Stephen Hawking's theory of black holes is well established.

One of Stephen Hawking's most famous theorems has been proven correct, using ripples in space-time brought about by the fusion of two distant black holes.

The black hole area theorem, which Hawking derived in 1971 from Einstein's theory of general relativity, states that the surface area of a black hole can't decrease over time. This rule interests physicists because it is closely related to another rule that appears to set time to run in a particular direction: the second law of thermodynamics, which states that the entropy, or disorder, of a closed system, must always increase. Since the entropy of a black hole is proportional to its size, the two always have to increase.

According to the new study, the researchers' confirmation of the area law seems to imply that the properties of black holes are significant clues to the hidden laws that govern the universe. Oddly, the area law seems to contradict another of the famous physicist's proven theorems: that black holes should evaporate over an extremely long time scale, so figuring out the source of the contradiction between the two theories could reveal new physics.

"The surface of a black hole cannot be diminished, which is similar to the second law of thermodynamics. It also has conservation of mass, as you can't reduce its mass, so that's analogous to the conservation of energy," lead author Maximiliano ISI, an astrophysicist at the Massachusetts Institute of Technology, told Live Science. "At first, people were like «Wow, that's a cool parallel», but we quickly realized that it was fundamental. Black holes have entropy, and it's proportional to how big they are. It's not just a funny coincidence, it's a profound fact about the world they tell.”

The surface of a black hole is bounded by a spherical limit called the event horizon – beyond that point, nothing, not even light, can escape its powerful gravitational attraction. According to Hawking's interpretation of general relativity, as the surface of a black hole increases with its mass, and because no object thrown in can get out, its surface cannot decrease. But a black hole's surface area also shrinks the more it spins, so researchers wondered whether it would be possible to throw an object hard enough to make the black hole spin enough to decrease its area.

"You're going to rotate it more, but not enough to counterbalance the mass you just added," Isi said. "Whatever you do, the mass and the rotation will make sure that you end up with a bigger area.”

To test out this theory, the researchers analyzed gravitational waves, or ripples in the fabric of space-time, created 1.3 billion years ago by two behemoth black holes as they spiraled toward each other at high speed. These were the first waves ever detected in 2015 by the Advanced Laser Interferometer Gravitational-Wave Observatory (LIGO), a 1,864-mile-long (3,000 kilometers) laser beam capable of detecting the slightest distortions in space-time by how they alter its path length.

By splitting the signal into two halves — before and after the black holes merged — the researchers calculated mass and the spin of both the two original black holes and the new combined one. These numbers were used to calculate the surface area of each black hole before and after the collision.

"As they spin around each other faster and faster, the gravitational waves increase in amplitude more and more until they eventually plunge into each other — making this big burst of waves," ISI said. "What remains to you is a new black hole that is in this state of excitement, which you can then study by analyzing how it vibrates. It's as if you ping a bell, the specific heights, and durations with which it sounds will tell you the structure of this bell, and also what it's made of."

The surface of the newly created black hole was above that of the first two handsets, confirming Hawking's zone law with a confidence level of over 95%. According to the researchers, their results more or less correspond to what they expected to find. The general relativity theory, where the domain law comes from, very effectively describes black holes and other large objects.

However, the true mystery starts when we try to integrate general relativity—the rules of large objects—with quantum mechanics—those of very small ones. Strange events are starting to take place, wreaking havoc on all our strict and speedy rules, and completely violating the law of the region.

This is because black holes may not narrow according to general relativity, but they may according to quantum mechanics. The iconic British physicist behind the surface area law also developed a concept known as Hawking radiation — where a fog of particles is emitted at the edges of black holes through strange quantum effects. This phenomenon results in the narrowing of the black holes and, ultimately, over a period of time several times longer than the age of the universe, evaporates. This evaporation may occur over time scales long enough not to violate local law in the short run, but it is a small consolation for physicists.

"Statistically, over an extended period of time, the law is broken,” Isi said. "It's like boiling water, you're getting steam evaporating from your pan, but if you only limit yourself to looking at the disappearing water inside of it, you might be tempted to say the entropy of the pan is decreasing. But if you take vapor into account as well, your general entropy has increased. It's the same with the blackouts and the Hawking radiation.”

With the area law established for short to medium time frames, the researchers' next steps will be to analyze data obtained from more gravitational waves for deeper insights that could be gleaned from black holes.

"I'm obsessed with these objects because of the paradox they have. They are extremely mysterious and confusing, but at the same time we know that they are the simplest things that exist," Isi said. "This, as well as the fact that they are where gravity meets quantum mechanics, makes them the perfect playground for our understanding of what reality is.”

Their findings were published in Journal Physical Review Letters on 26 May.

Original publication in Live Science.