Speed of Gravity Researcher Stands Behind Theory

(Hypography) Last year Ed Fomalont, an astronomer at the National Radio Astronomy Observatory, and Sergei Kopeikin, a physicist from the University of Missouri, teamed up to take measurements of the radio signals from a distant quasar as it passed behind Jupiter on September 8th. The measurements were performed using the Very Long Baseline Array, with radio telescopes stretching from the United States to Germany.

The recorded data was examined and in January the team announced that the results showed that indeed the speed of gravity was within +/-20% of the speed of light.

Stuart Samuel. Photo: Berkeley Labs.

However in a recent news release Stuart Samuel, a scientist at Berkley Lab, claims that while the speed of gravity may equal the speed of light, this was not proved because the experiment and data were in err. The news release can be found at: http://www.lbl.gov/Science-Articles/Archive/Phys-speed-of-gravity.html

When asked by Hypography, Dr. Kopeikin was kind enough to provide rebuttals to the claims of Mr. Samuel, which he states "is a [...] mixture of erroneous formulas and [incorrect] statements about the measuring procedure and essence of general relativity."

In the news release, Samuel is quoted say that "in effect, the experiment was measuring effects associated with the propagation of light, not the speed of gravity." This would definitely be a mistake in the original experiment, but Dr. Kopeikin explains the meaning of this statement with the following remarks. "This is common misinterpretation based on the statements: 'The letter c denotes the speed of light. Hence, every time when one has 'c' in equations it must be the speed of light. Kopeikin has in his equations 'c'. Consequently, he measures the speed of light.'

The mistake of this logic is that people does not take into account physical nature of equations. For example, the Einstein equations in vacuum have the letter 'c' in the wave operator, which describes propagation of gravitational waves. But it does not mean that we shall measure the speed of light when gravitational waves will be observed - we shall measure the speed of propagation of gravity. In other words, physical meaning of the letter 'c' depends on the nature of equations, which are solved. Mr. Samuel ignores this fact. So, he will measure the speed of light even in the gravitational wave experiments. My experiment approaches the problem of measuring the speed of gravity from different positions. But I operate with the solutions of the Einstein equations and consequently the speed 'c' in my solution of these gravitational field equations should be called "the speed of gravity". The purpose of the experiment was to prove that this 'c' in my solution is equal to the speed of light as predicted by general relativity. This was successfully achieved."

Dr. Kopeikin also goes on to say "Mr. Samuel mistakenly believes that we measured the angle between Jupiter and the quasar. This angle was not measured. VLBI measures phase of electromagnetic waves from [the] quasar. From measurement of this phase only the relative angular distances between the main quasar and two reference quasars are determined. Jupiter was not observed in the experiment at all. Its position on its orbit was determined later from the fitting procedure. […] From the paper of Mr. Samuel it follows that he does not know what exactly VLBI observes and how the data processing algorithm works."

Apparently Mr. Samuel's paper and alternate theories are mistaken in part due to a misunderstanding of the experiment that was being performed and of the procedure of the data processing. This does not mean, however, that the Speed of Gravity measurements have been confirmed as with many theories it takes years to prove, or disprove, what appears to be factual data.

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