Artist concept of Gravity Probe B orbiting the Earth to measure space-time, a four-dimensional description of the universe including height, width, length, and time. |
Stanford
and NASA researchers have confirmed two predictions of Albert
Einstein’s general theory of relativity, concluding one of the space
agency’s longest-running projects.
Known
as Gravity Probe B, the experiment used four ultra-precise gyroscopes
housed in a satellite to measure two aspects of Einstein’s theory about
gravity. The first is the geodetic effect, or the warping of space and
time around a gravitational body. The second is frame-dragging, which is
the amount a spinning object pulls space and time with it as it
rotates.
After
52 years of conceiving, building, testing and waiting, the science
satellite has determined both effects with unprecedented precision by
pointing at a single star, IM Pegasi, while in a polar orbit around
Earth. If gravity did not affect space and time, Gravity Probe B’s
gyroscopes would point in the same direction forever while in orbit.
But in confirmation of Einstein’s general theory of relativity, the
gyroscopes experienced measurable, minute changes in the direction of
their spin as they were pulled by Earth’s gravity.
The findings appear online in the journal Physical Review Letters.
“Imagine
the Earth as if it were immersed in honey. As the planet rotated its
axis and orbited the Sun, the honey around it would warp and swirl, and
it’s the same with space and time,” said Francis Everitt, a Stanford
physicist and principal investigator for Gravity Probe B.
A lasting legacy
“GP-B
confirmed two of the most profound predictions of Einstein’s universe,
having far-reaching implications across astrophysics research,” Everitt
said. “Likewise, the decades of technological innovation behind the
mission will have a lasting legacy on Earth and in space.”
Stanford
has been NASA’s prime contractor for the mission and was responsible
for the design and integration of the science instrument and for mission
operations and data analysis.
Much
of the technology needed to test Einstein’s theory had not yet been
invented in 1959 when Leonard Schiff, head of Stanford’s physics
department, and George E. Pugh of the Defense Department independently
proposed to observe the precession of a gyroscope in an Earth-orbiting
satellite with respect to a distant star. Toward that end, Schiff teamed
up with Stanford colleagues William Fairbank and Robert Cannon and
subsequently, in 1962, recruited Everitt.
NASA
came on board in 1963 with the initial funding to develop a relativity
gyroscope experiment. Forty-one years later, the satellite was launched
into orbit about 400 miles above Earth.
The
project was soon beset by problems and disappointment when an
unexpected wobble in the gyroscopes changed their orientation and
interfered with the data. It took years for a team of scientists to sift
through the muddy data and salvage the information they needed.
Despite
the setback, Gravity Probe B’s decades of development led to
groundbreaking technologies to control environmental disturbances on
spacecraft, such as aerodynamic drag, magnetic fields and thermal
variations. The mission’s star tracker and gyroscopes were the most
precise ever designed and produced.
Played a role in developing GPS
Innovations
enabled by GP-B have been used in the Global Positioning System, such
as carrier-phase differential GPS, with its precision positioning that
can allow an airplane to land unaided. Additional GP-B technologies
were applied to NASA’s Cosmic Background Explorer mission, which
determined the universe’s background radiation. That measurement is the
underpinning of the “big bang theory” and led to the Nobel Prize for
NASA’s John Mather.
“The
mission results will have a long-term impact on the work of theoretical
physicists for years to come,” said Bill Danchi, senior astrophysicist
and program scientist at NASA Headquarters in Washington. “Every future
challenge to Einstein’s theories of general relativity will have to seek
more precise measurements than the remarkable work GP-B accomplished.”
Over
the course of its mission, GP-B advanced the frontiers of knowledge and
provided a practical training ground for 100 doctoral students and 15
master’s degree candidates at universities across the United States.
Over 350 undergraduates and more than four dozen high school students
also worked on the project, alongside leading scientists and aerospace
engineers from industry and government.
Sally
Ride, the first American female astronaut in space, worked on GP-B
while studying at Stanford. Another was Nobel Laureate Eric Cornell,
who also studied at Stanford.
NASA’s
Marshall Space Flight Center in Huntsville, Ala., managed the Gravity
Probe-B program for the agency. Lockheed Martin Corporation of
Huntsville designed, integrated and tested the space vehicle and some of
its major payload components.
