LIGO’s capability to detect gravitational waves allows scientists to observe celestial mergers, like binary black hole coalescence.

The Laser Interferometer Gravitational-wave Observatory, more popularly known as LIGO, first detected gravitational waves on September 14, 2015.

LIGO is more a feat of engineering than one of science—this discovery was predicted by Einstein’s theory of relativity nearly a century ago. Attempts at detecting gravitational waves began with the work of Joseph Weber in the 20th century; he claimed to have developed a detector capable of detecting gravitational waves, and even looked into generating gravitational waves in a laboratory (Weber, 1960). Although his discoveries have been subject to scrutiny, with scientists accusing him of manipulating data, he is often credited for starting the effort to detect gravitational waves (Cho, 2016). After the National Science Foundation (NSF) began funding LIGO in 1987, nearly three decades passed before the first detection of a gravitational wave.

Figure 1. The setup employed by LIGO. The light from the laser source is split through the beam splitter; when the beams return, they recombine and hit the photodetector. Distortions in space time manifest when the light strikes the photodetector, allowing researchers to model observed gravitational waves or verify phenomena that are observable through other means. (Image Credit: B. P. Abbott et al., 2016)

The setup of the LIGO interferometer can be found in Figure 1. The LIGO interferometer utilizes distortions in relative distance between the two four-kilometer arms to realize a ”time delay.” The changing relative distance alters the time needed for the lasers to return to the beam splitter and re-combine. This delay manifests in a misalignment of the two laser beams when they recombine and hit the photo-detector. Rather than having two light waves in phase, they experience destructive interference.

When the photoreceptor absorbs the two light waves, if they were in phase, they would be expected to strike the some location, whereas if not, they would strike elsewhere. Then, through complex modeling, researchers can match the observed gravitational wave to reconstructed gravitational waves.

To accurately detect fluctuations in space of less than 10−19m, LIGO needs to account for a myriad of external variables that would appear as noise in their spectra. First, active damping is taken to prevent the effects of seismic movement. Even seismic vibrations produced by trucks on nearby roads can contribute noise to the gravitational wave.

To further reduce that noise, the two four-kilometer arms of the LIGO detector (Figure 1) are in a vacuum with a pressure equivalent to one trillionth of that at sea level because air particles can refract light. Other measures taken by LIGO to improve their signal to noise ratio include stabilizing the laser and creating pristine 40kg mirrors (LIGO, 2017a).

With the level of precision afforded by masterful engineering, there have been six LIGO detections to date, with one reported but yet to be published (in chronological order from newest to oldest: GW170817, GW170814, GW170608, GW170104, GW151226, GW150914. There have been five detections of binary black hole coalescence, and the most recent detection was the detection of a binary neutron star merger.)

Beyond confirming another aspect of Einstein’s general theory of relativity, an already monumental discovery, LIGO detections yield another means of observing massive celestial occurrences, especially unobservable events. For example, GW150914, the first LIGO detection, was unobservable, occurring at a distance of around 410 Mega-parsecs from our earth. For comparison, Andromeda is approximately 778,000 parsecs from Earth.

Figure 2. A binary neutron star merger observed by the Swope and Magellan Telescope in Chile. (Image Credit: Ryan Foley, UC Santa Cruz Carnegie Observatories)

They also serve as additional verification for observable phenomena: in the case of GW170817, the binary neutron star merger was observed by the Swope and Magellan Telescope (in Chile) in conjunction with LIGO. An image of the Swope and Magellan observation can be found in Figure 2. Through LIGO, astronomers and astrophysicists have a new lens to detect celestial events, providing more data for astronomers to understand physical phenomena like star evolution and binary systems.

Even with exciting, new results, LIGO begs the question: is investment in pure science valuable and worth it? The NSF has invested 1.1 billion dollars into the LIGO project over the span of nearly three decades (LIGO, 2017b), and for comparison, the human genome project cost 2.7 billion dollars in public funding. With projected decreases in scientific funding due to budget cuts, should research grants be given to more immediately applicable fields, like research into pressing diseases, or should the government continue to invest heavily in the pure sciences?