Gravitational wave astronomy has just given us another amazing gift: the first *Observation* Confirmation of one of Stephen Hawking’s predictions about black holes.

An analysis of the very first gravitational wave detection from 2015, GW150914, confirmed Hawking’s area theorem. It says that in classical physics the area of the event horizon of a black hole can only get larger – never smaller.

The work gives us a new tool to study these mysterious objects and to test the limits of our understanding of the universe.

“It is possible that there is a zoo with different compact objects, and while some of them are the black holes that obey the laws of Einstein and Hawking, others can be slightly different animals.” said astrophysicist Maximiliano Isi of the Kavli Institute for Astrophysics and Space Research at MIT.

“Well, it’s not like you take this test once and it’s over.

Hawking first proposed his theorem in 1971. It predicted that the surface area of a black hole’s event horizon should never decrease, only increase.

The event horizon is not the black hole itself, but the radius at which even the speed of light in a vacuum is insufficient to achieve the escape speed from the gravitational field created by the singularity of the black hole. It is proportional to the mass of the black hole; Since black holes can only gain mass, according to general relativity, the event horizon should only be able to grow.

(This increase-only model is also curiously similar to another theory, the Second Law of Thermodynamics. It states that entropy – the progress from order to disorder in the universe – can only increase. Entropy is also ascribed to black holes, and it’s directly proportional to their event horizon surface.)

Mathematically, the area theorem fits, but it was difficult to confirm from an observation point of view – mainly because black holes are extremely difficult to observe directly because they do not emit any detectable radiation. But then we discovered the gravitational waves propagating through spacetime of a collision between two of these enigmatic objects.

That was GW150914, and the short version *blossoms* the collision recorded by the LIGO interferometer changed everything. It was the first direct evidence of not one black hole, but two. They came together and formed a larger black hole.

This black hole then rang softly like a struck bell. In 2019, Isi and his colleagues found out how to recognize the signal of this ringdown. Now they have deciphered it and disassembled it to calculate the mass and spin of the last black hole.

They also performed a new analysis of the merger signal to calculate the mass and spin of the two black holes before the merger. Since mass and spin are related to the area of the event horizon, the event horizons of all three objects could be calculated.

If the event horizon could shrink, then the event horizon of the last merged black hole should be smaller than that of the two black holes that created it. By their calculations, the two smaller black holes had a total event horizon area of 235,000 square kilometers (91,000 square miles). The last black hole had an area of 367,000 square kilometers.

“The data show with overwhelming confidence that the horizon area has increased after the merger and that the law of area is met with a very high probability”, Isi said.

“It was a relief that our result was in line with the paradigm we expected and confirmed our understanding of these intricate black hole mergers.”

At least in the short term. In the context of quantum mechanics – which does not work well with classical physics – Hawking later predicted that black holes would lose mass over very long periods of time in the form of a type of black body radiation that we call today Hawking radiation. So it is still possible that a black hole’s event horizon will eventually decrease in area.

Of course, this will have to be investigated more closely in the future. Meanwhile, the work of Isi and his team has given us a new set of tools for studying other gravitational wave observations in the hope of gaining even more insight into black holes and the physics of the universe.

“It’s encouraging that we can think about gravitational wave data in new, creative ways and ask questions we thought we couldn’t before.” Isi said.

“We can always tease out information that speaks directly to the pillars of what we believe we understand.

The study was published in *Physical review letters*.