DUSK has descended on Rattlesnake Mountain. A thin crescent moon hangs over the ridge, while Venus shines through thin wisps of cirrus clouds. The Yakima people call Rattlesnake “the land above the water”, apparently because it once stood untouched while floods ravaged the plains below.
Today the treeless ridge stands 1000 metres high overlooking a silent sagebrush-covered steppe in the east of Washington state. Its silence holds secrets. “…The mountain’s folds and shadows / roll with stars, soft April greens, and lupine / belying missile silos hidden in catacombs / and the waste of 50 years of atomic bombs,” wrote Washington’s poet laureate, Kathleen Flenniken, who grew up nearby.
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In December 1942, US military personnel flying over Rattlesnake saw what they described as the perfect “isolated wasteland” in which to produce plutonium for the wartime push to build an atomic bomb. The fissile core of Fat Man, which laid waste to Nagasaki, was made here at the Hanford nuclear site. At the height of the cold war, it was home to nine nuclear reactors and five fuel-processing factories.
Fly over Rattlesnake today and two rather different, enigmatic features stand out: a pair of concrete pipes, kilometres long and metres in diameter, shooting off arrow-straight at right angles to one another on the stark plain below.
The ghosts of Hanford’s nuclear past are gathered in by the night, and the silence seems to intensify. But for what is going on inside those concrete tubes, no ordinary silence will be enough.
Albert Einstein’s shadow looms large over the past century of physics. Though he would have baulked at the association, the Hanford nuclear site owes its existence to E = mc2, his equation that describes how mass is just a concentrated form of energy waiting to be released.
Mass-energy equivalence was born of Einstein’s special theory of relativity of 1905. This was the theory that fixed the speed of light, c, as an ultimate cosmic speed limit, and began to distort long-held notions of an absolute, unchanging, separate space and time.
But special relativity was merely an hors d’oeuvre to general relativity. Laid out in 1915, Einstein’s masterwork fleshes out the full connections between mass, the force we call gravity, and a pliable fabric on which the universe’s events play out: space-time.
In Einstein’s universe, gravity is not a magical force that two objects instantaneously exert on one another, as Isaac Newton envisaged in his universal law of gravitation more than two centuries earlier. Instead, a massive object warps space-time around it, much as an iron ball would dent a stretched sheet of rubber. The distortions the sun introduces into the surrounding space-time forces Earth and our companion planets to run rings round it; Earth’s warpings of its environment keeps our feet pinned to the ground. Gravity just follows the curvature of space-time.
In the past century, general relativity’s predictions have been tested over and over again, from the British astronomer Arthur Eddington’s efforts in 1919 to show how the sun bent passing starlight during a solar eclipse, to modern measurements of galaxy clusters warping light from even-more-distant quasars. The theory has never been found wanting. But oddly, one of its central predictions has never been directly verified, even after all these years.
It was Einstein himself who first ventured it. In a paper called simply “Über Gravitationswellen” – “On Gravitational Waves” – he suggested that if matter warps space-time, perhaps moving matter shakes it so as to create waves. The equations of general relativity at least seemed to indicate it was possible: Einstein showed how a rotating oblong object – something like a very large American football – might generate ripples in the universe’s fabric.
But he wasn’t quite convinced. Almost two decades later, together with a young colleague, Nathan Rosen, he titled another paper sent to thePhysical Review “Do gravitational waves exist?”, only now to answer the question with a cautious “no”. The paper never appeared: the journal’s editor, John Tate, sent it on to an anonymous referee. . He did not take kindly to being peer-reviewed.
But that gave him time to realise the mistake in his calculations and reverse his prediction before the paper was eventually published in 1937 in the . The existence of ripples in the fabric of reality, extending every which way throughout the cosmos, finally had Einstein’s personal seal of approval.
“The master,” says Vernon Sandberg, proudly pointing to Einstein in a photograph taped to the corner of his whiteboard. It is . He sits at the centre of the front row, to present-day eyes an iconic figure outshining the anonymous-looking luminaries of the young science of quantum mechanics who surround him: Niels Bohr, Werner Heisenberg, Max Planck, Erwin Schrödinger.
Trained as a theorist, Sandberg now swims in a sea of experimentalists at the Hanford (LIGO), which stands beside the old nuclear site. Sandberg first fell in love with Einstein’s ideas about space and time in the early 1970s. It was around this time that Joseph Weber, a physicist at the University of Maryland, claimed the first detection of gravitational waves.
Weber’s detectors are now museum pieces: one literally, at the Smithsonian Institution in Washington DC. Another . Beautifully machined, giantly proportioned cylinders of aluminium, they were once suspended from steel wires some 1000 kilometres apart on the Maryland campus and the Argonne National Laboratory near Chicago, Illinois. In 1969, Weber reported that the cylinders had vibrated in tandem 17 times over an 81-day period. A seismic, acoustic or electromagnetic trigger for this sudden resonance was not apparent. “This is good evidence that gravitational radiation has been discovered,” he wrote in .
“In 1969, aluminium cylinders 1000 kilometres apart in Illinois and Maryland vibrated in tandem. A seismic, acoustic or electromagnetic trigger was not apparent”
He had enough reason to be optimistic. Follow the lessons of general relativity, and gravitational waves should be everywhere. The many violent events that define the cosmos should all generate ripples that spread through space-time at the speed of light, from the collapse of massive stars as they go supernova, to the spiralling-in and merging of dense objects such as neutron stars and black holes, to the violent convulsions of space-time soon after the big bang.
And yet the consensus today is that Weber was mistaken. To set the cylinders vibrating as he claimed, the nearby universe would have to be filled with extremely strong sources of gravitational waves, but that doesn’t seem to be the case.
To this day the only widely accepted evidence that gravitational waves exist is indirect. In 1974 the astrophysicists Joseph Taylor and Russell Hulse discovered a double-star system 21,000 light years away. At least one of them is a fast-rotating neutron star – a pulsar. Observations of the two bodies show that they are spiralling inwards towards mutual obliteration exactly as they should if they are losing energy by radiating gravitational waves.
A further indirect piece of evidence that does not yet have the benefit of decades of confirmation came in March this year, when astronomers at the BICEP2 experiment at the South Pole claimed to have seen the imprint of primordial gravitational waves on the cosmic microwave background, the frigid, all-pervading radiation left over from the big bang.
If we have yet to find gravitational waves directly, Sandberg has a reason. “Space-time is really, really stiff,” he says. Something like a thousand billion billion times stiffer than diamond, so even mighty cosmic events generate pitifully weak, murmuring waves in it. A gravitational wave from the death tango of two neutron stars each with a mass of 1.4 suns – the benchmark used by gravitational-wave astronomers – would squeeze or stretch the 4.3 light years between us and Alpha Centauri by less than half the width of a human hair.
That distortion would be eminently measurable, if we could build an instrument that stretched from here to Alpha Centauri. As things stand, LIGO’s rather more modest arms are probably our best chance.
Similar laser interferometers have been built in a handful of locations across the globe – in Germany, Italy and Japan – expressly to catch gravitational waves, but LIGO is the biggest. Its two arms, each 4 kilometres long, are identical in almost every respect. One extends out from the experiment’s main building towards Rattlesnake Mountain and the other lies at a right angle, pointing towards the decommissioned reactors of the Hanford nuclear site.
Each contains a steel pipe housing a vacuum through which one half of a split laser beam pings back and forth, bouncing off a mirror at the far end. Back at the main building, this reflected laser beam is reunited with its twin that has zipped up and down the other arm.
Any change in the length of either arm caused by a passing gravitational wave distorting space-time would alter the patterns of constructive and destructive interference that form when the two light waves recombine. Gravitational waves squeeze space in one direction while stretching it in the other, making the LIGO detector with its arms splayed at right angles doubly sensitive: one arm of the interferometer should lengthen and then shorten as a gravitational wave passes, while the other shortens and lengthens.
But forget a hair’s breadth. A standard-issue gravitational wave is expected to expand and contract LIGO’s arms by just 10-19 metres, a distance one ten-thousandth the width of a proton.
“Gravitational waves are expected to expand and contract LIGO’s arms by just one ten-thousandth of the width of a proton”
If you ever saw a movement that small in just one place, you could never be sure that it wasn’t just an innocuous blip, so just as with Weber’s bars, LIGO-Hanford is one of a pair. Its twin is 3000 kilometres away down by the bayou in Livingston, Louisiana. See the same signature in one and 10 milliseconds later in the other (the time it would take a wave to travel between the two) and then you would be talking.
Or perhaps not: to make that delicate a measurement, every possible disturbance, every tiniest reverberation that might put things out of kilter must be suppressed. Only absolute silence will do.
Winter’s first cold snap has hit as I arrive at Hanford, Rattlesnake looming to my left as I drive along Highway 240. The still-warm surface water of the Yakima river flowing alongside is evaporating and condensing, making the river look like a steaming hot spring. Every shrub and tree lining the river is covered in fine frost, a picture-postcard of winter stillness.
Parts of the Hanford nuclear site count as some of the most contaminated spots on Earth, dotted with underground tanks containing leftover plutonium, uranium, caustics and acids. Jokes abound of radioactive rabbits roaming the site. For LIGO, though, the problem is the 18-wheeler clean-up trucks that bump along the gentle undulations of Highway 240. “That’s a big low-frequency whomp into the Earth,” says .
Vibrations of air or earth is noise that might throw the LIGO instrument “out of lock”. It is a phrase I hear often from Landry, a rugged but soft-spoken Canadian physicist who came to Hanford in 2000 and never left. After a decade working on the basic instrument, he is overseeing an upgrade due to be ready at the end of the year. Like almost everyone else at LIGO, he spends his days preoccupied by noise.
To increase the detector’s sensitivity, light is made to bounce back and forth many times along the interferometer arms. In the upgraded “Advanced LIGO”, the aim is for 100 reflections, making each 4-kilometre arm effectively 400 kilometres long. For that to work, the reflecting mirrors must be held unerringly still.
The world conspires against that in an almost literal sense. The problems start with Earth’s ceaseless seismic groans. Then there are the distortions of its surface caused by the moon’s changing gravitational pull as it orbits. That alters the length of LIGO’s arms by about a tenth of a millimetre every 12 hours – a tiny amount, perhaps, but huge compared with what a passing gravitational wave is expected to be capable of. But this source of noise is, at least, predictable, and the frequency of the laser entering the interferometer, and the distance between its mirrors, is continuously adjusted to compensate for the passage of the tides.
The mirrors themselves hang free on wires of fused silica barely a millimetre thick, isolating them from motions of the ground as far as possible. A befuddling stack of blade springs and pendulums helps to filter out high-frequency noise. Servo motors pitch, roll and yaw the entire 5-tonne stack to counter lower-frequency ground motions. They are a product of harsh lessons learned in the early days of LIGO-Livingston, where the low-frequency din of heavy machinery working at logging sites in the surrounding forests was a constant and unpredictable bugbear. “It was like a continuous cacophony of noise punctuated by some falling trees,” says Landry.
His catalogue of unexpected and irritating noises is long. Mysterious, irregular, Doppler-shifted acoustic noise at LIGO-Hanford coincided with arrival and departure times at Richland airport some 20 kilometres away. Then there were the times every few nights in spring when a mysterious noise would roll to a sudden crescendo and ebb away slowly – the rumble of rushing meltwater released from nearby dams. Sometimes, the sources of noise were neither local, nor obvious. “When we were running, we saw storms in the North Sea,” says Sandberg. “We certainly see storms when they hit the coast at Seattle.”
For LIGO, Hanford’s winter stillness is only skin-deep.
I never saw LIGO’s mirrors, each one a refulgent 40-kilogram disc of highly polished fused silica. “If you move around them, you see this beautiful florid display of colours,” says Sandberg. “They are gorgeous.”
It is no mean feat to get the mirrors to that state. A company in Los Angeles first gives their surfaces a super-polish to make them glassy. Near San Francisco, material is etched away molecule by molecule so they assume the required curvature to within the width of an atom of silicon. From there, they are flown to France, where the surfaces are given a final coating to constrain the thermal jiggling of the mirror’s molecules to known frequencies. They vibrate together rather like the ringing of a crystal wine glass – just another source of noise to take into account.
“And what do we do with them?” says Sandberg. “We put them in a vacuum tank where no one will ever see them again.” LIGO’s mirrors and laser beams are protected within the largest volume of ultra-high vacuum anywhere below near-Earth orbit. About 10,000 cubic metres of it ensure that no stray molecules deflect the laser beams as they dart back and forth. The price of my entering even the vicinity of this vacuum is to stick my shoes in a rotating scrubber to rid them of dirt, slip on overshoe covers and cover my head in a bouffant-style cap, the better to contain my own unruly molecules.
“The mirrors and laser beams are protected by the largest volume of ultra-high vacuum anywhere below near-Earth orbit”
“Our asset,” says Sandberg, “is a whole lot of nothing.”
In the cliché of the Wild West, tumbleweed blows where a whole lot of nothing is going on. At Hanford, it skitters frequently across the steppe and piles up against LIGO’s concrete arms, creating a fire hazard. Besides noise, catching tumbleweed and baling it like hay is a constant preoccupation.
In theory, the basic LIGO detector was sensitive to waves from standard sources up to 65 million light years away. After a decade of operations in which no hint of a gravitational wave was seen, we now at least have an upper limit on how large such waves can be. (Taylor and Hulse’s presumed source, though in cosmic terms nearby, is rotating so slowly that its signal frequency is well below the threshold of LIGO’s hearing. Just before its two stars finally coalesce its chirping should become audible to gravitational wave detectors on Earth, in about 300 million years or so.)
The Advanced LIGO detector coming on stream this year will extend its cosmic reach tenfold, and the volume of space it can search a thousand-fold – a huge leap in this game of statistics. The past decade has also served to tame or characterise all known sources of conventional, or classical, noise that might move its mirrors and mimic or hide a gravitational-wave signal, from seismic disturbances to the thermal jostling of individual molecules. If a gravitational wave passes it will be seen, says Landry with quiet confidence. “We are not looking to push down upper limits any longer. The goal now is detection.”
Not so fast, says the quantum world.
Einstein never liked quantum theory. Even as he as posed for his picture with the stalwarts of quantum mechanics at the Solvay conference, he was arguing against their favoured depictions of reality as an arena governed by rules of chance. Over the years, that sentiment only grew. “God does not play dice with the world,” he took to saying.
In this Einstein seems to have been wrong. The quantum world is rife with dice-throwing deities. At the heart of our relationship with it lies a principle whose validity Einstein debated over decades with Bohr: quantum uncertainty.
According to the uncertainty principle, there are pairs of properties in the quantum world that we can never know simultaneously with complete accuracy. Position and momentum form one such pair: the better we know where a quantum particle is, the less we know where it is going. Energy and time are another. This sounds ominous for LIGO, since knowing the precise timing and energies of photons pinging on mirrors is the key to catching a gravitational wave.
Odder still, the noise of uncertainty smears not just light, but also its absence. A gravitational wave’s passage will ultimately be traced in the patterns of light and dark fringes where LIGO’s two laser beams recombine and interfere constructively and destructively. When there is no wave, the darkness should be complete. But quantum uncertainty says that even in darkness there will be light. The quantum fluctuations of the vacuum of space-time will generate photons where there should be none, and obscure the subtle shifts of light and shade that signal a passing wave.
So even when every source of noise in the classical world has been identified and accounted for, the effort to find the missing piece of Einstein’s relativity might ultimately be limited by the theory he struggled to accept. “I find it somehow a very beautiful twist of fate,” says physicist of the Massachusetts Institute of Technology. “It’s kind of a poetic injustice, if you will.”
“The effort to find the missing piece of Einstein’s relativity may ultimately be limited by the theory he struggled to accept”
The physicist Carlton Caves first understood this bizarre quantum noise in the early 1980s, and showed how you might eliminate some of it. Uncertainty does not limit how much we can know about single quantities, but pairs of them in tandem, so it implies a certain elasticity. If you need to measure a photon’s time of arrival, for example, you might constrain its uncertainty by agreeing to know less about its energy.
That is perfectly doable: starting with one photon you can squeeze it to make two whose uncertainty is concentrated in one of a pair of properties, allowing the other to be measured almost noise-free. Even odder, in theory at least you can do the same thing not to a photon of light, but to an absence of one. Gradually reduce the stream of light entering a squeezer to nothing, and a squeezed field of nothing will emerge from the other end, a vacuum whose nothingness has been manipulated in just the way you wanted. “It’s a pretty remarkable thing,” says of the Australian National University in Canberra. “It’s almost like magic; it’s vacuum but it’s not.” A few years back, his team actually built a nothing squeezer. By shining squeezed vacuum onto a photodetector, they reduced the detector’s intrinsic noise to below its natural floor.
“This is beyond spooky, right?” says Mavalvala. Together with her MIT colleague Sheila Dwyer, she has since sent squeezed vacuum into the LIGO interferometer, and seen its quantum noise drop, too. The detector becomes quieter than nothing.
It will be at least 2017 before squeezed light regularly enters the LIGO vacuum. Then, when the surrounds are quieter than quiet, our ear may finally be attuned to the gravitational whispers of space-time. Sandberg hesitates before letting his mind savour the implications. “We’ll be able see what goes on right up to the event horizon of a black hole, we’ll be able to see details inside of a neutron star exploding,” he says.
He pauses, unable to think up things more fantastic. “I am too impoverished in my imagination, and so is everybody else at this point, to figure what we are really going to see with this stuff, and what we are going to use it for,” he says. “It really will be exciting. It’ll represent over a 100 years’ worth of damn hard intellectual effort to try to figure out what the hell is going on with the most fundamental aspect of the relationship between matter, energy and fundamental concepts of distance and space. I can’t think of anything deeper and more profound.”
The floods of Native-American lore may never return to Hanford’s plains. But perhaps soon, we will become aware of a different sort of wave crossing them. That first detection will be an epochal event, says McClelland. “It will be like being able to hear for the first time.”
This article appeared in print under the headline “Einstein’s silence”
