In 2015, scientists at LIGO detected the first gravitational wave, a historic moment representing the first experimental observation of Einstein’s proposed gravitational wave. LIGO’s 4 km-long arms look like the letter L: one is at Hanford in the US, and the other is at Livingston. It is so sensitive that it will immediately detect any ripple in the space-time geometry coming from billions of light-years away. Scientists detected an unusual signal on September 14, 2015. The signal was first detected at Hanford and then, after 6 milliseconds, at Livingston.
After measuring the strength and clarity of that signal, the scientists discovered it came from a new black hole that is roughly 62 times heavier than the sun, formed by the merging of two smaller black holes that weighed about 36 and 29 times the sun. Where do the other three solar masses go? According to Einstein’s equivalence of mass and energy (E=mc^2), the missing mass must have transformed into energy, which is manifested as this gravitational wave. In 2017, Rainer Weiss, Barry C. Barish, and Kip S. Thorne were awarded the Nobel Prize in recognition of their contributions to gravitational wave detection.
After a century, scientists validated Einstein’s theory experimentally, launching a new field of study known as gravitational wave astronomy. Recently, LIGO detected GW250114, which is three times louder than GW150914, captured ten years ago. Detectors have improved over the past decade. But this time LIGO is no longer alone; now researchers in the joint venture of LVK (LIGO, Virgo, and KAGRA) published this work in *Physical Review Letters* on September 10. This groundbreaking discovery provides definitive proof of how black holes vibrate and release gravitational energy when disturbed. The researchers made two remarkable findings using this observation.
To begin with, it verified that the merging objects display characteristics typical of Kerr black holes (A rotating black hole possesses both mass and angular momentum but no electric charge; it is described by the Kerr metric, which is a solution to Einstein’s general relativity field equations). Secondly, it validated in a strong way Hawking’s area theorem. Hawking predicted in 1971 that when black holes merge, the daughter black hole’s event horizon surface area will be greater than or equal to the sum of the progenitor black holes’ initial areas, a theorem closely linked to physicist J. Bekenstein’s proposal that a black hole’s area is proportional to its “entropy.”
These findings represent a major leap forward in the ability of gravitational-wave observations to investigate some of the fundamental laws of nature. When two black holes collide, the young remnant makes a “ring down” sound like a bell. The black hole’s gravitational-wave signal solely contains the remnant’s mass and spin. The ring down signal should have “quasinormal modes.” The longest-lasting fundamental mode has the lowest frequency. Overtones at higher frequencies fade faster, like plucked string harmonics. Researchers can determine if the remnant is a Kerr black hole by its overtones, as predicted by general relativity.
Indeed, signals from black hole mergers enable scientists to test theoretical black hole physics principles, exposing an array of complex information about quantum gravity. Gravitational waves allow for the study of the area law by measuring the masses and spins of the two merging black holes from the premerger signal, as well as the remnant’s mass and spin from the ring down or post-merger signal. Data analysis enables researchers to convert black hole masses and spins into areas and assess whether the end black hole area is greater than the overall initial black hole area. The first Hawking’s area theorem validation claim was published in 2021. Reanalysis of GW150914 data suggested the signal contained the fundamentals initial overtone.
That claim sparked years of debate concerning the data analysis’s validity, stalling the field. The loud GW250114 signal should settle any concerns. This signal’s high SNR (signal-to-noise ratio) allowed for strong identification of the dominant ring down tone and its first overtone. Each mode’s frequency and damping rates matched the black hole remnant’s Kerr spectrum. Using separate studies of pre- and post merger signals, the researchers calculated the initial and final black hole areas and found that GW250114 agreed with Hawking’s area law with high credibility.
Actually, this work demonstrates that overtones are unambiguous with a high SNR, rendering them indisputable. It also improves thermodynamic tests by removing circular arguments that interpret premerger phase results. Their approach provides a genuine test of the area law or its underlying assumptions by conducting independent studies of the pre- and post merger phases. There has been a decades-long effort by thousands of scientists behind the detection of GW250114. This achievement is a major step forward for gravitational wave astronomy as LIGO celebrates entering its second decade of astronomical discovery.
(THE WRITER IS A SCIENCE WRITER AND RESEARCHER)