FEATURE ARTICLE
The Sounds of Spacetime
In the biggest events in the universe, massive black holes collide with a chirp and a ring. Physicists are finding ways to listen in
Craig Hogan
Picometers over Gigameters
When will LISA fly; when can we don our earbuds and listen to what's
happening out there? The instrument is challenging to build, but a
team of scientists and engineers from the United States and Europe
think they can do it.
The basic LISA concept is simple. The heart of the system, a
gold/platinum cube, floats freely within each spacecraft, not
touching anything. The cube is protected from all forces except
gravity; the spacecraft very gently senses its position and
maneuvers with tiny thrusters to avoid running into it. Laser light
reflects off the cubes and is sent by telescopes, over the 5 million
kilometers between the multiple LISA spacecraft, to measure the tiny
changes in distance between the cubes caused by gravitational waves.
The measured changes in distance are given by the fractional stretch
in spacetime, 10-23 times smaller than the distance
between them, or around 0.05 picometers. That distance is much
smaller than an atom—it is almost as small as the
nucleus of an atom.
It seems incredible to contemplate building an instrument that will
measure distances far larger than the distance to the Moon, to an
accuracy far smaller than a single atom. Among the many technical
challenges in making this work, a major one is to create an
environment for the cubes that is free of all but gravitational
forces. The spacecraft surrounding the mass must somehow sense its
position, without disturbing it, and follow it around as it follows
the wiggles of spacetime alone. The most sensitive accelerometers on
the planet—torsion balances that have also been used to search
for tiny forces from extra dimensions and new shapes to
gravity—are helping to find ways of minimizing forces.
Of course, one reason LISA goes into space is because of all the
gravitational noise on Earth. To test technology to the exquisite
precision required, especially after undergoing the rigors of rocket
launch, we must send machines into space. A satellite called LISA
Pathfinder will launch in a few years, to check the most sensitive
LISA technologies that can't be tested on Earth. It is just one
satellite, so it won't be able to detect gravitational waves, but
the proof masses and sensors on board, and the tiny micro-newton
thrusters that allow the spacecraft to maneuver delicately, will
have the same design as LISA. Engineering prototypes of these
systems already exist. As far as we know, no fundamental technology
hurdles exist to building LISA.
The actual launch of LISA itself is still many years away and will
take substantial and sustained commitments from the science and
engineering communities and from the agencies and taxpayers that
fund them from both sides of the Atlantic. At over a billion
dollars, it is a major undertaking, although it is not unprecedented
for an important science project: For example, this budget is still
much smaller than that of the largest particle accelerators, such as
the new Large Hadron Collider at the European particle-physics
laboratory CERN, or space telescopes, such as the Hubble Space
Telescope. It is unusual for the first step in such a new area to be
such a big one, but then it's also unusual for a science project to
probe the universe in such an entirely new way.
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