An international team of scientists excitedly announced that they had directly observed gravitational waves, often described as ripples in the fabric of spacetime. The discovery of gravitational waves confirms a prediction that Albert Einstein made nearly 100 years ago to shore up his general theory of relativity.

The detection was made by the twin Laser Interferometer Gravitational-wave Observatory (LIGO) detectors, an experiment led by researchers at Caltech and MIT that includes more than 1,000 affiliated scientists, including several Stanford physicists and engineers who have played key roles in the program since it was launched. The instrument systems that made the detection possible were built in part on a legacy of interdisciplinary technological advances made by Stanford scientists.

"LIGO is by far the most precise measurement machine that man has ever built," said Robert L. Byer, the William R. Kenan, Jr. Professor of Applied Physics at Stanford, and an original member of the LIGO team. "It's finally sensitive enough to see the bells that are ringing in the universe."

The ringing bell that LIGO heard early in the morning of Sept. 14, 2015, was the result of two massive black holes merging together 1.3 billion light-years away. As the two black holes spiraled around each other, they radiated energy in the form of gravitational waves. While they merged into one even more massive black hole, they released three solar masses of energy.

"This was a huge signal," said Martin Fejer, a professor of applied physics. "It's more energy than our sun will release in its entire lifetime, and it all happened in about a fifth of a second as these two massive black holes coalesced."

Although the peak power output of the event was about 50 times that of the whole visible universe, it required an extremely sensitive device to detect the ensuing gravitational waves. LIGO consists of twin instruments, located 1,865 miles apart in Louisiana and Washington. Each of these instruments involves a single laser, each directed into two 4-kilometer-long arms that run perpendicular to one another.

As a gravitational wave passes through a detector, it distorts spacetime such that one arm lengthens, and the other shortens. By comparing the disturbances at the two detectors, the scientists can confirm the direct detection of a gravitational wave.

The detection itself was something of a surprise; the detectors were undergoing final commissioning at the time, and weren't scheduled to enter full-time detection mode for a few days. Brian Lantz, the lead scientist for seismic isolation and alignment systems for Advanced LIGO, was sitting at his desk when the detection appeared in the experiment's online notebook, and it immediately caught his attention.

The scientists had also been on the lookout for a signal that matched gravitational waves from co-orbiting neutron stars, a more anticipated event. But this signal was significantly more energetic and shorter in duration than what would be expected from neutron stars.

"We spend a lot of time making sure that the instruments are very reliable, it doesn't have any funny noise glitches, so you wonder is this a noise glitch or is this a real signal?" said Lantz, who is a senior research scientist in the Gintzon Lab at Stanford. "We [the LIGO team] immediately went through all the things that we know can cause glitches. I started looking around for earthquakes anywhere in the world that might have triggered this, and for funny misbehaviors of our instruments. But we didn't see any."

After carefully considering and eliminating all other possibilities, the LIGO researchers came to the black hole conclusion.

Separating the signal from the noise
Making the detection is incredibly difficult, in part because the amount that a gravitational wave affects the detector arms is incredibly small, only about a thousandth the diameter of an atom's nucleus. To give that some perspective, it's comparable to being able to detect if the distance between the sun and Earth increased by the width of an atom. Eliminating noise from the system has been a central challenge since LIGO's inception, and one in which Stanford research has contributed in several ways.

First, the heart of the instrument, its 1-micron, solid-state laser, was developed through Stanford. Compared to technology being used at that time, these lasers were significantly more reliable but, more important, smaller. By scaling down to a single, monolithic chip, the researchers were able to greatly reduce the instabilities caused by acoustic noise.

Hydraulic and electromagnetic systems developed by Daniel DeBra, the Edward C. Wells Professor of Engineering, Emeritus, moves the mirrors to compensate for the tidal stretching of Earth's crust. Motion of the ground is another major source of noise affecting LIGO. DeBra and Lantz have taken the lead on helping implement measures to prevent these motions from influencing the detectors, reducing vibrational disruptions caused by passing trucks and trains, earthquakes and even the moon.

Another source of noise comes from the mirrors that reflect the lasers in the arms of the detectors. Thermal energy in the mirrors causes the mirror faces to vibrate, which can lead to motions that interfere with the observation of gravitational waves. Modeling these effects and finding materials that minimize these vibrations has been a major focus of Fejer's work, which contributed to the eventual design of the mirrors used in Advanced LIGO.

This work continues together with Jonathan Stebbins, a professor of geological sciences, and Riccardo Bassiri, a research associate, who are helping to identify and test high-performance glassy materials to further reduce this noise source in future detectors. Experiments on these materials are being conducted at the Stanford Synchrotron Radiation Lightsource at SLAC National Accelerator Laboratory, with staff scientist Apurva Mehta. Further reduction in the thermal vibrations was obtained through the design of pure silica wires to suspend the mirrors, developed by physicists Norna Robertson and Sheila Rowan.

"While this is a physics experiment, there were challenges in optics and photonics and precision controls engineering and materials science," Fejer said. "Faculty members from all over campus - Applied Physics, Mechanical Engineering, Aeronautics and Astronautics, SLAC and the School Earth, Energy and Environmental Sciences - all participated. The interdisciplinary nature of the faculty at Stanford who enjoy bringing their knowledge and tools to bear on broader problems made Stanford an excellent environment in which to make these contributions to the larger project."

As exciting as it is to finally directly observe gravitational waves, the researchers are already moving toward upgrading LIGO in hopes of probing known space phenomena. Targets include asymmetric neutron stars, compact binary black holes that could reveal clues about the universe's evolution and perhaps even the stochastic background of gravitational waves - those formed at the very beginning of the universe.

"It opens a whole new vista in astronomy," Byer said. "We have a new tool to look at the universe now. Gravitational waves are pure, and they can travel billions of light-years through the full length of the universe. We have some ideas of what we might find, but it also raises the potential to find totally new phenomena."