Astronomers have recently made a groundbreaking discovery by detecting a collision between two black holes with unprecedented detail. This event has provided the clearest insight into the nature of these cosmic oddities and confirmed long-held predictions by renowned physicists, including Albert Einstein and Stephen Hawking.
The event, referred to as GW250114, was first identified in January when researchers utilized the Laser Interferometer Gravitational-Wave Observatory (LIGO). This state-of-the-art facility consists of two identical instruments located in Livingston, Louisiana, and Hanford, Washington. LIGO detected gravitational waves, which are minute ripples in space-time created when the two black holes collided.
The search for gravitational waves stems from predictions made in 1915 as part of Einstein’s theory of relativity. Initially, Einstein believed these waves would be too weak to be detected by human technology. However, in September 2015, LIGO made history by recording gravitational waves for the first time, a feat that eventually earned three scientists a Nobel Prize for their critical contributions to this “black hole telescope.”
In this latest observation, the black holes were found to be approximately 30 to 35 times the mass of the sun and were spinning very slowly, according to Maximiliano Isi, an assistant professor of astronomy at Columbia University and an astrophysicist at the Flatiron Institute’s Center for Computational Astrophysics. Isi led a study on the GW250114 data published in the journal Physical Review Letters.
“The black holes were situated about 1 billion light-years away, orbiting each other in a near-perfect circle,” Isi explained. “The resulting black hole had a mass around 63 times that of the sun and was spinning at 100 revolutions per second.” These attributes make this merger a nearly exact replica of the first significant detection made a decade ago. “Thanks to advancements in our instruments, we can observe these two black holes with much greater clarity as they approached and merged,” Isi added.
LIGO, which also includes two smaller instruments — Virgo in Italy and KAGRA in Japan — is managed by a global network of about 1,600 researchers. It detects tiny distortions in space caused by gravitational waves, which are changes in distance that are 1,000 times smaller than the radius of an atom’s nucleus. So far, scientists have observed over 300 black hole mergers using this technology.
Earlier in the year, LIGO identified the most massive black hole collision to date, involving two black holes with masses of approximately 100 and 140 times that of the sun. Since its inception, LIGO has undergone significant upgrades, including enhancements to its lasers and mirrors, which have increased accuracy and reduced background noise. This improved performance has made the new observation over three times more precise than the initial detection.
The clarity of the GW250114 observation has allowed astronomers to validate predictions about black holes made by physicists decades ago. One key prediction, formulated by New Zealand mathematician Roy Kerr in 1963, builds on Einstein’s theory of general relativity. It posits that black holes are paradoxically simple objects describable by a single equation.
Isi elaborated, “While black holes are complex and mysterious, mathematically, they should be characterized by just two numbers: their mass and their rotation speed.” To test this theory, researchers focused on a unique feature of black hole collisions: a “ringing” or vibration, akin to a bell struck by a hammer, that the final black hole emits.
“When a bell rings, its pitch and duration convey information about its composition. Similarly, black holes emit gravitational waves that 'ring' in a way that encodes details about their structure and the surrounding space,” Isi noted. Although this phenomenon had been observed faintly before, GW250114 presented a signal with “two modes: a fundamental mode and an overtone,” offering much more clarity.
The second significant prediction confirmed by GW250114 is the surface area theorem proposed by Stephen Hawking in 1971. This theorem asserts that the surface area of a combined black hole must be equal to or greater than that of its progenitors. “It’s a profound yet straightforward theorem indicating that the total surface area of a black hole can only increase or remain constant,” Isi explained.
Although previous LIGO observations provided tentative confirmations of this theorem, the clarity of the latest signal offers researchers unprecedented confidence in their findings. “By identifying the portion of the signal from the black holes when they were still separate, we can infer their areas and compare that to the final black hole’s surface area,” Isi said.
As noted by Kip Thorne, one of the three Nobel Prize recipients for contributions to LIGO, Hawking expressed a keen interest in whether LIGO could test his theorem soon after the first gravitational wave detection in 2015. “If Hawking were alive, he would have been thrilled to see the area of the merged black holes increase,” Thorne remarked.
Gravitational waves are notoriously weak, making detection a monumental challenge. Emanuele Berti, a physics and astronomy professor at Johns Hopkins University, describes the LIGO detectors as “hearing aids” that enhance this process. “A large group of scientists has spent the last decade improving these hearing aids, allowing us to detect signals with much higher clarity than before,” he stated.
Among the principles now being tested is the notion that black holes are the simplest macroscopic objects in the universe. The detailed “ringing” from the GW250114 collision confirms that the final object aligns with predictions made by Einstein’s general relativity, a development Berti describes as “terribly exciting.”
Leor Barack, a mathematical physics professor at the University of Southampton, emphasized the significance of the latest study, stating it marks a long-awaited analysis of over 300 black hole merger events recorded by LIGO. Researchers successfully extracted two “pure tones” from the remnant black hole, including the first clear extraction of a fainter overtone, which is a remarkable achievement.
According to Macarena Lagos, an assistant professor at the Institute of Astrophysics of the Universidad Andrés Bello, this study represents a significant milestone in gravitational wave astronomy. Lagos believes that the detection of a second tone in the ringing black hole underscores LIGO’s continuous improvements and demonstrates that gravitational wave detections can test fundamental physics in unprecedented ways.
“While current tests of gravity still have broad uncertainties, this work sets the stage for future detections that promise to yield more precise insights into our understanding of spacetime and gravity,” Lagos added.
