How Louisiana's LIGO Proved Gravity

Several weeks ago, the word went out: gravitational waves, long predicted by physicists, had finally been detected. It might not have been earth-shaking news, but it was news that the earth was shaking, and the world’s eyes briefly turned to Livingston, Louisiana, home of one of the pair of detectors that had tracked the jiggle in the cosmos. I visited the Laser Interferometer Gravitational-Wave Observatory (LIGO), a Caltech project, in March, and was shown around by scientist Anamaria Effler and facility science educator William Katzman. I left with my head spinning, a reinforced respect for the awe-inspiring forces that govern our universe, and a renewed admiration for the people who spend their lives expanding human knowledge.

Why Livingston? First, and perhaps most practically, it was far enough from Hanford, Washington, where the other detector was to be built. Since gravitational waves travel at the speed of light, the detectors have to be far enough apart that a wave traveling at light speed doesn’t hit them at the same time; the tiny lag between detections helps researchers pinpoint the direction from which the wave came and allows them to check their data against one another. If both detectors are active, but only one comes up with a reading, there’s a good chance the signal came from a local source (for example, the Louisiana detector is uncomfortably close to an active logging operation). Second, there was a lot of buy-in from local stakeholders: state government was in favor, and LSU wanted to be involved. LSU had been part of an earlier, ultimately unsuccessful, effort to detect gravitational waves using supercooled aluminum bars, and so had built-in institutional knowledge and interest in the subject. Livingston is also far enough from the urban clamor of a major city (detectors need to run in quiet locations), but close enough to airports that scientists and equipment can easily arrive.

Gravitational waves are difficult to picture because they are very small and extremely counterintuitive. They don’t travel through space, like light or radio waves, and they don’t travel through matter, like sound waves do through air; they are waves in the very fabric of space-time itself. This is hard to envision, but the distinction is important in understanding how they’re detected and why doing so is so difficult. It turns out, probably luckily, that space is fairly rigid, so it takes a lot of gravitational energy to make it ripple; inconsistencies are evidence. For instance, if you build two objects that are exactly, perfectly the same length, to a fanatically precise degree, and you check them and check them and check them again, and one day they are briefly but demonstrably not the same length, something’s up.

The LIGO detector consists of two four-kilometer-long metal tubes set at right angles to one another and encased in larger concrete tubes that protect the sensitive detector from things like lightning and confused animals. A building containing offices, a control room, and detectors sit at the vertex. (If, like me, you’ve never learned to think in terms of kilometers, that’s about two and a half miles.) The metal tubes are the straightest human constructions in the world, to the extent that they’re elevated at the far ends to compensate for the curvature of the Earth. They are also, Effler explained, incredibly, fantastically empty—the closer to an absolute vacuum, the better the results. The vacuum-production process took months: after all the air was sucked out, the tube had to be heated in sections to evaporate every last bit of water; the resultant vacuum is emptier than the outer space between Earth and the Moon. 

When the detector is active, the lengths of the tubes are constantly measured by bouncing lasers off of specially calibrated mirrors at the far ends. These lasers should return to the vertex at exactly the same time, canceling each other perfectly and emitting no light. On September 14, 2015, they didn’t; a wobbly little burst of light didn’t cancel its counterpart out. The Hanford detector received the same signal, with the correct amount of time delay. The data were checked, and the signal was compared to Einstein’s predictions. The shape of the waves corresponded to the pattern two medium-sized black holes would produce upon colliding. 

A picture emerged: over a billion years ago, two medium-sized black holes, about twenty-nine and thirty-six times the mass of the Sun, passed fatally close to one another and began to spiral inward. In the dizzying final microseconds before they collided, they whirled around each other at over half the speed of light. Their collision converted about three solar masses of matter into gravitational waves, putting out a brief burst of energy more intense than the output of the entire visible universe. A new black hole consolidated out of the chaos, and the unfathomable energy of the collision rippled outward at the speed of light. In the meantime, intelligent life evolved on Earth, beings capable of encasing hypersensitive laser-mirror constructions in metal and concrete tubes built such detectors just in time to record the faint echoes of this cataclysm.

The operators present at both observatories during the crucial moment were Louisiana graduates, one from Southern University and one from LSU, reinforcing Louisiana’s role in this enormous, worldwide project. But the detection, while exciting, also contained an aspect of hurry-up-and-wait while researchers ensured that it was neither a planned test (occasionally run without warning, like a fire drill, to ensure the equipment is working) nor a false signal planted by outside pranksters. They then had to wait to collect more readings before sending the measurements to data scientists, who were able to confirm the suspected discovery. Over one thousand scientists eventually signed off on the final version of the paper announcing the news.

The Internet has already predicted that Nobel prizes will be awarded for this work. And whether this actually comes to pass or not, being widely assumed to deserve one is in itself an honor.

What does this mean for science? First of all, it proves that gravitational waves exist—they were widely believed to, but actual proof had been elusive. It also finally allows scientists to observe black holes; because even light can’t escape a black hole’s powerful gravitational force, so we can’t see them like we can other celestial bodies. Gravitational waves are also an early indicator of cosmic phenomena: if LIGO scientists get an interesting read, they can alert their peers in time to have light or radio telescopes positioned to detect whatever-it-is for themselves. With the planned addition of more detectors, this ability is expected to become significantly more powerful.

One of the great goals of modern physics has been to reach a Theory of Everything (TOE). The basic problem is that there are essentially two versions of physics: quantum mechanics governs the very small (atoms and sub-atomic particles) while general relativity takes the wheel for larger objects (things you can actually see, ranging up to galactic-scale objects). Both of these systems are internally consistent within their respective domains, and both have been proven and refined experimentally countless times—but they simply do not mesh. I asked Effler if the gravitational wave results would get scientists closer to a TOE. It wouldn’t immediately, she said, but might in time: Now that we can observe black holes and neutron stars, we can sort of “stress test” relativity to see what happens under extreme conditions. Katzman was more bullish: “We don’t know what this will let us do yet—the people who invented lasers didn’t know they’d be used for eye surgery and grocery checkouts.” 

On the third Saturday of every month, LIGO holds its Science Saturdays. From 1 pm–5 pm, LIGO staff members give tours of LIGO’s control room and open the exhibit hall to the public. There are also activities in the lobby centered on a LIGO-related theme, which change monthly. More details at ligo.caltech.edu/LA/page/Science-Saturdays.