Back in 1971, the late physicist Stephen Hawking made an intriguing prediction: The total surface area of a black hole cannot decrease, only increase or remain stable. So if two black holes combine, the newly formed black hole will have a larger surface area. This became known as Hawking's area theorem. Analysis of the gravitational signal from a black hole merger detected in January provides the best observational evidence to date in support of Hawking's theorem, according to a new paper published in the journal Physical Review Letters.
The breakthrough just happens to coincide with the 10-year anniversary of the LIGO collaboration's Nobel Prize-winning first detection of a black hole merger. A second paper has been submitted (but not yet accepted), placing theoretical limits on a predicted third tone at a higher pitch that could be lurking in the event's gravitational wave signal.
Now known as LIGO/Virgo/KAGRA (LVK), the collaboration searches the Universe for gravitational waves produced by the mergers of black holes and neutron stars. LIGO detects gravitational waves via laser interferometry, using high-powered lasers to measure tiny changes in the distance between two objects positioned kilometers apart. LIGO has detectors in Hanford, Washington, and in Livingston, Louisiana. A third detector in Italy, Advanced Virgo, came online in 2016. In Japan, KAGRA is the first gravitational-wave detector in Asia and the first to be built underground. Construction began on LIGO-India in 2021, and physicists expect it will turn on sometime after 2025.
Each instrument is so sensitive that it also picks up small ambient vibrations, like a rumbling freight train or natural thermal vibrations in the detectors themselves. So the LIGO collaboration goes to great lengths to shield its instruments and minimize noise in its data. On September 14, 2015, at 5:51 am EST, both detectors picked up signals within milliseconds of each other for the very first time. The waveforms of those signals serve as an audio fingerprint—in this case, evidence for two black holes spiraling inward toward each other and merging in a massive collision event, sending powerful shock waves across spacetime. Picking up the signals was a stunning achievement, and nobody was surprised when the first direct observation of gravitational waves won the 2017 Nobel Prize in Physics.