THE European Space Agency has an ambitious plan. 鈥淲e are talking about a
scientific instrument a thousand times bigger than the Earth, located as far
away as Venus, making measurements smaller than an atom,鈥� says Peter Bender, a
physicist at the Joint Institute for Laboratory Astrophysics in Boulder,
Colorado.
The project is called LISA鈥攍aser interferometer space antenna鈥攁nd
Bender is one of an international team of scientists who are backing the idea.
The project will be one of the most challenging and important experiments in the
history of space science 鈥攁nd the team hopes to build and launch it early
next century.
Once in space, LISA will spend two years listening for the distant rumblings
of black holes as they smash into one another and swallow stars in the far
reaches of the Universe. Black holes are so massive that when they collide, the
Universe itself begins to shake and rumble. These rumbles, known as
gravitational waves, propagate through the Universe like waves on a pond,
stretching and squeezing the fabric of space itself, and distorting any objects
in their path. Gravitational waves are predicted by Einstein鈥檚 general theory of
relativity, one of the cornerstone theories of modern physics, but they have
never been directly observed. Failure to spot them could strike a devastating
blow to the theory.
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But theoretical physicists and astronomers say they already have indirect
evidence that the waves exist and are confident of spotting them for real. These
scientists believe that LISA will reveal structures and events never seen
before. In short, they say that project will herald a new era for fundamental
physics and astronomy.
The instrument will consist of three pairs of spacecraft with each pair
sitting at the corner of a huge triangle in space. Because the distortions
produced by gravitational waves depend on the size of the detector, a bigger
instrument is more sensitive. With sides 5 million kilometres long, this
triangle will be so large that if the Earth sat at its centre, the Moon鈥檚 orbit
would not even come close to touching the sides. Were LISA any larger, however,
shorter gravitational waves would begin to slip through the array unnoticed.
鈥淎ll things considered, an arm length of five million kilometres is just about
ideal,鈥� says Jim Hough, who is leading the work to design LISA鈥檚 optical system
at Glasgow University.
Squeeze the triangle
The principle behind the instrument is disarmingly simple. As a gravitational
wave passes across the array it should distort the triangle, squeezing one side
while stretching another. All LISA has to do is measure the change. However, the
distortions will be so small that the length of a 5-million kilometre arm will
alter by less than the diameter of an atom. Distance measurements on this scale
can be made using lasers. But because the measurements are so sensitive, the
tiniest errors would create noise that overwhelms the signals the team will be
looking for.
So the big challenge facing the design team is to identify and minimise every
source of error so that the total noise is many times smaller than the signals
they hope to measure. And it is not a trivial task.
The entire experiment depends on a technique called laser interferometry
which can measure changes in distances with immense accuracy. The technique is
identical to the one that will be used in Earth-based gravitational wave
detectors that are now being built (see 鈥淕ravity鈥檚 Secret Signals鈥�, New
杏吧原创, 26 November 1994). These interferometers have two perpendicular
arms 4 kilometres long and work by splitting light from a laser into two beams
which travel down the arms and reflect off mirrors at the end.
Armed and adrift
On their return, the beams interfere to produce a pattern of light and dark
fringes. As the length of the arms change in response to gravity waves, the
pattern varies. By analysing these variations, researchers can work out exactly
what changes have occurred, and decipher the physical nature of the sources of
the gravitational waves.
LISA is a little more complicated. The triangular array has three arms, 5
million kilometres long, with 60掳 angles between them. Each corner of the
array acts both as the hub of an independent V-shaped interferometer and also as
a mirror to reflect light back to the other corners. Effectively, LISA consists
of three interferometers and because of this it can continue to operate even if
two spacecraft fail (provided they are not at the same corner). This redundancy
is an important factor in the design of a space experiment, explains the team
leader, Karsten Danzmann, a physicist at the University of Hannover and the
Max-Planck Institute for Quantum Optics in Garching, Germany. 鈥淭here is no way
to go out there with a screwdriver and fix things if they break down,鈥� he
says.
The obvious way to design a triangular interferometer is with three identical
spacecraft, one at each corner. However, each spacecraft must then be able to
aim laser beams at the two other corners simultaneously which is particularly
difficult when they drift around due to orbital motion. In fact, it is actually
easier to have two independent closely-spaced spacecraft at each corner and the
current design consists of six spacecraft arranged in three pairs. The
spacecraft at each corner are separated by about 200 kilometres and linked by an
optical beam.
All six spacecraft will be identical. Each one will be equipped with a laser
for generating an infrared beam and a telescope for focusing the incoming and
outgoing light. At the heart of each spacecraft is a reference mirror and an
optical system for performing the interferometry. Electric thrusters are used to
position the spacecraft relative to its internal cube. Each spacecraft has a
diameter of about 3 metres, and a mass of about 300 kilograms.
Building and launching six spacecraft is expensive. Preliminary ESA estimates
suggest a total mission price tag upwards of 700 million Ecus (拢850
million). In the current climate of shrinking science budgets, the LISA team is
eager to explore ways of reducing the mission cost. Last month some 100
colleagues from Europe and America, who have been working on the design since
1993, met at the Rutherford Appleton Laboratory in Oxfordshire to compare notes
and discuss ways to reduce the cost. The options range from simply reducing the
size of each spacecraft, to using a single, more complex spacecraft at each
corner. The implications of these ideas will be studied at length in the coming
year. Another option is to share the cost with another space agency.
Meanwhile, the LISA team must deal with other practical problems. A crucial
requirement for interferometry is that the beams in each arm originate from a
common source. In the ground-based interferometers this is simple to achieve
because the light from a single laser can easily be split into two identical
beams. And because the arms are short, conventional mirrors can be used to
reflect the beams back. But by the time a beam has travelled the long distance
along one LISA arm, it is too weak to be reflected back. Instead, the arrival of
each beam triggers the onboard laser which boosts the signal.
Each LISA spacecraft will be able to synchronise beams using a technique
known as phase-matched transponding. This involves using the incoming light from
a distant laser as a pacemaker for the onboard laser. When synchronised, the six
lasers produce beams that are identical in frequency and phase, as if they had
all been produced by the same laser. And, in effect, each spacecraft acts like
an amplifying mirror instead of a simple reflector.
Large, powerful lasers capable of generating high-intensity beams might seem
ideal for LISA but they would generate more waste heat than can be radiated
easily into space. To prevent the delicate optical systems from overheating,
each spacecraft will thus make do with a small laser which generates infrared
light with a power output of just 1 watt. The light will spread out during the
5-million-kilometre journey to the distant spacecraft, and only 30 picowatts, a
few hundred billionths, of the original 1 watt will arrive at its destination.
Nonetheless, the weak incoming beams will be easy to pick up, says Bernard
Schutz, the team鈥檚 theorist from the University of Wales and the Albert Einstein
Institute in Potsdam, Germany. If the lasers produced visible light, the
incoming laser beams would be clearly visible to the naked eye, he adds.
Golden cube
The light received by each spacecraft is focused by a telescope onto its
mirror. Each mirror is actually a small, highly polished solid cube made from an
alloy of platinum and gold which renders it immune to magnetic disturbances.
Although only 4 centimetres square, these mirrors will weigh just over a
kilogram. The spacecraft is designed so that each mirror will float in the
vacuum of space, whilst surrounded by a protective titanium box fitted with
quartz windows to admit the infrared beams. The mirrors are important because
they define where each arm begins and ends. The distance between them is the
crucial quantity that the experiment must measure. Isolating the mirrors from
forces and vibrations that could ruin the results is one of the biggest
challenges facing the team.
While orbiting the Sun, the spacecraft will experience temperature
variations. Because the titanium box and the optical components that carry out
the measurements are attached to the spacecraft鈥檚 structure, any thermal
expansion or contraction must be minimised. Consequently, most of the structure
will be built from carbon fibre. The frame that holds the box and optical
components will be constructed from fused silica (a type of quartz), and will be
protected from direct sunlight by layers of thermal shielding. Should any heat
seep through, both quartz and carbon fibre have extremely low coefficients of
thermal expansion. Given a change in temperature of one degree, a metre rod made
of either material would lengthen by only a few hundred nanometres.
Another force arises because spacecraft are continually bombarded by cosmic
rays, mainly protons, which cause them to become positively charged. This is a
well-known effect and is normally insignificant. But if a cube becomes charged,
the electrostatic forces between its surface and the sides of the box could ruin
the experiment. The electrostatic force created by the build up of even a
million charges is minute鈥攅quivalent to the gravitational attraction
between two people standing a few metres apart鈥� but even this is too
great. To neutralise this charge, the cube will be bathed in electrons produced
by an ultraviolet light shining on a nearby electrode.
Even the force on the spacecraft due to photons from the Sun must be taken
into account. Although the momentum associated with each solar photon is tiny,
the combined effect of large numbers of them is like a bombardment of peas and
can be significant. For each LISA spacecraft it amounts to an average pressure
of about 10-10 atmospheres which fluctuates by up to 1 percent due to internal
changes in the Sun.
Left to itself, this pressure would cause the spacecraft to drift. But
because the cube is shielded from the photons, it is unaffected by this force
and eventually the spacecraft would collide with the cube. Even before this
happens, small amounts of drift relative to the cube would ruin the results
because the gravitational attraction between different parts of the spacecraft
creates a nonuniform gravitational field. This is harmless as long as the cube
remains stationary relative to the field. However, any movement sets up forces
big enough to swamp the data.
To prevent this, the position of the cube inside the box must be constantly
monitored by measuring the electrical capacitance between the cube and
electrodes in its titanium housing. These measurements are fed to a control
system which continuously instructs the thrusters to centre the spacecraft on
the cube. The thrusters can only control the relative position of the cube and
the spacecraft to within about a nanometre but this is much larger than the
sub-atomic distances that the interferometer must measure. However, by arranging
the optical path so that the light reflects from both opposing faces on each
cube, small changes in position cancel each other out.
Another reason for keeping the spacecraft stable is to maintain the pointing
accuracy. Each spacecraft is fitted with a telescope that focuses the incoming
and outgoing beams. This ensures that each outgoing beam spreads by only 0.0001
degrees. Nevertheless, after a journey of 5 million kilometres the beam is 20
kilometres wide, so the task of hitting the receiving telescope is not too
difficult. But this is not the whole story.
Right attitude
If the telescope had no faults, the light would propagate outwards as a
perfect spherical wavefront. But because of unavoidable imperfections in the
optics, the wavefront becomes distorted. The receiving spacecraft must select a
small uniform portion of the distorted incoming beam, and remain locked on to
this small portion. Deviations in spacecraft attitude relative to this uniform
portion will cause erroneous signals.
Locking is achieved by measuring the attitude of the spacecraft relative to
the beam with a photodetector. 鈥淚t works like the telescopic sight on a rifle,
but with extremely high sensitivity to angular motion鈥� explains Hough. When it
senses a tiny change in attitude, the thrusters push the spacecraft gently back
into position. This is the same general principle used on conventional
spacecraft attitude control systems. However, LISA鈥檚 requirements are even more
demanding and the system must control the spacecraft鈥檚 attitude to within a
millionth of a degree.
To meet these unique demands, ESA is sponsoring the development of a new type
of electric thruster. These work by accelerating caesium ions in an electric
field creating a force in the opposite direction, a system known as field effect
electric propulsion (FEEP). Over the duration of the two-year mission each
spacecraft will use up only a few grams of caesium fuel. The force is controlled
by varying the voltage in the electric field and therefore the speed of the
ejected ions and can be as small as 0.1 micronewtons, thousands of times as weak
as the pressure of a human breath on a window pane.
The trick is to be able to change this force smoothly. The ions are produced
by exposing liquid caesium to a powerful electric field which effectively sucks
them up from the surface. But when the voltage changes, the number of ions
emitted does not vary smoothly. In fact, the number of ions emitted depends on a
large number of factors such as tiny but inevitable variations in the shape of
the thruster. Because of this, each thruster must be calibrated
separately鈥攁 difficult and time consuming task.
If the team can meet all of these technical challenges, the scientific
rewards will be huge because LISA will spot gravitational waves that cannot be
seen on Earth. The Earth-based detectors are designed to receive the signals
produced by spinning neutron stars, supernovae and the coalescence of compact
binary stars which have kilohertz frequencies.
But the waves from the most exciting events such as the collisions between
huge black holes have much lower frequencies. Earth-based detectors will never
be able to spot these signals because they are overwhelmed by tiny, unavoidable
disturbances on the ground. Even the gravitational field associated with the
mass of a passing car is enough to drown out these low-frequency waves.
In space, trailing the Earth by 50 million kilometres, LISA will be free from
these problems allowing it to tune in to the signals from black holes as they
roam the cosmos swallowing stars. LISA might even spot exotic objects that
theoretical physicists believe exist but have never seen. These objects include
naked singularities, points in the Universe where the laws of physics break down
entirely, fault lines in the structure of space called cosmic strings, and even
entirely new forms of matter.
New-age astronomy
None of these objects emit much electromagnetic radiation because their huge
gravitational fields prevent most light from escaping. 鈥淪ince almost no light
comes out, gravitational waves offer the only view in,鈥� explains Kip Thorne, a
theoretical physicist at the Californian Institute of Technology in Los Angeles
and a leading expert on gravitational waves. Thorne confidently expects that
gravitational waves will reveal objects that have never been seen before and
that LISA will herald a new era of experimental physics and astronomy. 鈥淭his
really is the dark side of the Universe,鈥� he says. Only time will tell if he is
right.