- Press Release
- Dec 1, 2022
CFA Scientists Help Test Einstein’s Theory on NASA’s Gravity Probe B
Cambridge, MA- When NASA’s Gravity Probe B (GP-B) satellite launches on
April 17th, it will begin a rigorous test of Einstein’s general theory of
relativity. The result will either support or challenge one of the
fundamental tenets of modern physics. This seminal test would not be
possible without a key contribution by scientists at the Harvard-Smithsonian
Center for Astrophysics (CfA). Their ground-based measurements will be
combined with those from the satellite to determine precisely how the
Earth’s mass and rotation affect the fabric of space and time.
“This NASA mission is the culmination of 45 years of work at Stanford
University. Here at the CfA, we are providing a key piece of astronomical
information that Stanford needs to complete its test of Einstein’s theory
with the full intended accuracy and reliability. The satellite measurements
have to be adjusted for at least one very significant astronomical effect.
We are using radio telescopes to measure the required adjustment,” says
Smithsonian radio astronomer Michael Ratner (CfA), who works with CfA
Director Irwin Shapiro on the project.
At the heart of the GP-B experiment are four gyroscopes, each holding a
precisely fabricated spinning mass the size of a ping-pong ball. They are
kept nearly free from disturbance, so that they provide an almost perfect
space-time reference system. By measuring, very precisely, tiny changes in
the direction of spin of those four gyroscopes, physicists will calculate
how space and time are warped by the presence of the Earth, and how the
Earth’s rotation drags space-time around with it. These effects, though
small for the Earth, have far-reaching implications for everything from the
nature of black holes to the structure and evolution of the universe.
“Our understanding of cosmology is based on the interpretation of
astrophysical data and assumes that general relativity is correct. If
general relativity were found to be substantially wrong, it would have a
profound effect on our description of the cosmos and its history. Even a
small discrepancy found in a local measurement, like the one to be made by
GP-B, could strongly affect our understanding of the universe,” says CfA
Associate Director Robert Reasenberg, who serves on the GP-B Science
Advisory Committee and has worked on tests of general relativity for three
According to general relativity, we live in a four-dimensional universe.
Space and time are interwoven and inseparable. The presence of any massive
object like the Earth will bend the space-time fabric, essentially warping
One prediction of the theory of general relativity is that starlight will be
bent by gravity. That effect was first measured in 1919 during a total solar
eclipse. Other predictions are not so easy to confirm.
One of the more unusual predictions of general relativity is the phenomenon
of frame dragging, in which the rotation of a massive object like the Earth
causes a twisting of the local space-time fabric. A visible result of this
effect is that the direction of the spin axis of each gyroscope on the
spacecraft turns very slowly toward the east.
Each spin axis also will appear to turn at a faster rate in the north-south
direction, as a result of the spacecraft’s polar orbit through the warped
space around the Earth. Gravity Probe B will measure both effects during its
approximately 1-1/2 year mission.
In order for GP-B to measure any “twist” or curvature of local space-time,
it must use gyroscopes that are nearly perfect, which will not wobble or
drift while spinning. Using ultrastable gyroscopes ensures that any angular
change to the gyroscope’s spin axis is due to relativistic effects.
Gravity Probe B carries a telescope that focuses on a guide star in order to
provide a reference point for measuring tiny deflections in the gyroscopes’
spin axes. The whole spacecraft is continually kept aligned to this star.
Yet the star shifts its apparent position as both it and the Sun
independently orbit the center of the Milky Way. As seen from the GP-B
spacecraft, the apparent position of the guide star also is affected by the
spacecraft’s orbit around the Earth and the Earth’s orbit around the Sun.
To compensate for those effects, Shapiro and his colleagues have monitored
the GP-B guide star for the past 7 years using a variety of radio
telescopes. That monitoring will continue through the lifetime of Gravity
Probe B. Only after all of the spacecraft data have been collected will the
calculations be made that will test Einstein’s theory.
“If the predictions of general relativity are confirmed, I personally would
feel a sense of satisfaction that not only Einstein’s historic theory of
gravity, but also the many years of work on the GP-B project, had all
succeeded spectacularly. Conversely, if the Gravity Probe B results are
inconsistent with Einstein’s theory, I would be excited about the prospect
of what might come next. It certainly would motivate a fresh look at the
foundations of physics,” says Ratner.
He adds, “No matter what the outcome, the Gravity Probe B program will stand
as a testament to the determination of scientists to subject even their
favorite theories to empirical tests. It is this process of testing that
gives science its ongoing validity, even while driving science unpredictably
NASA’s Marshall Space Flight Center, Huntsville, Ala., manages the GP-B
program. NASA’s prime contractor for the mission, Stanford University,
conceived the experiment and is responsible for the design and integration
of the science instrument, as well as for mission operations and data
analysis. Professor Francis Everitt (Stanford) is the GP-B Principal
Investigator. More information on Gravity Probe B is available online at
At CfA, Irwin Shapiro is the Principal Investigator of the Stanford
subcontract and heads the astronomical effort. CfA also is coordinating
ongoing observations by teams at York University (led by Norbert Bartel),
the University of Pittsburgh (led by George Gatewood), and at other
Headquartered in Cambridge, Mass., the Harvard-Smithsonian Center for
Astrophysics (CfA) is a joint collaboration between the Smithsonian
Astrophysical Observatory and the Harvard College Observatory. CfA
scientists, organized into six research divisions, study the origin,
evolution and ultimate fate of the universe.