A black hole collision sent something strange to Earth in 2015 – not light, not sound, but a ripple in space itself. That ripple had been traveling for 1.3 billion years before reaching two detectors in the U.S.

On September 14, 2015, humans picked up gravitational waves for the very first time. It wasn’t just a scientific milestone. For the first time, we were able to listen to the universe by feeling its motion.

More than just light and particles

Until that moment, the universe had only spoken to us through light – visible, ultraviolet, X-rays, radio – and through particles like cosmic rays and neutrinos. Gravitational waves gave us a brand-new way to listen in. It’s like adding a new sense.

The detectors that picked up the signal are part of LIGO, the Laser Interferometer Gravitational-Wave Observatory. LIGO has two stations – one in Washington state and the other in Louisiana.

Together with Virgo in Italy and KAGRA in Japan, these instruments now form a global network of gravitational wave detectors known as LVK.

The detectors don’t look into space. They feel it. And they’re incredibly sensitive – able to detect changes in distance smaller than one ten-thousandth the width of a proton. That’s about 700 trillion times thinner than a human hair.

Clearer voice from deep space

Over the last decade, this network has captured more than 300 black hole mergers. At the current rate, LVK sees about one of these cosmic collisions every three days.

The latest signal, named GW250114, arrived on January 14, 2025. It came from another pair of black holes that collided about 1.3 billion light-years away. What made this one special wasn’t what happened – but how clearly the signal came through.

“We can hear it loud and clear, and that lets us test the fundamental laws of physics,” said Katerina Chatziioannou, leading author of the new study.

Thanks to upgrades in technology and better noise reduction, the team was able to get the most detailed look yet at a process that Stephen Hawking predicted back in 1971.

Hawking’s black hole theory finally proven

Hawking’s idea, called the black hole area theorem, says the total surface area of black holes should never shrink. Even though black holes lose energy as gravitational waves and spin faster after merging, their overall area should still grow.

In this new event, the surface area of the two black holes before merging was around 240,000 square kilometers (92,600 square miles) – about the size of the United Kingdom.

After the collision, the final black hole had an area of about 400,000 square kilometers (154,000 square miles) – roughly the size of Sweden.

This marks the second time that scientists have tested the area theorem using gravitational waves. The first test, back in 2021, used data from the original 2015 signal and had a confidence level of 95 percent. This new one hit 99.999 percent.

Kip Thorne from Caltech, one of LIGO’s original founders, remembered getting a phone call from Stephen Hawking right after the first detection.

Hawking wanted to know if LIGO could test his theorem. He passed away in 2018 and never saw it proven.

“If Hawking were alive, he would have reveled in seeing the area of the merged black holes increase,” Thorne said.

Listening to the black hole’s final echo

One of the toughest parts of this kind of analysis is measuring what happens after the black holes merge.

As the new black hole settles down, it vibrates like a struck bell in what’s called the ringdown phase. Those vibrations, or modes, fade quickly and are hard to separate.

But the clarity of the GW250114 signal let scientists, for the first time, identify two distinct modes during the ringdown.

That helped them figure out the mass and spin of the final black hole – and confirm that the math behind these predictions holds up.

Black hole collision sounds

Another study, based on the same signal, searched for a possible third, higher-pitched tone. It didn’t find it, but the test pushed the limits of how well general relativity describes black holes.

“Analyzing strain data from the detectors to detect transient astrophysical signals, send out alerts to trigger follow-up observations from telescopes or publish physics results gathering information from up to hundreds of events is quite a long journey,” said Nicolas Arnaud, CNRS researcher in France and Virgo coordinator of the current science run.

“Out of the many skilled steps that such a complex framework requires, I see the humans behind all these data, in particular those who are on duty at any time, watching over our instruments. There are LVK scientists in all regions, pursuing a common goal: literally, the Sun never goes down above our collaborations!”

Beyond black hole collisions

Black holes aren’t the only stars of the show. LVK detectors have also picked up neutron star collisions.

These are the super-dense remains of stars that went supernova. In 2017, a neutron star merger sent gravitational waves and light flying across space.

That event, called a kilonova, helped scientists figure out where gold and other heavy elements come from.

It also marked the first time that both gravitational waves and light were captured from the same event – something called “multi-messenger astronomy.”

LVK now sends out alerts to astronomers around the world every time they think another neutron star collision might be happening. Telescopes then race to spot the light from these violent events.

“The global LVK network is essential to gravitational-wave astronomy,” said Gianluca Gemme, Virgo spokesperson and director of research at INFN.

“With three or more detectors operating in unison, we can pinpoint cosmic events with greater accuracy, extract richer astrophysical information, and enable rapid alerts for multi-messenger follow-up. Virgo is proud to contribute to this worldwide scientific endeavor.”

Bigger detectors, deeper space

In the coming years, the LVK network plans to keep improving its instruments, making them even more sensitive. They also aim to build new detectors – including one in India called LIGO India.

Longer term, scientists are planning even larger observatories. The European Einstein Telescope would be built underground with arms of more than six miles (9.7 kilometers) long.

In the U.S., a detector called Cosmic Explorer would stretch its arms to about 25 miles (40 kilometers).

These giant instruments could let us hear the earliest black hole mergers ever – and maybe even the faint tremors from the very first moments of the universe.

Massimo Carpinelli is a professor at the University of Milano Bicocca and director of the European Gravitational Observatory in Cascina.

“This is an amazing time for gravitational wave research: thanks to instruments such as Virgo, LIGO and KAGRA, we can explore a dark universe that was previously completely inaccessible,” Carpinelli concluded.

The scientific achievements of these 10 years are triggering a real revolution in our view of the Universe.

“We are already preparing a new generation of detectors such as the Einstein Telescope in Europe and Cosmic Explorer in the US, as well as the LISA space interferometer, which will take us even further into space and back in time.”

The full study was published in the journal Physical Review Letters.

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