A black hole illustration showing the extreme forces at play in deep space.
Credit : buradaki, Shutterstock
It’s not every day that something happening 1.3 billion light years away sends ripples that can be measured here on Earth.
Yet that’s exactly what happened on January 14 2025, when scientists picked up what they’re calling the clearest-ever signal of two black holes colliding – and the discovery has just confirmed one of Stephen Hawking’s most famous predictions.
The signal, known as GW250114, was recorded by the LIGO observatory in the United States. Although black hole collisions are among the most violent events in the universe, the waves they send out are incredibly faint by the time they reach us. But this time, the imprint was so sharp and so clean that researchers could see the moment the two black holes merged into one – with more detail than ever before.
More than 1,700 scientists, from 318 research groups worldwide, contributed to analysing the signal, which has now been published in Physical Review Letters.
A Ripple in space-time smaller than an atom
So what exactly did they detect?
When two massive objects – like black holes – spiral around each other and eventually smash together, they disturb the very fabric of space. Imagine dropping a stone in a pond: ripples move outward. Except here, the “pond” is space-time, and the ripple moves at the speed of light.
Albert Einstein predicted these gravitational waves more than a century ago. But even he didn’t think we’d ever be able to detect them, because the distortions are shockingly tiny. Even after a cosmic event involving unimaginable force, the wave that reaches Earth is so small that it changes distances by less than the width of a proton.
This is why scientists rely on laser interferometers – precision instruments that send beams of light down long tunnels, bounce them off mirrors, and look for the smallest possible shift in their alignment. LIGO in the United States has two such detectors. There’s also VIRGO in Italy and KAGRA in Japan, and all three collaborate to catch these fleeting signals.
Between them, they’ve identified around 300 black hole mergers in recent years — but none have been as clear as GW250114.
The collision: Two giants become one
From the signal, researchers determined that the two original black holes had masses somewhere between 30 and 40 times that of our Sun. As they spiralled closer, they dragged space-time into tighter and tighter curves, sending out stronger and stronger gravitational waves – like a cosmic crescendo.
Then, they merged.
The result was a single black hole weighing around 63 solar masses. The moment of fusion is chaotic, like the ringing of a struck bell. The new black hole vibrates, settles, and stabilises – all while sending out waves that slowly fade away.
This “after-ring” was captured more clearly than ever before, which is what allowed scientists to measure the final black hole’s properties with unusual precision.
And that’s where Stephen Hawking comes in.
A Theory from 1971 just got its strongest proof yet
Back in 1971, Hawking proposed something bold: that the total surface area of black holes can never decrease. In other words, if two black holes merge, the surface area of the resulting black hole should always be larger than the combined area of the two original ones.
The new data confirms exactly that.
Before the collision, the two black holes together had a surface area of about 240,000 square kilometres (roughly the size of the UK). After merging, the new black hole measured around 400,000 square kilometres (closer to the size of Sweden).
This isn’t just a match — it’s a match with 99.999% confidence. Scientists had suspected Hawking was right, but now they are about as certain as science ever allows.
Tragically, Hawking didn’t live to see this confirmation. He died in 2018. But his idea now stands on some of the strongest observational evidence ever collected.
The next step: A detector in space
The story doesn’t end here.
To study the largest black holes of all – the supermassive giants at the centres of galaxies – scientists are now planning a space-based detector, known as LISA. It will involve three spacecraft flying millions of kilometres apart, measuring distortions in space from orbit.
If successful, it could reveal collisions on a scale far bigger than anything LIGO can currently observe.
For now, GW250114 marks a milestone:
The clearest black hole merger ever recorded – and firm confirmation that Stephen Hawking was right all along.
