After ten years Perfecting the detection of gravitational wavesLigo sensors achieved such a precise observation which has allowed physicists to confirm one of Stephen Hawking’s most famous predictions: the black holes area theorem.
Ten years. A decade has passed since the scientists of the Ligo Observatory The universe listened for the first time in a completely new way: by detecting gravitational waves. On September 14, 2015, wrinkles in spacetime tissue Predicts by Albert Einstein a century earlier inaugurated a new era in astronomy.
What was then an almost imperceptible cosmic whisper, today has become a symphony that sensors can clearly hear. And on the tenth anniversary of that milestone, the Ligo-Virgo-Kagra (LVK) collaboration has captured the most clear gravitational wave signal to date.
GW250114. Detected on January 14, 2025, physicists believe that these gravitational waves were caused by the collision and subsequent fusion of two black holes to about 1.3 billion light years from the earth.
Interestingly, the event is almost a twin that ended using Ligo’s physicists the 2017 Nobel PrizeGW150914. In both cases, it was two black holes with masses between 30 and 40 times that of our sun. But there is an abysmal difference: the signal quality.
An unprecedented sharpness. Thanks to a decade of technological improvements and advances in quantum engineering, Ligo detectors are now almost four times more sensitive. While the first signal had a signal/noise ratio of 26, that of GW250114 has a 80.
“We can hear it high and clear, and that allows us Physical Review Letters. This sharpness has been key to unraveling the secrets that were hidden in the vibrations of the black hole resulting from the merger.
Hawking theorem. In 1971, Stephen Hawking proposed that the total area of the event horizon of a black hole can never be reduced. It can increase or remain the same, but never shrink. This, which is known as the Hawking area theorem, is analogous to the second law of thermodynamics, which says that the entropy (the disorder) of an isolated system always increases. Therefore, the area of a black hole is a measure of its entropy.
Trying it is complicated. When two black holes merge, part of their mass becomes an enormous amount of energy in the form of gravitational waves (the famous E = mc²). In addition, the new black hole can turn much faster, and a larger turn implies a minor area for the same dough. Does the increase in mass compensate for these losses so that the final area is always greater? The analysis of GW250114 has been settled by the matter bluntly.
Hawking was right. In this case, the two initial black holes had a combined area of about 240,000 square kilometers. After the merger, the new black hole, with a mass of about 63 times that of the sun, it had an area of 400,000 square kilometers.
If in 2021 a first test with the 2015 signal showed a 95%confidence, the new data raises that certainty to 99,999%. As Kip Thorne recalls, one of Ligo’s parents and Hawking personal friend, the British physicist called him right after the first detection in 2015 to ask if they could try his theorem. Hawking died in 2018but today his theory has been verified in a way that would have left him very satisfied.
Einstein too. Thanks to this new signal, scientists have been able to analyze the moment just after the merger in which the new black hole vibrates like a newly hit bell before stabilizing. The frequencies and speed with which these tones are attenuated.
It is the most solid test to the date that black holes are seemingly simple objects that can be completely described with only three properties: mass, spin and electric charge. All other information of the material that formed them is lost. But each detection of gravitational waves is one more piece in the puzzle of the cosmos. And as GW250114 demonstrates, to understand them we travel on the shoulders of giants such as Einstein and Hawking.
Image | Aurore Simonnet (SSU/edeon)/LVK/URI
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