April 20, 2004: NASA's Gravity
Probe B spacecraft left Earth today in search of a force
of nature, long suspected but never proven: gravitomagnetism.
Gravitomagnetism is produced by stars and planets when
they spin. "It's similar in form to the magnetic
field produced by a spinning ball of charge," explains
physicist Clifford Will of Washington University (St.
Louis). Replace charge with mass, and magnetism becomes
gravitomagnetism.
We don't feel gravitomagnetism as we go about our everyday
lives on Earth, but according to Einstein's theory of
General Relativity it's real. When a planet (or a star
or a black hole ... or anything massive) spins it pulls
space and time around with it, an action known as "frame
dragging." The fabric of spacetime
twists like a vortex. Einstein tells us
that all gravitational forces correspond to a bending
of spacetime; the "twist" is gravitomagnetism.
What does gravitomagnetism do? "It can make the
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.
Gravity Probe B, developed by scientists at Stanford
University, NASA and Lockheed Martin, will do the experiment
using gyroscopes.
The spacecraft circles Earth in a polar orbit 400 miles
high. Onboard are four gyroscopes, each one a sphere,
1.5 inches in diameter, suspended in vacuum and spinning
ten thousand 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 42 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. Consider
this: One arcsecond equals 1/3600th of a degree. One
milli-arcsecond is 1000 times smaller than that. The
half milli-arcsecond precision expected for Gravity
Probe B corresponds to the thickness of a sheet of paper
held edge-on 100 miles away.
Sensing wobbles so small is a great challenge, and
scientists working on Gravity Probe B had to invent
whole new technologies to make it possible.
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 42 milli-arcseconds.
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 10 km 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.
Black hole
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 19th 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? Is it just "another milestone on the
path of our natural quest to understand nature?"
wonders Will. Or something unimaginably practical? Time
will tell.
