Putting relativity to the test, NASA's Gravity Probe B experimentis 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