The signal arrived at LIGO on January 14, 2025. Two laser beams running down 4-kilometer vacuum tubes in Hanford, Washington and Livingston, Louisiana flickered by a distance roughly ten-thousandths the width of a proton, lasted a fraction of a second, and went still. The disturbance, catalogued as GW250114, turned out to be the loudest gravitational wave ever detected \u2014 about three times stronger than the chirp that opened gravitational wave astronomy a decade earlier. Hidden inside it was a signal no one had previously known how to read.<\/p>\n
That signal has now been pulled out. In a paper published in Nature on June 24, 2026<\/a>, Dr. Ling Sun and PhD candidate Neil Lu of the ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav) at the Australian National University, working with Sizheng Ma of the Perimeter Institute, Ornella Piccinni at ANU, Yanbei Chen at Caltech and colleagues in Spain, report the first direct measurement of two fundamental properties of a newly formed black hole\u2019s event horizon: its rotation frequency and its surface gravity.<\/p>\n A louder signal, a sharper window<\/p>\n The collision that produced GW250114 was the same kind that produced GW150914 a decade earlier \u2014 two stellar-mass black holes spiraling together at cosmological distance. The primary weighed about 33.6 solar masses, the secondary about 32.2. They fell into each other and left behind a remnant of roughly 62.7 solar masses, spinning at a dimensionless spin of 0.68, with the rest of the missing mass radiated outward as gravitational waves.<\/p>\n What was different was the detectors. Ten years of quantum-precision upgrades<\/a> have driven instrumental noise down to the point where LIGO can register space-time distortions on scales 10 thousand trillion times smaller than a human hair. That precision is why the same kind of event that once produced a faint chirp now produces a textbook-clean waveform, with a signal-to-noise ratio about three times that of the first detection.<\/p>\n The first detection, GW150914, was announced in February 2016<\/a> after a century of speculation and decades of instrument-building. By contrast, the LVK network now routinely observes roughly one black hole merger every three days<\/a>, and has logged around 300 binary black hole mergers across its four observing runs.<\/p>\n What the direct waves reveal<\/p>\n The new finding hinges on a small piece of the signal that earlier analyses had set aside. \u201cWe measured the last sound the black holes made when they crashed,\u201d Lu said in a statement released by ANU<\/a>. \u201cHidden within that signal is a small component, called direct waves, that had not previously been well understood. Our new analysis allows us to decipher this component and extract unique information from close to the event horizon.\u201d<\/p>\n The direct wave is gravitational radiation emitted from immediately outside the newly formed horizon, as material spirals across the boundary at the moment a single horizon takes shape from two. Ma, at the Perimeter Institute, proposed searching GW250114 for this signature based on theoretical work showing it should be visible in a loud enough signal. The wave oscillates at approximately twice the rotation frequency of the new horizon, and decays at a rate set by the horizon\u2019s surface gravity.<\/p>\n Those two numbers matter. In general relativity, a non-charged black hole is fully described by its mass and spin. Rotation frequency and surface gravity are the horizon-level expressions of that minimalism \u2014 what you would measure if you could hover at the boundary itself. Until now, gravitational wave astronomers inferred them indirectly from the inspiral and ringdown. The direct-wave channel reads them closer to the source.<\/p>\n Sun, the team co-leader, framed the result this way in comments to Space.com<\/a>: the exceptionally loud signal \u201ccan be used as a powerful probe of the remnant black hole\u2019s horizon, allowing us to measure its two fundamental properties: rotation frequency and surface gravity.\u201d<\/p>\n The horizon, briefly<\/p>\n The event horizon is a mathematical surface, not a physical membrane. It emerged from Karl Schwarzschild\u2019s solutions<\/a> to Einstein\u2019s 1915 field equations, worked out while Schwarzschild was serving with the German army on the Eastern Front during the First World War. He sent the manuscript to Einstein in December 1915 and died a few months later of an autoimmune skin condition contracted in the trenches.<\/p>\n The Schwarzschild radius for the Sun sits at about 2.95 kilometers \u2014 roughly 1.83 miles \u2014 from its center of mass. For Earth, it is roughly 9 millimeters, about the diameter of a small marble. For the Moon, about 0.1 millimeters. The radius scales linearly with mass; the physics does not change.<\/p>\n A real black hole\u2019s horizon is the surface beyond which no signal can escape to a distant observer. That makes it, in principle, impossible to image with light. Gravitational waves are different. They are emitted by the dynamics of spacetime itself, including the dynamics of the horizon as it forms, rings, and settles.<\/p>\n Black holes come in a vast range of scales. The stellar-mass objects detected by LIGO are tens of solar masses. The ultramassive quasar TON 618<\/a>, by contrast, has been estimated at roughly 66 billion solar masses based on a 2004 analysis of its H\u03b2 emission line, with more recent work suggesting the figure could be closer to 40 billion. The horizon physics is the same; only the radius changes.<\/p>\n Testing Einstein at the boundary<\/p>\n