Two black holes have just proven Stephen Hawking right. On 14 January, LIGO detectors picked up the last gravitational vibrations from a cataclysmic collision in the depths of the cosmos.
This merger, published in Physical Review Letters, confirms one of the fundamental laws of black holes formulated by the British physicist in 1971. This time, the figures speak for themselves.
This image of a massive black hole in the heart of deep space fuels research into quantum physics and the visionary work of Stephen Hawking.
When two cosmic giants merge and redraw the laws of gravity
Imagine two invisible monsters, each several times more massive than our Sun. They are hurtling towards each other at breakneck speed. When they collide, space-time begins to vibrate, like a stretched canvas shaken by a gust of wind. This is exactly what LIGO researchers observed during the GW250114 event.
But, and this is the surprise, the black hole born of this merger is larger than the sum of its two ‘parents’. In other words, the total surface area of the event horizon, the boundary beyond which nothing can escape, has increased. This seemingly insignificant detail confirms Stephen Hawking’s ‘second law of black hole mechanics’. According to this law, the surface area of a black hole can never decrease, a principle as elegant as it is formidable.
Hawking’s law: a visionary idea linking entropy, time and matter
In 1971, Hawking put forward a revolutionary idea: the horizon of a black hole functions like entropy in thermodynamics, it can only increase. The more matter a black hole swallows, the more its surface area expands. This idea, which was bold at the time, linked gravity, thermodynamics and quantum mechanics for the first time.
Today, more than fifty years later, the collision detected by LIGO finally confirms this theory. As Maximiliano Isi, a researcher at Columbia University, points out: ‘Even though this principle seems simple, it reveals profound truths about the quantum structure of space-time.’
So Hawking was right. Black holes are not simply passive abysses, but dynamic, almost ‘living’ systems. They obey laws that are as fascinating as they are implacable. In short, they breathe in time with the Universe.
The gravitational song of black holes: the silent music of the cosmos finally deciphered
What the researchers listened to were the gravitational waves emitted during the merger, a veritable cosmic symphony. Like a ringing bell, the newly formed black hole produced a series of characteristic vibrations called ‘ringdown’. By studying these signals, scientists were able to measure the surface area of the final black hole and confirm that it exceeds that of the two original black holes.
This ‘sound signature of the Universe’ solved the mystery. As a bonus, the discovery also validates Kerr’s metric, a mathematical description of rotating black holes. As Katerina Chatziioannou of Caltech explains, ‘at equal mass and rotation, two black holes are identical.’ In other words, nature is consistent.
A new era for gravitational astronomy: when humanity begins to listen to the Universe
Since the first detection of gravitational waves in 2015, LIGO has undergone a veritable revolution. Today, researchers detect a merger every three days, compared to just one per month ten years ago. This progress is spectacular, but it is only the beginning.
By 2030, new instruments such as LIGO-India, the European Einstein Telescope and the American Cosmic Explorer will be launched. Thanks to them, we will be able to hear the first mergers of the early Universe. Little by little, we will go back to the birth of the first stars and black holes.
So, yes, I like to imagine Stephen Hawking, somewhere in the vastness, smiling at the beauty of this confirmation. His intuitions continue to guide science, like a theoretical star that still illuminates our understanding of the cosmos.
