Hawking and Kerr black hole theories confirmed by gravitational wave

Scientists have confirmed two long-standing theories relating to black holes – thanks to the detection of the most clearly recorded gravitational wave signal to date.

Ten years after detecting the first gravitational wave, the LIGO-Virgo-KAGRA Collaboration has today (10 Sep) announced the detection of GW250114 – a ripple in spacetime which offers unprecedented insights into the nature of black holes and the fundamental laws of physics.

The study confirms Professor Stephen Hawking’s 1971 prediction that when black holes collide, the total event horizon area of the resulting black hole is bigger than the sum of individual black holes - it cannot shrink.

Research also confirmed the Kerr nature of black holes - a set of equations developed in 1963 by New Zealand mathematician Roy Kerr elegantly explaining what space and time look like near a spinning black hole. The Kerr metric predicts effects such as space being ‘dragged’ around and light looping to make multiple copies of objects.

Publishing their findings in Physical Review Letters, the international group of researchers – including experts from the University of Birmingham – note that GW250114 was detected with a signal-to-noise ratio of 80. This clarity enabled precise tests of general relativity and black hole thermodynamics.

Geraint Pratten, Royal Society University Fellow at the University of Birmingham and member of the LVK paper writing team commented: “GW250114 is the loudest gravitational wave event we have detected to date, it was like a whisper becoming a shout. This gave us an unprecedented opportunity to put Einstein's theories through some of the most rigorous tests possible - validating one of Stephen Hawking's pioneering predictions that when black holes merge, the combined area of their event horizons can only grow, never shrink.”

GW250114 was picked up by the twin detectors of the Laser Interferometer Gravitational-Wave Observatory (LIGO) in the USA. LIGO operates in coordination with two international partners, the Virgo gravitational-wave detector in Italy and KAGRA in Japan forming a gravitational-wave-hunting network, known as the LVK (LIGO, Virgo, KAGRA).

The LVK team, which includes members of the University of Birmingham, was able to establish that GW250114 was generated by the collision of two black holes with masses of about 32 times that of our Sun.

LIGO detects a gravitational wave passing through the Earth every few days, but GW250114, has turned out to be special. The data show that the initial black holes had a total surface area roughly the size of the United Kingdom (240,000 square kilometers) while the final area was about 400,000 square kilometers (roughly the size of Sweden).

In the 1970s, Hawking and physicist Jacob Bekenstein concluded that the black hole's area is proportional to its entropy, or degree of disorder, paving the way for later groundbreaking work in the field of quantum gravity, which attempts to unite two pillars of modern physics: general relativity and quantum physics.

After the black holes fused together, during what physicists call the ringdown phase, the final black hole vibrates emitting gravitational waves at specific frequencies, like characteristic sounds a bell would make when struck, the ‘voices’ of the black hole.

Roy Kerr’s solution predicts that a black hole, and its ‘voices’ when disturbed, are uniquely described just by two numbers: the mass and the spin of a black hole. The implications of this groundbreaking result single out black holes from any other celestial objects: a star can only be described by a very large set of complex properties, while even black holes heavier than a million times our Sun are astonishingly described just by two simple numbers, mass and spin.

Gregorio Carullo, Assistant Professor at the University of Birmingham and coordinator of one of the LVK analysis teams commented: “Given the clarity of the signal produced by GW250114, for the first time we could pick out two ‘tones’ from the black hole voices and confirm that they behave according to Kerr’s prediction, obtaining unprecedented solid evidence for the Kerr nature of black holes found in nature.”

The results are released almost exactly ten years after the first landmark observation of gravitational waves. On September 14, 2015, a signal arrived on Earth, carrying information about a pair of remote black holes that had spiralled and fused together.

This historic discovery, to which Birmingham’s researchers have made wide-ranging contributions by developing hardware for the LIGO detectors, highly accurate models of gravitational waves generated by merging black holes, and analysis techniques to tease out from the data the properties of black holes, meant that scientists could now sense the universe through three different means.

Patricia Schmidt, Associate Professor at the University of Birmingham and co-chair of LVK analysis team, commented: “The detection of a black hole binary with parameters similar to those of GW150914, but three times louder, only a decade after the breakthrough discovery is owed to the tremendous technological improvements of our instruments, paving the path for precision astronomy with gravitational waves.“

Amit Singh Ubhi, part of the Birmingham’s instrumentation team contributing key hardware that made such discovery possible, added: “The exceptional signal-to-noise ratio of GW250114 showcases the collective advances in gravitational-wave instrumentation across our community. This unprecedented clarity allows us to probe black hole evolution with unmatched precision, showcasing the impact of cutting-edge technology on our understanding of the fundamental laws of nature.”

ENDS

For more information, interviews or an embargoed copy of the paper, please contact the University of Birmingham press office on pressoffice@contacts.bham.ac.uk or +44 (0) 121 414 2772.  

Image captions and credits

1. Left panel: Frequency and decay time (half-life) of the different ringdown tones measured in GW250114. The black markers indicate the values predicted for a Kerr black hole. Right panel: gravitational-wave signal (bottom spiral) emitted by the remnant black hole (bottom sphere) into the different tones, for a numerical simulation matching the measured parameters of GW250114. Credits: Dr. Keefe Mitman (Cornell University), Prof. Harald Pfeiffer (Albert Einstein Institute, Potsdam).
2. Infografic showcasing the advancements of gravitational wave observatories -- among the most precise measuring machines ever built by humankind -- in observing black holes cosmic collisions, with the registered signals shown in the bottom panel. These events are embedded in a multitude of celestial objects, which modern telescopes are also observing with ever increasing precision, producing the beautiful images displayed in the top panel. Credits: Dr. Derek Davis (Caltech, LIGO Laboratory).
3. Artistic representation of a ringing rotating black hole. Credits: Aurore Simonnet (SSU/EdEon).

Notes to Editors

• The University of Birmingham is ranked amongst the world’s top 100 institutions, its work brings people from across the world to Birmingham, including researchers and teachers and more than 8,000 international students from over 150 countries.
• 'GW250114: testing Hawking’s area law and the Kerr nature of black holes' - A.G.Abac, et al is published by Physical Review Letters.
• Participating UK institutions include: University of Birmingham; University of Warwick; Queen Mary University of London; Cardiff University; University of Strathclyde; Royal Holloway; King’s College London; University of Portsmouth; University of Glasgow; University of the West of Scotland; University of Southampton; University of Cambridge; University of Lancaster; and Rutherford Appleton Laboratory, Didcot