MIT and other physicists have used gravitational wave to confirm Hawking's black hole area theory for the first time. This computer simulation shows how two black holes collided to produce the gravitational signal, GW150914. Credit: Simulating eXtreme Spacetimes project. LIGOEven the most extreme objects of the universe have to follow certain rules. The central law of black holes states that the area of their eventhorizonsthe boundary beyond where nothing can ever escapeshould not shrink. Hawking's area theory, named after Stephen Hawking, a physicist who discovered the law in 1971, is the basis of this law.Fifty years after Hawking's theory was first proved, physicists from MIT and other universities have confirmed it using observations of gravitational wave. Physical Review Letters today publishes their results.The researchers examine GW150914, which was the first detected gravitational wave signal by the Laser Interferometer Gravitational-wave Observatory in 2015. This signal was the result of two inspiraling dark holes, which created a new blackhole. The signal also contained a large amount of energy, which rippled through space-time as gravitational wave.Hawking's area theory holds that the horizon area for the new black hole must not be smaller than its parent black holes. The new study was based on the reanalysis of the signal from GW150914 prior to and after the cosmic collide. They found that the total event horizon area did indeed not decrease following the merger, a result they report with 95% confidence.These observations are the first observational confirmation of Hawking’s area theorem. Although mathematically proven, it has never been observed in nature. To confirm Hawking's theory or to show that law-bending physics is possible, the team will test future gravitational wave signals.Maximiliano is a NASA Einstein Postdoctoral Fellow at MIT's Kavli Institute for Astrophysics and Space Research. "It's possible that there's a zooof of different compact objects. While some of them follow Einstein and Hawking’s laws, other beasts may be slightly differently," Maximiliano Isi says. It's not as if you just do the test once and then it's over. This is the beginning.Isi's coauthors include Will Farr from Stony Brook University and Flatiron Institute's Center for Computational Astrophysics. Matthew Giesler of Cornell University, Mark Scheel at Caltech and Saul Teukolsky, both of Cornell University.The age of insightStephen Hawking's 1971 area theorem was a fundamental insight into black hole mechanics. According to the theorem, the area of all black holes in the universe and the event horizon of a black one will never decrease. This statement was an interesting parallel to the second law in thermodynamics which stated that the entropy (or degree of disorder) within an object should never decrease.Similarities between the theories led to the conclusion that black holes could be thermal and heat-emitting objects. This is a confusing proposition as black holes were believed to have a unique nature, which means they never let energy escape or radiate. In 1974 Hawking reconciled the two theories, showing that black hole could emit radiation and have entropy over very long timescales if quantum effects are taken into consideration. Hawking's "Hawking radiation", a phenomenon that was one of the most important revelations about black holes, was called this.Isi states that it all began with Hawking realizing that black holes have a total horizon area that can never decrease. "The area law captures a golden age of the 1970s when all these insights were being generated."Hawking and other researchers have shown that the area theory works mathematically. However, there was no way to verify it against nature before LIGO's detection of gravitational wave.Hawking immediately contacted Kip Thorne (Caltech's Feynman Professor in Theoretical Physics) after hearing the results. His question was: Does the detection confirm the area theory?Researchers did not have the capability to discern the information in the signal before and after the merger to determine if the final horizon area had not decreased, as Hawking's theory would suggest. It was not until many years later that the area law was tested by Isi and his collaborators.Before and afterIsi and his coworkers developed a technique to extract the reverberations that occurred immediately after GW150914's peak, the moment when the two black holes parent collided to create a new black hole. This technique was used by the team to identify specific frequencies or tones in the otherwise loud aftermath that could be used to calculate the final black holes mass and spin.Thorne recalls Hawking's question and asked them if they could use the same technique to check the signal before and during the merger to confirm the area theorem.Researchers took up the challenge and split the GW150914 signal again at its peak. To analyze the signal prior to the peak and identify the mass and spin before they merged, the researchers developed a model. They calculated their total horizon area, which was roughly equivalent to approximately 235,000 square kilometers or nine times Massachusetts.The "ringdown," or reverberations, of the newly formed black holes was then extracted using their previous technique. From this, they calculated the mass, spin, and horizon area. This gave them an equivalent of 367,000 km (approximately thirteen times that of the Bay State).Isi states that data shows with overwhelming confidence that the area of the horizon increased following the merger and that the area law has been satisfied with high probability. It was a relief to see that our results were consistent with our expectations and that they confirm our understanding of these complex black hole mergers.Hawking's "area theorem" and other theories of black hole mechanics are being tested by the team using data from LIGO, Virgo and its Italian counterpart, Virgo.Isi states, "It's encouraging to think in new and creative ways about gravitational wave data, and reach answers we didn't know we could before." We can continue to find pieces of information that directly relate to what we believe we know. This data might reveal something unexpected one day.Continue exploring Gravitational wave echos could confirm Stephen Hawking’s theory of quantum black holesThis story has been republished by MIT News (web.mit.edu/newsoffice/), a popular website that covers news related to MIT research, innovation, and teaching.
