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By measuring the shape of space with exquisite
precision, NASA's Gravity Probe B spacecraft aims to confirm Einstein's
theory of relativity ... or provide the first evidence against it
by Patrick L Barry and Dr Tony
Phillips
This year marks the
100th anniversary of a revolution in our notions of space and time.
Before 1905, when Albert
Einstein published his theory of special relativity, most people
believed that space and time were as Sir Isaac Newton described
them back in the 17th century: Space was the fixed, unchanging "stage"
upon which the great cosmic drama unfolded, and time was the mysterious,
universal "clock in the sky."
Even today, people
commonly assume that this intuitive sense of space and time is correct.
It's not.
Einstein's 1905 paper,
along with another one he published in 1915, painted an entirely
different and mind-bending picture. Space itself is constantly being
warped and curved by the matter and energy moving within it, and
time flows at different rates for different observers. Numerous
real-world experiments over the last 100 years indicate that, amazingly,
Einstein was right.
But scientists today
have reason to think that even Einstein's theory isn't the whole
story; another revolution seems inevitable.
The reason for doubt
is that Einstein's theory is incompatible with quantum mechanics,
another pillar of modern physics that describes the odd world of
subatomic particles. When the theories are used together, sometimes,
their combined equations produce nonsense. This leads scientists
to believe that current theories will eventually be replaced by
a single, elegant theory that explains all physical phenomena from
the subatomic to the cosmic, the so-called "Theory of Everything."
When will the first
shots of this physics revolution ring out? Perhaps when Einstein,
like Newton before him, is proven wrong - or at least not quite
right.
To hunt for flaws in
Einstein's theories, scientists are crafting experiments that can
measure the predictions of relativity with ever-greater precision.
One such experiment is NASA's Gravity Probe B (GP-B).
According to Einstein,
Earth makes a dimple in the spacetime around it--something like
a bowling ball sitting on a sheet of Spandex. Because Earth spins,
this "dimple" is twisted into a shallow vortex. Gravity
Probe B is orbiting Earth, right now, in search of these distortions.
GP-B senses the distortion
of spacetime around our planet using gyroscopes. (There are four
of them onboard the spacecraft.) Francis Everitt, principal investigator
for GP-B and a professor at Stanford University, explains:
More
Gravity
Probe B orbiting Earth: an artist's impression
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"Gyroscopes moving
through curved spacetime will gradually change their direction of
spin (i.e. tilt) with respect to the stars. GP-B will measure
that tilting motion with extraordinary precision and from that measurement
we can calculate the structure of space near the Earth."
Everitt will give a
presentation about Gravity Probe B in April at the "Physics
for the Third Millennium: II" conference hosted by NASA's
Marshall Space Flight Center in Huntsville, Alabama. The conference
is part of the World Year of Physics 2005, a United Nations-endorsed
series of events to recognize the 100th anniversary of Einstein's
seminal work and to raise public awareness of big issues in modern
physics.
In addition to giving
a status update on GP-B (in short: so far, so good), Everitt plans
to explain how GP-B will measure gamma, an important physics variable
used by scientists in their quest to go beyond Einstein's relativity.
Roughly speaking, gamma corresponds to the curvature of three-dimensional
space.
If Einstein's theory
matched reality perfectly, gamma ought to be exactly equal to one.
Measuring a value for gamma that's even slightly different from
one would be the "first shot" that physicists have been
waiting for.
"Gamma is the
most sensitive way of measuring any possible deviation from Einstein,
because it is sensitive to [any kind of unknown field]," says
Thibault Damour, a professor at the Institut des Hautes Etudes Scientifiques,
France, and an expert in theories that could replace relativity.
In the GP-B experiment,
gamma contributes to the slight tilt of the gyroscopes' spin axes,
which are expected to drift about 6.6 arcseconds (0.00183 degrees)
during the year-long data-gathering phase of the mission. This drift
should allow scientists to measure gamma within about 0.01% of its
true value -- and perhaps as good as 0.001%, Everitt says.
If gamma turns out
to be slightly less than one, it would support the idea that a new
force field exists, akin to gravity but much weaker. Physicists
call it a "scalar field." This new field is a feature
of some candidate Theories of Everything, including string theory.
String theory is popular because of its elegance in explaining all
known physical phenomena, from the subatomic to the cosmic. The
problem is that string theory is very hard to test in the real world,
and no experimental evidence of the unique predictions of string
theory has yet been found.
Einstein,
still making headlines...
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"Finding that
gamma is slightly less than one would support the idea of a scalar
field, and thus could provide some of the first experimental support
for string theory," Thibault says.
If gamma turns out
to be slightly greater than one, however, it would be "back
to the drawing board" for theorists. No existing theories predict
that gamma should be larger than one, so physicists would have no
idea how to explain such a finding. "Let's just say that every
time I ask theorists what it would mean if gamma were larger than
one, they change the subject," laughs Everitt, himself an experimentalist.
GP-B might also find
that, within the experiment's limits of precision, gamma is equal
to one--just as Einstein predicted. What would that mean? Perhaps
the flaw, if it exists, is smaller than GP-B can sense. Or maybe
the revolution's first shots will ring out elsewhere. No one knows.
Gravity Probe B is
half-way through its one-year mission. One hundred years down, six
months to go. Stay tuned for answers.
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