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New Gravitational Wave Detection Shows Shape Of Ripples From Black Hole Collision

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Astronomers have made a new detection of gravitational waves and for the first time have been able to trace the shape of ripples sent through spacetime when black holes collide.

The announcement, made at a meeting of the G7 science ministers in Turin, marks the fourth cataclysmic black-hole merger that astronomers have spotted using Ligo, the Laser Interferometer Gravitational-Wave Observatory.

The latest detection is the first to have also been picked up by the Virgo detector, located near Pisa, Italy, providing a new layer of detail on the three dimensional pattern of warping that occurs during some of the most violent and energetic events in the universe.

A tiny wobble in the signal, picked up by Ligo’s twin instruments and the Virgo detector on 14 August, could be traced back to the final moments of the merger of two black holes about 1.8bn years ago.




The black holes, with masses about 31 and 25 times the mass of the sun, combined to produce a newly spinning black hole with about 53 times the mass of the sun.

The remaining three solar masses were converted into pure energy that spilled out as deformations that spread outwards across spacetime like ripples across a pond.

Detecting these tiny distortions has required detectors sensitive enough to measuring a discrepancy of just one thousandth of the diameter of an atomic nucleus across a 4km laser beam.

What is a gravity wave?

Rippling out from a super- massive collision, for example between two black holes, gravity waves could be detected through the stretching and contracting of space and time.

How Ligo and Virgo’s detectors work?

  1. A single laser beam is split and directed down two identical tubes, 4km long
  2. Mirrors reflect the twin beams back to a detector
  3. Back inside the detector, the laser beams arrive perfectly aligned
  4. Recombined, they cancel each other out

How are gravity waves detected?

  1. When spacetime is distorted by a gravity wave, the two tubes change length. One tube stretches as the other contracts over and over until the wave has passed
  2. As the distances fluctuate the peaks and troughs of the two returning laser beams move in and out of alignment
  3. The recombined waves no longer cancel each other out. Light reaches the detector and the gravity wave can be measured

Ligo scientists’ historic observation of gravitational waves in September 2015, marked the first experimental proof of Einstein’s prediction a century ago that space itself can be stretched and squeezed.

However, the parallel orientation of the two Ligo detectors, one in Hanford, Washington state, the other in Livingston, Louisiana, has meant that scientists are effectively observing one flat plane through space, rather than getting a 3D picture.

It’s like if I give you just one slice of apple, you can’t guess what the fruit looks like,” said Prof Andreas Freise, a Ligo project scientist at the University of Birmingham.

This was intentional because it maximised the chances of detection – a discovery that is hotly tipped to be rewarded when the Physics Nobel Prize is announced next week.

However, the configuration made it impossible to test a second crucial prediction of Einstein’s theory – the shape of the path that the waves travel along.

Virgo’s arms are angled differently than the two Ligo detectors, allowing astronomers to extract new information about the polarisation of gravitational waves – essentially the path traced out by the vibrations.

Einstein’s theory predicts two polarisations of gravitational waves, but some competing theories of gravity predict up to six.

Prof Stefan Ballmer, a physics professor at Syracuse University, explains: “If you look at how you can bend the sheet of paper that spacetime is, there are many ways you can bend it. But if you look at [Einstein’s predictions], only two of those ways are present.

The new data – albeit based on a single detection – already appear to strongly favour Einstein’s predictions of how spacetime is expected to crumple.

Combining results from three detectors has also allowed scientists to more accurately triangulate the area of sky from which the waves are emanating.

In future, this could allow scientists to swing ground-based telescopes to the target locations to see whether there is any visible trace of the collision itself.

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