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NASA's Gravity Probe B spacecraft has gathered all the data
physicists need to check a bizarre prediction of Einstein's
relativity.
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November 16, 2005: Is Earth in a vortex of space-time?
We'll
soon know the answer: A NASA/Stanford physics experiment called
Gravity Probe B (GP-B) recently finished a year of gathering
science data in Earth orbit. The results, which will take
another year to analyze, should reveal the shape of space-time
around Earth--and, possibly, the vortex.
 Time
and space, according to Einstein's theories of relativity,
are woven together, forming a four-dimensional fabric called
"space-time." The tremendous mass of Earth dimples
this fabric, much like a heavy person sitting in the middle
of a trampoline. Gravity, says Einstein, is simply the motion
of objects following the curvaceous lines of the dimple.
If
Earth were stationary, that would be the end of the story.
But Earth is not stationary. Our planet spins, and
the spin should twist the dimple, slightly, pulling it around
into a 4-dimensional swirl. This is what GP-B went to space
to check
Above:
An artist's concept of twisted space-time around Earth. [More]
The
idea behind the experiment is simple:
Put
a spinning gyroscope into orbit around the Earth, with the
spin axis pointed toward some distant star as a fixed reference
point. Free from external forces, the gyroscope's axis should
continue pointing at the star--forever. But if space is twisted,
the direction of the gyroscope's axis should drift over time.
By noting this change in direction relative to the star, the
twists of space-time could be measured.
In
practice, the experiment is tremendously difficult.
The
four gyroscopes in GP-B are the most perfect spheres ever
made by humans. These ping pong-sized balls of fused quartz
and silicon are 1.5 inches across and never vary from a perfect
sphere by more than 40 atomic layers. If the gyroscopes weren't
so spherical, their spin axes would wobble even without the
effects of relativity.
According
to calculations, the twisted space-time around Earth should
cause the axes of the gyros to drift merely 0.041 arcseconds
over a year. An arcsecond is 1/3600th of a degree. To measure
this angle reasonably well, GP-B needed a fantastic precision
of 0.0005 arcseconds. It's like measuring the thickness of
a sheet of paper held edge-on 100 miles away.
GP-B
researchers invented whole new
technologies to make this possible. They developed a "drag
free" satellite that could brush against the outer layers
of Earth's atmosphere without disturbing the gyros. They figured
out how to keep Earth's penetrating magnetic field out of
the spacecraft. And they concocted a device to measure the
spin of a gyro--without touching the gyro.
Pulling
off the experiment was an exceptional challenge. A lot of
time and money was on the line, but the GP-B scientists appear
to have done it.
 Right:
One of the spherical gyroscopes of Gravity Probe B. [More]
"There
were not any major surprises" in the experiment's performance,
says physics professor Francis Everitt, the Principal Investigator
for GP-B at Stanford University. Now that data-taking is complete,
he says the mood among the GP-B scientists is "a lot
of enthusiasm, and a realization also that a lot of grinding
hard work is ahead of us."
A
careful, thorough analysis of the data is underway. The scientists
will do it in three stages, Everitt explains. First, they
will look at the data from each day of the year-long experiment,
checking for irregularities. Next they'll break the data into
roughly month-long chunks, and finally they'll look at the
whole year. By doing it this way, the scientists should be
able to find any problems that a more simple analysis might
miss.
Eventually
scientists around the world will scrutinize the data. Says
Everitt, "we want our sternest critics to be us."
The
stakes are high. If they detect the vortex, precisely as expected,
it simply means that Einstein was right, again. But what if
they don't? There might be a flaw in Einstein's theory, a
tiny discrepancy that heralds a revolution in physics.
First,
though, there are a lot of data to analyze. Stay tuned.
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Authors: Patrick
L. Barry and
Dr. Tony Phillips | Editor:
Dr. Tony Phillips | Credit: Science@NASA
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