Putting relativity to the test: was Einstein right?
Date: Tuesday, October 11, 2005 @ 19:56:46 GMT Topic: Science
http://www.physorg.com/news7143.html
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. Â… 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.
Source: Stanford University (by Bob Kahn)
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