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On September 14, 2015, a transmission reached Earth, bearing details about a duo of distant black holes that had rotated together and merged. The communication had journeyed approximately 1.3 billion years to arrive at our planet at the velocity of light—but it was not composed of light. It was a distinct form of transmission: a trembling of space-time termed gravitational waves that had been first forecasted by Albert Einstein a century earlier. On that date a decade ago, the dual detectors of the US National Science Foundation Laser Interferometer Gravitational-wave Observatory (NSF LIGO) accomplished the inaugural direct observation of gravitational waves, murmurs in the universe that had remained unheard until that instant.
The milestone finding indicated that scientists could now perceive the universe through three varied methods. Light waves, including X-rays, optical, radio, and other frequencies of light, in addition to high-energy particles known as cosmic rays and neutrinos, had been detected previously, but this marked the first occasion that anyone had witnessed a cosmic occurrence via the gravitational distortion of space-time. For this milestone, initially conceived over 40 years earlier, three of the project’s founders received the 2017 Nobel Prize in Physics: MIT’s Rainer Weiss, professor of physics, emeritus (who recently passed away at the age of 92); Caltech’s Barry Barish, the Ronald and Maxine Linde Professor of Physics, Emeritus; and Caltech’s Kip Thorne, the Richard P. Feynman Professor of Theoretical Physics, Emeritus.
In the present day, LIGO, which comprises detectors located in both Hanford, Washington, and Livingston, Louisiana, routinely detects roughly one black hole merger every three days. LIGO now functions in collaboration with two global partners, the Virgo gravitational-wave detector in Italy and KAGRA in Japan. Collectively, this network dedicated to hunting for gravitational waves, known as the LVK (LIGO, Virgo, KAGRA), has recorded a total of around 300 black hole mergers, some of which are confirmed while others are pending further scrutiny. During the network’s current scientific run, the fourth since the initial run in 2015, the LVK has identified over 200 candidate black hole mergers, more than double the figure observed in the first three runs.
The substantial increase in the number of LVK findings over the past decade is attributed to a variety of enhancements made to their detectors—some of which involve state-of-the-art quantum precision engineering. The LVK detectors remain, by a considerable margin, the most accurate instruments ever devised by humanity for the purpose of measurement. The space-time variations induced by gravitational waves are remarkably minuscule. For example, LIGO perceives alterations in space-time smaller than 1/10,000 the diameter of a proton. That equates to 700 trillion times smaller than the thickness of a human hair.
“Rai Weiss originated the notion of LIGO in 1972, and I thought, ‘This doesn’t stand much chance of succeeding,’” recounts Thorne, a specialist in the theory of black holes. “It took me three years of intermittent contemplation and discussions with Rai and Vladimir Braginsky [a Russian physicist] to become convinced this had a noteworthy possibility of success. The technical challenges of mitigating the extraneous noise that interferes with the intended signal were immense. We needed to invent an entirely new technology. NSF was exceptionally effective in guiding this project through technical evaluations and obstacles.”
MIT’s Nergis Mavalvala, the Curtis and Kathleen Marble Professor of Astrophysics and dean of the School of Science, states that the hurdles the team triumphed over to achieve the first discovery are still quite relevant. “From the remarkable accuracy of the LIGO detectors to the astrophysical theories related to gravitational-wave sources, to the intricate data analyses, all these challenges had to be met, and we continue to advance in all these fields,” Mavalvala asserts. As the detectors improve, we yearn for more distant, fainter sources. LIGO continues to be a technological wonder.”
The Most Distinct Signal Yet
LIGO’s enhanced sensitivity is illustrated in a recent finding of a black hole merger designated GW250114 (the numbers represent the date the gravitational-wave signal reached Earth: January 14, 2025). The event was not significantly different from LIGO’s first-ever detection (termed GW150914)—both involve colliding black holes approximately 1.3 billion light-years away with masses between 30 to 40 times that of our Sun. However, due to a decade of technological progress in diminishing instrumental noise, the GW250114 signal is markedly clearer.
“We can discern it clearly, which enables us to test the fundamental principles of physics,” remarks LIGO team member Katerina Chatziioannou, Caltech assistant professor of physics and William H. Hurt Scholar, and one of the authors of a recent study on GW250114 published in the Physical Review Letters.
By scrutinizing the frequencies of gravitational waves emitted from the merger, the LVK team supplied the finest observational evidence recorded thus far for what is termed the black hole area theorem, a concept proposed by Stephen Hawking in 1971, which asserts that the cumulative surface areas of black holes cannot diminish. When black holes merge, their masses amalgamate, leading to an increase in surface area. However, they also release energy in the form of gravitational waves. Additionally, the merger could cause the resultant black hole to increase its spin, which may result in it having a reduced area. The black hole area theorem suggests that despite these opposing influences, the total surface area must enlarge.
Subsequently, Hawking and physicist Jacob Bekenstein concluded that a black hole’s area is correlated with its entropy, or degree of disorder. These discoveries paved the way for subsequent revolutionary research in the domain of quantum gravity, which seeks to unify two foundational pillars of contemporary physics: general relativity and quantum theory.
Essentially, the LIGO detection enabled the team to “hear” two black holes growing as they merged into a single unit, validating Hawking’s theorem. (Virgo and KAGRA were inactive during this specific observation.) The original black holes had a cumulative surface area of 240,000 square kilometers (approximately the size of Oregon), while the final area was around 400,000 square kilometers (roughly the size of California)—a clear expansion. This represents the second verification of the black hole area theorem; an initial verification was conducted in 2021 utilizing data from the first GW150914 signal, but since that data was less pristine, the results had a confidence level of 95 percent compared to 99.999 percent for the new information.
Thorne recalls Hawking contacting him to inquire whether LIGO might be capable of testing his theorem immediately after he became aware of the 2015 gravitational-wave detection. Hawking passed away in 2018 and, regrettably, did not live to witness the validation of his theory.
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observationally confirmed. “Had Hawking been alive, he would have delighted in witnessing the expansion of the combined black hole’s area,” Thorne articulates.
The most challenging aspect of this form of analysis involved establishing the final surface area of the amalgamated black hole. The surface areas of black holes prior to merging can be more easily discerned as the duo spirals together, disturbing space-time and generating gravitational waves. However, once the black holes merge, the signal becomes less distinct. During this so-called ringdown phase, the resulting black hole resonates like a struck gong.
In the recent investigation, the scientists meticulously gauged the specifics of the ringdown phase, which enabled them to compute the mass and rotation of the black hole and subsequently ascertain its surface area. More exactly, they managed, for the first time, to confidently identify two separate gravitational-wave modes within the ringdown phase. The modes resemble the distinctive tones a bell produces when struck; they share somewhat similar frequencies but diminish at varied rates, making them difficult to recognize. The enhanced data for GW250114 permitted the team to extract these modes, confirming that the black hole’s ringdown transpired exactly as mathematic models based on the Teukolsky framework—formulated in 1972 by Saul Teukolsky, who currently holds a professorship at Caltech and Cornell.
Another research from the LVK, submitted to Physical Review Letters today, establishes constraints on a theorized third, higher-pitched tone within the GW250114 signal, and performs some of the most rigorous tests thus far concerning the accuracy of general relativity in detailing merging black holes.
“A decade of advancements empowered us to conduct this remarkable measurement,” Chatziioannou expresses. “It required both of our detectors in Washington and Louisiana to accomplish this. I can’t predict what will occur in another 10 years, but in the initial decade, we have achieved significant enhancements in LIGO’s sensitivity. This not only accelerates the rate at which we uncover new black holes but also allows us to collect intricate data that broadens our understanding of the fundamental characteristics of black holes.”
Jenne Driggers, the lead senior scientist for detection at LIGO Hanford, mentions, “It requires a global collaboration to realize our scientific ambitions. From our sophisticated instruments to the precise calibration of data, validating and assuring the quality of the data, searching the information for astrophysical signals, and packaging it into a format that telescopes can swiftly interpret and act upon, there are numerous specialized tasks that come together to ensure LIGO’s success.”
Pushing the Boundaries
LIGO and Virgo have also revealed neutron stars over the last decade. Like black holes, neutron stars arise from the explosive demise of massive stars; however, they are less massive and emit light. Notably, in August 2017, LIGO and Virgo observed a monumental clash between a pair of neutron stars—a kilonova—that hurled gold and other heavy elements into space and attracted the attention of numerous telescopes worldwide, capturing light from high-energy gamma rays to low-energy radio waves. This “multi-messenger” astronomy event marked the inaugural occasion that both light and gravitational waves were recorded from a single cosmic event. Currently, the LVK continues to notify the astronomical community about possible neutron star collisions, prompting telescopes to scan the heavens for signs of kilonovae.
“The LVK has made significant progress in recent years to ensure we’re providing high-quality data and alerts to the public in under a minute, enabling astronomers to search for multi-messenger signs from our gravitational-wave candidates,” Driggers states.
“The global LVK network is vital for gravitational-wave astronomy,” asserts Gianluca Gemme, Virgo spokesperson and research director at the National Institute of Nuclear Physics in Italy. “With three or more detectors working in harmony, we can identify cosmic events more precisely, extract richer astrophysical insights, and facilitate swift alerts for multi-messenger follow-ups. Virgo is proud to contribute to this global scientific initiative.”
Other scientific achievements by LVK include the first identification of collisions between a neutron star and a black hole; asymmetric mergers, where one black hole significantly outweighs its companion; the identification of the lightest black holes known, contesting the notion of a “mass gap” between neutron stars and black holes; and the most massive black hole merger observed to date, with a combined mass of 225 solar masses. For context, the previous record holder for the most massive merger had a total mass of 140 solar masses.
Even in the years preceding LIGO’s data collection, researchers were laying the groundwork that enabled the field of gravitational-wave science to flourish. Breakthroughs in computational simulations of black hole mergers, for instance, allow the team to extract and scrutinize the faint gravitational-wave signals produced across the cosmos.
LIGO’s technological advancements, dating back to the 1980s, encompass several transformative innovations, such as a novel method to stabilize lasers through the so-called Pound–Drever–Hall technique. Conceived in 1983 and named after contributing physicists Robert Vivian Pound, the late Ronald Drever from Caltech (one of LIGO’s founders), and John Lewis Hall, this technique is currently utilized in various fields, including the creation of atomic clocks and quantum computers. Additional innovations involve cutting-edge mirror coatings that nearly perfectly reflect laser light; “quantum squeezing” tools that permit LIGO to overcome sensitivity limits set by quantum physics; and novel AI techniques that could further minimize certain types of undesirable noise.
“Ultimately, what we are doing within LIGO is safeguarding quantum information and ensuring it remains undamaged by external factors,” Mavalvala explains. “The methods we are developing serve as foundations of quantum engineering and have applications across a wide array of devices, including quantum computers and quantum sensors.”
In the years to come, the scientists and engineers of LVK aspire to continue refining their instruments, extending their reach deeper into the cosmos. They also intend to leverage the knowledge gained to construct another gravitational-wave detector, LIGO India. Establishing a third LIGO observatory would significantly enhance the accuracy with which the LVK network can pinpoint gravitational-wave sources.
Looking further into the future, the team is exploring a concept for an even larger detector, referred to as Cosmic Explorer, which would feature arms measuring 40 kilometers in length (the twin LIGO observatories each have 4-kilometer arms). A European initiative, called the Einstein Telescope, also has ambitions to develop one or two vast underground interferometers with arms exceeding
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10 kilometers in length. Observatories of this magnitude would enable researchers to detect the earliest black hole mergers in the cosmos.
“Merely a decade ago, LIGO presented us with a fresh perspective on gravitational waves and transformed humanity’s understanding of the universe,” states Aamir Ali, a program manager in the NSF Division of Physics, which has backed LIGO since its beginning. “There exists an entire cosmos to discover through this entirely novel viewpoint, and these recent findings indicate that LIGO is merely at its inception.”
The LIGO-Virgo-KAGRA Partnership
LIGO receives funding from the US National Science Foundation and is managed by Caltech and MIT, which together envisioned and constructed the initiative. Financial backing for the Advanced LIGO project was primarily provided by NSF, with contributions from Germany (Max Planck Society), the United Kingdom (Science and Technology Facilities Council), and Australia (Australian Research Council) playing a pivotal role. Over 1,600 researchers from across the globe engage in the undertaking through the LIGO Scientific Collaboration, which encompasses the GEO Collaboration. Additional collaborators can be found at my.ligo.org/census.php.
The Virgo Collaboration presently comprises around 1,000 members from 175 organizations in 20 distinct (predominantly European) nations. The European Gravitational Observatory (EGO) accommodates the Virgo detector near Pisa, Italy, and is financed by the French National Centre for Scientific Research, the National Institute of Nuclear Physics in Italy, the National Institute of Subatomic Physics in the Netherlands, The Research Foundation – Flanders, and the Belgian Fund for Scientific Research. A comprehensive list of the Virgo Collaboration groups is available at: https://www.virgo-gw.eu/about/scientific-collaboration/. Further details can be accessed on the Virgo site at https://www.virgo-gw.eu.
KAGRA is the laser interferometer featuring a 3-kilometer arm length located in Kamioka, Gifu, Japan. The hosting organization is the Institute for Cosmic Ray Research at the University of Tokyo, with co-hosting from the National Astronomical Observatory of Japan and the High Energy Accelerator Research Organization. The KAGRA partnership consists of over 400 members from 128 organizations across 17 countries/regions. Information about KAGRA for the general public is available at the site gwcenter.icrr.u-tokyo.ac.jp/en/. Resources for scholars can be found at gwwiki.icrr.u-tokyo.ac.jp/JGWwiki/KAGRA.
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