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When black holes collide

The discovery of gravitational waves, announced on 11 February 2016 by scientists from the Laser Interferometer Gravitational-wave Observatory (LIGO), has made headline news around the world. One UK broadsheet devoted its entire front page to a image of a simulation of two orbiting black holes on which they superimposed the headline “The theory of relativity proved.”

This is of course inaccurate. Scientific theories are never proved (though they can be refuted), and the new results did not really make any significant change to the credibility of Einstein’s theory amongst physicists. If anything, it is the other way around, and the agreement between the details of the signal observed by the LIGO team and Einstein’s predictions lend significant credibility to the new results. After years of searching, the two LIGO interferometers (one in Hanford, Washington, and the other in Livingston, Louisiana) have both picked up a remarkable signal that originates from the collision of two black holes.

One UK tabloid relegated the story to page 21 and their headline, “Einstein, Einstein, give us a wave!”, is perhaps closer to the truth. Einstein’s theory of General Relativity predicts that accelerating mass produces gravitational waves which radiate away at the speed of light, rippling the very fabric of space-time. Gravitational waves are usually very weak and difficult to detect, so you need a really dramatic event to produce a significant source of them.

Two black holes in stable orbit around each other do produce a continuous stream of gravitational waves which gets stronger as the black holes get closer to each other, but even these are too weak to detect. However, when they get really close, and start to spiral into each other, the orbits become faster and faster, the acceleration increases and, as they dramatically plunge headlong into collision a ‘chirp’ of intense gravitational waves are emitted. This is exactly what was detected on 14 September 2015 and, using Einstein’s theories, it has been possible to deduce that the chirp originated from the collision, 1.3 billion light-years away, of two black holes one having a mass 36 times that our of Sun and the other of 29 solar masses. These two black holes merged to form a spinning, 62-solar-mass black hole (the difference in mass between 62 and 36+29 being converted into energy and radiated away in the form of gravitational waves).

Thus the real significance of the LIGO results, quite apart from the wonderful discovery of gravitational waves (long expected by physicists), is that this is the first time we have been able to watch two black holes collide and merge. Almost all our information in astronomy comes from photons. We routinely study the visible light, x-rays, infrared and radio waves emitted by the objects in the Universe. But now we have a new type of observatory, the gravitational wave observatory that is uniquely sensitive to very dramatic events that results in the emission of gravitons; and now we know they work!

Featured image credit: Best-Ever Snapshot of a Black Hole’s Jets by NASA Goddard Space Flight Center. CC-BY-2.0 via Flickr

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