‘Routine’ detection of space ripples

Graphic of black holes

Gravitational waves have been picked up from another black hole merger.

It is the fifth time such an event has been validated, and the sixth occasion overall that ripples in space-time have been detected from far-off phenomena.

The LIGO-VIRGO collaboration, whose laser labs sense the waves, issued the news via a simple press release.

Previous events have had the fanfare of major international media briefings, which suggests the detections are almost now being seen as routine.

That in itself should be regarded as remarkable.

For decades, science chased the possibility that these very subtle signals might be observable, with a good many people doubting it would ever be achieved.

So to have arrived at a situation where the astonishing accomplishment is bordering on the ordinary is noteworthy in itself.

“I think we feel now that with the black hole binaries – unless we come across something that is qualitatively different then it really has started to become cataloguing if you like,” commented Prof Ken Strain, a collaboration member from Glasgow University, UK.

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The new merger was picked up on 8 June by two of the three laser interferometer labs in the collaboration – the LIGO facilities in the US states of Washington and Louisiana.

The third station, the Italian VIRGO establishment near Pisa, was still being commissioned and therefore gathered no data.

But the LIGO pair were able to sense two objects, with masses about seven and 12 times that of our Sun, colliding at a distance of about a billion light-years from Earth.

The result was a black hole roughly 18 times as massive as our Sun, meaning energy equivalent to one solar mass was radiated across space in the form of gravitational waves.

It is the lightest merger of the five black hole binaries sensed so far and assumes the catalogue number GW170608.

A minimum of two labs must detect an event for it to be validated, and this observation was somewhat fortuitous in that one of the US labs was just returning to observations from a period of maintenance when the trigger occurred.

The absence of VIRGO’s involvement this time meant a tight triangulation of the location on the sky of the collision was not possible.

Gravitational waves – Ripples in the fabric of space-time

Image captionArtwork: Black hole merger generates gravitational waves
  • Gravitational waves are a prediction of the Theory of General Relativity
  • It took decades to develop the technology to directly detect them
  • They are ripples in the fabric of space-time generated by violent events
  • Accelerating masses will produce waves that propagate at the speed of light
  • Detectable sources ought to include merging black holes and neutron stars
  • LIGO/VIRGO fire precision lasers into long, L-shaped tunnels
  • The beams sense the way the waves stretch and squeeze the tunnels
  • Detecting the ripples opens up the Universe to completely new investigations

The very first gravitational waves detection was made in September 2015. All observations since have also been black hole mergers, apart from the event picked up on 17 August this year.

That was a collision of two neutron stars – the compact, rapidly rotating remnants from exploded stars (supernovas).

All the labs are now offline for improvements. They should come back online next October with the ability to sense twice the distance, with hopefully therefore eight times the detection rate.

“Roughly speaking that means that instead of one event a month, we should see perhaps two a week,” said Prof Strain. “Those would be ‘candidates’ that would have to be followed up and confirmed. That’s the broad expectation but it depends on the upgrades going as planned.”

Interesting targets not yet observed, but which should trigger the laser interferometers, include the supernova explosions themselves and “lopsided” neutron stars.

The latter is a particularly fascinating possibility. Neutron stars are expected to be nearly perfectly spherical, but if they have tiny “mountains” on their surfaces they ought to generate gravitational waves as they spin.

“One day we will find one of these pulsars (a special class of neutron star) where we actually see the gravitational wave signal synchronised to the rotation rate of the pulsar because this mountain, which may be a millimetre or so high, is going around and around,” Prof Strain told BBC News.