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NASA\'s Gravity Probe B spacecraft left Earth on April 19^th^ in search
of a force of nature, long suspected but never before measured.
Scientists call it \"gravitomagnetism.\"

Gravitomagnetism is to gravity as magnetism is to electricity. \"Moving
electric charges produce a magnetic field,\" explains physicist Cliff
Will of Washington University (St. Louis). \"Likewise, moving masses
should produce a gravitomagnetic field.\"

Or so the theory goes.

The theory in question is Einstein\'s General Relativity\--the modern
theory of gravity that replaced Newton\'s in the early 20^th^ century.
Written out in full glory, the equations of General Relativity are
intensely complicated. Indeed, they have been solved in only a few
special cases. One of them is the case of weak gravity, like we
experience here on Earth.

In the \"weak field limit,\" Einstein\'s equations reduce to a form
remarkably like Maxwell\'s equations of electromagnetism. Terms appear
that are analogous to the electric field caused by charges and the
magnetic field produced by the flow of charge. The \"electric terms\"
correspond simply to the gravity that keeps our feet on the ground. The
\"magnetic terms\" are wholly unfamiliar; we don\'t sense them in
everyday life. \"But that doesn\'t mean they\'re not there,\" notes
Will.

The best place to measure gravitomagnetism is in Earth orbit. Just as a
spinning ball of electric charge produces a well-defined magnetic field,
a spinning mass such as Earth is expected to produce a well-defined
gravitomagnetic field. What effect does this gravitomagnetic field have?
\"It can make orbits of satellites precess,\" says Will, \"and it would
cause a gyroscope placed in Earth orbit to wobble.\"

Both effects are small and difficult to measure.

Researchers led by D. Ciufolini have tried to detect the gravitomagnetic
precession of satellite orbits. For their study, they used the Laser
Geodynamic Satellites, LAGEOS & LAGEOS II, two 60 cm diameter spheres
studded with mirrors. Precise laser ranging of the pair allows their
orbits to be monitored. They did find a small amount of precession
consistent with gravitomagnetism. But there\'s a problem: Earth\'s
equatorial bulge pulls on the satellites, too, and causes a precession
billions of times greater than gravitomagnetism. Did Ciufolini et al.
subtract that huge pull with enough precision to detect
gravitomagnetism? Many scientists are skeptical, says Will.

Gravity Probe B, designed and built by scientists and engineers from
Stanford University, the NASA Marshall Space Flight Center and
Lockheed-Martin, does the experiment differently. Gravity Probe B uses
gyroscopes. There are four of them onboard the spacecraft. Each
gyroscope is a sphere, 1.5 inches in diameter, suspended in vacuum and
spinning 10,000 times per minute. If Einstein\'s equations are correct
and gravitomagnetism is real, the spinning gyroscopes should wobble as
they orbit the earth. Their spin axes will shift, little by little,
until a year from now they point 43 milli-arcseconds away from where
they started. Gravity Probe B can measure this angle with a precision of
0.5 milli-arcseconds, or about 1%.

Any angle measured in milli-arcseconds is tiny. One arcsecond equals
1/3600^th^ of a degree. One *milli*-arcsecond is 1000 times smaller. The
0.5 milli-arcsecond precision expected for Gravity Probe B corresponds
to the thickness of a sheet of paper held edge-on 100 miles away.

The spherical gyroscopes are the roundest objects ever made on Earth.
Engineers at the NASA Marshall Space Flight Center polished them to
within 0.01 microns (less than 40 atom-widths) of perfect sphericity.
Irregularities must be eliminated; otherwise the gyroscopes will wobble
on their own without any help from gravitomagnetism.

Sensing wobbles as small as a milli-arcsecond is a great challenge.
Engineers working on Gravity Probe B had to invent whole new
technologies to make it possible. \"In the course of its design work on
GP-B, the team has made brilliant and original contributions to basic
physics and technology,\" wrote a panel of reviewers, including Cliff
Will, from the National Academy of Sciences in 1998. \"Its members were
among the first to measure the London moment of a spinning
superconductor, the first to exploit the superconducting bag method for
excluding magnetic flux, and the first to use a \'porous plug\' for
confining superfluid helium without pressure buildup. They invented and
proved the concept of a drag-free satellite, and most recently some
members of the group have pioneered differential use of the Global
Positioning System (GPS) to create a highly reliable and precise
aircraft landing system.\"

Not bad for basic research.

Physicists are both anxious and excited by Gravity Probe B. They\'re
anxious because gravitomagnetism might not be there. Einstein\'s theory
could be wrong (a possibility held unlikely by most), and that would
spark a revolution in physics. They\'re excited for the same reason.
Everyone wants to be on hand for the next great advance in science.

Near Earth, gravitomagnetism is weak. That\'s why the Gravity Probe B
gyroscopes wobble only 43 mas. But gravitomagnetism could be powerful in
other parts of the Universe\--for instance, \"near a spinning black hole
or a neutron star,\" says Will. A typical neutron star packs more mass
than the Sun into a ball only 10km wide, and it spins a hundred thousand
times faster than Earth. The gravitomagnetic field there could be very
strong.

Astronomers might have already observed the effects of gravitomagnetism.
Some black holes and neutron stars shoot bright jets of matter into
space at nearly light speed. These jets come in pairs, oppositely
directed, as if they emerge from the poles of a rotating object.
Theorists think the jets could be powered and collimated by
gravitomagnetism.

In addition, black holes are surrounded by disks of infalling matter
called \"accretion disks,\" so hot they glow in the x-ray region of the
electromagnetic spectrum. There\'s mounting evidence, gathered by X-ray
telescopes such as NASA\'s Chandra X-ray Observatory, that these disks
wobble, much like the gyroscopes on Gravity Probe B are expected to do.
Gravitomagnetism again? Perhaps.

Here in our solar system gravitomagnetism is, at best, feeble. This
raises the question, what do we do with gravitomagnetism once we\'ve
found it? The same question was posed, many times, in the 19^th^ century
when Maxwell, Faraday and others were exploring electromagnetism. What
use could it be?

Today we\'re surrounded by the benefits of their research. Light bulbs.
Computers. Washing machines. The Internet. The list goes on and on. What
will gravitomagnetism be good for... ? Time will tell.

CAPTION: Einstein\'s General Relativity explains gravity and motion by
uniting space and time into a 4-dimensional, elastic fabric called
space-time. A stationary mass causes a dimple in the fabric, like a
heavy person sitting at the center of a trampoline. A spinning mass
causes the fabric to twist like a vortex. The \"twist\" is
gravitomagnetism, also known as \"frame dragging.\"

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Cliff Will: 011-33-1-4432-8130

I don\'t think Einstein ever thought about it in that way; I don\'t know
who the first person to think about it in those terms. The first people
who exploited the idea and talked about it a lot Kip Thorne abd Vladamir
V , paper in the 1970\'s. They wrote out the equations in those forms.
Even before GR, people tried to think about ways of making Newtonian
gravity compatioble with Lorentz invariance. If you want to make gravity
compatible with that, you need a magnetic term.

Right hand rule, there\'s a curl; there\'s a left hand rule, because of
the difference in sign. There\'s a basic sign between the phenama;
There\'s also a factor 4 difference because gravity is a tensor.

V/c is the ratio; take the velocity of the equator going around the
Earth, that in wires the electrons are moving at the speed of light.
There are these models that attempt to account for jets of matter from
quasars, to mg effect nears a spinning black hole; how controversial are
those idea? Blanford; everyone thinks; gravitomagnetic effects near
spinning black holes; another place: hints: accretion disks around bh
and pulsars; look at periodicity of signals we see the effects of
precession produced by the dragging of space time.

x-ray observations are still not good enough;

This is a question that\'s been asked for a long time; I\'ve been on two
review panels; recommend go forward or killed; that\'s been a part of
the controversy; first conceived in the early 1960s; there\'s was lot of
uncertainty about GR; everyone\'s favorite theory! But tests were weak;
we\'ve come along way in 40 years; tests in the solar system and the
binary pulsar; no one expects to find the deviation;

On the other hand...

The basic parameters that govern gravity have neeb so pinned down with
such precision; yet all those experiments involve other sorts of tests.
Even though I\'m a theorist, I spend time talking to experimentalists;
\"Has this effect been measured?\" Maybe we\'ll prove all the theorists
wronf

If it disagrees, it will be suchg a catastrophe, it will overthrow
things; It would disprove every metric theory of gravity ever devised.

It\'s such a weak phenomenon it won\'t be relevant, gravito electric
effects are important to GPS. The effects of gravitomagnetism are too
weak.

These kind of phenomena will play a role in GW detection: LIGO: LISA:
end spiral of two compact object: merge into one object: binary neutron
stars or black holes: high-speed orbits: if one or both are spinning, GM
effects will cause the orbital plane of the companion to wobble around
the central force: this will have an observable effect on the Gwform:
this is becoming a amajor issue:

Geodetic precession has been seen in Joe Taylor\'s binary pulsar; in 100
years we\'ll no longer see the pulsars; We were lucky to see it. A
couple of hundred years that it was pointing.

How does the gravitomagnetic force compare to the gravitational force?

How does this ratio compare to electricity and magnetism?

What will it mean if we can find this force? Who knows?

A spinning ball of electrical charge produces a well-prescribed static
magnetic field, and correspondingly a spinning mass such as the Earth is
expected to produce a static gravitomagnetic field. What does a
gravitomagnetic field do? For one thing, it makes a gyroscope wobble. A
gyroscope in Earth orbit (600 km polar) will wobble 42 mas per year. The
spin of the Earth causes the gyroscope to wobble! What else? It affects
the orbits of satellites. Astrophysical examples: black holes, pulsars?

Even before Einstein, people thought about gravitomagnetism. Can you
tell me about that? What experiments have been tried?

LAGEOS a pair of satellites orbiting Earth analyzed for frame dragging;
claimed to have measured the frame dragging effect to a precision of
20%, that method is very unlikely that it get better than 5 to 10%
precision; the orbital plane of the satellites by the oblateness of the
Earth; it\'s enormous 100,000+ times bigger than the frame dragging
effect; GP-B will do 1%, that\'s what NASA is paying for.

Newton\'s theory discrepancy of Mercury\'s orbit; Leverier noted that
Mercury perihelion was precession, 140 ar per century, perturbing
effects of all the other planets; but the numvers didn\'t add up; this
was a huge crisis in the 19^th^ century; GR came along and explained it
right away;

Beyond framedragging: look for any kinds of deviations that could come
from string theory and branes, etc., these BETTER TESTS OF the PRINCIPLE
OF EQ: STEP, there\'s some idea of violations 10-17, lots of people
would be pushing for such experiments; short distance variuations from
Newtonian gravity; extra dimensions; the

Does graviton have a mass? Speed of gravitational waves; gravitational
wave regime;

Chair the NASA independent panel; science advisory committee not totally
disinterested; our charge is to advise the project on how to get the
best science from the experiment; we do advise them.