March 2022

Essays

Founding NEMO

By Paul Davies
An photograph of the spacewarping effect, taken with the NASA/ESA Hubble Space Telescope, and depicting GAL-CLUS-022058s, located in the constellation of Fornax.

An example of the spacewarping effect, taken with the NASA/ESA Hubble Space Telescope (depicting GAL-CLUS-022058s,
located in the constellation of Fornax). The image of the more-distant galaxy is distorted by the gravity of the intervening galaxy to form
a smeared-out arc. ESA/Hubble & NASA, S. Jha. Acknowledgement: L. Shatz

Gravitational waves and the secrets of the universe

Gold has long exercised a peculiar fascination for mankind. The dream of alchemists was to transmute base metals into gold, an endeavour that proved fruitless. The origin of gold remained a mystery until well into the 20th century, when physicists came to realise that this precious metal, like almost all the chemical elements, was made by stars. But the details remained hazy until August 2017, when astrophysicists witnessed, in a mere fraction of a second, the creation of enough gold to outweigh the entire Earth. The source of this astonishing stellar alchemy was an event of almost unimaginable violence: the collision of two neutron stars in a faraway galaxy.

Neutron stars are the remnants of large stars that run out of fuel, resulting in their cores imploding catastrophically. They typically possess a mass of about one and a half suns, but squashed into a ball the size of Adelaide, a density so great that even atoms are crushed by the intense gravitational force to form neutrons. Sometimes a pair of neutron stars are locked in orbit around each other, entering a death spiral that terminates in a monstrous encounter when the two objects smash together and then collapse, in an instant, into a black hole.

Spurred on by the events of 2017, a group of Australian scientists is now proposing to build a giant instrument dubbed NEMO – Neutron Star Extreme Matter Observatory – to detect the fine details of neutron star collisions. In the split second it takes for the stars to coalesce, they re-create the conditions that would have prevailed in the universe just after the big bang, so NEMO should provide clues about the very birth of the cosmos. But neutron star mergers happen so fast that studying them demands a highly specialised instrument with a $100 million price tag.

The key to observing these awesome encounters is to make use of one of the strangest phenomena known to science – gravitational waves. That story begins with Albert Einstein’s theory of relativity, published in 1905, with its weird space- and time-warping predictions. In a sweeping extension to his work, outlined in a series of lectures in Berlin in 1915, Einstein unveiled his “general theory of relativity”, regarded by many as the finest intellectual achievement of mankind.

General relativity, as it is usually called, is a theory of gravitation. It toppled Isaac Newton’s account of gravity, formulated more than 200 years previously and used by generations of astronomers to work out the orbits of planets and comets. Einstein’s iconoclastic theory conceived of gravitation in a completely novel way. Newton, famously inspired by the sight of a falling apple, treated gravity as a force of attraction between material bodies that reaches across space and weakens with distance. According to general relativity, however, gravity isn’t a force at all but a distortion in the geometry of space and time. The Sun, for example, creates a space-and-time warp around it and the reason the Earth orbits the Sun along a curved path is not because the Sun pulls on our planet, as Newton described it, but because Earth follows the shortest possible path through the warped geometry.

We are familiar with material objects that bend, such as blocks of rubber. But the idea that space can bend takes some getting used to; most people think of empty space as simply a featureless vacuum. The illusions resulting from warped space are similar to that created by fish-eye lenses and fairground mirrors. General relativity predicts a similar effect, but with the lensing done by space itself. To get some idea of space warping effects, look at the featured photograph. It shows a spacewarp created by the enormous mass of a galaxy, which causes images of more distant objects to be sculpted into distinctive arcs. The light beams from these far-flung objects are bent as they pass through the curved space around the intervening galaxy.

The reason that space can bend is because it is elastic. Nobody noticed that before Einstein, as space is incredibly stiff: it takes an enormous mass to bend it by just a smidgeon. And space can not only bend. It can stretch, shrink, twist and buckle too. The expansion of the universe, for example, can be envisaged as the space between galaxies swelling or stretching. Every day, a hundred billion billion cubic light years of additional space appears within the observable universe. The twisting of space is also observable: Earth’s rotation very slightly drags space around with it, an effect that has actually been measured using gyroscopes carried aboard a satellite called Gravity Probe B, launched in 2004.

Given the elastic nature of space, it is no surprise that it can vibrate too. That much was predicted by Einstein already in 1918. He noticed that the equations of general relativity could describe undulating spacewarps that travel at the speed of light. The best way to envisage these ripples is to imagine their effect on matter. Suppose a gravitational wave were to come at you face on. The distortion of the space inside your body means you could be stretched vertically and squeezed horizontally. A moment later that would be reversed – with vertical compression and horizontal extension – as the wave passed through its cycle.

Needless to say, nobody has ever experienced the physical effects of a passing gravitational wave directly in this manner. That’s because such waves’ traction on matter is extremely small. Even if Jupiter smashed into Saturn, nobody on Earth would feel a gravitational wobble. Contrast this with more familiar electromagnetic waves: stand in front of a military radar antenna and you’d be fried. Electricity is far more powerful than gravity, which is why you can make a balloon stick to the ceiling simply by rubbing it.

For a long time, it was assumed that gravitational waves would be too feeble to ever be directly observed. But, in the 1960s, a handful of visionary physicists thought it might be possible to detect gravitational ripples from exploding stars by carefully suspending a bar of metal in a vacuum chamber and looking for minute vibrations using electronic sensors. By the 1970s, several such bars were in operation, including one at the University of Western Australia designed by physicist David Blair. Nothing was picked up. Ideally, the scientists wanted a supernova explosion in the Milky Way. But the last one to be observed was in 1604, so we might be in for a long wait for the next.

Meanwhile, attention shifted to another source of gravitational waves: neutron stars. Thousands are known to astronomers from the radio pulses they emit. When they occur in pairs, they orbit each other, and in so doing they should emit a steady stream of low-frequency gravitational waves, according to the theory. The waves rob orbital energy from the stars, causing them to spiral together and move faster and faster until they hurtle round each other at a substantial fraction of the speed of light. The equations of general relativity enable scientists to calculate exactly how much energy is radiated in this manner and thus how quickly the orbits shrink. In 1974, two astrophysicists at the University of Massachusetts Amherst, Joseph Taylor and Russell Hulse, began monitoring a binary neutron star system in the constellation of Aquila using the giant Arecibo radio telescope in Puerto Rico (now decommissioned having fallen into disrepair following hurricane damage). Over several years they were able to measure slight orbital changes and compare their observations with the theoretical prediction. The match was perfect. The result was stunning because it proved that gravitational waves really existed and could transport prodigious amounts of energy across space. It provided a huge fillip to the efforts to detect these elusive waves on Earth.

By this stage, the focus had shifted from ringing bars to a completely different method of detection, involving very precise laser measurements that would reveal telltale changes in distance due to the stretching and shrinking of space as a gravitational wave passes. Suppose you fire a laser pulse at a distant mirror and time how long it takes to return after reflection. Knowing the speed of light, that will tell you how far away the mirror is located. Now imagine that a gravitational wave sweeps by and stretches the space between the laser and the mirror. The pulse will get back slightly late. Conversely, if the space is shrunk, the pulse will get back early. That simple notion forms the basis of laser gravitational wave detection.

In practice, timing the pulses individually isn’t necessary; a comparison of laser light travelling in different directions is enough. In the set-up now widely used, the system is L-shaped. Laser beams are sent out perpendicular to each other down each arm of the L and reflected back from distant mirrors. When the light returns, the two reflections are merged. A passing gravitational wave will stretch one arm of the L but shrink the other. If such a change occurs, it can be detected by carefully examining the relative alignments of the peaks and troughs of the light waves from each beam as they are brought together. It is a time-honoured procedure in physics known as wave interference, so the system is known as a laser interferometer.

In 1999, two such interferometers were completed in the US: one at Hanford, Washington and the other at Livingston, Louisiana. The reason for building two widely separated interferometers is because vibrations can be caused by many sources: earthquakes, waves pounding on sea shores, even freeway traffic or cows. To distinguish between a gravitational wave and, say, a horse galloping in a nearby field, the scientists looked for coincident disturbances in both locations, which could only come from space. The whole system is known as the Laser Interferometer Gravitational-Wave Observatory (LIGO).

The precision engineering involved is mind-boggling. The arms – the sides of each L – are 4-kilometre long high-vacuum tubes. The mirrors suspended at the remote ends are manufactured to extraordinary smoothness and reflectivity. The laser beams are allowed to pass many times back and forth down the tubes before being brought together for comparison, thus increasing the effective arm length. But the really stunning statistics come with the sensitivity attained. Gravitational waves have such a feeble effect that even a powerful pulse of them would be unlikely to shift the mirror by as much as an atom’s width. So LIGO was designed to detect changes in the mirror’s position thousands of times smaller than an atomic nucleus (a 10 trillionth of a centimetre) over a distance of many kilometres. For comparison, it is like being able to detect a change of one hair’s breadth in the distance from Earth to the star Alpha Centauri. The sought-after shudders are so tiny that even quantum effects have to be factored in.

For a few years, LIGO saw absolutely nothing and it began to look like the instruments might be a white elephant. The scientists went back for more money to do an upgrade and further tweak the sensitivity of the system. Many fingers were crossed. Then, on September 14, 2015, bingo! A distinct judder was detected in both LIGO detectors with identical characteristics. Furthermore, the disturbance bore all the hallmarks of a burst of gravitational waves emanating from two astronomical bodies spiralling into each other. Calculations soon showed that these objects were too massive to be neutron stars; they had to be black holes, of 36 and 29 solar masses respectively, located some 1.3 billion light years away. The resulting merged black hole had a mass of 62 suns, so the equivalent of about three solar masses had been converted into gravitational wave energy – that’s hundreds of times the total energy our sun has ever emitted as heat during its 4.5-billion-year lifetime, all emitted in a fraction of a second! It was spectacular. All those years of effort had at last paid off. The scientific community was ecstatic. Stephen Hawking told the BBC the detection had “the potential to revolutionise astronomy”. His sentiments were echoed by US President Barack Obama, who tweeted: “a huge breakthrough in how we understand the universe”.

LIGO finally dispelled any doubts about the reality of gravitational waves, almost a century after Einstein first predicted them. It was a tribute to the tenacity of the scientific community and the confidence they placed in the underlying theory. But the 2015 detection was not the end of the project; rather, it was just the beginning. The ability of laser interferometers to act as super-sensitive ears that can listen to the vibrations of the universe ushered in a brand-new era of astronomy. Since ancient times, astronomers have studied heavenly bodies from the light they emit. Then, in the 1950s, radio telescopes were built, followed by satellites that could detect everything from gamma rays, through x-rays and ultraviolet waves, to infrared rays and microwaves. Astronomers have now covered the entire electromagnetic spectrum. But gravitational waves are a completely new spectrum, providing a novel window on the universe.

LIGO soon began detecting other binary mergers, and a European system known as VIRGO also began operations in Italy. Data from both was used to detect and pinpoint the location of a collision of two neutron stars in August 2017, a remarkable event confirmed when a NASA satellite called Chandra almost simultaneously detected a burst of gamma rays emanating from a source 6.6 billion light years away in the same patch of sky. Conventional telescopes spotted a rapidly fading luminous source there with the distinct spectral signature of gold. Astronomers now think impacts between neutron stars have created most of the gold in the universe.

By late 2019, LIGO was running smoothly, and over a period of four months it detected no less than 35 gravitational wave events – a bonanza that had scientists reaching for the champagne. Of these, most were black hole mergers, a few involved neutron stars, and in one case a neutron star was observed being swallowed by a black hole. That brought the total number of events to 90, making it possible to draw some statistical conclusions about the range of black hole masses, their rotation rates and how they formed from the burnt-out cores of massive stars. Since that run, LIGO has been undergoing servicing. Scientists hope that with upgrades and the addition of new laser interferometers in India and Japan, gravitational wave events will be observed on a weekly, if not daily, basis.

Australia has played a key role in the birth of this new discipline ever since the pioneering work of David Blair on resonant bar detectors. The Australian Research Council funds a centre of excellence for gravitational wave discovery, called OzGrav, a consortium involving the University of Western Australia, the Australian National University, Monash University, the University of Melbourne, Swinburne University and the University of Adelaide. In 2020, the Prime Minister’s Prize for Science was awarded to now Emeritus Professor Blair and three colleagues for their critical contributions to the field.

Now that gravitational wave astronomy is a reality, scientists are keen to take the next step. The proposed new Australia-based instrument NEMO will focus on gravitational waves of much higher frequency, which will enable astronomers to follow the messy details of neutronic matter sloshing about as pairs of neutron stars scrunch together and gyrate frenetically in the fraction of a second before they plunge down a black hole.

The other end of the spectrum – ultra-low frequency gravitational waves – is not being neglected either. Ambitious plans are afoot to detect them using a giant space-based interferometer that spans 2.5 million kilometres. Known as the Laser Interferometer Space Antenna (LISA), it is being designed by the European Space Agency. The mission will consist of three spacecraft orbiting the Sun in triangular formation, each containing two telescopes, two lasers and two gold-coated test masses. Each spacecraft will be a zero-drag satellite in which the test masses float freely inside a container that shields them from non-gravitational disturbances such as solar wind.

The events detected by LIGO have frequencies in the range of tens to hundreds of hertz (cycles per second) and wavelengths of a few thousand kilometres. The frequency of the peak power radiated by a gravitational source depends on its rate of change. For two stellar-mass black holes spiralling together, each a few kilometres in size, they end up circling at close to the speed of light, which means they orbit each other hundreds of times a second before they merge.

However, black holes come in a variety of sizes. The Milky Way has a large black hole near its centre with a mass of about 4 million suns. Some galaxies are known with still larger black holes containing the mass equivalent of billions of suns. The gravitational waves generated by these objects are much longer and emitted at lower frequency, perhaps taking many minutes to complete just one cycle of the wave. Although collisions between supermassive black holes will be rare, LISA would be sensitive enough to monitor the entire observable universe for such events. More common would be a supermassive black hole swallowing a neutron star or a stellar mass hole, which should also be detectable.

The history of astronomy has shown that each time a new window has been opened on the universe, unexpected discoveries follow. Radio astronomy, developed in the 1950s, led to the accidental discovery of neutron stars in 1967 by a student, Jocelyn Bell, at Cambridge University. Bell was alerted to something unusual: highly regular radio pulses, which turned out to be the signature of spinning neutron stars (and is the reason these objects are referred to as pulsars). The launch of x-ray satellites in the early 1970s led to the first identification of black holes, after it was found that when a black hole is in orbit around a normal star, it drags away and swallows some of the stellar material, heating it so much on the way that it emits x-rays. It is a similar story for infrared-, ultraviolet- and gamma-ray orbiting observatories.

Few doubt that the same will prove true of the new gravitational window. Because they interact so weakly with matter, gravitational waves can emanate from regions that are shrouded from the view of optical telescopes and other electromagnetic observations. To date, most of the action has centred around black holes and neutron stars, but any movement of large masses will generate gravitational waves. Theoretical astrophysicists have a long list of exotic hypothetical cosmic objects, any of which could be copious sources of gravitational waves displaying distinctive features. For example, some astrophysicists believe that as the universe cooled from the heat of the Big Bang, thread-like entities formed, concentrating enormous mass into infinitesimally thin tubes. These hypothetical “cosmic strings” would be strong sources of gravitational waves as they thrash about. And the most violent event in cosmic history was the Big Bang itself – the explosion that marked the birth of the whole universe. It would have filled the cosmos with gravitational waves that are rumbling through space still. If these primordial disturbances could be detected, they would provide vital clues to the earliest epoch of physical existence and the forces that shaped the universe we see today.

However fertile the imagination of theoretical astrophysicists, there are certain to be phenomena that nobody has yet thought of. Because every physical process is a source of gravitational waves, the promise of gravitational astronomy is that it can, in time, help compile the most comprehensive catalogue of all the objects and systems out there in the million trillion trillion cubic light years of space that make up the observable universe. Armed with this inventory of astronomical entities, scientists will be able to reconstruct the complete narrative of the birth, evolution and likely death of the universe. The obscure vibrations of space predicted mathematically by Einstein a century ago serve as a key to unlocking the secrets of the universe, and this generation will go down in history as the one that began that quest.

Paul Davies

Paul Davies is a physicist and astrobiologist at Arizona State University, where he is Regents’ Professor. His latest book is What’s Eating the Universe? And Other Cosmic Questions.

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