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Gravitational waves: Third detection of deep space warping

2017-06-02 05:20:46

By Jonathan Amos BBC Science Correspondent

Scientists are reporting yet another burst of gravitational waves.

The signals were picked up by the Advanced LIGO facilities in the US and are

determined to have come from the merger of two huge black holes some three

billion light-years from Earth.

It is the third time now that the labs' laser instruments have been perturbed

by the warping of space-time.

The detection confirms that a new era in the investigation of the cosmos is now

truly under way.

"The key thing to take away from this third, highly confident event is that

we're really moving from novelty to new observational science - a new astronomy

of gravitational waves," said David Shoemaker, spokesperson for the LIGO

Scientific Collaboration.

The latest detection, which was made at 10:11 GMT on 4 January, is described in

a paper accepted for publication in the journal Physical Review Letters.

Once again, it is a merger of black holes, and once again the energy scales

involved are extraordinary.

The analysis suggests the two black holes that coalesced had starting masses

that were just over 31 times and 19 times that of our Sun. And when they

finally came together, they produced a single object of a little under 49 solar

masses.

It means the unison radiated a simply colossal quantity of pure energy.

"These are the most powerful astronomical events witnessed by human beings,"

explained Michael Landry, from the LIGO lab in Hanford, Washington State.

"In this case two times the mass of the Sun were converted into deformations in

the shape of space. This energy is released in a very short space of time, and

none of this comes out as light which is why you have to have gravitational

wave detectors."

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 produce waves that propagate at the speed of light

Detectable sources should include merging black holes and neutron stars

LIGO fires lasers into long, L-shaped tunnels; the waves disturb the light

Detecting the waves opens the cosmos to completely new investigations

As with the two previous observations - in September and December 2015 - the

scientists are uncertain about where on the sky the 4 January event occurred.

From the three millisecond gap between the signal being picked up first at

Hanford and then at the second lab in Livingston, Louisiana, researchers can

only specify a large arc of possibility for the source.

Conventional telescopes were alerted to go look for a coincident flash of

light, but they saw nothing that could be confidently ascribed to the black

hole merger.

The LIGO collaboration will only solve this triangulation problem when a third

station called VIRGO, in Italy's Pisa province, starts work alongside the US

pair this summer.

'Every 15 minutes'

The detection of gravitational waves has been described as one of the most

important physics breakthroughs in recent decades.

Being able to sense the distortions in space-time that occur as a result of

cataclysmic events represents a transformative step in the study of the

Universe, one that does not depend on sensing electromagnetic radiation in any

of it forms - from radio and optical light through to X-rays and gamma rays.

Now, as well as trying to "see" far-off events, scientists can also "listen" to

those events as they vibrate the very fabric of the cosmos.

And immediately this approach is telling researchers new things. One simple

discovery is the recognition of a totally new class of black holes. Before

LIGO's discoveries, orbiting pairs of these objects, some of 25 solar masses

and greater, were completely unknown.

"In two years, we've gone from not knowing these systems existed to being

really confident there's a whole population of them out there," commented

Sheila Rowan, a collaboration team member from Glasgow University, UK. "And

it's all consistent with gravitational waves from one of these systems passing

through us about once every 15 minutes, from somewhere in the Universe," she

told BBC News. The quest for the future is to get LIGO to sensitivities where

more of these events can be detected.

Unbroken Einstein

Also possible now are new investigations of the properties of black holes. The

scientists can tell from the nature of the 4 January signal that the spins of

the objects were not fully aligned when they came together.

This suggests they were not created from a pair of previously orbiting stars

that exploded and then collapsed into black holes. Rather, their origin was

more probably as stars that led independent lives and only at some end stage

fell in as a duo.

"In that first case, we would expect that the spins would stay aligned," said

Laura Cadonati, the collaboration's deputy spokesperson. "So, we have found a

new tile to put in the puzzle of understanding formation mechanisms."

In addition, gravitational wave astronomy permits new tests of Einstein's

theories. Because of the greater distance to this merger (twice the distance to

the 2015 events), researchers could more easily look for an effect called

"dispersion".

For light, this describes how electromagnetic radiation of different

frequencies will travel at different speeds through a physical medium - to

produce a rainbow in a glass prism, for example.

Einstein's general theory of relativity forbids any dispersion from happening

in gravitational waves as they move out from their source through space towards

Earth.

"Our measurements are really very sensitive to minute differences in the speeds

of different frequencies but we did not discover any dispersion, once again

failing to prove that Einstein was wrong," explained Bangalore Sathyaprakash, a

LIGO team member from Penn State, US, and Cardiff University, UK.

In a poignant coincidence, 4 January was also the day that Heinz Billing, a

pioneer of gravitational wave science, died aged 102.

The German physicist and computer expert built one of the first laser

interferometers - the instruments now used to detect gravitational waves. His

early work is credited with making crucial contributions to the development of

the eventual LIGO systems.

"His group started in about 1975, just before we did it here in Glasgow,"

recalled LIGO collaborator Jim Hough. "They were following the idea that the

American Rai Weiss had had of using multiple beam delay lines, and of course

the German detector was absolutely superb. They did fabulous work that has

continued in Germany to this day."

A laser is fed into the machine and its beam is split along two paths

The separate paths bounce back and forth between damped mirrors

Eventually, the two beams are recombined and sent to a detector

Gravitational waves passing through the lab should disturb the set-up

Theory holds they should very subtly stretch and squeeze its space

This ought to show itself as a change in the lengths of the light arms

The photodetector captures this signal in the recombined beams