The Rotating Graviometer*

A Device and Experiment Designed for Measuring the Speed of Gravity

C. Towne Springer

 

Background: In 1970 I was a Freshman physics major at Washington State University. One day I ran across some discarded papers with intriguing titles like Photon Noise Limited LASER Transducer for Gravitational Antenna from Hughes Research and written by Robert L. Forward. The combination of physics and engineering in the papers was fascinating and I followed the work of Dr. Forward.

In 1975, I was continuing my physics major at the University of Puget Sound. I had seen more of Forward's work and corresponded and spoken on the phone several times. In a 1973 Hughes paper Review of Artificial Satellite Gravity Gradiometer Techniques for Geodesy, he described his 1966 vibrating string Gradiometer and his rotating cruciform Gradiometer. His rotating Gradiometer was analyzed in Gravitation by Misner, Thorne, and Wheeler, (MTW) in which they mentioned it was sensitive enough to measure the space-time curvature produced by a fist. I was impressed that there were only three instruments described in the massive text: The Cavendish Experiment, The Eatvos Experiment (or another equivalence experiment), and Forward's rotating gradiometer!

I was involved with seismometry and Worden and LaCoste & Romberg gravity meters and looking forward to a course in general relativity. I was an emerging experimentalist and had been fascinated by difference and lock-in methods and other ways to extract weak signals from noise. n fact, I had constructed an electronic analog of the driven oscillator with op-amps and then drove it with the seismograph output. A clock motor on the shaft of a 20 turn pot changed the resonant frequency of the circuit. The output was recorded on a paper plotter over a ten hour period with the notion of finding components of the microseismic noise. This was before there was easy access to recording and digitizing equipment (especially for undergrads) that could be used for a numerical Fourier Transform.

The combination of the elegance of the cruciform gradiometer and the role of phase in lock-in amplifiers led me to this idea for measuring the speed of gravity. In its simplest form, a massive dumbbell is set rotating in space (like a baton) at some angular rate w, and a Gradiometer is set some distance, x, from the dumbbell and in the plane of rotation. The Gradiometer rotates at a multiple nw of the dumbbell's rate and is adjusted to peak when the masses are aligned. In other words, they are synchronous. A LASER range finder is used to tell the distance form the center of the dumbbell to the gradiometer. The Gradiometer is translated in the plane of the dumbbell's rotation. The output signal is analyzed for a shift in the time of the peak, a phase shift produced by a finite speed of gravity. The speed of gravity is calculated from the distance translated and the phase shift.

I was very excited about this and could not find any evidence of previous proposals of this nature. I wrote to Kip Thorne describing the experiment in 1976 but received no answer. However, I did mention it to Robert Forward a few times and he liked the idea. In 1980 we exchanged publications. I sent him copies of a couple of letters to editors of technical journals and he sent me a very big box of Hughes and NASA papers and prepublication copies of everything except his fiction and Omni articles.

In the meantime I had revised the experiment with an Earth based test mass, which has potential problems with deformation and seismic noise generation. I say may have been, because an ideal cruciform Gradiometer has complete rejection of accelerations and only detects gradients. Rotating the "dumbbell" horizontally so that the forces on the structure are static was the obvious way to go.

I simulated the process with some computational physics and that is where it has sat since about 1982. In 1991 I again wrote to Kip Thorne and again received no answer.

What follows is a detailed description of the proposed experiment, differing from the 1975 version only in details of spinning the test masses.

*Graviometer is the name I coined for this gravity speedometer around 1980. At the time, there was no use of this word. In 2004, a web search found no uses on the Internet. In 2007, there are 4 pages of hits on Google. They result from two mistakes. First, there are many younger researchers who are using the term "graviometer" to refer to simple gravimeters like Worden or LaCost-Rhomberg gravity meters used in geological survey. The other examples are from people using the word "graviometer" instead of "hydrometer" as a device to measure the specific gravity of a liquid.

** At the time, it was generally accepted that the speed of gravity was the speed of light and there was no interest in experiments to measure the obvious. Since then a raging debate has formed between those who think the speed of gravity is infinite, or nearly so, and those who think it is the same as 'c'.

 

- Charles Towne Springer ---- June, 2004