Gravitational waves finally prove Stephen Hawking’s black hole theorem

Recent advancements in gravitational wave astronomy have successfully tested Stephen Hawking’s black hole area theorem, originally proposed over 50 years ago. Scientists have captured waves produced by a remarkable collision of black holes as they raced toward Earth at light speed.

Hawking’s black hole area theorem, introduced in 1971, posits that the event horizon of a newly formed black hole—after merging two black holes—cannot have a surface area smaller than the sum of the event horizons of the original black holes. This principle resonates with the second law of thermodynamics, which asserts that the entropy, or disorder, in a closed system never decreases.

The violent merging of black holes warps space-time, generating minute fluctuations known as gravitational waves. These waves travel through the cosmos at light speed. Five observatories globally are tuned to detect these imperceptibly faint waves, which are 10,000 times smaller than an atomic nucleus. The observatories include the two American facilities of the Laser Interferometer Gravitational-Wave Observatory (LIGO), the Virgo detector in Italy, KAGRA in Japan, and GEO600 in Germany, all part of an international collaboration known as LIGO-Virgo-KAGRA (LVK).

The latest significant collision, referred to as GW250114, closely resembled a previous event that led to the observation of the very first gravitational waves back in 2015. Both events involved black holes with masses ranging between 30 and 40 solar masses and occurred approximately 1.3 billion light-years from Earth.

With upgraded LIGO detectors sensitive to three times the magnitude of those operational in 2015, scientists were able to record gravitational waves from this merger with unprecedented clarity. This allowed a precise calculation of the event horizon area, confirming that it indeed expanded post-merger, validating Hawking’s theorem.

When black holes collide, they generate distinctive gravitational waves reminiscent of a bell ringing, as explained by Laura Nuttall, an astronomer at the University of Portsmouth and member of the LVK team. Until now, the overtones from these events dissipated too rapidly to allow for adequate observation, making it impossible to measure the areas of the event horizons before and after the collisions. A 2021 study on the initial detected collision supported the theory with a confidence level of 95%, but this new investigation raises that confidence to a staggering 99.999%.

Over the past decade, gravitational wave observatories have documented around 300 black hole mergers, but none have been as resonant as GW250114, which recorded waves twice as loud as any previously detected gravitational signal.

“The truly nearby mergers—those exceptionally loud in our data—are immeasurably valuable for probing the fundamental mechanics of the universe due to the clarity of their signals and minimal uncertainties,” Nuttall asserts. “We eagerly await the next significant observations nature has to offer.”

At the time GW250114 was observed, LIGO was the only operational detector; other facilities within the LVK were offline. While this didn’t hinder the verification of Hawking’s theorem, it did present challenges in accurately localizing the origin of the gravitational waves in the celestial sphere.

Future upgrades to LIGO and the introduction of new observatories are set to enhance sensitivity, facilitating deeper investigations into the physics surrounding black holes. “We may not capture every event, but we can anticipate another occurrence like this in the future,” explains Ian Harry, also of the University of Portsmouth and part of the LVK collaboration. “Perhaps during subsequent upgrades, we will witness an event of similar magnitude and sensitivity that allows us to explore the finer details of these extraordinary occurrences.”

These findings open avenues for novel research in quantum gravity, which seeks a synergy between general relativity and quantum mechanics. The latest observations indicate that while general relativity and quantum theories currently align well, future anomalies are anticipated.

“As our instruments achieve greater sensitivity, we may reach a point where quantum mechanics and general relativity begin to exhibit discrepancies in their interactions during highly energetic events,” Nuttall remarks.

The recent data from LVK has also affirmed long-standing predictions by mathematician Roy Kerr from the 1960s, elucidating that black holes can be delineated solely by two traits: their mass and spin. Thus, pairs of black holes with equivalent mass and spin are mathematically indistinct. The observations stemming from GW250114 have provided substantial evidence supporting this assertion.