Alpha Centauri is the closest star system and the closest planetary system to our own. Consisting of three stars, its members include: Rigil Kentaurus, designated α Centauri A; Toliman, α Centauri B; and the famous Proxima Centauri, α Centauri C. The three stars and all the planets surrounding them are around 4.37 light-years, or more than 41 trillion kilometers (25.7 trillion miles), away from all of us here on Earth. (A light-year is just the distance light travels in a year through a vacuum, like space; it’s around 9.46 trillion kilometers, or 5.88 trillion miles.) Now, imagine measuring all that distance with so much precision that you’re only allowed to miss it within the width of a single strand of human hair; that’s the level of precision needed to create and operate the Laser Interferometer Gravitational-Wave Observatory, or LIGO for short, then measure the gravitational effect of two black holes colliding, more than a billion light-years away.
Gather the Brightest Minds
LIGO is a multi-phase collaboration project, and was started by the brightest minds in astrophysics between the California Institute of Technology (Caltech) and the Massachusetts Institute of Technology (MIT), while being funded by the U.S. National Science Foundation (NSF). Later steps of the collaboration included scientists from all over the world—even including contributions from at-home computational projects. At the time of detection of the first gravitational wave back in 2015, its chief collaborators were physicists Barry Barish, Rainer Weiss, and Kip Thorne—with Thorne perhaps best recognized as the conceptual adviser for Christopher Nolan’s 2014 film Interstellar, as well as an adviser for Nolan’s 2020 film Tenet. The three would be awarded the 2017 Nobel Prize in Physics for this very feat.
Build and Operate Your Machine(s)
With planning beginning as early as 1980, LIGO actually consists of a pair of facilities, both located in the U.S.A.; one east of Baton Rouge in Livingston, Louisiana, and another inside desert land in Hanford, Washington, with both sites being constructed identically. They constructed two identical facilities to remove any effects on data from isolated events that only happen to one location (such as tremors), compared to events that happen to both (such as gravitational waves passing through both locations).
The facilities are giant, L-shaped buildings with each “arm” of the L being around 4 kilometers (2.49 miles) long, both containing tubes within which lasers pass through. Since laser light travels in a straight line, both arms of each “L” must be extremely straight too; they even had to raise the ends of each L by about 0.9 meters (1 yard) off the ground to account for the Earth curving beneath them. Each facility is equipped with vibration dampeners and isolation technology so that the scientists can remove any noise in the gathered data from the effects of tremors, passing nearby traffic, and even animals in the vicinity. During operation, a beam of 1-MW (1-megawatt) laser light of a single wavelength is shot from the “corner” of the facility, with a beam each travelling through the two arms of each “L.” To remove the effects of air interfering with the lasers, both 4-kilometer (2.49-mile) arms must be evacuated of air—so much so that they whittle internal pressures down to just a trillionth of normal atmospheric pressure by constantly pumping air out of them over the course of forty (40) days. At the ends of each “arm” sits a big mirror, the smoothest ever created at the time of construction, suspended by two fused silica threads that would shatter like glass at the slightest perturbation. (Fused silica was chosen since it vibrates in such a way that computers in the facility can neglect its vibrating effect on the lasers.) Finally, the lasers reflect from these giant mirrors then travel all the way back to the corner of the “L,” with the lasers from the two arms meeting up again.
Of course, they need all this preparation and precaution; the changes in spacetime from the passing of gravitational waves are proportional to the energy they contain, and they dissipate over time. Mind you, these waves have been travelling through space for at least a billion years before ever reaching Earth. Their effects on spacetime to us are so miniscule that they only “wobble” space with as much change as one ten-thousandth the diameter of a single proton.
Sprinkle In a Little Luck
LIGO, being a developing project with several phases, was of course not immediately constructed as the paragon of science and technology that it was at the time of detection; LIGO actually started with a small proof-of-concept project, with an operational scale model to start things off and help gather funding for itself. It took several pitches to funding committees before LIGO finally got off the ground, and even in those trials they had adjustments to make before settling in on the design that actually detected something pertinent. Yet, despite all these precautions and purposeful overengineering, sometimes you just encounter a little bit of good timing.
On September 13, 2015, scientists in both LIGO sites were finishing up some tests that simulated different sources of interference—shouting employees, magnetic interference, and even vibration testing. Naturally, while testing for these interferences, LIGO must be “turned off,” and cannot gather data. As the tests took too longer than initially planned, they ended up testing into the early hours of the morning. With one test left to do—a test that simulates a truck driver hitting their brakes near the facility—the two teams from both facilities, without communicating with each other, decided to call it a day and pack it up. They returned the two LIGO sites to a data-gathering state, then went home for the night. Their non-coordinated early pack-up might have been their best-timed decision yet, as around 50 minutes later, the gravitational wave passed through both the Livingston and Hanford LIGO sites, stretching spacetime within one ten-thousandth the diameter of a proton. That very small change in distance changed how the two lasers from each “arm” of the “L” arrived back at the “corner” after reflecting off the mirrors at the end, which ended up being detected by the very precise machinery within the site.
At first, the scientists were baffled at how serendipitous these events were. What were the chances that less than an hour after testing preemptively ended, with four days to go before the first official run of that specific phase of LIGO, a gravitational wave would pass through? Had they pushed through with the final test before going home, they never would have detected the wave that just passed through. They double-checked, triple-checked, and quadruple-checked their results—as good scientists ought to do—to make sure that the data gathered were not some anomalous finding or some disturbance that somehow got through all the noise-reducing technology. LIGO even had an internal committee assigned with feeding the team fake data in secret, just to keep the team on their toes and keep them looking out for false positives; that very same team swore they had nothing to do with the data that they just got. It’s for this reason that the time gap between actually detecting the wave then announcing it to the world took so long; after rigorous testing and confirmatory tests, the LIGO team announced the first-time detection of gravitational waves (designated GW150914, Gravitational Wave 2015-09-14) five months later, on February 11, 2016. That same year, we celebrated the 100th-year anniversary of Albert Einstein’s publication of his General Theory of Relativity, which predicted these very same gravitational waves.
Change the World
The detection of gravitational waves served as a vindication for Einstein, as he predicted their existence decades before LIGO even started as an idea in someone’s head. Einstein actually thought they couldn’t possibly exist twice, reversing then re-reversing his own statement. He’d thought we’d never be able to detect something so miniscule in effect—perhaps the most precise measurement humanity had done at that time—and that it would be an impossible task; yet here we are, nearly 5 years after its first detection. LIGO has actually detected 49 other gravitational waves since then, in a total of three operational runs. Operations only just stopped during March 2020 due to the COVID-19 pandemic.
Due to relentless efforts from the brightest minds in the field of theoretical physics and engineering, along with a collaborative effort from other scientists the world over, we now have a new way to peer into the inner workings of the universe. They’re not even done improving LIGO itself; future improvements to its components can make LIGO even more sensitive, as these gravitational-wave events (like black hole mergers) always go off at least somewhere in the universe. Regardless, efforts like these—much like the Large Hadron Collider just an ocean away—serve as markers of humanity’s endless pursuit of understanding the world around us and beyond.
Also, it kind of makes you think what Einstein would have felt had he been alive to see all this happen.
Bibliography
- González, G. (2017, October 5). The story behind LIGO’s detection of gravitational waves. Ideas.TED.com. Retrieved September 1, 2021, from https://ideas.ted.com/the-story-behind-ligos-detection-of-gravitational-waves/
- Twilley, N. (2016, February 11). How the First Gravitational Waves Were Found. The New Yorker. Retrieved September 1, 2021, from https://www.newyorker.com/tech/annals-of-technology/gravitational-waves-exist-heres-how-scientists-finally-found-them
