LISA is On! Gravitational Wave Detection is Going to Space

The discovery of gravitational waves by the LIGO experiment in 2015 sent ripples through the scientific community. Originally predicted by Einstein’s Theory of General Relativity, the confirmation of these waves (and two subsequent detections) solved a long-standing cosmological mystery. In addition to bending the fabric of space-time, it is now known that gravity can also create perturbations that can be detected billions of light-years away.

Seeking to capitalize on these discoveries and conduct new and exciting research into gravitational waves, the European Space Agency (ESA) recently green-lighted the Laser Interferometer Space Antenna (LISA) mission. Consisting of three satellites that will measure gravitational waves directly through laser interferometry, this mission will be the first space-based gravitational wave detector.

This decision was announced yesterday (Tuesday, June 20th) during a meeting of ESA’s Science Program Committee (SPC). It’s implementation is part of the ESA’s Cosmic Vision plan – the current cycle of the agency’s long-term planning for space science missions – which began in 2015 and will be running until 2025. It is also in keeping with the ESA’s desire to study the “invisible universe“, a policy that was adopted in 2013. 

To accomplish this, the three satellites that make up the LISA constellation will be deployed into orbit around Earth. Once there, they will assume a triangular formation – spaced 2.5 million km (1.55 million mi) apart – and follow Earth’s orbit around the Sun. Here, isolated from all external influences but Earth’s gravity, they will then connect to each other by laser and begin looking for minute perturbations in the fabric of space-time.

Much like how the LIGO experiment and other gravitational wave detectors work, the LISA mission will rely on laser interferometry. This process consists of a beam of electromagnetic energy (in this case, a laser) being split in two and then recombined to look for patterns of interference. In LISA’s case, two satellites play the role of reflectors while the remaining one is the both source of the lasers and the observer of the laser beam.

When a gravitational wave passes through the triangle established by the three satellites, the lengths of the two laser beams will vary due to the space-time distortions caused by the wave. By comparing the laser beam frequency in the return beam to the frequency of the sent beam, LISA will be able to measure the level of distortion.

These measurements will have to be extremely precise, since the distortions they are looking for affect the fabric of space-time on the most minuscule of levels – a few millionths of a millionth of a meter over a distance of a million kilometers. Luckily, the technology to detect these waves has already been tested by the LISA Pathfinder mission, which deployed in 2015 and will conclude its mission at the end of the month.

Artist’s concept of the LISA mission. Credit: AEI/Milde Marketing/Exozet

In the coming weeks and months, the ESA will be looking over the design of the LISA mission and completing a cost assessment. If all goes as planned, the mission will be proposed for “adoption” before construction begins and it is expected to be launched by 2034. In the same meeting, the ESA also adopted another important mission that will be searching for exoplanets in the coming years.

This mission is known as the PLAnetary Transits and Oscillations of stars, or PLATO, mission. Like Kepler, this mission will monitor stars within a large sections of the sky to look for small dips in their brightness, which are caused by planets passing between the star and the observer (i.e. the transit method). Originally selected in February of 2014, this mission is now moving from the blueprint phase into construction and will launch in 2026.

It’s an exciting time for the European Space Agency. In recent years, it has committed itself to multiple endeavors in the hope of maintaining Europe’s commitment to and continued presence in space. These include studying the “invisible universe”, mounting missions to the Moon and Mars, maintaining a commitment to the International Space Station, and even building a successor to the ISS on the Moon!

Further Reading: ESA

Astronomy Without A Telescope – Gravitational Waves

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Gravitational waves have some similar properties to light. They move at the same speed in a vacuum – and with a certain frequency and amplitude. Where they differ from light is that they are not scattered or absorbed by matter, in the way that light is.

Thus, it’s likely that primordial gravitational waves, that are speculated to have been produced by the Big Bang, are still out there waiting to be detected and analyzed.

Gravitational waves have been indirectly detected via observations of pulsar PSR 1913+16, a member of a binary system, the orbit of which decays at the rate of approximately three millimetres per orbit. The inspiraling of the binary (i.e. the decay of its orbit) can only be explained by an invisible loss of energy, which we presume to be the result of gravitational waves transporting energy away from the system.

Direct observation of gravitational waves currently escapes us – but seems at least feasible by monitoring the alignment of widely separated test masses. Such monitoring systems are currently in place on Earth, including LIGO, which has test masses separated by up to four kilometres – that separation distance being monitored by lasers designed to detect tiny changes in that distance, which might result from the passage of a gravitational wave initiated from a distant point in the universe.

The passing of a gravitational wave should stretch and contract the Earth. This is not because it strikes the Earth and imparts kinetic energy to it – like an ocean wave hitting land. Instead, the Earth – which sits within space-time – has its geometry altered, so that it continues to fit the momentarily stretched and then contracted space-time within which it sits, as a gravitational wave passes.

The Laser Interferometer Gravitational-Wave Observatory (LIGO) Hanford installation. When you are talking gravitational wave astronomy, big is good. Credit: Caltech.

Gravitational waves are thought to be unaffected by interaction with matter and they move at the speed of light in a vacuum, regardless of whether or not they themselves are in a vacuum. They do lose amplitude (wave height) over distance, but only through attenuation. This is similar to the way that a water wave, emanating from the point of impact of a pebble dropped into a pond, loses amplitude proportionally to the square of the radius of the growing circle that it forms.

Gravity waves may also decline in frequency (i.e. increase in wavelength) over very large distances, due to the expansion of the universe – in much the same way that the wavelength of light is red-shifted by the expansion of the universe.

Given all this, the exceedingly tiny effects that are expected of the gravitational waves that may routinely pass by Earth create a substantial challenge for detection and measurement – since these tiny space-time fluctuations must be distinguished from any background noise.

The noise background for LIGO includes seismic noise (i.e. intrinsic movements of the Earth), instrument noise (i.e. temperature changes that affect the alignment of the detection equipment) and a quantum-level noise, also known as Johnson-Nyquist noise – which arises from the quantum indeterminacy of photon positions.

Kip Thorne, one of the big names in gravity wave theory and research, has apparently ironed out that last and perhaps most troublesome effect through the application of quantum non-demolition principles – which enable the measurement of something without destroying it, or without collapsing its wave function.

Nonetheless, the need for invoking quantum non-demolition principles is some indication of the exceedingly faint nature of gravitational waves – which have a generally weak signal strength (i.e. small amplitude) and low frequency (i.e. long, in fact very long, wavelength).

Where visible light may be 390 nanometres and radio light may be 3 metres in wavelength – gravitational waves are more in the order of 300 kilometres for an average supernova blast, up to 300,000 kilometres for an inspiraling black hole binary and maybe up to 3 billion light years for the primordial echoes of the Big Bang.

So, there’s a fair way to go with all this at a technological level – although proponents (as proponents are want) say that we are on the verge of our first confirmed observation of a gravitational wave – or otherwise they reckon that we have already collected the data, but don’t fully know how to interpret them yet.

This is the current quest of citizen science users of [email protected] – the third most popular BOINC distributed computing project after [email protected] (spot an alien) and [email protected] (fold a protein).

This article follows a public lecture delivered by Kip Thorne at the Australian National University in July 2011 – where he discussed plans for LIGO Australia and also the animated simulations of black hole collisions described in the paper below – which may provide templates to interpret the waveforms that will be detected in the future by gravitational wave observatories.

Further reading: Owen et al (including Thorne, K.) Frame-Dragging Vortexes and Tidal Tendexes Attached to Colliding Black Holes: Visualizing the Curvature of Spacetime.