Black holes may be the fiercest objects in the universe, yet we still know very little about them. It has only been a decade since we confirmed their existence by detecting gravitational waves: ripples in the fabric of spacetime.
Authors
- Patricia Schmidt
Associate Professor of Physics, University of Birmingham
- Geraint Pratten
Royal Society University Research Fellow Assistant Professor of Physics, University of Birmingham
Since then, gravitational waves from colliding black holes have unveiled insights into their hidden physics and the theories that support them. On January 14, 2025, the loudest gravitational-wave signal ever detected, known as GW250114, was observed by the two Laser Interferometer Gravitational Wave Observatories (LIGO).
This remarkable event provided our international team of scientists with a golden opportunity to test two major predictions of Albert Einstein's theory of general relativity with unprecedented precision: the nature of black holes and something called Hawking's area law theorem.
The results, published in Physical Review Letters , mark a significant step forward in our understanding of gravity and black holes.
Black holes are a key prediction of general relativity, our leading theory for describing gravity. Astrophysical black holes form when a massive star reaches the end of its life and collapses under its own gravity, exceeding a certain mass called the Chandrasekhar limit (approximately 1.44 times the mass of the sun).
What remains is a region of spacetime completely disconnected from communication with the rest of the universe, bounded by a surface known as the event horizon - from which nothing can famously escape, not even light. But if black holes cannot send signals or light beyond this boundary, how can we be sure they exist and that they behave as predicted?
The power of gravitational waves
General relativity actually predicted the existence of gravitational waves in the first place. Any massive object that accelerates through spacetime will generate tiny distortions that propagate away at the speed of light. These waves encode a wealth of information about the source and the nature of gravity itself.
To produce gravitational waves strong enough to be detected, we need systems where massive objects undergo sustained and intense acceleration. One of the most powerful sources is a binary black hole, where two black holes orbit each other under the influence of gravity. As gravitational waves carry energy away from the system, the orbit will gradually shrink until the black holes eventually merge into a single, larger black hole.
Hence, by analysing gravitational waves from black hole binaries we can probe whether astrophysical black holes truly behave as predicted by general relativity.
Gravitational waves were first observed by the LIGO detectors on September 14, 2015, when they captured the collision of two black holes known as GW150914. Due to the rapid improvements in detector technology , we are now able to observe binary black hole mergers in ultra-high definition, enabling the single most stringent tests of general relativity and black hole physics to date.
Hawking's theorem
Despite their mathematical complexity, black holes are surprisingly simple objects entirely characterised by their mass, rotation and (possibly) electromagnetic charge.
In 1972, Stephen Hawking published a seminal study showing that as two black holes merge, the surface area of the final event horizon must be larger than the sum of the surface areas of the two initial black holes. This is known as the area law.
One way to understand this is to realise that the surface area of the event horizon scales with the mass and spin of the black hole in very particular ways. If we double the mass of a black hole, its event horizon becomes four times larger. If we make the black hole spin faster, the event horizon will become more oblate (think of a rugby ball) and the surface area will decrease. For merging black holes, Hawking demonstrated that despite the loss of energy and angular momentum to gravitational waves, it will always result in a final black hole that has a larger event horizon.
GW250114 provided us with a golden opportunity to test Hawking's predictions. By analysing the gravitational-wave data with the best available models, our team has now validated the area law to high significance. This was possible through the detailed modelling of the " ringdown ", the final stage after the merger during which the remnant black hole emits gravitational waves in a characteristic pattern (known as quasi-normal modes).
This process is similar to striking a bell: the tones emitted depend on the material and shape of the bell, with the ringing of the bell encoding this information. By analysing the emitted sound, we can figure out the shape and material of the bell. For rotating black holes, we can do something similar using gravitational waves.
By analysing the emitted waves and all its harmonics, we can reconstruct the mass and spin of the black hole, and hence the surface area of the horizon. With a signal this strong, we were able to carry out a comprehensive suite of tests probing different aspects of Einstein's theory. In every case, the predictions of general relativity held firmly.
The observation of GW250114 offers the clearest validation yet of Einstein's theory, validating some of its most profound predictions, including Hawking's area law. This is still just the beginning, and the next decade promises to revolutionise our understanding of gravity and black holes even further.
Patricia Schmidt receives funding from UK Research and Innovation (UKRI) through grants ST/V005677/1 and ST/Y00423X/1, and The Royal Society through a Research Grant RGR1241327.
Geraint Pratten is supported by The Royal Society through a University Research Fellowship (URFR1221500 and RFERE221015), the UK Space Agency (grant ST/Y004922/1), and UKRI (grants ST/V005677/1 and ST/Y00423X/1).
