The sound of colliding black holes

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


’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 on gravitational wave astronomy on 1 February. On 14 September 2015,

the Laser Interferometer Gravitational-wave Observatory (LIGO) detected gravitational waves from two black holes merging, 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 | @electrooptics

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 other wavelengths. ‘The event was dark! The most

“What we have created is the stillest place in the universe”

gigantic event mankind has ever detected was dark! It only consisted of spacetime curvature,’ Danzmann remarked. 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, and have been measured with a laser interferometer and shown to be on course. ‘What we have created is the

stillest place in the universe,’ Danzmann stated. He said that the measuring capabilities between the two free-flying cubes is so sensitive that there would be a huge signal if even g

March 2017 Electro Optics 11

Pius Lee/

Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36  |  Page 37  |  Page 38  |  Page 39  |  Page 40  |  Page 41  |  Page 42  |  Page 43  |  Page 44