Our weighty new view of the universe
Italian physicist Marco Drago is unlikely to have many days in his career quite like the one he had on 14 September 2015. The 33-year old postdoctoral researcher from Padua is one of more than 1,000 scientists around the world who make up the LIGO collaboration, which uses two enormous detectors in the U.S. to hunt for ripples in the structure of space-time known as gravitational waves. The existence of those waves was predicted nearly 100 years earlier by Albert Einstein, who said they should be generated by any accelerating mass – most notably the extremely large masses of distant bodies in the universe. But until last year, LIGO, like earlier generations of gravitational-wave detectors, had drawn a blank.
Sitting in his office at the Max Planck Institute for Gravitational Physics in Hannover, Germany, Drago was monitoring data that are automatically dispatched to a number of computer centres around the world. Just before noon, a message flashed on his screen telling him that data gathered by LIGO (which stands for Laser Interferometer Gravitational-Wave Observatory) only three minutes earlier contained what could have been a signal. All previous such alerts that he and his colleagues had received had turned out to be duds – caused by terrestrial vibrations or other sources of interference rather than any cataclysmic event in space. But looking at the putative signal himself he realised that this time it could be real. “It’s difficult to describe the excitement I felt,” he says. “We’d been waiting for this for years.” After thoroughly checking their equipment and analysing their data, the LIGO collaboration announced to the world on 11 February 2016 that they had, at long last, discovered gravitational waves.
As LIGO spokesperson Gabriela González of Louisiana State University explains, the discovery is of interest to physicists because it provides yet further evidence in support of Einstein’s general theory of relativity. However, she says, no one really doubted that the discovery would eventually be made, particularly since physicists had already obtained indirect evidence for gravitational waves in the 1970s through careful measurements of the radio waves emitted by a type of star known as a pulsar.
Instead, what really excites González and her colleagues is the potentially revolutionary impact of the discovery on astronomy – being able to observe the most extreme, violent phenomena in the universe via the gravitational radiation that they produce. “To actually see a space-time fluctuation is very cool,” she says. “But most if not all of the scientific community believes in general relativity. What is really important is that we have a new way of looking at the sky.”
Listening with laser light
Until last autumn, astronomers looked at the universe almost exclusively via electromagnetic radiation. Using visible light, radio, X-ray and waves at other frequencies, they had built up an incredibly rich knowledge of the heavens, from Galileo’s discovery of the moons of Jupiter to the latest observations of the afterglow of the Big Bang. But electromagnetic radiation has its limitations. It interacts strongly with matter, making it easy to detect, but at the same time it is easily absorbed by matter as it travels to Earth from deep space.
Gravitational radiation is fundamentally different. While electromagnetic waves travel in space-time, gravitational waves are distortions of space and time themselves. Gravitational waves also interact with matter very weakly (owing to the feebleness of gravity itself), which makes them incredibly hard to detect but also very valuable as messengers of processes taking place in some of the universe’s most extreme environments. Also, while electromagnetic waves give us images of objects, gravitational waves are more like sound waves in that they give us information about objects’ overall motion.
To detect these waves, scientists build “interferometers” such as LIGO, which consists of two such devices, one in Louisiana and the other in Washington State. Each interferometer consists of two long, evacuated hollow tubes, or “arms”, positioned at right angles to one another. A single laser beam emitted from the arms’ vertex is split and sent down each arm, within which it bounces back and forth between mirrors positioned very precisely at the arms’ extremities.
Peaks and troughs
Normally the two beams arriving back at the vertex precisely cancel one another out – the peaks of one wave line up with the troughs of the other – which leads to no light at the output. But any passing gravitational wave will distort space, first stretching one of the arms and squeezing the other, before reversing this change as its own peaks and troughs propagate. This causes a corresponding mismatch in the peaks and troughs of the interfering laser beam, resulting in a cyclic brightening and dimming at the output.
Exactly how that light brightens and dims tells scientists about the kind of object or event that generated the gravitational wave passing through the apparatus. The wave detected last September contained a “chirp” – a sudden increase in frequency over a fraction of a second before it died off – which is characteristic of a binary system, in this case two black holes merging to create one bigger, spinning black hole. Aside from telling us that black holes definitely exist, the waveform showed that the merging black holes had about 29 and 36 times the mass of the Sun and that they merged some 1.3 billion years ago.
Ewald Mueller, a physicist at the Technical University of Munich and the Max Planck Institute for Astrophysics outside Munich, praises the discovery but also looks forward to other kinds of observations from LIGO and fellow gravitational-wave observatories, particularly the Virgo interferometer near Pisa. Among the most interesting potential quarries, he says, will be merging neutron stars – whose gravitational waves should tell us how matter behaves at extremely high densities – and collapsing stars, or “supernovae”, whose gravitational waves are emitted ahead of electromagnetic radiation, potentially providing an earlier and more direct view of stars’ “supernova engine”.
“This discovery has opened a new window on the universe,” says Mueller. “It allows us to look inside objects that previously we couldn’t study directly.”
The reason gravitational waves have only now been detected, even though scientists have been trying to snare them since the 1960s, is the incredible weakness of their interaction with matter. Indeed, Einstein thought they would never be discovered. The wave registered last September distorted space by just one part in a thousand billion. The arms of LIGO and Virgo are very long – measuring 4 and 3 km respectively – in order to convert such relative changes into as large an absolute change as possible, but the resulting contraction and expansion of LIGO’s arms was still minute – about one thousandth the width of an atomic nucleus.
Unfortunately, such changes are far smaller than those caused by many common sources of background “noise”, such as seismic waves, temperature fluctuations or even a car passing close by. When LIGO first started operating, between 2002 and 2010, it used relatively simple techniques to reduce noise and as a result detected nothing. But over the following five years it was refitted with more advanced components, and the improvement was dramatic – it detected the merging black holes just two days after restarting normal operations.
The European connection
LIGO owes much of its success to a smaller interferometer located outside Hannover known as GEO600. Run by a collaboration of research groups from Germany and the UK, GEO600 has become a kind of technology incubator, having in particular pioneered advanced techniques for isolating the mirrors inside an interferometer’s arms. LIGO’s mirrors were initially hung using a single piece of steel wire, but the scientists of GEO600 developed a cascade of three pendulums, each one hanging from the one above and with !the mirror at the bottom.
Next year, according to GEO600 research group leader Hartmut Grote of the Albert Einstein Institute in Hannover, the LIGO collaboration should install another technology that has been running on the German-based machine: one that reduces interference from the fluctuating electromagnetic fields that quantum mechanics tells us emerge even in an otherwise perfect vacuum. Grote and colleagues can reduce the most harmful aspect of this “quantum noise” – its phase (how much it shifts the laser beams’ peaks and troughs) – at the expense of increasing a more benign property, its amplitude. Another key to the gravitational-wave hunt is Virgo, which is run by a European collaboration under Italian-French leadership. Like LIGO, this device is also being refitted. According to collaboration spokesperson Fulvio Ricci of the University of Rome “La Sapienza”, the upgraded machine should have a sensitivity to gravitational waves that is comparable to that of the American facility. Ricci says that the new-look Virgo should start operating by the end of 2016, a couple of months after LIGO restarts.
Several European countries are also planning a very ambitious mission known as the Laser Interferometer Space Antenna (LISA) that would consist of three spacecrafts flying in a triangular formation to form three pairs of virtual arms each several million kilometres in length, along which they would fire laser beams. The scientists involved don’t envisage launching the roughly €1 billion mission until 2034, but the European Space Agency did launch a precursor spacecraft called LISA Pathfinder last December to test some of the mission’s technologies.
Not to be outdone, a collaboration of European physicists is planning a next-generation gravitational wave observatory known as the Einstein Telescope that would be 100 times more sensitive than existing facilities. It would comprise three pairs of 10 km-long arms in a triangular shape buried some 100 m underground to limit seismic noise, and would also employ cryogenically-cooled mirrors to reduce thermal interference. It would cost at least €1 billion and would not be ready for well over a decade, says Ricci, who adds that the collaboration hopes to start work on a technical design next year.
But is there really a need for such large, expensive devices? Grote has no doubt. “We are just at the start of gravitational wave astronomy,” he says. “The development of telescopes didn’t stop with Galileo, since it is always possible to look more precisely at the stars. So it is with gravitational wave detectors. Science will go there as long as society gives us the funding.”