Polished
Quartz for Gravity Probe B
University of Arizona News Services
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From:.........Lori Stiles, UA News
Services, 520-621-1877, lstiles@u.arizona.edu
Contact(s): Martin J. Valente,
520-621-2341, mvalente@ccit.arizona.edu
..................Scot A. Sumner,
520-621-4384, spindog@azstarnet.com
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February 9, 1998
UA optical scientists polish quartz blocks for Gravity Probe B
Optical Sciences Center Web site: http://w3.opt-sci.Arizona.EDU/
Gravity Probe B Web site: http://einstein.stanford.edu/
Optical scientists at The University of Arizona in Tucson have
successfully polished a 75-pound quartz block that is a critical
piece in a science mission to test two remarkable predictions of
Albert Einstein's general theory of relativity. The quartz block
is for the Gravity Probe B experiment that Stanford University
and NASA will fly. The predictions are that massive bodies such
as Earth warp space and time, and that such bodies drag
space-time with them as they rotate.
Martin J. Valente, manager of the Optical Sciences Optics
Laboratory, will deliver the block to scientists from Stanford
University and the Naval Research Labs next Thursday, Feb. 12, in
Tucson. The block then will be shipped to the W.W. Hansen
Experimental Physics Labs at Stanford for integration into the
space experiment, in development for launch in 2002.
First conceived 40 years ago, Gravity Probe B is NASA's longest running astrophysical development program.
Valente, UA senior optician Scot Sumner and others from the UA Optics Shop earlier polished and delivered another identical quartz block, the flight backup quartz block for the Gravity Probe B experiment. The experiment will check, very precisely, tiny changes in the direction of spin of four gyroscopes contained in an Earth satellite orbiting at 400-mile altitude directly over the poles. The 22-inch long, 7 1/4-inch diameter quartz block is the stable mechanical structure within which the four, perfectly spherical gyroscope "rotors" will spin. The block will be married to a quartz telescope fabricated by Tucson Optical Research Corp., a Tucson firm owned by Joseph Appels, a scientist formerly with UA Optical Sciences. The quartz block and telescope will be contained in a helium-cooled dewar.
The telescope will point at a distant, seemingly
fixed-position reference star, and the gyroscopes will be set
spinning with their axes pointed at this object. According to
Einstein's theory, a
massive body (Earth, in this case) spinning in space would drag
space and time around with it. This "frame-dragging
effect" should be most noticeable close to the rotating
Earth. Researchers will measure the extremely slight change in
the direction predicted for the spinning gyroscopes. Scientists
predict the change in gyroscope spin during one year will be 42
thousandths of an arc second, or the width of a single human hair
seen from a quarter mile away.
The experiment will measure both how space and time are warped by Earth's mass and how Earth's rotation drags space-time around with it.
Stanford awarded UA Optical Sciences the job of polishing the
quartz block and its backup twin after two previous polishing
attempts at other facilities failed. It took the UA researchers
14
months to grind and polish the blocks to incredibly exacting
specifications. The block face to which the telescope will be
mounted has been polished to within 2.5 millionths-of-an-inch,
peak
to valley. The other block faces were polished to a smoothness of
7 millionths-of-an-inch, Valente said.
But the greatest challenge was to hold very tight angular
tolerances between faces of the block, Valente said. Sumner had
to control extremely small angles between block faces in two
planes:
Two side surfaces had to be within 5 arc seconds of each other
and within one arc second of the front face. (An arc second is
one-3,600th of a single angular degree; there are 360 angular
degrees in a circle.)
The UA team achieved this precision by measuring angles with a strange-looking, reflective device called an autocollimator and some auxiliary optics called optical squares.
The polishing had to be done by hand because machines are not precise enough. Sumner used polishing tools as small as 1/4-inch in diameter to stroke seemingly imperceptible bumps to smoothness. Temperatures in the lab are kept to within a single degree Fahrenheit during polishing operations.
The fused quartz material used in the blocks presented no unknown challenges to the team, Valente said. Previously, the Optical Sciences Optics Shop had collaborated with UA optical scientist Steven Jacobs in supplying test pieces of the fused quartz material for the Stanford experiment.
Valente's group is currently involved in other spaceborne projects. Given their success with the Gravity Probe B science mission blocks, "I really want to pursue more projects like this," he said. "There is definitely a need out there. Now that we've done this successfully, we realize that we're the ones developing this technology."
(EDITORS NOTE: Valente also has a plexiglass model of the
Gravity Probe B quartz block. It was used for general reference
in this program.)
Andrew Yee
ayee@nova.astro.utoronto.ca