In 1609 Galileo trained his newly improved telescope on the heavens and revolutionised our understanding of the universe. By grinding two glass lenses and positioning them according to his mathematical calculations, he had improved the telescope’s magnification 20-fold.
It was enough to see the four moons of Jupiter and reach an inescapable conclusion. If another planet had moons, Earth could not be the centre of the Universe.
Four hundred and six years later, an instrument billions of times more sensitive than Galileo’s telescope was trained on the universe. The Laser Interferometer Gravitational-Wave Observatory, better known as LIGO, was operated by an army of 1,006 scientists mostly from the US, but also from Germany, the UK and Australia. On 11 February 2016, they announced they had detected gravitational waves and a never-before-witnessed event: the merger of two black holes. These colliding black holes are the 21st century’s equivalent of the moons of Jupiter – the harbinger of the next revolution in our understanding of the Universe.
From Galileo’s time until now, we have tuned in to the Universe’s broadcasts on the electromagnetic spectrum. But colossal events like the merger of black holes or the Big Bang have been eerily silent. A new generation of gravitational wave telescopes promises to change that. “It’s as if we have just been given a new pair of ears,” says David Blair, from the University of Western Australia, a 40-year veteran of the hunt for gravitational waves. “As we open the window to gravitational waves, we may hear things we never saw,” says David Reitze, director of the LIGO lab.
Each time technology gives us a new way to peer at the universe, our perception of it changes.
Optical telescopes opened our eyes to the solar system, to twinkling faraway stars and nebulous dust clouds. Then, from the 1930s, radio telescopes peered into the dust, detecting the signature of hydrogen atoms. They saw the birth and death of stars, and in the late 1950s, something even more violent – a jet shooting out of a galaxy. It was named “3C 273”, the 273rd object in the Third Cambridge Catalogue of Radio Sources. It turned out to be a quasar, the brightest object ever seen, brighter than the entire galaxy it resided in.
The best explanation was that this was the work of a supermassive black hole. As it sucked in matter, particles were accelerated to near light speed causing them to radiate a jet of radio waves.
Each time technology gives us a new way to peer at the universe, our perception of it changes.
Radio telescopes also accidentally discovered the radiation echoing from the Big Bang in 1964, the so called Cosmic Microwave Background (CMB).
Again by accident in 1967, they uncovered the lighthouses of the Universe – neutron stars that can spin hundreds of times per second while emitting a powerful beam. Known as pulsars, their precision in some cases rivals that of atomic clocks.
Putting X-ray eyes on the Universe offered more evidence for black holes. In 1964 a rocket bearing a Geiger counter detected a strong X-ray source in the constellation Cygnus. Many astronomers, not least young Caltech astrophysicist Kip Thorne, thought the signal was the work of a black hole, the X-rays being released by matter as it was sucked in. Others, including Stephen Hawking, disagreed. In 1975 he and Thorne famously placed a bet on it. In 1990, Hawking conceded.
That was also the year the Hubble space telescope launched clear of Earth’s atmosphere to deliver a bonanza of surprises. The high resolution, optical telescope, could take in a view of frequencies from ultraviolet to infrared and peer almost to the edge of the Universe. It revealed a breathtaking density of galaxies in what had hitherto been thought of as empty space. It told us our Universe was 13.7 billion years old, and found evidence that it was not only expanding but accelerating!
“Through optical telescopes the Universe looks serene; through radio and X-ray telescopes it looks tremendously violent. I think we will see even bigger surprises” with gravitational waves, says Thorne, one of the inventors of LIGO, but probably best known as the consultant on black holes for the movie Interstellar. He is hotly tipped to be one of those who will receive the Nobel for detecting gravitational waves.
A gravitational wave is not so different from other waves. Water waves ripple water, sound waves ripple air, light waves ripple an electromagnetic field. The weird thing about gravitational waves is they ripple empty space, “like an angel flapping his wings”, in the words of University of Pisa astrophysicist Federico Ferrini.
Since Einstein’s 1915 Theory of General Relativity, we’ve known that empty space is not nothing – it’s a four-dimensional fabric where space has been interwoven with time. Einstein showed that this fabric can be stretched and shaped by the matter in the Universe. The classic metaphor is a bowling ball dimpling a trampoline and causing objects in the vicinity to fall irresistibly towards it – the effect we experience as gravity.
Just as dropping the bowling ball would ripple the trampoline, so fast-moving massive objects cause ripples in the fabric of space-time. And just like the trampoline, when a gravitational wave passes, the space-time fabric is stretched in one dimension and compressed in the other. If one travelled through the building you’re in at the moment, the room around you would become a little longer in, say, the north-south direction and slightly narrower in the east-west direction. A moment later, the room dimensions would oscillate the other way: shorter in the north-south, longer in the east-west. But as waves go, gravitational ones are extremely weak, so weak Einstein thought they would be impossible to detect.
Yet in the 1950s one brave American scientist set out to try. Joseph Weber at the University of Maryland suspended large cylinders of aluminium, about two metres in length and one metre in diameter in a vacuum chamber, to detect vibrations caused by a passing gravitational wave. He chose aluminium because, like a well-cast bell, it would vibrate cleanly when struck by the wave. These vibrations could be detected with piezoelectric sensors which are able to convert movement into electrical signals. If the signals were genuine, he reasoned, he would simultaneously detect vibrations in two cylinders 1,000 kilometres apart. By 1970 Weber claimed to have detected hundreds of coincident signals.
Within two years, duplicates of the Weber “resonant bar” appeared in Moscow, Munich, Paris, Oxford, Glasgow, China and in the IBM and Bell labs in the US, recalls Blair. But none were able to snare a gravitational wave. Weber’s coincident results were probably just “coincidence”.
“Though brave and ingenious, Weber suffered from a common problem,” says Blair. “He believed his data too strongly.” But he adds, “I expect he’ll come to be seen as more and more of a hero.” Weber not only rushed in where others feared to go, he laid down the basic principles by which gravitational waves might be found. One was simultaneous detection by distant instruments. The other was that the best chance of detecting one would be from the cosmic tsunami created when two black holes collide.
Blair was one of those inspired to join the hunt. During a stint at Louisiana State University in the 1970s, he improved upon Weber’s resonant bar by using the metal niobium, which loses 100 times less energy than aluminium when it vibrates. And to detect the tiny signal, Blair used microwave sensors that were cryogenically cooled to quieten down the thermal noise.
Back at the University of Western Australia, he built a three metre by 30 centimetre cylinder of niobium. Collaborators in Italy, the US and Switzerland, also built resonant aluminium bars with cryogenic detectors. Using Einstein’s equations, they calculated they had the sensitivity to detect gravitational waves from black holes colliding anywhere within our Milky Way galaxy – a total span of 100,000 lightyears.
For a decade they waited, but no wave came.
In 2000, the resonant bars were consigned to history. The conclusion? The Milky Way did not have any merging black holes. “We went out on a hunch even though there was no evidence for them,” explains Blair.
The next hunt would have to be less of a wild goose chase. It needed a quarry that was actually known to exist. And it needed an instrument with the range to snare it.
Binary neutron stars fit the bill. Circling each other like dancers, they are the densest objects in the Universe after black holes. A merger of these behemoths would also roil space-time enough to create detectable gravitational waves. But the instrument to capture the event would have to reach 64 million light years into space, about 1,000 times further than the range of the resonant bars.
Physicists were already developing such an instrument. Working in parallel to the resonant bar community, MIT physicist Rainer Weiss and Caltech’s Ron Drever and Thorne came up with the idea of using lasers to detect gravitational waves in the 1970s. The then newly invented laser was a beam of light waves whose normally choppy crests and troughs had been trained to oscillate in sync. Perhaps they could be used to measure the stretching of space-time?
In the 1980s, the US National Science Foundation started funding the construction of two LIGOs, one in the swamps of Livingstone, Louisiana, the other 3,000 kilometres away in the high desert near Hanford in Washington State.
LIGOs create giant L-shapes on these remote landscapes. Each arm of the “L” houses a four-kilometre, vacuum tunnel with a mirror at each end. An incoming laser beam is split between the two arms. The polished mirrors reflect the light back so that the split beams are re-joined and picked up by a detector at the intersection of the two arms.
Normally the beams are perfectly out of phase, the troughs of one are perfectly aligned with the peaks of the other, so they cancel each other out before hitting the detector. But a passing gravity wave will change the relative length of the arms, disrupt the perfect alignment of the peaks and troughs, and create a signal. LIGO was designed to detect a length change of 10-19 of a metre, 10,000 times smaller than the nucleus of an atom.
“For years, the thought of that made me think: why am I wasting my time?” admits David McClelland, whose team at Australian National University developed mirror suspension systems to steer LIGO’s laser light with extreme accuracy.
The original LIGOs ran from 2002 to 2010 and detected nothing. But Advanced LIGO was in the wings. An international team worked to finesse every component of the machine – the noise-cancelling microphones, the ultra-reflective coating of the mirrors, the computer algorithms to pick out the signal from the noise – to increase sensitivity and triple the device’s reach into the cosmos to 190 million light years.
‘It’s been like a military assault… we have pushed back the limits of technology’
“It’s been like a military assault. We’ve relentlessly pushed back the limits of technology,” says Peter Veitch, whose contingent at the University of Adelaide contributed by correcting distortions in the LIGO mirrors due to the heat of the high-powered laser.
Advanced LIGO, developed at the cost of more than a billion dollars, is by far the most precise measuring device mankind has ever built. And on 14 September 2015, it was poised to detect the flapping of an angel’s wings.
At 5:51 that morning the instrument was still undergoing tests known as “engineering runs”. It had only been operating for an hour or so when something altered the position of the mirrors of the Louisiana instrument for a tenth of a second. Seven milliseconds later, the Washington instrument saw the same signal. An email was dispatched around the world with the subject: “Very interesting event on ER8”.
Most of the recipients assumed it was a fake signal used to test the system. It wasn’t.
“I looked at this thing and thought my God, this looks like it’s it,” says Thorne.
Nevertheless 1,006 researchers around the world proceeded extremely cautiously, trying to keep a veil of secrecy over their finding. The hunt for gravitational waves had a long history of coming up with phoney findings. Only a year and a half before, a team from the Harvard Smithsonian Centre for Astrophysics claimed that their radio telescope at the South Pole had detected gravitational waves from the Big Bang in the form of swirls in the CMB. But those swirls turned out to be caused by dust in the Milky Way. With that fiasco haunting them, the physicists brutally interrogated their data for five months to see if it stood up.
And so it was that on 11 February 2016, David Reitze fronted a press conference in Washington, and with barely suppressed emotion announced, “Ladies and Gentlemen, we have detected gravitational waves. We did it!”
The wave was detected first at Livingstone, Louisiana “above the ever present rumbling of the detectors”, said Gabriela Gonzalez, the Louisiana State University-based spokesperson for LIGO. “We know it’s real because seven milliseconds later we saw the same thing at the Hanford detector.”
The signals had oscillations that grew faster and higher before settling down. It was exactly the pattern predicted for the merging of two black holes.
The frequencies were also in the audible range. An electronically amplified playback makes a sound like a whistle that begins low pitched and steadily grows to higher and higher frequencies before abruptly ending. Physicists refer to it as a “chirp”.
That chirp was packed with information. As Gonzalez explains, it reveals the colliding black holes had initial masses equivalent to 36 and 29 Suns. When they merged, they formed a body of 62 solar masses – three Suns worth of energy was radiated in a tsunami of gravitational waves. It was the most energetic event we humans have ever witnessed. Yet being black holes, they emitted no light or other kind of electromagnetic radiation.
The height of the waves also carried information. It told us these black holes collided 1.3 billion years ago. For billions of years prior to that, they must have circled each other, spiralling ever closer and speeding up as they did. In the final second, before they consummated their union with that high pitched chirp, they were travelling at half the speed of light.
And because the two LIGOs were spaced 3,000 kilometres apart, physicists could roughly trace back the source of the cataclysm to somewhere in the direction of the Magellanic clouds. Once LIGO gets a third ear from the advanced Virgo detector near Pisa, Italy, due to start operations later this year, astronomers will be able to triangulate future signals to pinpoint their location.
Many have noted the staggering series of coincidences that led to the detection. The tsunami gravitational wave began its journey 1.3 billion years ago when life on Earth was ruled by single-celled life forms. Einstein predicted its existence 100 years ago; 40 years ago physicists set out to build a detector and, just in the nick of time, advanced LIGO came online last September to catch the passing wave!
For the moment the only cosmic event LIGO has spotted has come from this one pair of colliding black holes (although rumours abound of other sightings). But it is also designed to detect other dense, fast-moving galactic beasts that roil space-time. So far we’ve known them only superficially. “Electromagnetic waves show us the surface of things but gravitational waves go through anything,” explains Blair. “We’re like the explorers who just discovered the coast of the Great South Land. Now we can explore what lies within.”
‘Electromagnetic waves show us the surface of things but gravitational waves go through anything.’
Besides colliding neutron stars, LIGO should also detect single rotating neutron stars. Perfectly spherical spinning objects do not generate gravitational waves, but neutron stars about 10 kilometres in diameter, are believed to carry millimetre-high “mountains”. Rather than a chirp, the whizzing neutron star would emit something more like a steady-pitched whistle.
Advanced LIGO should also detect the signature of supernovae. The final explosion of a large dying star should be detectable as a crackling explosion.
Gravitational waves also promise to show us the beginning of creation in the Big Bang.
So far we can only peer back to 380,000 years after the event. Before that time, the plasma that was our baby Universe was opaque and impenetrable to radiation. As it coalesced to form hydrogen atoms, the first light from the Universe emerged in the form of the CMB. But what of the first few fractions of a second after the Big Bang when the Universe suddenly inflated into existence?
We should be able to tune into the spectrum of gravitational waves that were released at that moment. LIGO might detect some of the shorter waves. But most will have stretched during the expansion of space itself and are too long to be detected by LIGO’s four-kilometre arms. Detecting them would require arms too long for any Earth-based observatory.
The European Space Agency (ESA) already has a space-based telescope on the drawing board with arms a million kilometres long. Known as eLISA – evolved Laser Interferometer Space Antenna – it consists of three separate spacecraft that will orbit the Sun as if they were the three points of an equal-sided triangle with sides a million kilometres long.
Laser beams will shine back and forth between them providing the same kind of laser ruler used by LIGO. It is due to be launched in 2034, a date that might be on track given the success ESA had with its pilot craft. The “eLISA pathfinder”, launched in December 2015, showed it was possible to keep two masses inside a space ship floating in perfect free fall. This is essential for the detectors in the ships since they must be perfectly unperturbed if they are to measure ancient gravitational waves.
And eLISA is only the beginning of space-based gravitational wave telescopes. There are plans afoot for a majestic instrument called the Big Bang Observer, made up of four triangular set-ups, each like eLisa. The hope is this super-instrument will detect the gravitational waves from a trillionth of a trillionth of a trillionth of a second after the big bang.
Besides these space telescopes, the BICEP astronomers at the South Pole are devising ever better methods to clear away the dust and to search for the signature of the longest gravitational waves of all, in the swirls of the CMB. These waves from the beginning of time are as long as the Universe – “more like a tide than a wave”, says Blair.
And over at the Parkes radio telescope in New South Wales, a team which has spent the past decade timing pulsars to catch a gravitational wave, continues its search for the waves caused by mergers of supermassive black holes. Blair and Thorne have no doubt that they, too, will catch a gravitational wave in the next few years.
Our Universe is awash with gravitational waves – tiny ripples, shore waves, huge swells, tidal waves, and tides. “Until now, we’ve only seen warped space-time when it’s very calm, as if we’d only spied the glassy surface of the ocean on a calm day. Now we see it as a rolling storm,” says Thorne.
“In a short time, we are going to map the shape of the Universe in gravitational waves, ” says Blair. And, he adds, “it’s going to be very musical”.
Black holes really exist!
Not only was this the first discovery of Einstein’s century-old prediction of gravitational waves, it was the first direct evidence that black holes exist. Despite being popular in fiction, until now we couldn’t be certain that black holes were actually real. That’s because these perfectly black bodies can’t be seen, so we’ve relied on circumstantial evidence to infer their existence – such as the bursts of radiation from matter being sucked into them, or the motions of surrounding stars. But these gravitational waves came directly from the black holes themselves.
The surprise for the scientists was that the discovery was made so soon after LIGO was switched on. Either they were incredibly lucky or these massive never-before-witnessed invisible explosions are relatively common. And LIGO is still only running at about one-third strength. It will be operating at full sensitivity in 18 months, and then it will be able to see three times further into the Universe than it currently can. That means by 2018, discoveries like this could be made on a weekly basis. That will allow us, for the first time, to estimate just how many black holes are out there.