Fully opening this new window on the universe will take decades—even centuries
Editor’s Note (10/3/17): This year’s Nobel Prize in Physics was awarded to Rainer Weiss, Barry C. Barish and Kip S. Thorne “for decisive contributions to the LIGO detector and the observation of gravitational waves.” This article is being resurfaced to highlight scientists’ plans to hunt for the elusive spacetime ripples in the near and far future.
A century ago, when Albert Einstein first predicted the existence of gravitational waves —subtle ripples in spacetime produced by massive objects hurtling through the cosmos—he also guessed they could not ever be seen. Although the echoes of distant celestial symphonies must ripple through the very fabric of reality, Einstein thought their ethereal harmonies were destined to remain eternally unheard.
On Thursday, scientists using the Laser Interferometer Gravitational-Wave Observatory (LIGO) proved Einstein both right and wrong, announcing their detection of the first note in a cosmic symphony he predicted no one would ever hear. It was a burbling chirp of gravitational waves produced by the cataclysmic birth of a black hole from the merger of two smaller ones. Emitted in a distant galaxy when multicellular life was just beginning to populate Earth, the waves traveled at the speed of light for more than a billion years to at last wash over our planet last September, taking just seven milliseconds to traverse the distance between LIGO’s twin listening stations in Louisiana and Washington State.
Now, unlike Einstein a century ago who could scarcely imagine gravitational waves ever being seen, the scientists hunting the elusive spacetime ripples already have big plans for more detectors and observatories in the near and far future.
“Imagine light having never been collected in a photograph,” says Janna Levin, an astrophysicist at Barnard College of Columbia University and author of a forthcoming book about LIGO. “The first thing people want to do is just to capture the recording, which is what LIGO has done.”
Soon, astronomers say, LIGO will record and unveil far more than the birth cries of newborn black holes. It and other operational observatories are already looking for ripples from the violent death throes of massive stars and from collisions of city-size orbs of degenerate matter called neutron stars. Current observatories could also help reveal what makes spinning neutron stars called pulsars tick, mapping their starquake-shaken interiors and any centimeters-high “mountains” (which would weigh roughly the mass of a planet because of neutron stars’ extreme density) that could pop up on their surfaces.
Decades from now new generations of space telescopes could capture the mergers of supermassive black holes and glimpse pulsars spiraling to doom down their maws, or see snapping “cosmic strings,” proton-thin intergalactic defects in spacetime that may have been stretched across the infant universe during an inflationary growth spurt. Tracked and timed by radio telescopes, rapidly spinning pulsars can themselves be transformed into galaxy-spanning detectors sensitive to spacetime ripples with wavelengths measured in light-years. Ultimately, the most ambitious gravitational wave observatories astronomers can presently conceive might someday record the hiss of waves emitted in the first fractions of a trillionth of a second after the big bang. Then, cosmologists could watch—could listen—as the first seeds of cosmic structure crystallized from a seething quantum fog.
What most excites scientists, though, is the unknown. “Are there things out there that we’ve never even wrapped our heads around with telescopes?” Levin wonders. “Seeing black holes collide is a golden discovery, but we expected that. What else is out there? I want to see something dark.”
“The skies will never be the same,” says Szabolcs Márka, a physicist and LIGO team member at Columbia University. “Imagine you can touch, smell, taste, and see—and one day you can hear. That day is a glorious day. This is what has happened to us, as humanity. From today, we can hear the cosmos. We can see the unseen.”
The most expensive “nothing” ever made.
Having found its first signal, LIGO is now gearing up to transform them into routine tools for astronomy. The twin LIGO stations each pass laser light back and forth between mirrors along perpendicular four-kilometer-long arms arranged in an L. An incoming wave would slightly warp these arms so that one became longer or shorter than the other by only a few thousandths the radius of a single proton, altering the flight time of the light and triggering a detection. Any number of background noises can scuttle the delicate measurement—LIGO can also hear ocean waves pounding distant coastlines, airplanes flying overhead and even the seismic hum from washing machines.
“We’re trying to detect something smaller than the atoms our detector is built of,” says Imre Bartos, a LIGO member and lecturer at Columbia University. “It doesn’t sound believable, to be honest with you.” That LIGO can hear gravitational waves at all over the cacophony of background noise is due to both stations recently receiving a series of noise-canceling “Advanced LIGO” upgrades that will soon make them 10 times more sensitive than they were during a fruitless first-generation search that occurred between 2002 and 2010. All told, the upgrades bring the experiment’s total cost to more than $1 billion, most of it paid by the National Science Foundation. LIGO’s ultra-high-vacuum hermetic perfection, Márka quips, is “the most expensive ‘nothing’ ever made.”
As Advanced LIGO reaches its maximum sensitivity and plans a third listening station in India, it will work in tandem with other European laser interferometers such as GEO600 and Advanced VIRGO to rapidly make the detection of gravitational waves routine. “We’ve just made a machine that has given humanity a new sense, beyond the usual five,” says LIGO team member Rana Adhikari, a Caltech physicist who helped develop the upgrades. “We’re going to have to learn what it is like to feel the burbling of space with these brand-new gravitational fingers.”
With gravitational waves set to soon ripple through several high-precision laser interferometers on Earth, astronomers will be also be able to locate where exactly each set of the ripples is coming from. LIGO’s first detection of colliding black holes, by contrast, could only be traced to a huge arc of sky over the Southern Hemisphere. Pinpointing the sources of gravitational waves will allow astronomers to point other telescopes their way, boosting the chances of learning more about them via x-rays, gamma-rays, radio waves, neutrinos and more.
Burying the noise
The current generation of ground-based laser interferometers can only go so far, however. The length, number and location of an interferometer’s arms intimately influence its resilience against background noise and the varieties of gravitational waves it can probe. LIGO’s four-kilometer-long arms are the biggest in the world, but at the project’s Louisiana site the encroaching, noisy sprawl of nearby Baton Rouge is getting too close for comfort to the delicate detectors.
Researchers are now planning and building a next generation of even bigger and more isolated detectors deep beneath the ground where hundreds of meters of overlying rock shield against most anthropogenic noises and seismic stresses. In the Kamioka mine in Japan, the Kamioka Gravitational Wave Detector (KAGRA) is already taking shape as workers construct twin sets of three-kilometer arms in newly bored tunnels. Slated to enter operation in 2018, KAGRA will use cryogenically cooled mirrors of sapphire to deliver LIGO-like sensitivity.
After KAGRA, a consortium of European partners is forming tentative plans for an even more ambitious subterranean laser interferometer, the Einstein Telescope, which could come online in the late 2020s at a cost of $1 billion or $2 billion. Although it presently lacks funding and a construction site, its conceptual design calls for dual cryogenic and room-temperature beam lines running through three 10-kilometer arms arranged in the shape of an equilateral triangle rather than an L. That configuration would help it pinpoint the sources of gravitational waves on the sky and allow it to see the longer-wavelength ripples from a wider range of sources including binary white dwarfs, slower-spinning pulsars and intermediate-mass black holes weighing hundreds or thousands of suns. It would also begin to construct a reasonably detailed map of “background” sources of gravitational waves—the accumulated ripples from all the messy, violent mergers and explosions all across the sky.
“People wonder why we are not content with one gravitational-wave detector, why we wish to build bigger ones,” says Harald Lück, a physicist at the Max Planck Institute for Gravitational Physics in Hannover, Germany who is a member of the GEO600 and Einstein Telescope teams. “Like electromagnetic radiation, gravitational waves cover an incredibly large range of wavelengths, and you can’t catch all of them with any single facility.” On the ground, Lück says, the arms of laser interferometers are unlikely to ever exceed 50 kilometers—past that, seismic noise, Earth’s curvature, imperfect optics and the great expense of digging deep tunnels would outweigh any conceivable scientific gains.
Sooner or later, it will simply be cheaper to forsake Earth, to build and operate truly giant gravitational-wave observatories in space.
Space is the place
Whenever the first gravitational wave mission launches into space, radio astronomers will wryly say they were there first. Using radio telescopes on the ground, researchers have already devised space-based gravitational wave detectors using nature’s resources: large numbers of pulsars spread throughout space that spin once every few milliseconds, sending out lighthouse-like beams of light that reach us in regular beats.
