Einstein's
general theory of relativity predicts that accelerating masses should
radiate gravity waves in a roughly similar way that accelerating
electrical charges radiate electromagnetic (light) waves. One notable
difference is that the gravitational force is intrinsically about
trillion trillion trillion times weaker than the electromagnetic force,
and so gravity waves are phenomenally weak. In fact, none has ever been
measured in the laboratory, although their presence has been reliably
inferred from the decaying orbital energy of binary stars as they
radiate these waves into space. Astronomers expect that new generations
of gravity wave detectors being built for space will be able not only
to test relativity in new regimes, but also measure many important
astrophysical phenomenon that are otherwise mysterious, from the motion,
growth, and evolution of black holes to binary star evolution, and even
details of the early universe.
The primary space gravity-wave mission planned is called eLISA
("evolved Laser Interferometer Space Antenna"). It is not scheduled for
launch for another decade or more, but a proof-of-concept mission is
scheduled for launch in 2015. One of the many issues facing the new
gravity-wave instruments is their accurate testing and calibration.
White-dwarf binary stars are expected to play this role. When a star
like our Sun gets to be old, in another seven billion years or so, it
will no longer be able to sustain burning its nuclear fuel. With only
about half of its mass remaining it will shrink to a fraction of its
radius and become a white dwarf star. White dwarfs are common, either
isolated or in a multi-star system of some kind, the most famous one
being the companion to the brightest star in the sky, Sirius.
Scientists estimate that there are about one hundred million
detectable white dwarf stars in binary systems in our galaxy. As they
orbit around each other they should emit considerable gravitational
radiation. Today, however, only eight confirmed candidates are known.
CfA astronomers Warren Brown and Scott Kenyon and four colleagues used
the MMT and Gemini telescopes to identify a new member of this select
group, a pair of white dwarf stars that orbit each other in only about
20 minutes (recall that the Earth’s orbital period is one year!), making
it the second fastest known system. A third star appears to be near
them and may be physically related; further observations are needed to
resolve this possibility, but if it is part of the system, then the
predicted gravity waves from the new triple source will make it one of
the strongest prospective emitters of gravity waves.
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