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The subsequent article is modified from an announcement released by the Laser Interferometer Gravitational-wave Observatory (LIGO) Laboratory. LIGO receives funding from the National Science Foundation and is managed by Caltech and MIT, which envisioned and constructed the project.
On September 14, 2015, a signal reached Earth, carrying details about a duo of distant black holes that had spiraled together and unified. The signal had traversed approximately 1.3 billion years to arrive here at the speed of light — yet it was not comprised of light. It was a unique type of signal: a trembling of space-time known as gravitational waves, initially postulated by Albert Einstein a century earlier. On that day a decade ago, the twin detectors of the U.S. National Science Foundation Laser Interferometer Gravitational-wave Observatory (NSF LIGO) achieved the first-ever direct observation of gravitational waves, whispers from the universe that had been unnoticed until that instant.
The momentous finding meant that scientists could now perceive the cosmos through three distinct modalities. Light waves, including X-rays, optical, radio, and other spectra, along with high-energy particles known as cosmic rays and neutrinos, had been recorded previously, but this was the inaugural occasion anyone had observed a cosmic phenomenon through the gravitational distortion of space-time. For this milestone, initially envisioned over 40 years earlier, three of the team’s founders were awarded the 2017 Nobel Prize in Physics: MIT’s Rainer Weiss, professor emeritus of physics (who recently passed away at 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.
Currently, LIGO, which includes detectors in both Hanford, Washington, and Livingston, Louisiana, routinely monitors roughly one black hole merger every three days. LIGO now collaborates with two international partners, the Virgo gravitational-wave detector located in Italy and KAGRA in Japan. Collectively, this gravitational-wave-hunting network, referred to as the LVK (LIGO, Virgo, KAGRA), has documented approximately 300 black hole mergers, some of which are verified while others await further examination. During the network’s current scientific operation, the fourth since the inaugural run in 2015, the LVK has identified over 200 potential black hole mergers, more than double the total from the first three runs.
The remarkable surge in the quantity of LVK discoveries in the last decade can be attributed to numerous enhancements made to their detectors — some of which involve state-of-the-art quantum precision engineering. The LVK detectors remain, by far, the most accurate instruments for taking measurements ever created by humans. The space-time distortions caused by gravitational waves are exceedingly tiny. For example, LIGO senses alterations in space-time smaller than 1/10,000 the diameter of a proton. That’s 1/700 trillionth the diameter of a human hair.
“Rai Weiss proposed the notion of LIGO in 1972, and I thought, ‘This doesn’t have much likelihood of functioning at all,’” recollects Thorne, a specialist on black hole theory. “It took me three years of pondering it intermittently and discussing concepts with Rai and Vladimir Braginsky [a Russian physicist] before I was convinced this had a considerable chance of success. The technical complexity of minimizing the undesired noise that contaminates the wanted signal was immense. We had to develop an entirely new technology. NSF was simply outstanding at guiding this project through various technical evaluations and challenges.”
Nergis Mavalvala, the Curtis and Kathleen Marble Professor of Astrophysics at MIT and the dean of the MIT School of Science, states that the obstacles the team surmounted to achieve the initial discovery are still very relevant. “From the exquisite precision of the LIGO detectors to the astrophysical theories of gravitational-wave origins, to the intricate data analyses, all these barriers had to be addressed, and we continue to ameliorate in all these dimensions,” Mavalvala states. “As the detectors enhance, we yearn for more distant, fainter sources. LIGO remains a technological wonder.”
The most distinct signal to date
LIGO’s enhanced sensitivity is exemplified in a recent identification of a black hole merger named GW250114. (The numbers indicate the date the gravitational-wave signal reached Earth: January 14, 2025.) This incident was not vastly different from LIGO’s initial detection (designated GW150914) — both involve colliding black holes approximately 1.3 billion light-years distant with masses between 30 to 40 times that of our sun. However, due to 10 years of technological advancements reducing instrumental noise, the GW250114 signal is significantly clearer.
“We can perceive it loud and distinctly, which allows us to test the fundamental principles of physics,” expresses LIGO team member Katerina Chatziioannou, Caltech assistant professor of physics and William H. Hurt Scholar, who is one of the authors of a recent study on GW250114 published in the Physical Review Letters.
By examining the frequencies of gravitational waves emitted during the merger, the LVK team provided the most substantial observational evidence gathered so far for what is referred to as the black hole area theorem, an idea proposed by Stephen Hawking in 1971 suggesting that the total surface areas of black holes cannot diminish. When black holes merge, their masses combine, resulting in an increase in surface area. However, they also lose energy in the form of gravitational waves. Additionally, the merger can cause the resultant black hole to increase its spin, leading it to possess a smaller area. The black hole area theorem asserts that despite these opposing factors, the total surface area must grow.
Subsequently, Hawking and physicist Jacob Bekenstein concluded that a black hole’s area correlates with its entropy or degree of disorder. These findings paved the path for subsequent groundbreaking work in the domain of quantum gravity, which attempts to unify two fundamental principles of modern physics: general relativity and quantum mechanics.
Essentially, the LIGO detection enabled the team to “hear” two black holes expanding as they merged into one, confirming Hawking’s theorem. (Virgo and KAGRA were inactive during this particular observation.) The original black holes possessed a combined surface area of 240,000 square kilometers (approximately the size of Oregon), while the final area measured around 400,000 square kilometers (approximately the size of California) — a definitive increase. This marks the second examination of the black hole area theorem; an initial examination was conducted in 2021 using data from the first GW150914 signal, but due to the data being less clear, that result had a confidence level of 95 percent compared to 99.999 percent for the new findings.
Thorne recalls Hawking contacting him to inquire whether LIGO might be able to validate his theorem shortly after he became aware of the 2015 gravitational-wave detection. Hawking passed away in 2018 and, regrettably, did not live to witness his theory being observationally validated. “If Hawking were alive, he would have delighted in observing the surface area of the merged black holes increase,” Thorne states.
The most challenging aspect of this type of analysis involved determining the final surface area of the merged black hole. The surface areas of pre-merger black holes can be further…
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easily perceived as the duo intertwine, tumultuous space-time and generating gravitational waves. However, once the black holes merge, the signal becomes less distinct. During this so-called ringdown stage, the resultant black hole oscillates like a tolled bell.
In the recent investigation, the scientists meticulously measured the specifics of the ringdown stage, enabling them to compute the mass and rotation of the black hole and, subsequently, ascertain its surface area. More notably, they were able, for the first instance, to reliably distinguish two separate gravitational-wave modes during the ringdown phase. The modes resemble the unique sounds a bell generates when struck; they possess somewhat similar frequencies but diminish at varying rates, complicating their identification. The enhanced data from GW250114 allowed the team to extract these modes, validating that the ringdown of the black hole transpired precisely as anticipated by mathematical models derived from the Teukolsky formalism — conceived in 1972 by Saul Teukolsky, currently a professor at Caltech and Cornell University.
Another research effort from the LVK, submitted to Physical Review Letters today, imposes constraints on a theorized third, higher-frequency tone in the GW250114 signal, and conducts some of the most rigorous evaluations yet of general relativity’s precision in depicting merging black holes.
“A decade of advancements facilitated this extraordinary measurement,” Chatziioannou remarks. “It required both of our detectors, located in Washington and Louisiana, to achieve this. I can’t predict what will unfold in another 10 years, but in the initial decade, we have made remarkable enhancements to LIGO’s sensitivity. This not only implies we are hastening the pace at which we identify new black holes, but we are also gathering intricate data that broadens our understanding of the fundamental characteristics of black holes.”
Jenne Driggers, senior scientist in charge of detection at LIGO Hanford, adds, “It requires a global community to realize our scientific ambitions. From our sophisticated instruments to precisely calibrating the data, ensuring the quality of the data, scanning the data for astrophysical hints, and assembling all that into a format that telescopes can quickly comprehend and act upon, numerous specialized tasks converge to make LIGO the remarkable success that it is.”
Pushing the boundaries
LIGO and Virgo have also unveiled neutron stars over the past decade. Like black holes, neutron stars arise from the explosive demise of massive stars, yet they possess lesser mass and emit light. Notably, in August 2017, LIGO and Virgo observed an extraordinary clash between a pair of neutron stars — a kilonova — that propelled gold and other heavy elements into space and attracted the attention of numerous telescopes worldwide, which recorded light ranging from high-energy gamma rays to low-energy radio waves. The “multi-messenger” astronomy event represented the inaugural occasion that both light and gravitational waves were detected in a singular cosmic occurrence. Currently, the LVK continues to notify the astronomical community of potential neutron star collisions, prompting telescopes to scan the skies for indicators 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 less than a minute, enabling astronomers to search for multi-messenger signatures from our gravitational-wave candidates,” Driggers expresses.
“The global LVK network is crucial for gravitational-wave astronomy,” asserts Gianluca Gemme, spokesperson for Virgo and director of research at the National Institute of Nuclear Physics in Italy. “With three or more detectors functioning in harmony, we can localize cosmic events with greater precision, extract richer astrophysical data, and facilitate rapid alerts for multi-messenger follow-ups. Virgo is proud to contribute to this international scientific venture.”
Other scientific achievements by the LVK include the initial detection of collisions between a neutron star and a black hole; asymmetrical mergers where one black hole is significantly more substantial than its counterpart; the identification of the lightest black holes known, challenging the notion of a “mass gap” between neutron stars and black holes; and the most significant black hole merger observed yet with a combined mass of 225 solar masses. For reference, the previous record for the most massive merger was 140 solar masses.
Even in the years prior to LIGO commencing data collection, researchers were establishing the groundwork that rendered the field of gravitational-wave science feasible. Innovations in computer simulations of black hole mergers, for instance, permit the team to extract and evaluate the faint gravitational-wave signals emanating throughout the universe.
LIGO’s technological breakthroughs, dating back to the 1980s, encompass several wide-ranging innovations, including a novel method for stabilizing lasers utilizing the so-called Pound–Drever–Hall technique. Invented in 1983 and named for physicists Robert Vivian Pound, the late Ronald Drever of Caltech (a co-founder of LIGO), and John Lewis Hall, this technique is now broadly employed in other domains, like the development of atomic clocks and quantum computers. Additional advancements incorporate state-of-the-art mirror coatings that nearly perfectly reflect laser light; “quantum squeezing” devices that empower LIGO to exceed sensitivity limits imposed by quantum physics; and novel artificial intelligence techniques that could further mitigate specific types of unwanted noise.
“Ultimately, what we are accomplishing within LIGO is safeguarding quantum information and ensuring it remains unaffected by external disturbances,” Mavalvala explains. “The methods we are cultivating serve as foundational elements of quantum engineering and have applications across a wide specter of devices, including quantum computers and quantum sensors.”
In the upcoming years, the scientists and engineers of LVK aspire to further refine their instruments, broadening their capacity deeper into space. They also plan to leverage the knowledge acquired to construct an additional gravitational-wave detector, LIGO India. Establishing a third LIGO observatory would significantly enhance the accuracy with which the LVK network can pinpoint gravitational-wave origins.
Gazing further ahead, the team is devising a concept for an even larger detector, dubbed Cosmic Explorer, which would feature arms extending 40 kilometers. (The twin LIGO observatories have 4-kilometer arms.) A European initiative, named Einstein Telescope, also intends to create one or two massive underground interferometers with arms exceeding 10 kilometers. Facilities of this magnitude would enable scientists to detect the earliest black hole mergers in the universe.
“Merely a decade ago, LIGO opened our eyes for the first time to gravitational waves and transformed the way humanity perceives the cosmos,” remarks Aamir Ali, a program director in the NSF Division of Physics, which has supported LIGO since its inception. “There exists an entire universe to explore through this newfound perspective, and these recent discoveries indicate that LIGO is just beginning.”
The LIGO-Virgo-KAGRA Collaboration
LIGO is financed by the U.S. National Science Foundation and managed by Caltech and MIT, which collectively conceived and constructed the project. Financial backing for the Advanced LIGO project was spearheaded by NSF, with significant contributions from Germany (Max Planck Society), the United Kingdom (Science and Technology Facilities Council), and Australia (Australian Research Council). Over 1,600 scientists from around the globe participate in this endeavor through the LIGO Scientific Collaboration, which includes the GEO Collaboration. Additional collaborators are listed at my.ligo.org/census.php.
The Virgo Collaboration presently consists of approximately 1,000 members from 175 institutions across 20 different (primarily European) nations. The European Gravitational Observatory (EGO) hosts the Virgo detector near Pisa, Italy, and is funded by the French National Center 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 catalog of Virgo Collaboration groups can be found on the project website.
KAGRA is the laser interferometer with a 3-kilometer arm length located in Kamioka, Gifu, Japan. The host institution is the Institute for Cosmic Ray Research of the University of Tokyo, and the project is co-hosted by the National Astronomical Observatory of Japan and the High Energy Accelerator Research Organization. The KAGRA collaboration comprises more than 400 members from 128 institutions in 17 countries/regions. KAGRA’s information for general audiences can be found at the website gwcenter.icrr.u-tokyo.ac.jp/en/. Resources for researchers are available at gwwiki.icrr.u-tokyo.ac.jp/JGWwiki/KAGRA.
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