Putting relativity to the test, NASA's Gravity Probe B experiment is one step away from revealing if Einstein was right (Forwarded)

News Service
Stanford University
Stanford, California

Contact:
Bob Kahn, Gravity Probe B Public Affairs
(650) 723-2540

Comment:
Francis Everitt, Gravity Probe B Principal Investigator
(650) 725-4104

October 3, 2005

Putting relativity to the test, NASA's Gravity Probe B experiment is one
step away from revealing if Einstein was right

BY Bob Kahn

Almost 90 years after Einstein postulated his general theory of relativity
-- our current theory of gravity -- scientists have finally finished
collecting the data that will put this theory to an experimental test. For
the past 17 months, NASA's Gravity Probe-B (GP-B) satellite has been
orbiting the Earth using four ultra-precise gyroscopes, about a million
times better than the finest navigational gyroscopes, to generate the data
required for this unprecedented test. As planned, the helium that cooled
the experiment and powered its micro-thrusters has run out, ending the
data-collection and final instrument calibration phase of the experiment.
All the data -- 50 weeks' worth -- has been downloaded from the spacecraft
and relayed to computers in the GP-B Mission Operations Center at Stanford
University, where GP-B scientists have begun the final painstaking task of
data analysis and validation. Was Einstein correct? They won't know for
another 15 months, when the analysis has been completed, but physicists
around the world are eagerly awaiting the results.

"This has been a tremendous mission for all of us," said Stanford's
Francis Everitt, GP-B's principal investigator. "Gravity Probe B presented
many challenges along the way and the team rose magnificently to every
occasion. With all the data now gathered, we are now proceeding very
deliberately over the next 15 months to make sure that everything is
checked and re-checked in as many ways as possible. NASA and Stanford can
be proud of what has been achieved so far."

This year, physicists celebrate the 100th anniversary of Einstein's
"miraculous year," in which he received his doctorate in physics from the
University of Zurich and published four seminal papers, including the
special theory of relativity and a paper on light that garnered him the
Nobel Prize in 1921. But Einstein's crowning achievement came in 1916,
with his publication of the general theory of relativity, in which he
expanded the special theory of relativity to include the elusive concept
of gravity. With general relativity, Einstein forever changed our
Newtonian view of gravity as a force, postulating rather that space and
time are inextricably woven into a four-dimensional fabric called
spacetime, and that gravity is simply the warping and twisting of the
fabric of spacetime by massive celestial bodies. Even though it has become
one of the cornerstones of modern physics, general relativity has remained
the least tested of Einstein's theories. The reason is, as Caltech
physicist Kip Thorne once put it: "In the realm of black holes and the
universe, the language of general relativity is spoken, and it is spoken
loudly. But in our tiny solar system, the effects of general relativity
are but whispers." And so, any measurements of the relativistic effects of
gravity around Earth must be carried out with utmost precision. Over the
past 90 years, various tests of the theory suggest that Einstein was on
the right track. But, in most previous tests, the relativity signals had
to be extracted from a significant level of background noise. The purpose
of GP-B is to test Einstein's theory by carrying out the experiment in a
pristine orbiting laboratory, thereby reducing background noise to
insignificant levels and enabling the probe to examine general relativity
in new ways.

Deceptively simple

Launched on April 20, 2004, from Vandenberg Air Force Base on the
California coast, GP-B has been using four spherical gyroscopes to measure
precisely two extraordinary effects predicted by Einstein's theory. One is
the geodetic effect -- the amount by which the Earth warps the local
spacetime in which it resides. The other effect, called frame-dragging, is
the amount by which the rotating Earth drags local spacetime around with
it.

How does GP-B measure these effects? Conceptually, the experiment is
simple: Place a gyroscope and a telescope in a satellite orbiting the
Earth. (GP-B uses four gyroscopes for redundancy.) At the start of the
experiment, align both the telescope and the spin axis of the gyroscope
with a distant reference point -- a guide star. Keep the telescope aligned
with the guide star for a year as the spacecraft orbits the Earth more
than 5,000 times. According to Einstein's theory, over the course of a
year, the geodetic warping of Earth's local spacetime should cause the
spin axis of the gyroscope to drift away from its initial guide star
alignment by a minuscule angle of 6.6 arcseconds (0.0018 degrees).
Likewise, the twisting of Earth's local spacetime should cause the spin
axis to drift in a perpendicular direction by an even smaller angle of
0.041 arcseconds (0.000011 degrees), about the width of a human hair
viewed from 10 miles away.

As the late Stanford physicist and GP-B co-founder William Fairbank once
put it: "No mission could be simpler than Gravity Probe B. It's just a
star, a telescope and a spinning sphere." However, it took the exceptional
collaboration of Stanford, NASA, Lockheed Martin and a host of other
physicists, engineers and space scientists almost 44 years to develop the
ultra-precise gyroscopes and the other cutting-edge technology necessary
to carry out this deceptively "simple" experiment. The
ping-pong-ball-sized gyroscope rotors, for example, had to be so perfectly
spherical and homogeneous that it took more than 10 years and a whole new
set of manufacturing techniques to produce them. They're now listed in the
Guinness Database of Records as the world's roundest objects. Similarly,
it took two years to make the flawless roof prisms in the GP-B science
telescope that tracks the guide star. Some scientists have mused about how
Einstein, himself once a patent clerk, would have enjoyed reviewing these
extraordinary technologies.

Stanford's Bradford Parkinson, GP-B's co-principal investigator and winner
of the 2003 Draper Prize in Engineering, said: "Optimism was rampant [in
1960, when GP-B began]. We didn't have any idea how hard this was, and I
would contend it was probably not until 30 years later that we brought
[into existence] the technology to make perfect spheres, the coating
technology, the readout technology, the cryogenic technology, the
[telescope] pointing technology. None of this was possible in 1960."

Running on empty

At launch, the Dewar, a giant Thermos bottle that comprises most of the
body of the spacecraft, contained approximately 650 gallons of helium,
cooled to a superfluid state just above absolute zero. The helium in the
Dewar served two vital functions: First, it was the superfluid bath that
kept the four gyroscopes at a superconductive temperature, required for
the readout of their spin axes. Second, helium gas that constantly
evaporated from the bath was reused as the propellant for the spacecraft's
micro-thrusters to maintain both its proper orientation and roll rate in
orbit and to keep it pointed at the guide star. When designing the Dewar,
the team carefully calculated that 650 gallons of helium would be adequate
to sustain the GP-B mission for at least 16 months, and that a Dewar large
enough to hold that amount would just barely fit in the nose of the Boeing
Delta II rocket that would launch the experiment. When the helium in the
Dewar was depleted on Sept. 29, it had outlived the team's initial
calculations by more than three weeks.

Mac Keiser, GP-B chief scientist who heads the data analysis team at
Stanford, said: "Getting 50 weeks of data from the satellite has been
particularly important -- not only because it will allow us to reduce our
statistical errors but also because the Earth has made almost a complete
revolution around the sun. This complete cycle will allow us to take full
advantage of one of our calibrating signals and eliminate potential
sources of systematic error."

Next-to-last milestone

The completion of data collection marks the last milestone prior to
announcing and publishing the results of this historic 44-year program. It
is a time of both triumph and emotion for the GP-B team. Some team members
have been working together on the program for more than 15 years. As the
focus of the mission shifts from spacecraft operations to data analysis,
it is time for many of the team's engineers and mission operations
specialists to move on, and this naturally brings a note of sadness into
the otherwise joyful spirit of accomplishment.

"It's a bit like sending your kid off to college," said GP-B Program
Manager Gaylord Green. "Our operations team became a family accomplishing
this mission, and after a good job the members will be departing to the
next phase of their lives."

Added Tony Lyons, NASA's GP-B program manager from Marshall Space Flight
Center in Huntsville, Ala.: "The completion of the GP-B mission is the
culmination of years of hard work, training and preparation by the GP-B
team. Every team member should feel proud of this accomplishment."

It will take the GP-B science team more than a year to complete the data
analysis, followed by up to six months of preparing and submitting papers
to major scientific journals detailing the experimental results. Following
NASA protocols used for other missions with precise quantitative
measurements, there will be no preliminary announcements of results nor
any speculation about the data before a formal announcement and
publication of results, expected early in 2007.

[Bob Kahn is the public affairs coordinator for Gravity Probe B at
Stanford.]

Editor Note: Photos and graphics are available on the web at
http://einstein.stanford.edu/pao/newspix/hires

Relevant Web URLs:

* http://einstein.stanford.edu
* http://www.gravityprobeb.com


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