Stephen Hawking’s theorem, established over 50 years ago, has aided astronomers in detecting waves produced by extraordinarily powerful collisions as they traverse Earth at light speed, shedding light on the merging of black holes thanks to significant advancements in gravitational wave astronomy.

In 1971, Hawking introduced the Black Hole Area theorem, which posits that when two black holes combine, the resultant event horizon cannot be smaller than the combined size of the original black holes. This theorem aligns with the second law of thermodynamics, which asserts that the entropy of a system cannot decrease.

The merging of black holes warps the structure of the universe, generating tiny ripples in space-time known as gravitational waves that move through the cosmos at the speed of light. Five gravitational wave observatories on Earth search for waves that are 10,000 times smaller than an atom. These include two detectors in the US—LIGO, a laser interferometer, alongside Italy’s Virgo, Japan’s Kagura, and Germany’s GEO600.

The recent event, named GW250114, mirrors the event that first detected gravitational waves in 2015.

Now, the upgraded LIGO detector is three times more sensitive than it was in 2015, enabling the capture of waves from collisions with remarkable detail. This has allowed scientists to confirm Hawking’s theorem, proving that the size of the event horizon actually increases following a merger.

When black holes collide, they generate gravitational waves with overtones akin to the sound of a ringing bell, as noted by Laura Nuttall, a member of the LVK team at the University of Portsmouth, UK. Previously, these overtones were too rapid to be detected clearly enough to assess the area of the event horizon before and after a merger, a crucial requirement to test Hawking’s theory. The initial 2021 study supporting the theory confirmed it at a 95% confidence level, but the latest findings suggest an impressive 99.999% confidence.

Over the past ten years, scientists have witnessed approximately 300 black hole collisions while observing gravitational waves. However, none have been as strong as GW250114, which was twice as powerful as any previously detected gravitational wave.

“What we are discovering in our data has tremendous implications for understanding basic physics,” remarked a researcher. “We’re eager for nature to provide us with further astonishing revelations.”

Only LIGO was operational when GW250114’s waves reached Earth; other detectors in the LVK collaboration were not active. This did not affect the validation of Hawking’s theory but limited researchers’ ability to pinpoint the waves’ origins more precisely.

Future upgrades to LIGO and upcoming observatories are anticipated to enhance sensitivity, offering deeper insights into black hole physics, according to Ian Harry, also from the University of Portsmouth and part of the LVK team. “We may miss some events, but we will certainly capture similar phenomena again,” Harry expressed. “Perhaps with our next set of upgrades in 2028, we might witness something of this magnitude and gain deeper insights.”

These findings pave the way for future research into quantum gravity, a field where physicists aim to reconcile general relativity with quantum mechanics. Nuttall stated that the latest results indicate that both theories remain compatible, although inconsistencies are expected in future observations.

“At some point, discrepancies are likely to emerge, especially when close signals appear noisy as the detector’s sensitivity improves,” Nuttall explained.

Moreover, the recent data from LVK enabled scientists to confirm equations proposed by mathematician Leakir in the 1960s, which suggested that black holes could be described by two key metrics: mass and spin. Essentially, two black holes with identical mass and spin are mathematically indistinguishable. Observations from GW250114 have verified this assertion.

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