One of Stephen Hawking’s most famous ideas has been proven to be right thanks to the ripples in space-time that were caused when two black holes far away merged. Hawking got the black hole area theorem from Einstein’s theory of general relativity in 1971. It says that a black hole’s surface area can’t go down over time. The second law of thermodynamics says that the entropy, or disorder, of a closed system must always go up. This rule is important to physicists because it seems to tell time to go in a certain direction. Since a black hole’s entropy is related to its surface area, both must always go up.
According to the new study, the fact that the researchers confirmed the area law seems to show that the properties of black holes are important clues to the hidden laws that run the universe. Surprisingly, the area law seems to go against one of the famous physicist’s proven theorems, which says that black holes should evaporate over very long periods of time. This suggests that figuring out why the two theories don’t match up could lead to new physics.
“The surface area of a black hole can’t get smaller, which is similar to the second law of thermodynamics. It also has a conservation of mass, which is similar to the conservation of energy, said the lead author, an astrophysicist from the Massachusetts Institute of Technology named Maximiliano Isi. “At first, people were like, ‘Wow, that’s a cool parallel,’ but we quickly figured out that this was very important. The amount of entropy in a black hole is equal to its size. It’s not just a funny coincidence; they show something important about the world.” The event horizon is the point beyond which nothing, not even light, can get away from a black hole’s strong gravitational pull. Hawking’s understanding of general relativity is that a black hole’s surface area goes up as its mass goes up. Since nothing that falls into a black hole can get out, its surface area can’t go down. But because a black hole’s surface area shrinks as it spins, researchers wondered if it would be possible to throw something into it hard enough to make it spin fast enough to shrink its surface area. Isi said, “You will make it spin faster, but not enough to make up for the weight you just added.” “No matter what you do, the mass and spin will make the area bigger no matter what.” To test this idea, scientists looked at gravitational waves, which are ripples in the fabric of space-time. These ripples were made 1.3 billion years ago when two very large black holes were moving very fast toward each other. In 2015, the first gravitational waves were found by the Advanced Laser Interferometer Gravitational-Wave Observatory (LIGO), which uses a laser beam split into two 2,485-mile-long (4-kilometer) paths that can detect even the smallest changes in space-time by how they change the length of its paths. By splitting the signal into two parts, before and after the black holes merged, the researchers were able to figure out the mass and spin of both the two black holes that were there before and the new black hole that was made from them. With these numbers, they could figure out how big each black hole was before and after the collision.
Isi explained, “As they spin around each other faster and faster, the gravitational waves get bigger and bigger until they crash into each other, making this big burst of waves.” “What you’re left with is a new black hole that’s in this excited state, which you can then study by looking at how it’s vibrating. It’s like how if you ring a bell at different pitches and for different lengths of time, you can learn about its shape and what it’s made of. Hawking’s area rule is proven with more than 95% certainty by the fact that the new black hole has a bigger surface area than the first two combined. The researchers say that these results are mostly in line with what they expected to find. The theory of general relativity, which is where the area law came from, is a great way to describe black holes and other big things. The real mystery, though, starts when we try to combine general relativity, which is about how big things work, with quantum mechanics, which is about how small things work. Strange things start to happen, breaking all of our strict rules and the law in the area as a whole. This is because quantum mechanics says that black holes may shrink according to general relativity. The same famous British physicist who came up with the surface area rule also came up with the idea of Hawking radiation. This describes how strange quantum processes cause black holes to release a fog of particles at their edges. Because of this, the black holes shrink and eventually disappear after a time that is many times longer than the age of the universe. This evaporation may take long enough to not break the law in the short term, but that doesn’t help physicists feel better about it. “Statistically, the law is broken over a long period of time,” Isi said. “It’s like when you boil water and steam comes out of your pan, but if you only look at the water leaving the pan, you might be tempted to say that the entropy of the pan is getting worse. But if you also think about the steam, your total entropy has gone up. Black holes and Hawking radiation are both the same.”
After figuring out the area rule for short to medium time periods, the scientists will look at more data from gravitational waves to learn more about black holes.
“I can’t stop thinking about these things because of how strange they are. “They are very mysterious and hard to understand, but we also know that they are the simplest things in the world,” Isi said. “This, along with the fact that they are where quantum mechanics and gravity meet, makes them the perfect places to learn about what reality is.”