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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