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LISA 22/8/00 9:19 Am Page 2 LISA 22/8/00 9:19 am Page 2 r bulletin 103 — august 2000 LISA – Detecting and Observing Gravitational Waves R. Reinhard Space Science Department, ESA Directorate of Scientific Programmes, ESTEC, Noordwijk, The Netherlands In Newton’s theory of gravity, the gravitational electromagnetic waves, created by the interaction between two bodies is acceleration of electric charges, propagate in instantaneous, but according to Einstein’s the framework of space and time, gravitational theory of gravity this should be impossible waves, created by the acceleration of masses, because the speed of light represents the are waves of the spacetime fabric itself. Unlike limiting speed for all interactions. If a body charge, which exists in two polarities, mass changes its shape, the resulting change in the always comes with the same sign. This is why force field will make its way outward at the the lowest order asymmetry producing speed of light. In Einstein’s theory of gravity, electromagnetic radiation is the dipole moment massive bodies produce ‘indentations’ in the of the charge distribution, whereas for ‘fabric’ of spacetime and other bodies move in gravitational waves it is a change in the this curved spacetime taking the shortest path. quadrupole moment of the mass distribution. If a mass distribution moves in a spherically Hence those gravitational effects that are asymmetric way, then the spacetime indentations spherically symmetric will not give rise to travel outwards as ripples in spacetime called gravitational radiation. A perfectly symmetric ‘gravitational waves’. collapse of a supernova will produce no waves, while a non-spherical one will emit gravitational Gravitational waves are fundamentally different radiation. A binary system will always radiate. from the familiar electromagnetic waves. While Gravitational waves are a direct consequence The primary objective of the LISA (Laser Interferometer Space of Einstein’s theory of General Relativity (GR). If Antenna) mission is the detection and observation of gravitational Einstein’s theory is correct, gravitational waves waves from massive black holes and galactic binaries down to a must exist, but up to now they have not been gravitational wave strain sensitivity of 10-23 in the frequency range detected. However, there is strong indirect 10-4 – 10-1 Hz (Fig. 1). This low-frequency range is inaccessible to evidence for the existence of gravitational ground-based interferometers because of the unshieldable waves: the binary pulsar PSR 1913 +16 loses background of local gravitational noise. energy exactly at the rate predicted by GR by emitting gravitational radiation. Gravitational waves distort spacetime; in other words they change the distances between free macroscopic bodies. A gravitational wave passing through the Solar System creates a time-varying strain in space that periodically changes the distances between all bodies in the Solar System in a direction that is perpendicular to the direction of wave propagation. These could be the distances between spacecraft and the Earth, as in the case of Ulysses or Cassini (attempts were and will be made to measure these distance fluctuations) or the distances between shielded proof masses inside spacecraft that are Figure 1. The target sensitivity curve of LISA and the strengths of expected gravitational wave sources 36 LISA 22/8/00 9:19 am Page 3 lisa separated by a large distance, as in the case of between the spacecraft – the interferometer LISA. arm length – determines the frequency range in which LISA can make observations; it has been The main problem is that the relative length carefully chosen to allow for the observation of change due to the passage of a gravitational most of the interesting sources of gravitational wave is exceedingly small. For example, the radiation. The centre of the triangular formation periodic change in distance between two proof is in the ecliptic plane, 1 AU from the Sun and masses, separated by a sufficiently large 20° behind the Earth. The plane of the triangle distance, due to a typical white dwarf binary at is inclined at 60° with respect to the ecliptic. a distance of 50 pc, is only 10-10 m. This is not These particular heliocentric orbits for the three to say that gravitational waves are weak in the spacecraft have been chosen such that the sense that they carry little energy. On the triangular formation is maintained throughout contrary, a supernova in a not too distant the year, with the triangle appearing to rotate galaxy will drench every square metre here on about the centre of the formation once per year. Earth with kilowatts of gravitational radiation. The resulting length changes, though, are very While LISA is basically a giant Michelson small because spacetime is an extremely stiff interferometer in space, the actual implemen- elastic medium, so that it takes extremely large tation in space is very different from a laser energies to produce even minute distortions. interferometer on the ground and is much more reminiscent of the technique called spacecraft It is because of the extremely small distance tracking, but here realised with infrared laser changes that gravitational waves have not yet light instead of radio waves. The laser light been detected. However, with the LISA space going out from the centre spacecraft to the interferometer, orbiting the Sun at 1 AU, millions other corners is not directly reflected back of sources will be detected in one year of because very little light intensity would be left observation with a signal-to-noise ratio of 5 or over in that way. Instead, in complete analogy better. with a radio-frequency transponder scheme, the laser on the distant spacecraft is phase- The LISA mission consists of three identical locked to the incoming light, providing a return spacecraft located 5 x 106 km apart, forming beam with full intensity again. After being an equilateral triangle (Fig. 2). The distance transponded back from the far spacecraft to Figure 2. Artist’s concept of gravitational waves emitted by a binary system, with the three-spacecraft LISA configuration and the Earth- Moon system 37 LISA 22/8/00 9:19 am Page 4 r bulletin 103 — august 2000 the centre spacecraft, the light is superposed sensor) and a system of electrical thrusters. with the on-board laser light serving as a local Capacitive sensing is used to monitor the oscillator in a heterodyne detection. relative motion between each spacecraft and its test masses. These position signals are used Each spacecraft contains two optical in a feedback loop to command micro-Newton assemblies (Fig. 3). The two assemblies on one ion-emitting proportional thrusters to enable spacecraft each point towards an identical the spacecraft to follow its test masses assembly on each of the other two spacecraft. precisely and without introducing disturbances A 1 W infrared laser beam is transmitted to the in the bandwidth of interest. The same corresponding remote spacecraft via a 30-cm thrusters are used for precision attitude control aperture f/1 Cassegrain telescope. The same relative to the incoming optical wave fronts. telescope is used to focus the very weak beam (a few pW) coming from the distant spacecraft Each of the three LISA spacecraft has a launch and to direct the light to a sensitive photo- mass of about 460 kg (incl. margin). Ion drives detector, where it is superimposed with a are used for the transfer from the Earth orbit to fraction of the original local light. At the heart of the final position in interplanetary orbit. All three each assembly is a vacuum enclosure spacecraft can be launched by a single Delta-II containing a free-flying polished platinum-gold 7925H vehicle. cube, 4 cm in size, referred to as the ‘proof mass’, which serves as an optical reference Several large interferometers for the detection (‘mirror’) for the light beams. A passing of gravitational waves are currently under gravitational wave will change the length of the construction on the ground: two LIGO optical path between the proof masses of one interferometers in the USA (each 4 km), the arm of the interferometer relative to the other French-Italian collaborative VIRGO in Europe arm. These distance fluctuations are measured (3 km), GEO 600 in Germany (0.6 km), AIGO to sub-Ångstrom precision which, when 500 in Australia (0.5 km) and TAMA 300 combined with the large separation between in Japan (0.3 km) (interferometer baseline the spacecraft, allows LISA to detect lengths in parentheses). The ground-based gravitational-wave strains down to a level of interferometers and the LISA interferometer in order ∆ l/l =10-23 in one year of observation (l is space complement each other in an essential the baseline length of 5 x 106 km). way. Just as it is important to complement the optical and radio observations from the ground The spacecraft mainly serve to shield the proof with observations from space at submillimetre, masses from the adverse effects of the solar infrared, ultraviolet, X-ray and gamma-ray radiation pressure, and the spacecraft’s wavelengths, so too is it important to position does not directly enter into the complement the gravitational-wave observations measurement. It is nevertheless necessary to made by the ground-based interferometers in keep all spacecraft moderately accurately the high-frequency regime (10 to 103 Hz) with (10-8 m Hz-1/2 in the measurement band) observations in space in the low-frequency centred on their respective proof masses to regime (10-4 to 10-1 Hz). reduce spurious local noise forces. This is achieved by a ‘drag-free’ control system, Ground-based interferometers can observe the consisting of an accelerometer (or inertial bursts of gravitational radiation emitted by Figure 3.Left: Cut-away view of one of the three identical LISA spacecraft.
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