On its own, a black hole is remarkably easy to describe. The only observable properties of a black hole are its mass, its electrical charge (usually zero), and its rotation or spin. It doesn’t matter how a black hole forms. Ultimately, all black holes have the same general structure. Which is strange when you think about it. Throw enough iron and stone together and you will have a planet. Throw hydrogen and helium together and you can make a star. But you could throw clippings of grass, chewing gum, and old Harry Potter books together and you’d end up with the same kind of black hole that you would get if you just used pure hydrogen.

This strange behavior of black holes is known as the no-hair theorem and relates to what is known as the information paradox. In short, since everything in the universe can be described by a certain amount of information and objects cannot simply disappear, the total amount of information in the universe should be constant. But when you throw a chair into a black hole it only increases the mass and spin of the black hole. All information about the color of the chair, whether made of wood or steel, large or small, is lost. Where did this information go?

A black hole appears to remove information from objects. Recognition: [email protected] – Gravitation @ University of Aveiro

A solution to this information paradox could be possible thanks to Stephen Hawking. As early as 1974 he showed that the event horizon of a black hole may not be absolute. Because of the quantum uncertainty, black holes should emit a tiny amount of light, known today as Hawking radiation. Hawking radiation has never been observed, but if it does exist, the information lost when objects enter a black hole could be transported out of the black hole via this light. Thus, the information is not really lost.

If the Hawking radiation is real, it also means that black holes obey the laws of thermodynamics. It’s an idea first suggested by Jacob Bekenstein. When black holes emit light, they must be at a thermal temperature. Based on Bekenstein’s idea, several physicists have shown that there are a number of laws governing black holes known as black hole thermodynamics.

Since you are reading this article, you are probably familiar with the second law of thermodynamics, which states that the entropy of any system must increase. Because of this, a cup of hot coffee will cool down over time and heat the room slightly until the coffee and the room are all the same temperature. You never see a cold cup of coffee warming up spontaneously and at the same time cooling the room slightly. Another way of formulating the second law is that heat flows from a hot object to cooler objects around it.

Gravitational wave data shows an increase in black hole area. Photo credit: Isi, Maximiliano, et al

For black holes, the second law of thermodynamics applies to the area of the event horizon of a black hole. The Hawking temperature of a black hole is related to this area. The bigger the black hole, the lower its Hawking temperature. The second law of thermodynamics of black holes says that for every merging of black holes, the entropy must increase. This means that the surface area of the resulting black hole must be larger than the surface area of the two original black holes combined. This is known as the Hawking area theorem.

All of this, of course, is a bunch of mathematical theory. It is what we expect given our understanding of physics, but proving it is a different matter. Now a study in Physical Review Letters has shown us that this is true.[^1] The team examined the very first observation of two merging black holes. The event is known today as GW150914 and was a merger of a black hole with 29 solar masses and one 36 solar masses. Using a new method of analyzing the gravitational waves they generated, the team was able to calculate the event horizon surfaces for the original black holes. When they compared it to the surface of the last black hole with 62 solar masses, they found that the total area has increased.

The results have a 97% confidence level, which is good but not strong enough to be clinch proof. But this method can be applied to other black hole mergers, and it is the first real evidence that black hole thermodynamics is more than just a theory.

**Reference:** Isi, Maximiliano, et al. “Testing the Black Hole Area Law with GW150914.” Physical Review Letters 127.1 (2021): 011103.

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