Go to my Home page.
Go to my Joy of High Tech page.

The Joy of High Tech


Rodford Edmiston

Being the occasionally interesting ramblings of a major-league technophile.

Gravity Probe B

            In a previous column I mentioned a project which involves launching a satellite to orbit the Earth and measure its frame drag. That is, the infinitesimal distortion our planet's rotating mass produces on the structure of space around it. For decades most scientists thought this subtle effect would have to be measured indirectly, by making observations of matter falling into a black hole. But a few individuals thought otherwise, and have built a space-rated clock, super gyroscopes and other instruments sensitive enough to perform the task with the relatively paltry mass of the Earth.

            If all goes well Gravity Probe B (Gravity Probe A was a 1976 NASA redshift experiment which was quite successful) will launch in April of 2004, culminating over 40 years of work.

            The original idea for the mission was developed by three Stanford University professors while they were skinny-dipping in the campus pool in 1959. Two of them have since died, but the work has been taken up by others. One man, Francis Everitt, has worked full-time on it since 1962, writing over 100 associated research papers through the decades. And there has been plenty to write about. Entire new methods of making precision objects had to be developed for this project. Everitt states that he has never been bored while working on Gravity Probe B. There was simply too much to learn, and too much to invent which wasn't already available.

            The heart of Gravity Probe B is a set of four small quartz spheres, roughly golf ball-sized. These are the four smoothest objects ever produced by humans. Of what we know of the universe, only neutron stars are rounder. The machines for making them had to be built from scratch, and are housed in the project's dedicated building at Stanford. These are fitted very closely into a single, solid block of quartz, leaving a space far narrower than a human hair between the sphere and the wall. Helium jets are then used to spin up the spheres to ten thousand revolutions per minute. The entire package is housed in a single, giant helium Dewar and cooled to 1.8 Kelvins. Once the spheres are rotating at full speed, the helium is pumped out, leaving a pressure even lower than the near-vacuum in the satellite's low orbit.

            Just figuring out the requirements for the instruments and determining that fused quartz would do the job took 21 years. Building the machines to build and test the equipment also took decades, but fortunately much of the development was concurrent with the research.

            The heart of the structure of the experiment is made of fused quartz. This reliable old standby of telescope mirrors is strong, rigid, has low thermal expansion and contraction and can be made with great purity and accuracy. No glue or brackets are used to hold the components together. As with Johansson Gauges, simply placing the ultra-flat, ultra-clean surfaces firmly in contact causes them to be held there by molecular attraction. The entire, multi-part instrument in effect becomes a single block of fused quartz. Among other things, this assures that the alignment telescopes (used to keep track of a reference star) are always perfectly referenced to the gyroscopes.

            Each of the fused quartz spheres is coated in niobium, which becomes superconducting at liquid helium temperatures. This allows the spheres to be suspended electrically, and also allows sensitive magnetic equipment to measure the changes which result from the effect of relativity on the spheres' rotation.

            Placed in a polar orbit 650 kilometers above the surface of the Earth the satellite is scheduled to gather data for 16 months. Analysis of this data will most likely take far longer.

            A rarity among work involving relativity, GBP will be an active physics experiment, instead of a passive observation of natural phenomena. The data it returns will help in verifying and quantifying both frame drag (think of spacetime as a slightly viscous fluid which is distorted in the direction of the Earth's rotation by the movement of our planet’s mass around its axis) and the amount of distortion in spacetime produced directly by the Earth.

            I won't go into all the details, but great pains have been taken to isolate the instruments from outside influences, provide a stable frame of reference, and separate the influences of the various effects of relativity from each other and compensate for the distortion they would have on each other.

            There will be those who see the project as an enormous boondoggle, an exercise in ivory tower persistence rather than anything beneficial. They are wrong. Ignoring the spin-offs, the advancement of state of the art, the challenges and rewards this has presented theorists and experimentalists, professors and grad students, the project is still justified. Because it will teach us something fundamental and important about how the universe works. And isn't the whole point of science?

            Addendum: Gravity Probe B successfully completed its mission, and analysis of its data confirmed frame drag.

          This document is Copyright 2005 Rodford Edmiston Smith. Anyone wishing to repost it must have permission from the author, who can be reached at: stickmaker@usa.net