronomy Observatory and Sergei Kopeikin from the University of Missouri were quite excited. They were attempting what-has-been-called the first direct measurement of the speed of the effect of gravity.
In January 2003, at the Seattle meeting of the American Astronomical Association, Dr. Kopeikin announced the results of the measurement: The speed of gravity, cg, is within 20% of the speed of light. Reporters who attended the meeting immediately publicized the result in newspapers with titles such as "Einstein Right about the Speed of Gravity!" Word soon spread around the world in print, by radio and on TV. Unfortunately, this is another example of the media hastily announcing a scientific result before it should.
Not only has the manuscript by Drs. Fomalont and Kopeikin not yet been accepted for publication, but it has also met some difficulties in the peer-review process. Several scientists have even stated that the results are wrong: One claims cg is infinite, another argues that the experiment was not testing the speed of gravity but the speed of light, and still another says that data were not measuring the speed of gravity but other gravitational effects. Hence, there is considerable controversy about both the theory and the measurement. Given the confusion, it is apparent that the media is announcing to the public that Einstein is right based on a result that might be wrong!?
Quasars are among the most powerful sources of electromagnetic energy in the Universe. They are powered by giant black holes at the centers of newly forming galaxies. Material falling into the black hole accelerates and radiates. Some of the radiation is emitted in directions opposite to the black hole and escapes.
Due to their unimaginable brilliance, quasars are the most distant observable objects. QSO J0842+1835 is about nine billion light-years from Earth, meaning that it takes the electromagnetic waves nine billion years to reach us. In other words, the radio waves observed from the quasar on September 8 were emitted not only before the Earth existed but about twice its age ago! (The Earth is about 4.5 billion years old.)
Radio telescopes, which are large parabolic antenna dishes, collect radio waves from outer space and focus them at a detector. The QSO J0842+1835 experiment used the huge 100-meter radio telescope in Effelsberg, Germany and the Very Long Baseline Array. The latter, known as VLBA, consists of ten, 240-ton, 25-meter-diameter dishes spread across the United States and its territories from St. Croix in the Caribbean to Mauna Kea in Hawaii. Signals from the various instruments are combined using interference. This very sensitive technique can now measure time delays from radio sources to an incredible accuracy of about a trillionth of a second, that is a picosecond (10-12 seconds). This is good news because, according to calculations by Dr. Kopeikin, the speed-of-gravity effect due to the conjunction of Jupiter and QSO J0842+1835 is just a few picoseconds.
Einstein's general theory of relativity assumes that the effects of gravity propagate at the speed of light. For instance, if the Sun were suddenly and magically to disappear then the Earth would not feel the absence of the Sun's gravity until about 8.3 minutes later. Likewise, we would not observe the disappearance of the Sun for 8.3 minutes because this is the time that it takes sunlight to reach us. In other words, the sky would go dark at the same time that the Earth would stop orbiting in a circle. Such a disappearance cannot happen since it violates the conservation of energy.
Nevertheless, there are situations in which the finite propagation of the effect of gravity manifests itself. Suppose, for example, another force began to cause the Sun to move toward the Earth. Then, the gravitational force on the Earth would strengthen but the strengthening effect would not take place until 8.3 minutes later.
On the other hand, if some force caused the Earth to begin to move toward the Sun, then the gravitational force on the Earth would strengthen instantly. The difference between these two situations depends on which body begins to change its motion. In general, when the Earth-Sun distance is decreasing, the increase in the strength of gravity must be computed using Einstein's theory of gravity.
In Einstein's theory of gravity, a massive body creates a curvature of spacetime. Another body moving in this curved spacetime undergoes motions that mimic Newton's force of gravity. See Jupiter Scientific's page on How General Relativity Produces Gravity. In Einstein's theory of gravity, the speed of gravity, cg, is the speed of light, c; indeed, no parameter cg appears in the theory, only c does. The effect of gravity and the speed of gravitational waves is equal to the speed of light.
Difficulties with the Speed of Gravity Concept
When electromagnetic waves pass by a heavy body such as the Sun, two general relativistic effects occur: (1) there is a delay in time in the propagation of the waves compared to the situation in which the heavy body is absent, and (2) there is a small angular deflection the waves bend slightly around the body as if being attracted to it. Both these effects can be understood intuitively within Einstein's theory: (1) The "well" created in space by a heavy body requires light to travel an extra distance leading to a delay; (2) Light is affected by the curvature of space thereby causing it to change direction. These two effects have been observed many times and agree with theoretical calculations based on Einstein's theory of gravity to within experimental errors. They are among the classic tests of general relativity.
Hence, when the radio waves from the quasar passed by Jupiter, they were delayed and bent. The strength of gravity, which determines how long the delay is and how much bending occurs, depends on the speed v of Jupiter. This is the motivation of the Fomalont/Kopeikin measurement.
They were attempting
the first direct measurement of
the speed of the effect of gravity.
Jupiter Scientific has done an independent analysis of the Jupiter/quasar measurement and has arrived at the following conclusion. It does not make sense to change Einstein's theory so that the speed of gravity cg is an adjustable parameter. In other words, the speed of gravity must be the speed of light. If one attempts to extend general relativity so that cg differs from c then measurements and their physical interpretation become inconsistent: They depend on the inertial frame that is used to make the measurements, a violation of special relativity. Observers that are moving at constant speeds relative to one another would not agree on the amount of the time delay or on the angle that light bends. The details of this analysis are presented here.
The confusion in the scientific community that has arisen is due to the problems generated when the concept of the speed of gravity is introduced and differs from c. This ill-defined situation significantly diminishes the usefulness of the Jupiter/quasar test as a measurement of cg. In this sense, the goal of the work of Fomalont/Kopeikin is misdirected.
However, the two scientists have actually measured and verified the v/c effects of Einstein's theory for the time delay and the deflection of light! This is a nice precision test of general relativity and should have been the main point of the research.
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