An extremely strong gravitational wave measured at LIGO has made it possible for the first time to experimentally verify the principle that the area of a black hole's event horizon cannot decrease. Concern: The government wants to close the LIGO project
Nearly a decade after the first gravitational waves were detected, scientists have announced the detection of the strongest signal yet – produced by the collision of two black holes in a distant galaxy. The merger, recorded by the Laser Interferometer Gravitational-Wave Observatory (LIGO), has allowed Albert Einstein’s general theory of relativity to be tested with greater precision than ever before, and has provided the first clear confirmation of Stephen Hawking’s area theorem, which states that the area of a black hole’s event horizon can only grow and never shrink.
The signal was detected on January 14, 2025, by two L-shaped interferometers located in Louisiana and Washington state. Laser beams echoing in 4-kilometer-long arms are used to detect gravitational waves, which stretch space by less than 1/100th the width of a proton. The data show that the signal was created when black holes with masses of 33.6 and 32.2 solar masses drifted toward each other and merged into a single body. Thanks to LIGO’s sensitivity upgrades, the new signal was three times clearer than the first event detected in 2015, with an unprecedented signal-to-noise ratio of 80.
The most dramatic phase of the merger occurred after the merger, when the event horizon of the new black hole vibrated like a bell being struck hard. The oscillation, known as the “ringdown,” was measured at a frequency of 247 Hz and lasted only about 10 milliseconds. In addition to the main tone, an overtone—a kind of secondary oscillation—appeared, visible and clear for the first time. The fact that the frequencies and decay rates of the main tone and the overtone matched the mathematical predictions of general relativity provided strong evidence that black holes are described by just two parameters: mass and spin.
The researchers calculated the area of the event horizon of the final black hole and found it to be about 400 square kilometers – about the size of Japan. Although the new black hole was less massive than the sum of the original two (because some of the mass was emitted in gravitational waves), its area was larger than the sum of all the initial areas – exactly as predicted by Hawking's area theorem. This provided experimental proof of one of the most fundamental theoretical principles in black hole physics.
The discovery illustrates how improvements in LIGO sensitivity have transformed the study of black holes from a purely theoretical field to a well-established experimental field. As MIT physicist Scott Hughes has noted, “Black holes don’t ring—they bang.” It was the strength of the latter signal that first made it possible to clearly detect the sub-oscillation. In the future, as LIGO continues to be upgraded and new observatories are built around the world, researchers expect to identify additional states of the oscillations, allowing for even more rigorous tests of general relativity. Some hope that eventually some anomaly will be discovered, opening the door to new physics.
However, there is also a threatening political-budgetary dimension: the US government has considered closing one of the LIGO stations as part of deep cuts to the National Science Foundation's budget. Such a move could significantly reduce the sensitivity of the experiment and reduce the chance of further groundbreaking discoveries in the field.
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