BEYOND GRAVITATIONAL WAVES
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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-beforewitnessed 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.
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
EACH TIME TECHNOLOGY GIVES US A NEW WAY TO PEER AT THE UNIVERSE, OUR PERCEPTION OF IT CHANGES.
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 northsouth, 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.