GRAVITATIONAL WAVES

Gravity Probe B measured exactly how much Earth’s gravity warped
the space-time surrounding it, ultimately vindicating Einstein’s
calculations.
LEFT: NASA. RIGHT: TED KINSMAN/SCIENCE SOURCE
IT’S ALL EQUIVALENT
Researchers see the equivalence principle, a central
tenet of general relativity, as a promising avenue of
attack that could steer them toward an ultimate theory
of everything. Simply put, the equivalence principle
holds that all bodies under the influence of the same
gravitational field experience the same acceleration,
regardless of their mass or composition.
One advantage of this strategy is that, so far, every
credible attempt at crafting a unified theory introduces
new forces that would cause ever-so-slight changes in
the way matter interacts with gravity. If these theories
are correct, and we look closely enough, we should see
“EP violations,” tiny departures from the equivalence
principle. In other words, a gold brick should fall
somewhat differently from one made of silver, and a
detailed analysis of those differences could provide
valuable hints for physicists trying to construct a
correct unified theory.
“We don’t know the level at which a violation [of the
equivalence principle] will show up, but we do believe
there should be one,” says Thibault Damour, a theorist
at IHES (Institut des Hautes Études Scientifiques) in
France. The reason, Damour adds, is that “all attempts
to unify Einstein’s theories with the other forces”
— an endeavor he considers essential to the field —
“lead to EP violations.”
Experiments conducted on Earth have shown
the principle to be valid to a precision of 1 part in
10 trillion. But a space-based experiment called STEP
The equivalence principle in action: Because the feather falls in an
airless tube, it experiences no wind resistance, meaning it and the
heavier apple fall at exactly the same rate in Earth’s gravity field.
(Satellite Test of the Equivalence Principle) could
pose a much sterner challenge, boosting the accuracy
of these measurements by a factor of 100,000. That
kind of precision could be enough to show physicists
just where Einstein’s theories start to miss the
mark — assuming they do.
THE NEXT STEP
STEP began in 1971 as a thesis project by thengraduate
student Paul Worden, with Stanford physicist
Francis Everitt serving on the thesis committee and
then as the project’s chief scientist soon afterward.
Everitt has devoted a half-century of his life to testing
general relativity and was the principal investigator
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