More than 50 years ago, acclaimed theoretical physicist Stephen Hawking theorized that the area of a black hole could never decrease. On Sept. 10, the LIGO-Virgo-KAGRA Collaboration, a global network of scientists, published a paper that verified his theorem using data from gravitational wave detectors.
Hawking derived the theorem in 1971, stating that the total area of the black hole’s event horizon — the boundary of a black hole at which nothing can escape — cannot ever decrease, only increase or stay the same. Prior to the Sept. 10 paper, the theorem had only been proven mathematically, but never through observation of black holes.
“These are the reasons why I came into physics,” said Ish Gupta, one of 11 coauthors of the study who are associated with Northwestern. “Because it has always been extremely fascinating to me how a person can just sit on a chair with a table in front of them, with a pen and paper… and you come up with a theorem that those two black holes follow to the T.”
Gupta is a University of California, Berkeley postdoctoral fellow, a visiting scholar at NU’s Center for Interdisciplinary Exploration and Research in Astrophysics and a member of the LVK Collaboration.
The LVK Collaboration was able to confirm Hawking’s theorem by using interferometers to measure the gravitational waves produced from the merging of two black holes.
Interferometers work by splitting a laser in half, each half going in perpendicular directions. The beams are then reflected by a mirror and reunited.
Scientists can then detect if the lasers passed a gravitational wave, such as those produced by two black holes merging. The collected data can be analyzed to calculate the area of the black hole.
The instruments’ measurements gave the researchers the area of the two original black holes and the area of the merged one. They then compared these values and found that the area of the merged black hole was larger than the sum of the areas of the original two.
Michael Zevin is another coauthor of the study and has been a member of the LVK Collaboration since 2015. At NU, he is a visiting scholar at CIERA and astronomer at the Adler Planetarium.
“Most black holes, we believe, form when big stars end their lives; so, stars that are ten or so times more massive than our sun,” Zevin said. “When they die, they can leave behind a black hole, which effectively is just this entire star collapsing in on itself and nothing being able to hold up the gravitational collapse.”
Anarya Ray, a CIERA postdoctoral associate and coauthor of the study, described black holes as “a particular form of space-time geometry that comes out of Einstein’s equations.” He explained that these equations describe how matter alters space-time and that one of the solutions to the equations is a black hole.
Hawking’s theorem was derived from Albert Einstein’s theory of relativity. This theory describes, among other things, gravity and time. The theory of relativity, in turn, comes from two related theories by Einstein: the special theory of relativity, published in 1905, and the general theory of relativity, published in 1915.
Ray explained how the verification of Hawking’s theorem by LVK further proves Einstein’s theory.
“The reason we care about that is because the correct description of gravity is crucial to our understanding of the universe and also for our daily lives,” Ray said. “For example, if Einstein’s theories were not correct, then the GPS in our phones would not work because GPS works based on very precise measurements of time.”
The verification of this theory also comes ten years after gravitational waves were detected for the first time ever in 2015 by the LIGO detectors. Gupta explained that the improved sensitivity of their detectors contributed to the verification.
“It just shows how much progress we have made in 10 years and how much progress we are going to make in another 10 years,” Gupta said. “The verification of Hawking’s area theorem is one of many landmark discoveries that are to come.”
Email: williamfisher2028@u.northwestern.edu
Related Stories:
—Northwestern astronomers detect massive black hole as part of international collaboration
—CIERA facilitates interdisciplinary research in astrophysics
—NU astrophysicist awarded top prize from American Physical Society
