Speed of Gravity Controversy: An Update

Speed of Gravity Measurement Discredited

Earlier this year (February 2003), Jupiter Scientific reported on the speed of gravity measurement made by Drs. S. Kopeikin and E. Fomalont. That report argued that measurements of the effects of Jupiter's motion on the direction and time delay of radio waves from quasar QSO J0842+1835 could not be used to extract the speed of gravity c_g: In one reference frame, it appears that velocity-dependent effects might be related to c_g, while in another reference frame they could only be attributed to the speed of light c. Consistency of Einstein's gravity theory therefore requires c_g = c. A calculation using general relativity by Dr. C. Will (astro-ph/0301145) supports Jupiter Scientific's conclusions.
     Now an article has appeared that completely discredits the Jupiter/Quasar measurement of c_g. It was published in June in Physical Review Letters and is available online at astro-ph/0304006. The article calculates the v/c corrections to the gravitational time delay and finds them to be at least 100 times smaller than the Very Long Baseline Interferometry stations could have measured. Here, v is the speed of Jupiter. In short, it is impossible that Drs. Kopeikin and Fomalont measured the speed of gravity.
     The Physical Review Letter shows what Dr. Kopeikin did wrong in the data analysis: He used the angle between Jupiter and the quasar at the time of the measurement on Earth rather than the physical angle. These two angles differ by a v/c term. The problem is that Jupiter moves a significant distance during the period in which quasar radio waves travel to Earth. This misuse of angles leads to v/c dependence that is actually absent but appears to be enhanced by a factor of about 1000. Furthermore, the artificially generated effect involves v/c rather than v/c_g because it is related to the time that it takes for quasar waves to arrive at Earth (they, of course, propagate at the speed of light from Jupiter to Earth). Therefore, when data is parameterized to measure the artificial term, Drs. Kopeikin and Fomalont are guaranteed to find c_g to be c to within experimental errors. For more information, we refer web readers to the press release put out by the Lawrence Berkeley National Laboratory.
     The relation c_g = c in Einstein's gravity theory is fundamental. Theorists believe it to be true. Unfortunately, it has not yet been established experimentally. The most likely way that his equality will be checked will be by a gravity wave detector such as LIGO (Laser Interferometer Gravitational Wave Observatory) or its successors. If a faraway catastrophic astrophysical event occurs that sends out both light and gravity waves and both are detected to arrive at Earth at the same time, then c_g = c can be verified.

To the Original Report on the Speed of Gravity
To the Jupiter Scientific's Science Information Page

This update was prepared by the staff of Jupiter Scientific, an organization devoted to the promotion of science through books, the internet and other means of communication.

This web page may NOT be copied onto other web sites, but other sites may link to this page.