Feature Nov. 23, 2010
Tuning an 'Ear' to the Music of Gravitational Waves
The full version of this story with accompanying images is at:
http://www.jpl.nasa.gov/news/news.cfm?release=2010-394&cid=release_2010-394
A team of scientists and engineers at NASA's Jet Propulsion
Laboratory has brought the world one step closer to "hearing"
gravitational waves -- ripples in space and time predicted by Albert Einstein
in the early 20th century.
The research, performed in a lab at JPL in Pasadena, Calif.,
tested a system of lasers that would fly aboard the proposed space mission
called Laser Interferometer Space Antenna, or LISA. The mission's goal is to
detect the subtle, whisper-like signals of gravitational waves, which have yet
to be directly observed. This is no easy task, and many challenges lie ahead.
The new JPL tests hit one
significant milestone, demonstrating for the first time that noise, or random
fluctuations, in LISA's laser beams can be hushed enough to hear the sweet
sounds of the elusive waves.
"In order to detect
gravitational waves, we have to make extremely precise measurements," said
Bill Klipstein, a physicist at JPL. "Our lasers are much noisier than what
we want to measure, so we have to remove that noise carefully to get a clear
signal; itâs a little like listening for a feather to drop in the middle of a
heavy rainstorm." Klipstein is a co-author of a paper about the lab tests
that appeared in a recent issue of Physical Review Letters.
The JPL team is one of many groups working on LISA, a joint
European Space Agency and NASA mission proposal, which, if selected, would
launch in 2020 or later. In August of this year, LISA was given a high recommendation
by the 2010 U.S. National Research Council decadal report on astronomy and
astrophysics.
One of LISA's primary goals is to detect gravitational waves
directly. Studies of these cosmic waves began in earnest decades ago when, in
1974, researchers discovered a pair of orbiting dead stars -- a type called
pulsars -- that were spiraling closer and closer together due to an
unexplainable loss of energy. That energy was later shown to be in the form of
gravitational waves. This was the first indirect proof of the waves, and
ultimately earned the 1993 Nobel Prize in Physics.
LISA is expected to not only "hear" the waves, but
also learn more about their sources -- massive objects such as black holes and
dead stars, which sing the waves like melodies out to the universe as the
objects accelerate through space and time. The mission would be able to detect
gravitational waves from massive objects in our Milky Way galaxy as well as
distant galaxies, allowing scientists to tune into an entirely new language of
our universe.
The proposed mission would amount to a giant triangle of
three distinct spacecraft, each connected by laser beams. These spacecraft
would fly in formation around the sun, about 20 degrees behind Earth. Each one
would hold a cube made of platinum and gold that floats freely in space. As
gravitational waves pass by the spacecraft, they would cause the distance
between the cubes, or test masses, to change by almost imperceptible amounts --
but enough for LISA's extremely sensitive instruments to be able to detect
corresponding changes in the connecting laser beams.
"The gravitational waves will cause the âcorksâ to bob
around, but just by a tiny bit," said Glenn de Vine, a research scientist
and co-author of the recent study at JPL. "My friend once said it's sort
of like rubber duckies bouncing around in a bathtub."
The JPL team has spent the last six years working on aspects
of this LISA technology, including instruments called phase meters, which are
sophisticated laser beam detectors. The latest research accomplishes one of
their main goals -- to reduce the laser noise detected by the phase meters by
one billion times, or enough to detect the signal of gravitational waves.
The job is like trying to find a proton in a haystack.
Gravitational waves would change the distance between two spacecraft -- which
are flying at 5 million kilometers (3.1 million miles) apart -- by about a
picometer, which is about 100 million times smaller than the width of a human
hair. In other words, the spacecraft are 5,000,000,000 meters apart, and LISA
would detect changes in that distance on the order of .000000000005 meters!
At the heart of the LISA laser technology is a process known
as interferometry, which ultimately reveals if the distances traveled by the
laser beams of light, and thus the distance between the three spacecraft, have
changed due to gravitational waves. The process is like combining ocean waves
-- sometimes they pile up and grow bigger, and sometimes they cancel each other
out or diminish in size.
"We can't use a tape measure to get the distances
between these spacecraft," said de Vine, "So we use lasers. The
wavelengths of the lasers are like our tick marks on a tape measure."
On LISA, the laser light is detected by the phase meters and
then sent to the ground, where it is "interfered" via data processing
(the process is called time-delay interferometry for this reason -- there's a
delay before the interferometry technique is applied). If the interference
pattern between the laser beams is the same, then that means the spacecraft
haven't moved relative to each other. If the interference pattern changes, then
they did. If all other reasons for spacecraft movement have been eliminated,
then gravitational waves are the culprit.
That's the basic idea. In reality, there are a host of other
factors that make this process more complex. For one thing, the spacecraft
don't stay put. They naturally move around for reasons that have nothing to do
with gravitational waves. Another challenge is the laser beam noise. How do you
know if the spacecraft moved because of gravitational waves, or if noise in the
laser is just making it seem as if the spacecraft moved?
This is the question the JPL team recently took to their
laboratory, which mimics the LISA system. They introduced random, artificial
noise into their lasers and then, through a complicated set of data processing
actions, subtracted most of it back out. Their recent success demonstrated that
they could see changes in the distances between mock spacecraft on the order of
a picometer.
In essence, they hushed the roar of the laser beams, so that
LISA, if selected for construction, will be able to hear the universe softly
hum a tune of gravitational waves.
Other authors of the paper from JPL are Brent Ware; Kirk
McKenzie; Robert E. Spero and Daniel A. Shaddock, who has a joint post with JPL
and the Australian National University in Canberra.
LISA is a proposed joint NASA and European Space Agency
mission. The NASA portion of the mission is managed by NASA's Goddard Space
Flight Center, Greenbelt, Md. Some of the key instrumentation studies for the
mission are being performed at JPL. The U.S. mission scientist is Tom Prince at
the California Institute of Technology in Pasadena. JPL is managed by Caltech
for NASA.
Whitney Clavin 818-354-4673
Jet Propulsion Laboratory, Pasadena, Calif.
Whitney.clavin@xxxxxxxxxxxx
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