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Got them! Gravitational waves detected – at last


After a false alarm in 2014, those slippery ripples in space-time have been finally snared. Astrophysicist Alan Duffy explains why it's such a momentous achievement.


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In physics, gravitational waves are ripples in the curvature of spacetime that propagate as a wave, travelling outward from the source. Predicted to exist by Einstein in 1915 on the basis of his theory of general relativity, gravitational waves theoretically transport energy as gravitational radiation.
DAVID PARKER / Getty Images

The final piece of Einstein's general theory of relativity, which has stubbornly evaded detection since his predictions a century ago, has been detected.

Scientists announced today at a press conference they’ve successfully picked up gravitational waves, formed during the cataclysmic collision and fusion of two mammoth black holes 1.3 billion light-years away.

Not only does this confirm Einstein’s predictions, it gives astronomers a new method of “seeing” the Universe.

Nailing down gravitational waves – ripples through the fabric of space-time – has not been an easy task. Even Einstein was pessimistic about finding the miniscule vibrations.

Indeed, in 2014 the astrophysics world was elated with the announcement that the Background Imaging of Cosmic Extragalactic Polarisation (BICEP2) telescope at the South Pole had picked up faint echoes of the Big Bang, only to have those hopes dashed when the signal turned out to be dust in our own galaxy.

This time, though, it’s different. The work was peer-reviewed and accepted for publication in the journal Physical Review Letters.

A team from Caltech, MIT and the Laser Interferometer Gravitational-Wave Observatory (LIGO) Scientific Collaboration reports success on 14 September 2015, just days after their recommissioned facilities were switched on.

But to set the scene, we need to turn back the clock 1.3 billion years.

Two enormous black holes (one 36 times the mass of the Sun, the other 29 times), snared in each others’ gravitational pull, spiralled towards each other and finally merged to produce a single black hole.

But the single black hole weighed only 62 times that of the Sun – the remaining three Suns’ worth of mass was converted to pure energy. According to Einstein’s famous energy-mass equivalence equation E = mc2, this equals around 5.4x1047 joules, or 8.5 billion trillion trillion Hiroshima atomic bombs.

The energy was emitted as gravitational waves, which have been hurtling outwards since, and wobbling space-time as they go.

Three Suns’ worth of mass was converted to pure energy – the equivalent of 8.5 billion trillion trillion Hiroshima atomic bombs.

Like ripples in a pond after a rock is thrown in, gravitational waves fade the further they travel from the source of the splash. But while floating leaves bob as ripples pass through, gravitational waves don’t “pick up” and move objects up and down – rather, they stretch and compress matter, even us.

That you don’t notice this is because each time a gravitational wave passes through, you’re stretched and squashed by less than the nucleus of an atom.

And thanks to the incredible precision of the twin L-shaped detectors of the advanced Laser Interferometer Gravitational-Wave Observatory (aLIGO) in the United States, this incredibly tiny change is measurable.

Lasers bounce inside the two four-kilometre “arms”, and are timed so when they meet each other at the corner of the L, they cancel out.

But should the length of an arm change – such as when its stretched or squashed by a passing gravitational wave – then not all the laser light will cancel out, and an “interference pattern” is detected instead.

Today’s direct detection discovery is of Nobel Prize-winning potential, but will not be the first Nobel award for gravitational waves.

In 1993, American astrophysicists Russell Hulse and Joseph Taylor shared the physics Nobel for their 1974 work, when they found superdense dead stars known as pulsars spinning inwards together at just the rate predicted by general relativity if they were emitting gravitational waves.

Although confident that the calculations were real, astrophysicists needed to directly detect them – as dense as they are, those pulsars simply weren’t massive enough or orbiting close enough to each other to generate detectable waves.

Two massive black holes orbiting each other, though, produced waves aLIGO could detect. And the remarkable serendipity of aLIGO finding gravitational waves so soon after its $200m upgrade is a testament to the enormous leap in sensitivity to this tiny stretching, allowing even more distant collisions to be seen.

Each time a gravitational wave passes through, you’re stretched and squashed by less than the nucleus of an atom.

aLIGO initially trebled its sensitivity over its predecessor LIGO, meaning it could see the same collision three times further away – up to 225 million light-years for collisions between objects such as neutron stars that are only a few times more massive than the Sun. (The black holes that caused the gravitational ripples picked up by LIGO were much, much bigger, and could be detected from further away.) This meant the search volume increased 27 times.

Unless black holes spiralled towards each other differently from that expected from Einstein’s predictions, the astrophysical community was confident to see a result from aLIGO within the year, especially as it would continue to improve its technology to end up ten times more sensitive, with a thousand times more search volume, out to 650 million light years in each direction.

Thankfully, sometimes nature is kind and the wait isn’t long.

The search for black holes, such as those a million times the mass of the Sun or more nestled in the heart of galaxies including our own, continues.

Parkes Telescope in New South Wales, operated by Australia’s peak government science agency CSIRO, aims to measure how many supermassive black holes (and the galaxies that host them) collide and merge, allowing us unique insights into how galaxies grow by cannibalism of other, smaller, galaxies.

Gravitational waves can let us see objects that are fundamentally invisible to our telescopes that can only detect light, or electromagnetic waves. As more detectors come online, we can more accurately triangulate the location of the cosmic explosion in the sky.

It will soon be routine for astronomers to use the arrival of gravitational waves to direct the world's telescopes to see the most extreme events in the Universe.

Now that the first gravitational wave has been detected, it is guaranteed that we won’t have to wait a century until seeing another.

Related reading:
Gravitational waves – your questions answered
How does LIGO look for gravitational waves?

Further reading:
Einstein's gravitational waves remain elusive
Star dust and gravitational waves
Ripples in the fabric of space-time
General Relativity – still ahead of its time
New model of the cosmos: a Universe that begins again

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Alan Duffy is an astrophysicist at Swinburne University of Technology, Melbourne. Twitter | @astroduff
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