Why Gravitational Physicists Don't Sleep at Night
Meeting the people behind the gravitational wave discovery.
"The best time to run an experiment," said Aaron Buikema, a PhD student in physics at MIT, "is between 8:30 in the evening and 4 in the morning."
It does lead to an unusual sleep schedule, Buikema admitted, but he's been working on something that requires his full attention—improving the detection of gravitational waves as part of the Laser Interferometer Gravitational-Wave Observatory, or LIGO—and the experiments are incredibly sensitive to outside interference. The rumble of mass transit, the chatter of thousands of feet trembling, and other ambient activity creates enough noise to be a nuisance at the prototyping facility located on campus. Late at night, however, that rumble tends to die down.
Buikema told me this on a frigid January day in Cambridge, Massachusetts, roughly a week after Lawrence Krauss, a theoretical physicist at Arizona State University, Tempe, tweeted a rumor that Buikema's lab had observed a gravitational wave. (Disclosure: I'm a student in MIT's science writing program.)
The rumors went viral. Science and Scientific American addressed Krauss's tweet in their respective publications while Sky and Telescope magazine noted that, "A Nobel Prize probably awaits the first direct observation."
Gravitational waves are everywhere
"I'm a little surprised by the attention myself," admitted Peter K. Fritschel, a senior scientist with LIGO at MIT, while the rumors were still flying furiously.
When LIGO confirmed at a press conference at the National Press Club on February 11 that the rumors were true, the public's reaction was intense. Journalists had speculated that this press announcement was a hoax, and there was good reason: In 2011 LIGO had deliberately inserted a fake signal to test the "data pipeline" and guard against human bias. The confirmation garnered praise from even President Obama, who tweeted, "Einstein was right! Congrats to @NSF and @LIGO on detecting gravitational waves—a huge breakthrough in how we understand the universe."
How to Hunt for Gravitational Waves
Gravitational waves were first posited in 1916 by Albert Einstein in his theory of general relativity. The theory is a way of mathematically expressing gravity, that tug of forces between two objects which among other things helps keep us planted on terra firma. In it, Einstein uses a set of coordinates that describe space and time and together which, as any fan of Star Trek can tell you, is known as the space-time continuum. Gravitational waves differ from electromagnetic waves—the waves that make up X-rays, light, and radio waves—because the latter is formed by oscillating electric and magnetic fields. Gravitational waves are made from the vibration of the fabric of space and time itself.
Stitched together in this way, space-time becomes like a mattress or a trampoline. Matter and energy warp the surface like a person jumping on it. This warping is gravity: gravitational waves are the ripples formed along the surface that are cause by this warping of space-time. And, like a pebble dropped into a still pool, these ripples carry energy across the universe. Though postulated in Einstein's theory of relativity, gravitational waves had never before been directly observed. In fact, in the 1960s, when Raener "Rae" Weiss first conceived of the design of what would eventually become LIGO, gravitation had become a mathematical pursuit. Because so few experiments related to gravitation were occurring it had become theoretical as opposed to experimental. That changed as technology shifts, such as improvements in laser technology, made experimental forays, such as LIGO, into gravitational experiments more achievable. Still, it's been a search that has spanned decades. Some LIGO researchers have devoted their entire professional careers to the search.
While the word "observatory" may trigger images of craned necks scouring the stars, LIGO's observatories which there are two, one in Livingston, Louisiana, the other in Hanford, Washington—are firmly earthbound. In fact, the Japanese have one located underground. The reason is simple: Gravitational waves are everywhere.
Each time you move, you create gravitational waves, albeit ones too small to measure. But black holes, made from a neutron star roughly 1.4 times the size of our sun colliding into another neutron star, even a few thousand light years away, will eventually send ripples big enough that can be felt here on Earth—if you have a detector that can measure them.
LIGO looks for gravitational waves using interferometers, devices that measure displacement in waves. If you send a light beam down two corridors of exactly the same length, at exactly the same time, with a mirror at the end of them, the light that returns to you should return at exactly the same time, Fritschel told me. If a gravitational wave passes through those tubes, however, and if the waves are big enough, your mirror is reflective enough, and your light beam fast enough, you'll notice that the laser beam will get slightly longer. This is the signal of a gravitational wave.
But before they could tell anyone, they had to be sure
There's a surprising amount of engineering in quantum physics. LIGO is looking for incredibly small measurements, as when a gravity wave passes the laser grows by only 1/10,000th the diameter of a proton, according to LIGO. If an atom were the size of a football field, its nucleus would be smaller than a marble. The proton located inside the nucleus would be even smaller. The technology that makes these kinds of discoveries possible are not available off the shelf. The laser that LIGO uses is infrared because it has longer wavelengths which are more likely to register a disturbance, and goes through a four stage process to make it powerful enough to detect gravitational waves. Developing it has pushed the limits of available tech to develop a laser that could meet their needs. The mirror they use, though more than 99 percent reflective to the specific wavelength of light that their laser emits, looks clear to the human eye.
According to University of Florida Physics Professor Guido Mueller who worked on the LIGO optics, LIGOs mirrors are layers of precise coating on a base of fused silica. Like LIGO's lasers, it first had to be conceived, designed, and tested before being implemented. It's a setup so sensitive that something as seemingly miniscule as how much epoxy is used to hang the mirrors in the vacuum tubes can impact the result. In fact, LIGO has the two facilities, partly as insurance. If one location detects a disturbance, it could be due to something other than a gravitational wave, but only a gravitational wave could cause a disturbance in facilities located 2,300 miles apart.
LIGO's inception was happenstance. Weiss, an experimental physicist, had been tapped to teach a course on gravitational physics. "I didn't know much about gravitational physics," he told me from his office a gray day in late January. The space, roughly 10 feet by six feet, is teeming with books, and papers, evidence of his long tenure at MIT. "It was all I could do to keep ahead of the students, and sometimes they were ahead of me."
At the time, in the early 1960s, the biggest experiment in detecting gravitational waves involved metal rods. The theory went that when they detected gravitational waves, the rods would effectively "sing" from the vibration. Weiss, who needed to present the research to his class, found that he didn't fully understand the concept experimentally. Rather than continue to struggle with the concept, he devised a method that he felt was theoretically more rigorous, using a clock and laser beams. The concept would undergo a few more rounds of refinement, with the clocks being substituted for mirrors, and as the project grew in complexity, more and more researchers.
When the NSF approved the LIGO construction project in 1990 as part of joint Caltech/MIT Project founded in 1984 with Kip Thorne, Ronald Drever, and Weiss, it was done with the understanding that the resulting detectors likely would not be sensitive enough to detect these elusive gravitational waves. Building it was the only way to get the kinks out, however.
In 2010, LIGO was taken offline for updates and repairs. When it was rebooted in the fall of 2015 for an engineering test a few days before the official search was to begin, and while the world was celebrating the 100th year anniversary of Albert Einstein's general theory of relativity, this upgraded LIGO, called Advanced LIGO, found a gravitational wave.
But before they could tell anyone, they had to be sure. In 2014, a team of astronomers using the Background Imaging of Cosmic Extragalactic Polarization 2 (BICEP2) telescope—which also hunts for gravitational waves, albeit slightly different ones than LIGO—announced that they had found gravitational waves. Six months later, they had to retract those results. They'd found not gravitational waves, but dust. LIGO needed to be sure.
Months went by as the LIGO team analyzed and reanalyzed the data from the September detection. A paper was put together, and then torn apart line by line. Though LIGO Laboratory spans three institutions, the California Institute of Technology (Caltech), and the Massachusetts Institute of Technology (MIT) and the National Science Foundation, the LIGO Scientific Collaboration is much larger. The collaboration includes 1,004 scientists at 37 different institutions, from 16 countries, a testament to the complexity of the endeavor
When I asked Weiss how it felt to have made such a historic discovery, he said it was a relief.
"I feel like I finally have this monkey off of my back," he said. "All of these people have devoted their lives, their careers to this and we weren't finding anything. To find something makes their work mean something."