In 1971, the renowned physicist Stephen Hawking made a groundbreaking prediction regarding black holes: the total surface area of a black hole can never decrease; it can only increase or remain constant. This prediction, known as Hawking's area theorem, has profound implications for our understanding of black holes and the universe. Recent analysis of a gravitational signal from a black hole merger, detected in January, provides the most compelling observational evidence yet supporting this theorem, as outlined in a new study published in the journal Physical Review Letters.
The timing of this revelation coincides with the tenth anniversary of the LIGO collaboration's historic Nobel Prize-winning detection of a black hole merger, marking a decade of significant advancements in astrophysics. An additional paper has been submitted that aims to place theoretical limits on a predicted third tone at a higher pitch, which may be hidden within the gravitational wave signal of the merger event.
The collaboration, now known as LIGO/Virgo/KAGRA (LVK), actively searches the cosmos for gravitational waves produced by the mergers of black holes and neutron stars. The LIGO detectors, located in Hanford, Washington, and Livingston, Louisiana, utilize laser interferometry to detect these waves by measuring minute changes in distance between objects placed kilometers apart. The sensitivity of these instruments is critical for capturing the faint signals from such cosmic events.
In 2016, the Advanced Virgo detector in Italy joined the effort, and Japan's KAGRA became the first underground gravitational-wave detector in Asia, with its construction starting in 2021. The anticipated LIGO-India facility is set to come online after 2025. Each of these detectors is engineered to minimize noise and ambient vibrations, ensuring that the data collected is as clean as possible. This meticulous attention to detail was evident on September 14, 2015, when both LIGO detectors recorded gravitational wave signals almost simultaneously, providing the first direct evidence of two black holes merging.
The initial detection led to a Nobel Prize in Physics in 2017 and marked the dawn of a new era in multi-messenger astronomy (MMA). Early observations primarily involved mergers of two black holes or neutron stars. However, in 2021, the LIGO/Virgo/KAGRA collaboration made history by confirming two mixed mergers between black holes and neutron stars. This breakthrough allowed astronomers to observe the accompanying “kilonova” events, which are characterized by massive bursts of energy that illuminate the cosmos.
Additionally, the collaboration has documented asymmetrical mergers where one black hole is significantly more massive than the other, challenging existing theories about the mass gap between black holes and neutron stars. This summer, the team detected the gravitational wave signal GW231123, the most massive merger of two black holes recorded to date, resulting in a new black hole with a mass 225 times that of our Sun.
Thanks to advancements in technology, LIGO is now nearly four times more sensitive than when it first detected gravitational waves. This enhanced sensitivity enabled the recording of the sharpest gravitational wave signal to date, GW250114, which closely mirrored the 2015 event. Both events involved two black holes of approximately 30 solar masses, and the resulting merger produced a black hole of about 63 solar masses.
The improved fidelity of GW250114's signal allowed researchers to isolate specific frequencies or tones, providing a more accurate calculation of the new black hole's properties and validating theoretical predictions. This analysis reinforces earlier findings regarding the so-called no-hair theorem, which posits that black holes can be fully described by just three properties: mass, spin, and electric charge.
The detailed analysis of GW250114 revealed that the initial black holes had a total surface area of approximately 240,000 square kilometers, comparable to the size of the United Kingdom. Post-merger, the new black hole's surface area expanded to around 400,000 square kilometers, roughly the size of Sweden. This increase in surface area is a direct confirmation of Hawking's theorem, which states that the areas of black holes can only increase.
According to physicist Maximiliano Isi of Columbia University, who has previously explored the implications of Hawking's area theorem, this finding is significant. He noted that Hawking and fellow physicist Jacob Bekenstein demonstrated that a black hole's area is proportional to its entropy, which must also increase according to the second law of thermodynamics. This relationship is crucial in the ongoing quest to develop a quantum theory of gravity.
Caltech physicist Kip Thorne, a close friend of Hawking, reminisced about the time Hawking inquired whether LIGO could test his theorem after the first gravitational wave detection. Hawking passed away in 2018, but Thorne believes he would have celebrated the confirmation of the increase in black hole areas. The research findings in Physical Review Letters underscore the enduring impact of Hawking’s contributions to science and our understanding of the universe.
