IT RESEMBLED the Oscars, only with physicists rather than actors. Three hundred of them were gathered in a ballroom in Arcadia, California; another 100 were connected by video link. All of them were waiting for the opening of an envelope.
What the event might have lacked in glamour (sorry, physicists), it made up for in drama. In contrast to the Hollywood awards, the note inside would either make them all winners, or all losers.
The drama had begun six months earlier when scientists around the world had noticed a peculiar signal.
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They were looking for gravitational waves – ephemeral ripples in the fabric of the universe that are the last untested prediction of Einstein’s general theory of relativity. It is thought they can be sparked by the collision of stars, the formation of black holes and the great violence of the big bang itself. By the time they travel enormous distances across space and reach Earth, however, the disturbances are only about one-thousandth the width of the smallest atomic nucleus, making them fiendishly difficult to detect. Still, their discovery would revolutionise the way we study the universe.
With that as a goal, it is no wonder a whole generation of gravitational wave researchers have spent their careers developing equipment capable of picking up these ultra-subtle signals. Long on ambition but short on sensitivity, the detectors have seen more than half a century of innovation and improvements to isolate them from the slightest disturbances. The effort has brought us to the threshold of detection, but the dogged pursuit has come at a price. “An awful lot of people have become conditioned into thinking that no matter what signal we think we see, it is bound to be just noise,” says , a gravitational wave physicist at the University of Glasgow, UK.
Which is why the upper echelons of the endeavour occasionally “inject” false signals into the data to keep the rank and file on their toes. Although no one could entirely suppress the suspicion that the peculiar signal they had spotted was one of these tests, the researchers set to work with vigour.
After six months of painstaking analysis, everyone was convinced that what they were seeing were gravitational waves from two dead stars colliding to create a black hole. It had come from the direction of the constellation Canis Major, so researchers referred to it as Big Dog. Having been forbidden from discussing it with the outside world until the analysis was complete, they gathered in California in March 2011 to decide what to do next.
“The collaboration took the decision that it was going to go forward with a paper claiming that it had evidence of a detection,” recalls Hendry, who was in the room for the final hurdle: the opening of the envelope to discover whether the signal had been placed in the system by management. “I remember thinking just how different the world was suddenly going to become if it turned out to be a real signal,” he says.
“The collaboration was going to go forward with a paper claiming detection of a gravitational wave. But the signal was a fake”
When the envelope was opened – in reality a PowerPoint presentation – the world remained the same. Knowing what the gravitational waves from a black hole should look like as they ripple across Earth, the management had secretly arranged to shake the detectors in just the right way and at just the right time. Big Dog was an artificial injection.
But one day soon, it could be a real signal. Gravitational wave detectors on both sides of the Atlantic are currently undergoing major upgrades to heighten their sensitivity. And when the big day comes, it will be as if astronomers have developed a new sense.
So far, the vast majority of our cosmic knowledge has come by looking – using telescopes to collect light and other forms of electromagnetic radiation, including radio, infrared and X-rays. “Now we want to listen to the universe as well,” says Stefano Vitale at the University of Trento in Italy.
Catch a wave
Just how do you detect a ripple in space-time? The , Italy, is at the forefront of gravitational wave detection. It consists of two arms at right angles that each stretch for 3 kilometres across the countryside. Laser light travels simultaneously along the two arms, using mirrors at either end to increase the distance travelled to about 100 kilometres.
Ordinarily, the laser beams should take the same time to complete their journeys. This is checked by bringing the beams together at the end of their respective journeys to see if they remain in synchrony with each other. But if a gravitational wave passes through, it will minutely alter the path length of the laser, first in one arm and then in another. This means that the signals will fall out of step and then return to normal.
The same technique is used at the Gravitational-Wave Observatory (LIGO) in the US. It has two detectors at Hanford, Washington, and one at Livingston, Louisiana. Gravitational wave hunting at the Virgo and LIGO detectors stopped in late 2010, to make way for upgrade work. All are being enhanced to become about 10 times more sensitive. By comparing the signals between the European and American detectors, physicists can triangulate the direction of the incoming gravitational wave and pinpoint its source.
When operations recommence at the three sites in 2016, the scientists involved believe that it will be only a matter of time before the first detection. “Late 2016 or early 2017, that’s about when we expect to have sufficient sensitivity that the odds in favour of detecting something should start to get rather good,” says Hendry.
As joyous as the first detection will be, the work will be far from over. In the same way that the electromagnetic spectrum covers a huge range of wavelengths, from radio waves and microwaves to light and gamma rays, the gravitational wave spectrum is as diverse (see diagram). There are many possible wavelengths given out by different celestial objects. Not all of them are detectable from Earth’s surface.
The ground-based detectors can only pick up the higher frequency waves, which are expected to come from the formation of small black holes caused by either exploding stars or the collisions of dead stellar cores.
If we want to detect the gravitational waves caused by supermassive black holes feeding, or merging with others during galaxy collisions, then we need to move our detectors out into space. Up there, the lower frequency gravitational waves will not be masked by the seismic grumbling of Earth’s interior.
In July 2015, the European Space Agency (ESA) will launch the spacecraft. Built to test the detection technology, it is the crucial first step needed for a wave mission in space.
At the heart of LISA Pathfinder will be two identical cubes of metal, each made from 2 kilograms of gold and platinum. These metal hearts will not be beating; quite the reverse. “They are going to be the stillest thing in the solar system, and I mean really still. Nothing as still as this has ever been made before,” says Vitale, who is principal investigator for the mission.
The test masses will be clamped tightly for launch and then released once the spacecraft is in orbit. After that, they will float freely inside, separated by a distance of about 35 centimetres.
Modified gravity
An onboard laser mechanism will monitor them for changes in their relative motion. It will detect movement as small as one billionth of a millimetre – a picometre (10-12 metres). This is a thousand times more precise than the last such sensor ESA built, which flew on its GOCE mission to measure Earth’s gravitational field from orbit.
If all goes well with LISA Pathfinder, the next time such test masses fly, they will be on the full-blown LISA mission with a date in 2028 or 2034. Then, instead of being just 35 centimetres apart, they will be on three different spacecraft up to 5 million kilometres apart. At this distance, they will be able to use their lasers to detect the slight jostling of the spacecraft as a gravitational wave ripples by.
That would make every gravitational physicist extremely happy; not to mention give them a lot of work to do in analysing the signals. However, in a twist, there may be more science to LISA Pathfinder than anyone originally imagined.
If a small but growing team of scientists and engineers get their way, LISA Pathfinder could transform itself into more than a technology demonstrator. It could become the greatest gravitational experiment since 1919, when British astrophysicist Arthur Eddington confirmed Einstein’s general theory of relativity, the work that extended Isaac Newton’s law of gravity to include Einstein’s own special theory of relativity.
“LISA Pathfinder could become the greatest gravitational experiment since 1919, when Arthur Eddington confirmed general relativity during a solar eclipse”
During a solar eclipse, Eddington showed that light from a distant star cluster was bent on its way to Earth as it passed by the sun. This “lensing” of starlight is something that only happens where gravity is exceptionally strong. It was not anticipated by Newton in his law of gravity but was correctly predicted by Einstein. Eddington’s confirmation was possible because telescopes had become precise enough to measure the effect.
Most physicists assume that Newton’s law only breaks down in strong gravity, near a massive celestial body such as a star, galaxy cluster or a black hole. Elsewhere, they assume that gravity behaves as Newton prescribed: dropping by a quarter every time you double the distance from the centre of a galaxy. This thinking has led to the introduction of dark matter, invisible stuff that provides the gravitational glue keeping individual galaxies rotating, and binds galaxies to one another in clusters. Although no one knows what dark matter is, most astronomers believe it exists.
Yet many observations can be explained if instead a galaxy has a stronger grip on some stars than Newton predicts. The most famous of these “modified gravity” scenarios is called modified Newtonian dynamics or MOND. Proposed in the 1980s by Mordechai Milgrom, then at Princeton University, MOND only applies in places where the acceleration due to gravity falls below a certain minuscule value.
The locations of such places were pinpointed in 2006 by Jacob Bekenstein of the Hebrew University of Jerusalem in Israel and João Magueijo of Imperial College London. Their calculations revealed certain regions in the solar system, known as saddle points, where the gravity of all the planets, the moons and the sun would cancel out (see “Back in the saddle“).
If LISA Pathfinder can be made to pass through or close to one of these saddle points, then its system could test Newton’s law of gravity down into the modified gravity regimes. “If we see something we don’t expect, we can pretty concretely say gravity is not what we thought it was; there’s ‘gravity plus’,” says Ali Mozaffari, also at Imperial College.
Together with Magueijo and others, Mozaffari is investigating a possible extension to the LISA Pathfinder mission. The closer the spacecraft gets to a saddle point, the greater the precision of the test that involves monitoring the separation of LISA Pathfinder’s metal cubes. If the instrument works as expected and the spacecraft can approach to within 50 kilometres of a saddle point, a MOND-like modification of gravity would stand out like sore thumb. Even with a catastrophic miss by about 400 kilometres, you would still get a strong enough signal to claim a discovery, says Mozaffari who published his latest insights last year ().
Since Milgrom’s original proposal, many versions of modified gravity theory have appeared. Whether a signal can be used to pin down a particular one is a question of ongoing work. “A lot of people have provided predictions about what their theories would say,” says Mozaffari. But to actually tie a signal to a particular theory is harder, he says. “It’s work in progress.”
Getting close to a saddle point and seeing nothing unexpected would be easier, because it would allow tight constraints to be placed on modified gravity theories, perhaps even ruling some of them out completely.
To do this experiment, ESA would have to authorise an extension to the LISA Pathfinder mission for when the gravitational wave technology has been completely demonstrated and understood. Vitale says the demonstration must remain the mission’s top priority. “I wouldn’t say that I am ready to go to the saddle points yet,” he says. “If we have to do extra tests for our gravitational wave detector that will be our highest priority.”
But he does feel the lure of the saddle points. “This is certainly a unique instrument and when you put a unique instrument in a unique place you are always going to learn something,” says Vitale.
Without doubt, there is a palpable excitement about both LISA Pathfinder and the chances of a gravitational wave detection with a ground-based instrument. “I’m still as enthusiastic as day one,” says Vitale, “You have to be. If you don’t believe, it doesn’t happen.”
As for when that historic first detection will be made, it came up as the final question for debate at a conference in July. The notional date the gathered scientists plumped for was 1 January 2017.
Hendry hopes this is not the case. It may not be the Oscars but, as a Scot, he’s got another party to go to the night before. “I pointed out that I for one would not expect to be anywhere near a computer on that date,” he says.
Back in the saddle
A gravitational saddle point is the place part way between two celestial objects where their gravitational fields cancel out each other. It is where the balancing point between two objects would be if they were placed on an enormous set of scales.
Saddle points are similar to the , which are better known because they provide a good location for astronomical instruments. Indeed, the European Space Agency’s LISA Pathfinder mission is destined for a Lagrangian point 1.5 million kilometres away from Earth towards the sun. Spacecraft placed at one of these points will feel a force of gravity that keeps them in position relative to Earth as our world orbits the sun. What makes saddle points different is that they are where the gravitational fields cancel out. Pass through a saddle point and the force of gravity would momentarily drop to zero.
As LISA Pathfinder approaches the Earth-sun saddle point, its payload will detect this drop and see whether it follows Newton’s law of gravity (see main text).
This article appeared in print under the headline “Ear on the universe”
