Gravitational Waves Confirm Stephen Hawking's Famous Black Hole Theory

A new gravitational wave analysis has put forward strong evidence.
Derya Ozdemir
Spiraling Supermassive Black HolesNASA Goddard/YouTube

Another of the famous theories on black holes put forth by theoretical physicist Stephen Hawking has been confirmed with a more than 95 percent level of confidence using gravitational waves, an MIT team reports in a paper to appear in Physical Review Letters.

The team's observational confirmation is based on data from GW150914, the first gravitational waves ever spotted. This new analysis puts forward strong evidence supporting Hawking's black hole area theorem, which he proposed in the early 1970s based on his interpretation of general relativity.

According to the theorem, black holes cannot decrease in surface area over time and the surface area of a black hole rises with its mass, mirroring the law of entropy, which states that in a closed system entropy always increases. Since the entropy of a black hole is proportional to its surface area, both must always increase.

However, Hawking also said that a black hole's surface area also slowly shrinks the more it spins, due to the loss of tiny amounts of energy known as Hawking radiation, and while no emissions have been spotted yet, most physicists believe they exist.

MIT researchers Maximiliano Isi, Will M. Farr, and others wondered whether it would be possible to toss an object inside a black hole hard enough to make it spin enough to decrease its area. According to the area theorem, the increase in the surface area caused by more mass will always outweigh the reduction in surface area caused by more spin, per Live Science.

To test this out, researchers looked at data from the first gravitational waves ever detected, which were produced during the final fraction of a second of two black holes merging to form a single, more massive spinning black hole, as an example visualized by NASA in the video below. Those gravitational waves were discovered by the Advanced Laser Interferometer Gravitational-Wave Observatory, or LIGO, in 2015.

The researchers split the data into two halves, before and after the mergers, and calculated the surface areas of the black holes. The newly created black hole's surface area was bigger than the combined surface areas of the two originating black holes, confirming the area rule with a 95 percent confidence level. The researchers say this is "the first time that we can put a number on this," per Science News.

According to the researchers, their findings are generally in accordance with their expectations, and their next steps will be to examine data from more gravitational waves to discover more about black holes.

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