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The sound of colliding black holes

Greg Blackman listens in to Professor Dr Karsten Danzmann giving his plenary session on gravitational wave astronomy at Photonics West

'I've been waiting 27 years of my professional career for this sound,’ declared Professor Dr Karsten Danzmann, director of the Max Planck Institute for Gravitational Physics. The sound – a little like a slide whistle going up in tone with a small pop at the end – was that of a dying black hole system, two black holes spiralling ever closer together until their event horizons touched and they merged into a single, more massive spinning black hole.

Danzmann played the audio clip – a translation into sound of the gravitational waves thrown out by the colliding black holes – during a plenary session at SPIE Photonics West on 1 February in San Francisco.

On 14 September 2015, the Laser Interferometer Gravitational-wave Observatory (LIGO) detected gravitational waves from the merger of two black holes, each around 30 times the mass of the sun. The event took place 1.3 billion years ago, producing a peak power output 50 times that of the entire visible universe. Einstein's general theory of relativity predicted such an event, but Einstein never believed that gravitational waves would be detected, Danzmann said.

Then, on 26 December 2015, LIGO detected a second collision, this time from two lighter black holes. This one lasted for 55 oscillations, and the scientists could watch 27 orbits of the dying system.

With only two detectors – one in Hanford, Washington and the other in Livingston, Louisiana – the event could only be localised to a few hundred square degrees. Danzmann said that there were 20 searches for optical counterparts for what LIGO observed, in the visible, infrared, radio frequencies, and others. 'The event was dark! The most gigantic event mankind has ever detected was dark! It only consisted of spacetime curvature,' Danzmann commented.

The future of gravitational wave astronomy, however, is bright, according to Danzmann. Plans for a third generation of detectors, like the Einstein telescope, are underway. What the ground-based telescopes will not be able to do is listen to low frequency gravitational waves, frequencies in the millihertz range. For that you have to go into space.

The Pathfinder mission for the Laser Interferometer Space Antenna (LISA) was launched on 3 December 2015, one day after the 100th anniversary of the publication of the written version of Einstein's general theory of relativity. This was accidental; ‘no PR agency could have done this better,' Danzmann remarked. Its mission is to test the LISA technology in space, and so far it is working extremely well. With a detector like LISA, scientists will be able to detect the collisions of whole galaxies.

The LISA mission will consist of three satellites a million kilometres apart that form the three arms of a laser interferometer. At this kind of distance between satellites, a watt of laser power will be reduced to picowatts at the detector. LISA will be placed in a solar orbit 50 million kilometres behind the Earth and orbit the Sun like a rigid body, like a cartwheel, Danzmann said, rolling around its centre once per year while it passively listens to the universe.

The LISA Pathfinder mission is operating two test masses, with the distance between them reduced from a million kilometres to 35cm. At the heart of LISA Pathfinder are two free-flying gold cubes that travel inside the satellite – the satellite is just there to protect them. These gold cubes follow a purely geodesic curve, only influenced by gravity.

The gold cubes have been measured with a laser interferometer and have been shown to be flying on a geodesic curve, unaffected by anything else.

'What we have created is the stillest place in the universe,' Danzmann stated. He said, the measuring capabilities between the two free-flying cubes is so sensitive that there would be a huge signal if a virus were to land on one of them.

On 1 March 2016, when the scientists took control of the satellites, LISA Pathfinder was already operating better than the requirements for the mission, and the latest readings from January 2017 show that it is about 50 per cent below the full LISA requirement at all frequencies, according to Danzmann.

The importance of LISA is that it will cover a region of detection not possible with ground-based instruments. Danzmann said the space-based detector will cover everything, out to the complete universe with extremely high signal-to-noise ratio.

Ten years ago, the two black holes detected on the 14 September 2015 were orbiting each other very slowly at low frequency. Danzmann explained that this is right in the middle of the LISA sensitivity. If LISA had been up there 10 years ago, he said, the collision could have been predicted in advance, the timing to fractions of a second and the direction in the sky to fractions of a square degree. 'I can guarantee you that every large piece of glass on this planet would have looked in that direction during that second,' he said, to see if they could detect any electromagnetic radiation left over from the event.

The LISA consortium has advanced its schedule. The call for mission concepts closed in the autumn of 2016 and the decision on implementation will follow in 2020.

Danzmann closed his presentation by paying tribute to Professor Dr Heinz Billing, Danzmann's predecessor at the Max Planck Institute for Gravitational Physics, who died on 4 January 2017 aged 102. Billing was one of a handful of scientists studying gravitational waves in the 1970s, work that has led to detectors like LIGO.

'He's the most modest scientist I've ever met,' Danzmann said. When Danzmann started working for the Max Planck Institute for Gravitational Physics 27 years ago, Billing promised to stay alive until gravitational waves were discovered. 'We did our best. I promised it to him, but I had no idea it would take 27 years to fulfil this. But, he was there to see them, to hear them, to enjoy it.'

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