One of Stephen Hawking’s most important predictions about black holes has finally been observationally confirmed by ripples in the fabric of spacetime, reports a new study. The milestone not only validates the theories of the influential physicist, who died in 2018, it also provides a new means to test some of our most fundamental assumptions about the universe.
Black holes are famous for extremely strange behaviors, such as the capacity to trap anything, including light, inside the event horizon that marks their borders. Hawking added to this list of black hole oddities in 1971 by predicting that the surface area of the event horizon should never shrink over time, which is now known as the area theorem.
The idea echoes the second law of thermodynamics, which states that entropy can only ever increase in a closed system, providing yet another hint that black holes are important windows into broader laws of the universe.
A team led by Maximiliano Isi, a NASA Einstein Postdoctoral Fellow in MIT’s Kavli Institute for Astrophysics and Space Research, now presents the first “observational confirmation of Hawking’s black-hole area theorem” with a confidence of 95 percent, according to a study published on Thursday in Physical Review Letters. The researchers achieved this feat by closely examining the first gravitational waves ever captured on Earth, which were created by the collision of two black holes some 1.3 billion years ago.
“It’s neat to be inscribed in this whole thread of insights and discovery by putting a little bit of a more experimental and observational spin on this field that, for many years, has been purely theoretical, very abstract, and mathematical,” Isi said in a call.
Gravitational waves cause space itself to undulate, but they are so subtle that we can’t perceive them on Earth without extremely sensitive instruments, such as the Laser Interferometer Gravitational-Wave Observatory (LIGO). On September 14, 2015, LIGO detected these waves for the very first time, ushering in a new era of astronomical discovery and earning the 2017 Nobel Prize in Physics.
The waves from this event, known as GW150914, were created by a collision between two black holes that were about 36 times and 29 times the mass of the Sun. Since that detection, dozens of gravitational waves have been captured, allowing scientists to collect a wealth of new data about the cataclysmic events that created them.
Researchers have been able to calculate certain basic properties of wave-producing mergers since GW150914, but Isi and his colleagues have pushed the field forward with a new technique that can reveal finer details about the objects involved in these events.
“Our innovation here is to develop a way to actually split the data so that we can distinguish the ‘before’ and the ‘after’ of the collision of the two black holes,” Isi explained. “In analyzing those two sets of data independently, we can therefore obtain independent estimates of the properties of the black holes before, and the black hole produced after.”
After the researchers debuted this approach in 2019, they were contacted by theoretical physicist Kip Thorne, one of the recipients of the Nobel Prize for gravitational wave astronomy, who encouraged them to apply it to Hawking’s area theorem. The results reveal that the unified black hole created by the GW150914 merger is larger in area than the sum of the black holes that formed it, in keeping with Hawking’s prediction.
By validating the area theorem, Isi’s team has also bolstered the proportional relationship between the surface area of black holes and their entropy, a measurement of the degree of disorder or randomness in a system. However, this connection also raises an interesting paradox: If black holes have entropy, they must have a temperature, which would mean that something can escape a black hole: radiated heat.
Hawking suggested that escaping heat, now known as Hawking radiation, could cause black holes to slowly evaporate over extremely long timescales, which would mean that their surface area could decrease over the lifespan of the universe. This weird disjunct between the area theorem and Hawking radiation is a microcosm of a larger mystery: Can Einstein’s theory of general relativity, which governs the universe on large scales, agree with the laws of quantum mechanics, which governs it on small scales?
“Black holes are weird because they are very abstract, but at the same time, they're very simple—at least Einstein’s classical part and not the quantum part, which is very hard,” Isi said. “They just have a mass and they have a spin, and they follow these simple rules that seem mystical but are simple to write down.”
“They are these paradoxical objects,” he added. “That's why I work on this.”
To that point, Isi and his colleagues plan to follow up on their findings by applying their technique to other gravitational wave events. This process will not only shed more light on the area theorem, it could reveal countless new insights about the colossal events and exotic objects that create these ripples in spacetime.
“The kind of precision we have now is going to be put to shame with future observations,” Isi said. “The tests, and the quality of the experiments that we can carry out with the data, are going to improve drastically.”
“This is just showing the potential,” he concluded. “It’s the seed showing that we can think creatively and put our data to use to actually learn something that, for so long, had been just pen and paper.”