Hunting Ripples in the Fabric of Space-Time – The Trials and Tribulations of LISA

Artwork for "Hunting Ripples in the Fabric of Space-Time – The Trials and Tribulations of LISA" showing the LISA space array, with the caption: "Think Again!" Artwork: NaturPhilosophieMeet LISA!

Erm…No.  Not Mona Lisa!  (Rolls eyes.)  Think again!!  This is LISA – the Lisa Pathfinder satellite, the key element for a grand new project: a space-based gravitational observatory. 

Since their existence was confirmed at the Advanced LIGO facilities for the first time, there has been enormous interest around gravitational waves – the ripples in space-time generated in cataclysmic cosmic events, such as the merger of black holes and the explosion of giant stars.

Artwork for "Hunting Ripples in the Fabric of Space-Time – The Trials and Tribulations of LISA" showing a close-up portrait of Leonardo da Vinci's Mona Lisa with the caption "Erm... No." Artwork: NaturPhilosophieLast September 2015, LIGO registered a very subtle disturbance in their laser interferometers as waves from far-distant, two distant coalescing black holes passed through the Earth.  A decades-long scientific quest had detected the warping of space created by the merger of black holes 29 and 36 times the mass of our Sun.

The successful first detection of gravitational waves was hailed as one of the greatest scientific breakthroughs of the past decades, and the international LIGO collaboration is broadly expected to be recompensed by a Nobel prize for their achievement.

 

A photograph showing one of the LISA Pathfinder test masses. The shiny golden cube is compared to the size of a 1 Euro coin.
One of the LISA Pathfinder test masses: a metallic cube made from an alloy containing 75% gold and 25% platinum, coated with gold. The prism-like impression at the centre of the top face takes the plunger of the caging system, and allows for the centring of the test mass. Edges are chamfered to prevent damage during caging. Source: Thales Alenia Space.

A Prediction of Einstein’s General Relativity

Researchers would like to take LIGO’s capability into space, to enable the observations of waves generated by these types of events, but are too far beyond the sensitivities of ground-based Earth laboratories.  Before such an expensive endeavour is even attempted, however, the European Space Agency’s (ESA) Lisa Pathfinder mission must demonstrate that the key technologies can work on the scale proposed.

The LISA Pathfinder satellite was launched in December 2015.  It features a single instrument, designed to measure and maintain a 38 centimetres separation between two small 4.5 cm gold-platinum blocks.

The test masses were released once the satellite reached its orbit, and were allowed to go into free-fall inside the spacecraft.  The results of this space metrology experiment have now been published in a paper in the journal Physical Review Letters.

Gravitational waves are a prediction of the Theory of General Relativity.

 

A 3D computer model of gravitational waves in space. Source: ESAThe Lowdown on Gravitational Waves

 

  • The existence of gravitational waves was inferred by Einstein’s Theory of General Relativity, but they were only very recently directly detected by Advanced LIGO
  • It took decades for scientists to develop the technology that was able to detect them
  • Adanced LIGO fires lasers into long L-shaped underground tunnels and gravity waves, if they are present, disturb the light within the apparatus
  • Accelerating masses produce waves that propagate at the speed of light
  • Gravitational waves are ripples in the fabric of space and time – the so-called construct of spacetime
  • They are produced by violent cosmic events, like black holes merger and colliding galaxies
  • Potential detectable sources include merging black holes and neutron stars in theory
  • The detection of gravitational waves opens up the Universe to completely new investigations.

 

A schematic diagram illustrating the set-up for a classic Michelson interferometer.
A classic Michelson interferometer.

The Ultimate Space Metrology Experiment

Building the instrument was a painstaking process.  LISA Pathfinder’s own gravity could have disturbed the experimental blocks.  Hence the layout had to be very carefully designed, in order for the tugging force exerted by the on-board equipment to be evenly balanced in all directions.

Lisa Pathfinder’s payload is a laser interferometer – it is essentially a very precise high-tech ruler.  It was designed to track the behaviour of the two free-falling cubes made from a platinum-gold alloy.

Placed 38 centimetres apart, the test masses are inside cages that are very precisely engineered to insulate them against all disturbing forces.  In this super-quiet environment is maintained, the falling blocks will follow a “straight line”, defined only by gravity.

A diagram showing the falling blocks (test masses) inside the LISA Pathfinder satellite. Source: ESA
ESA’s LISA Pathfinder mission is a technology demonstrator that will pave the way for future space-borne gravitational-wave observatories. It will operate about 1.5 million km from Earth towards the Sun, orbiting the first Sun–Earth ‘Lagrangian point’, L1. Source: ESA/ATG medialab.

Under these conditions, a passing gravitational wave would be noticed by ever so slightly changing the separation of the blocks.

The requirement was to measure the position of the test masses to nine picometres per root hertz, and the on-orbit performance is actually about 30 femtometres – better by a factor of 300 than required.

Relative accelerations lower than a 10th of a millionth of billionth of Earth’s gravity have been detected.

 

A computer model image showing the inners of the LISA Pathfinder satellite.
Inside LISA Pathfinder:  This system of floating cubes and the laser that monitors them is one of the technologies that is to be tested on LISA Pathfinder, along with the thrusters and associated systems that will allow the spacecraft’s position to be held with the precision necessary to achieve its goal.  The spacecraft is use the thruster plarform onto which it was mounted in Stevenage to fly 1.5 million km to the First Libration Point (L1), a position in space where the gravity of the Earth, Sun and Moon cancel each other out. The only external force acting on it there will be the radiation pressure from the Sun. Source: TheEngineer

In layman friendlier terms, one picometre (1 pm) is a millionth of a millionth of a metre.  Thirty femtometres is an tinier figure still, equivalent to the diameter of a couple of gold atom nuclei.

The laser instrument can witness the smallest of accelerations in the test masses, such as those resulting from the impact of residual gas molecules still bouncing around inside Pathfinder in the vacuum of space – relative accelerations lower than a 10th of a millionth of billionth of Earth’s gravity.  In the ultra quiet environment of space, there is no seismic noise, no machinery switching on and off, no-one walking past to disturb the equipment, scientists seem to have found the perfect lab setting.

A perfect lab setting for the ultimate venture.

 

Two graphs showing the gravity waves sources for targets with LIGO and LISA.
The ESA/NASA collaborative space mission LISA (Laser Interferometer Space Antenna), which is planned to be launched 2018, aims to detect gravitational waves in the frequency range from 0.1 to 100 millihertz, a frequency band where the universe produce is richly populated in strong and periodic sources. This band is not accessible by ground-based gravitational wave detectors (e.g. LIGO Laser Interferometer Gravitational-wave Observatory or GEO600) utilizing laser interferometry due the gravity-gradient noise and arm-lengths limited to few kilometres. The main sources of gravitational radiation in the LISA frequency band are the galactic binaries and the massive black holes. Source: physik.hu-berlin.de

The Ultimate Venture and Why?

Astrophysicists and cosmologists have been looking to exploit this ultra-quiet configuration for LISA’s future mission – to detect the long-wavelength, low-frequency signals that are beyond the range of Earth-based facilities, like Advanced LIGO.

On Christmas Day 2015, scientists collected a second burst of gravitational waves sweeping through the Earth.  The warping of space-time was sensed at the Advanced LIGO laboratories – the same facilities that made the first historic detection in September 2015 in the United States.

The latest set of waves is thought to have its origins in another black hole merger, albeit a smaller one than the first.  Reporting the event in the journal Physical Review Letters, the international collaboration that operates Advanced LIGO says the two objects involved had masses that were 14 and eight times that of our Sun.  And the data indicates the merger produced a single black hole of 21 solar masses, meaning they radiated pure energy to space equivalent to the mass of one star of Sun size.

The detection of a second event was evidence that the first one was NOT merely an isolated incident.

The Universe is filled with black holes, spiralling and merging together, and emitting these bursts of gravitational waves.  As entire galaxies come crashing together, the merging of supermassive black holes is the kind of source the future mission would target.

Sensing the gravitational ripples very delicate disturbances would require the gold blocks in LISA’s detectors to be separated not by a mere 38 centimetres, as in Pathfinder, but by a few million kilometres, across three spacecrafts flying in formation. Before attempting such an expensive venture, it had to be shown that the key technologies can work on the scale proposed. Hence, the European Space Agency’s (ESA) Lisa Pathfinder mission.

Lisa Pathfinder was designed to demonstrate picometre sensitivity, but the satellite cannot itself make a detection of the ripples.  Now, things need to be scaled up!

An artist's impression for the future LISA Gravity wave hunting space observatory.
The future LISA gravity wave hunting mission. Source: space.com

Universities in Glasgow, Imperial College London, and Birmingham provided the core parts of the instrument, while the satellite itself was assembled at Airbus Defence and Space in Stevenage, U.K.  The success of Lisa Pathfinder is a testament to the skills of modern British academia and industry.  And NASA is considering a collaboration with ESA on the next project.

All this gives confidence that a mission proper to measure gravitational waves in space will work.

A space-borne observatory would need to reproduce the same performance with blocks positioned over a million kilometres apart.

LISA (Laser Interferometer Space Antenna) is tentatively scheduled for launch in 2034.