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- Herausgeber: Icon Books
- Kategorie: Wissenschaft und neue Technologien
- Serie: Hot Science
- Sprache: Englisch
- Veröffentlichungsjahr: 2018
On 14 September 2015, after 50 years of searching, Gravitational Waves were detected for the first time and astronomy changed for ever. Until then, investigation of the universe had depended on electromagnetic radiation: visible light, radio, X-rays and the rest. But Gravitational Waves – ripples in the fabric of space and time – are unrelenting, passing through barriers that stop light dead. At the two 4-kilometre long LIGO observatories in the US, scientists developed incredibly sensitive detectors, capable of spotting a movement 100 times smaller than the nucleus of an atom. In 2015 they spotted the ripples produced by two black holes spiralling into each other, setting spacetime quivering. This was the first time black holes had ever been directly detected – and it promises far more for the future of astronomy. Brian Clegg presents a compelling story of human technical endeavour and a new, powerful path to understand the workings of the universe.
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GRAVITATIONAL WAVES
How Einstein’s Spacetime Ripples Reveal the Secrets of the Universe
BRIAN CLEGG
For Gillian, Chelsea and Rebecca
ACKNOWLEDGEMENTS
My thanks to the team at Icon Books involved in producing this series, notably Duncan Heath, Simon Flynn, Robert Sharman and Andrew Furlow. I’ve had great support in writing this from LIGO – notably Michael Landry – the ESA and the Max Planck Institute. A particular thank you to Kip Thorne for his wonderfully informative Andrew Chamblin Memorial Lecture at the Department of Applied Mathematics and Theoretical Physics in Cambridge.
CONTENTS
TIMELINE
1846 – Faraday gives his ‘Thoughts on Ray-Vibrations’ lecture in which he speculates in passing that gravitation may involve some kind of travelling wave
1916 – Einstein writes his first paper on gravitational waves as an outcome of his general theory of relativity
1918 – Einstein issues a second paper correcting an error in the first
1922 – Arthur Eddington suggests Einstein’s waves are imaginary, created by the way the mathematics was used
1936 – Einstein writes to Max Born, telling him that after work with Nathan Rosen, he no longer believes gravitational waves exist
1937 – After corrections to his paper with Rosen, Einstein revives gravitational waves, but believes they are so weak they will never be observed
1955 – Joseph Weber studies gravitational radiation with John Wheeler at the Institute for Advanced Study
1957 – Richard Feynman shows that gravitational waves could do work and hence be detected
1958–60 – Weber begins construction of resonant bars in an attempt to detect gravitational waves
1962 – Mikhail Gertsenshtein and Vladislav Pustovoit publish the first paper on the theoretical use of interferometers in gravitational wave detection
1967 – Rainer Weiss publishes first practical design for gravitational wave interferometers
1968 – Kip Thorne begins theoretical work on gravitational wave detection
1969 – Weber claims the first detection
1972 – A detector based on Weber’s principles travels to the Moon on Apollo 17
1974 – Weber’s findings are largely dismissed
1974 – Hulse and Taylor make indirect observation of gravitational waves due to their impact on orbital decay
1975 – Prototype interferometer with 3-metre arms built by German/UK team in Garching
1980 – Funding obtained for planning for a large-scale interferometer in the US
1981 – Prototype with 40-metre arms built at Caltech
1986 – The LIGO project gets its first unifying project director, Rochus Vogt
1991 – The first major LIGO funding obtained
1994 – Ground broken on building the Hanford observatory
1995 – Ground broken on the Livingston observatory
1995 – Work on the European GEO600 detector started
2002 – Initial LIGO goes live and runs to 2010 – no waves detected
2005 – Useable models of black hole interaction and the waves they would produce developed
2006 – GEO600 reaches expected sensitivity – no waves detected
2014 – BICEP2 project team claim to have detected gravitational waves in the cosmic microwave background radiation – later retracted
2015 – Advanced LIGO goes live
14 September 2015 – First gravitational wave detection
11 February 2016 – LIGO first gravitational wave detection made public
1 June 2017 – Detection announcements continue with the third detection with a high confidence
1
14 SEPTEMBER 2015
There are times when those working on a major science project receive public accolades. Typically, it’s when the data from a live science run is released, and what has been an intense period of private work becomes public property, to be dissected by the researchers’ scientific peers and celebrated by the world’s press. But on 14 September 2015, the huge team working on LIGO – more formally, the Laser Interferometer Gravitational Wave Observatory – had no such expectations. No one realised that 50 years of fruitless work was about to be rewarded in an unexpected fashion.
The immense LIGO experiment, covering two sprawling sites in the US and supported by over 1,000 scientists working around the world, was undergoing an engineering run. This was routine technical testing before the gravitational wave observatory would go live a few days later. It was the eighth and final cycle of fine-tuning before things might get interesting. Yet around 7.00am Eastern Standard Time – midday in the UK – a first email was sent out to interested parties that signalled the beginning of the biggest change to astronomy since the introduction of telescopes.
On that day, our understanding of the universe took a leap forward.
The gravity detectives
To call LIGO an observatory appears to be a dramatic understatement, though that is exactly what it is. It comprises two vast sites over 3,000 kilometres (1,865 miles) apart. Each of the near-identical facilities, one based in Livingston, Louisiana and the other at Hanford, Washington state, is home to a pair of 4-kilometre (2.5-mile)-long tubes, 1.2 metres across, set at right angles to each other to form an L-shape. At each site, a laser passes along the pair of tubes to reflect off mirrors at the ends many times before the beams are brought together to form an optical interference pattern, a tiny set of fringes that gives a visible warning of incredibly small changes. The slightest variation in the length of the beams will produce a detectable effect, a change that was expected to happen in the presence of gravitational waves – ripples in the fabric of space and time that had been predicted by Albert Einstein back in 1916, but had never been detected.
The vast twin systems, including those 4-kilometre lengths of metal tubing, contain hardly any air. The presence of vibrating air molecules would scatter the laser beams, introducing ‘noise’ into the carefully monitored signal. Any sound vibrations and air currents buffeting the delicately suspended mirrors located at the ends of the tubes would equally destroy the detection process.
The Livingston detector site, Louisiana.Caltech/MIT/LIGO Laboratory
The Hanford detector site, Washington state.Caltech/MIT/LIGO Laboratory
The pressure inside those tubes is a remarkable trillionth of the atmospheric level. This took 40 days of gradual pumping to achieve, during which time the tubes were heated to over 150°C to expel as much gas as possible from the metal surfaces.
Just getting the tubes ready for that evacuation took immense care. Establishing delicate equipment in remote areas of the United States was not without its problems. The tubes are big enough, and took long enough to construct, for the local wildlife to take up residence. When a member of the team walked through the near-completed tubes at Livingston, he discovered that wasps, black widow spiders, mice and snakes had all moved in. And that meant acid-bearing urine leaving stains on the pristine stainless steel that would release vapour when the air was removed, requiring a major cleaning effort before that vacuum could be established. (That word ‘stainless’ in ‘stainless steel’ doesn’t apply once acids are involved.)
Despite the intense vacuum within the operational tubes, their metal walls are just 3 millimetres thick – around the same as 50 sheets of standard A4 paper. Without the frequent reinforcing loops along the length of the tubes, the outside air pressure would crush them. The exterior of each tube is cased in concrete, not to resist the vacuum, but to cushion any outside impact. This is just as well, as a security truck collided with one of the tubes of the Hanford observatory at night. The driver suffered a broken arm, but the tube stayed intact. A damaged tube, allowing air at atmospheric pressure to pour in, would have been catastrophic. The resulting blast of air would have destroyed most of the detection system, causing many millions of dollars’ worth of damage.
Because the arms extend for such a distance, their supports have to gradually increase in height along their length to cope with the curvature of the Earth. From one end to the other, there is more than a metre difference in height, needed to keep the tube perfectly straight. And this was just a small consideration in ensuring that the detectors can function properly. A far bigger issue was vibration.
To deal with the inevitable environmental vibrations, LIGO has a whole host of feedback systems, which monitor position and make tiny movements of the arms and components to compensate for changes. Positions are monitored 983,000 times a minute – once every 0.000061 seconds. The ‘seismic isolation platforms’ deal with the larger vibrations down to around 1 million times larger than the waves that LIGO has to detect. The remaining reduction is achieved by the remarkable suspension systems used to keep LIGO’s mirrors from moving due to anything other than gravitational waves. These use four separate pendulum suspensions to dampen movement, dangling the mirrors from glass fibres just twice the thickness of a human hair, keeping the 40-kilogram ‘test mass’* mirrors as stable as possible.
One of LIGO’s test masses installed in its quad suspension system. The 40-kg test mass is suspended below the metal frame above by four silica glass fibres.Caltech/MIT/LIGO Laboratory
Business as usual
During the engineering run in September 2015, all of LIGO’s detection systems were in play, bringing the light beams into alignment and testing their functionality, with no thought of capturing a breakthrough observation of a gravitational wave. For over 50 years, scientists had been looking for the tiny distortions in space and time caused by a distant cosmic event that would add a new, powerful approach to the astronomer’s armoury. They had never achieved a single result. Some even suggested that gravitational waves would be impossible to detect unless we could take the leap of building an observatory in space, as the tiniest local tremor was enough to confuse the incredibly delicate instruments. But for now, these worries were put to one side. No careers were at risk of yet another failed detection of these elusive waves on this run. It was simply a matter of ensuring that the technology behaved as it should.
However, just because this was an engineering run did not mean that the observatory was inactive. Unlike the dome of a traditional telescope, with shutters to prevent light coming in, there is nothing that can stop gravity getting through. Gravitational waves may be incredibly weak and difficult to detect, but nothing can hinder their progress across the universe. And the 14 September email told the members of the LIGO collaboration that an unexpected event had occurred.
We still tend to think of astronomers peering directly through telescopes – but even most traditional optical observatories are now automated, their observers located anywhere in the world. Detection in the case of gravitational wave observatories is not about seeing something in the sky, but about pinpointing subtle changes in a stream of data from the instruments. The origins of those first, few cautious emails on 14 September emphasise how far this kind of science has moved the work away from on-the-spot observers. There are people stationed at Hanford and Livingston, but they are mostly engineers, involved in the day-to-day running of the equipment. The earliest email comments from gravitational physicists originated in Hanover, Melbourne, Paris and Florida – the only one from the US (where most of the collaboration were still asleep), and that located far away from either detector.
There was a time when data like this would have to be searched by eye, giving a team of grad students sleepless nights as they worked through page after page of computer printouts, fighting their way along mind-numbing strings of numbers. Now, though, much of that initial sifting is done using computer algorithms. Some of these systems look for specific patterns that models predict will be produced by natural phenomena expected to generate gravitational waves. But the system that flagged up the event on 14 September, the cWB or ‘coherent wave burst’ pipeline, had no such preconceptions. It was merely looking for near-simultaneous bursts of activity recorded at the two facilities. And cWB flagged up that a strong wave pattern had been received at Livingston, followed by a remarkably similar burst of activity at Hanford, 7 milliseconds later.
Event alert
The first response to this alert was to check for hardware injections. During the engineering run, it is normal practice to produce artificial signals to test whether or not the detection systems at the two sites pick them up. But there were no known planned injections made in the period when the detection occurred.
That didn’t mean that the event was certainly a real sighting of a gravitational wave. All kinds of checks still had to be made. After all, this wasn’t supposed to be an observation run; many oddities could occur in the systems as they were being fine-tuned. For that matter, there was always the possibility that what was being recorded was a large-scale seismic vibration that had been picked up by both observatories – or even that two totally separate vibrations had just happened to occur at the same time. And the LIGO team were always aware of the unnerving possibility of a blind injection.
Although the scientists could confirm that there were no routine hardware injections planned, what was being detected could have been an artificial event that had been intentionally triggered without the scientists’ knowledge. Such ‘blind injections’ play an important role in the operation of a complex set of instruments like LIGO. They make sure that those involved aren’t allowing their own prejudices and desires to influence their interpretation of the results. After all, the observations they make are simply variations in an ever-changing data stream. How that data is interpreted is crucial, and because the scientists never know whether an event is artificial or real until they have fully analysed it, they can’t be biased by wishful thinking.
Blind injections had already been used in previous runs of LIGO, raising hopes on two occasions when it appeared more and more likely that an observation of gravitational waves had been made, only to have those expectations crushed when the secrecy was lifted to reveal a fake event. In theory, blind injections weren’t needed during an engineering run, as no one was intending to take the data seriously, so it seemed unlikely that this was the case on 14 September – but at this stage of the process, the scientists had no way of knowing for sure.
Over the next two days, excitement grew. The event seemed more and more likely not only to be a real one, but also to provide a very significant discovery. No one had expected gravitational waves to be obvious in the data stream, but these were clear, visible signals – so strong that, were this a real detection, they had both found gravitational waves and made the first-ever direct observation of black holes. In which case, the team was surely looking at a Nobel Prize. More than that, their work – which some still believed was pointless, because they thought that LIGO wasn’t sensitive enough – would have been the first step into accessing the mysteries of the universe in a way that had never been possible before. They were genuinely at the frontier of science. Yet it would be many months before the details could be made public. Months during which the teams had to lie to colleagues and repeatedly try to quash the rumours that began to fly around the scientific community. The countdown to gravitational wave astronomy had begun.
Before we can follow how the LIGO discovery unfolded in detail, we’ve some groundwork to do. We’ll see how Einstein predicted the existence of gravitational waves almost exactly a century before the discovery (a coincidence that would itself make some wonder if the whole thing was a hoax). We will uncover the controversy surrounding early attempts to detect gravitational waves using massive metal bars, explore the brave step into the dark that led to LIGO despite, rather than thanks to its management, and discover the remarkable cosmological events involving black holes and neutron stars that make gravitational wave detection possible.
First, though, we ought to sort out the most fundamental aspect of the whole discovery. This is all about gravitational waves – but what do we actually mean by a ‘wave’?
* The LIGO mirrors are known as ‘test masses’, as it is the effect of gravitational waves on their mass that is used in detection.
2
WHAT IS A WAVE?
Everyone has come across waves – those ripples on the surface of the water that you see if you drop a stone into a pond, or the moving walls of foam and brine that come crashing onto a beach, sometimes with devastating force. But to get the hang of gravitational waves we need to take a step back from the specific examples and understand what’s going on beneath.
The anatomy of a wave
At its most basic, a wave is a movement in a substance, where that movement changes cyclically as it travels forward. The most familiar form, like those waves on the beach, are known as transverse waves – their cyclic motion alternates at right angles to the direction the wave is travelling – up and down in the case of water waves, or side-to-side when we send a wave along a rope by flicking it.
A very simple transverse wave looks like this:
The wave is moving left to right, with the distance covered by a complete cycle of the wave known as the wavelength, and the number of such cycles that occur in a second being its frequency. A wave needs a ‘medium’ – stuff to actually do the waving, though in some cases, such as light, the nature of that medium is not immediately obvious. In the case of ocean waves, the medium is straightforwardly the water. A frequent misunderstanding is to assume that it is the water that moves forward – or more generally the medium – but actually it is the wave. Think of a Mexican wave travelling around a stadium (a Mexican wave is a transverse wave as the cycle of the motion is up and down, while the wave travels at right angles to that direction, round the stadium). The medium here is the mass of spectators who bob up and down. But they stay in their seat positions – they don’t move forward around the stadium, only the wave does.
The other common form of wave is the longitudinal or compression wave. Perhaps the most familiar form of a longitudinal wave is sound, or the kind of wave you can send down a Slinky spring by giving its end a quick push. Here the cyclic motion isn’t at right angles to the direction the wave is travelling, but back and forth in the same direction. Going at right angles wouldn’t work for a sound wave, as it goes through the middle of the medium – the air. If it tried to go side-to-side, it would quickly lose its energy battling against the other air molecules. Transverse waves usually have to travel along the edge of the medium – for example, on the top of the water that the wave passes through. For a longitudinal wave, the regular cycle is in the same direction as the wave moves forward, not at right angles. The medium is repeatedly squashed up and relaxed like a concertina, so what travels through it is a pattern of compression and rarefaction.
A simple longitudinal wave looks like this:
When you speak to someone, your vocal cords start a compression wave in the air that spreads out from your mouth until those compressions and rarefactions reach the listener’s ear. There, they vibrate the hair-like structures in the ear, producing the sensation of hearing. But the link between you and the listener is the longitudinal waves that pass through the air.
Waves are very common occurrences in nature. Apart from waves on water and in sound, we find them, for example, travelling through the ground as a result of earthquakes. And you may well have been taught at school that light is a wave. In that example, though, we have to be a little more careful. Light certainly can act like a wave, but it’s a little more tricky to pin down exactly what it is. It’s worth getting an understanding of light, though, because it is the basis for almost all current astronomy – the discipline that gravitational waves have the potential to transform.
The model wave
For centuries, there have been arguments about the nature of light. We all are familiar with it, but it’s intangible. It’s difficult to pin down its nature. Some early scientists, such as Isaac Newton, argued that light was made up of a stream of particles. This would make sense of its value to astronomers. A stream of particles can flow across the vacuum of space to reach our eyes and telescopes. But a light wave shouldn’t be able to cross totally empty space, because there’s no medium to do the waving. Despite this restriction, others, notably Newton’s contemporary Christiaan Huygens, thought that the light was a wave. Increasingly, over time, the wave theory of light became stronger, notably when it was observed that light displayed a common behaviour of waves called interference, which would prove hugely important in the gravitational wave story.
Imagine simultaneously dropping two stones, a few centimetres apart, into a still pond. The ripples – waves – that the stones create will head outwards from the two locations that the stones hit the water until those waves meet. When they do, there will be points on the surface of the water where both waves are rippling in the same direction (up or down) at the same time. Here, the waves will reinforce each other, becoming stronger than before. At other points on the surface, the waves will be rippling in opposite (vertical) directions at any point in time. Here the waves will cancel each other out, leaving relatively still patches of water. This effect, producing a distinctive pattern on the surface, is known as interference.
An interference pattern in water.Shutterstock
In 1801, English scientist and polymath Thomas Young showed that light behaved exactly the same way as those ripples, apparently proving that it was a wave. When two beams of light were sent through nearby slits and the resultant beams overlapped, the result was an interference pattern of dark and light fringes. But there was a problem. As we have seen, unlike sound, light happily travels through the vacuum of space, where there is no medium for it to wave in. So how could it work?
Initially, the only possible explanation was that there was some kind of invisible, undetectable material that filled all space. This was known as the ether. But this would have to have been a very strange material – so insubstantial that we can’t directly detect it, yet so rigid that light could travel through it for vast distances without losing energy to the floppiness. The remarkable Scottish physicist James Clerk Maxwell worked out in the early 1860s that light was an interaction between electricity and magnetism. And this meant that in principle, you could have an electric wave creating a magnetic wave, creating an electric wave and so on, hauling itself through empty space by its own bootstraps without any material medium required – it is the electromagnetic field that acts as the material.
This was the position in the early years of the twentieth century. However, quantum theory would blow a hole in comfortable Victorian assumptions. The work of Max Planck, Albert Einstein, Niels Bohr and others showed that light appeared to be both a wave and a stream of particles. Although it was convenient for many purposes to think of light as behaving like a wave, the particle idea explained more phenomena. As the great American physicist Richard Feynman would later put it: ‘It is very important to know that light behaves like particles, especially for those of you who have gone to school, where you were probably told about light behaving like waves. I’m telling you the way it does behave – like particles.’
If you ask a physicist today what light is, they may well say that it is a travelling excitation in a quantum field. This is also a valuable analogy for light – though none of these descriptions provides us with its true nature. It’s not that any are wrong – but each is just a kind of analogy. Light isn’t a wave, or a stream of particles, or a disturbance in a quantum field, it’s light. Each of these is a useful way of thinking about light in some circumstances. It’s what’s called in science a ‘model’ – not an actual description of reality, but a way of describing it that makes useful predictions.
So, we can say that light (sometimes) behaves like a wave, but not that it is a wave. This is in contrast to gravitational waves, which if they were to exist would actually be waves. Models are immensely useful, because we can rarely examine nature perfectly. We have to make do with what we can measure and detect and from that we build a model to describe how it behaves. But what is important for us in the story of gravitational waves is that light gives us a way to access distant parts of the universe. Unless it interacts with matter, light will carry on travelling indefinitely. There is light out there that has been travelling through space for billions of years. This makes it possible to examine different parts of the universe, and to see what it was like in the past. Light takes time to reach us, so the further it comes, the further back in time we are looking.
