This ingenious idea could help us understand the strange physics that reigns inside black holes

At the beginning of time and in the center of each black hole is an infinite point of infinite density called singularity. To explore these enigmatic phenomena, we take what we know about space, time, gravity and quantum mechanics and apply it to a region where all this is simply broken. Maybe there is nothing in the universe that challenges the imagination more. Physicists still believe that if they achieve a coherent explanation of what really happens inside and around singularities, something revealing will emerge, perhaps a new understanding of the composition of space and time.

At the end of the 1960s, some physicists speculated that singularities could be surrounded by an agitated region of chaos, where space and time grow and contract randomly. Charles Misner, from the University of Maryland, called it “Mixmaster Universe”, in honor of what was then a popular line of kitchen appliances. If an astronaut fell into a black hole, “one can imagine how the parts of his body would be mixed in the same way that a whisk stir the yolk and the clear of an egg,” Kip Thorne later wrote, a physicist winner of the Nobel Prize.

Einstein’s general theory of relativity, which is used to describe the seriousness of black holes, uses a single field equation to explain how space is curved and matter moves. However, this equation uses a mathematical abbreviation called tensioner to hide 16 different and interrelated equations. Several scientists, including Misner, had devised useful simplifiers that allowed them to explore scenarios such as the Mixmaster Universe.

Without these assumptions, the Einstein equation could not be resolved analytically, and even with them it was too complex for the numerical simulations of the time. Like the apparatus that gave them name, these ideas went out of style. “This dynamic is supposed to be a very general phenomenon of gravity,” said Gerben Oling, postdoctoral researcher at the University of Edinburgh. “But it is something that was lost on the map.”

In recent years, physicists have been reexamining chaos around singularities with new mathematical tools. Its objectives are double. One is to demonstrate that the approaches of Misner and others are valid for Einsteinian gravity. The other is to approach singularities in the hope that their extremes will help reconcile general relativity with quantum mechanics in a theory of quantum gravity, an objective that physicists have pursued for more than a century. As Hartnoll said, from the University of Cambridge: “The time has come for these ideas to develop fully.”

Thus the Mixmaster Chaos was born

Thorne described the end of the 60s as a “golden age” for the investigation of black holes. The term had barely widespread. In September 1969, during a visit to Moscow, Thorne received a manuscript from Evgeny Lifshitz, an outstanding Ukrainian physicist. Together with Vladimir Belinski and Isaak Khalatnikov, Lifschitz had found a new solution to Einstein’s severity equations near a singularity, using assumptions that the three had devised. Lifshitz feared that Soviet censorship delay the publication of the result, since it contradicted an earlier evidence of which he had been co -author, so he asked Thorne to share it in the West.

The previous models of black holes assumed perfect symmetries that are not found in nature, postulating, for example, that a star was a perfect sphere before collapsing in a black hole, or that had no net electric charge. (These assumptions allowed Karl Schwarzschild to solve Einstein’s equations, in their simplest form, shortly after Einstein would publish them). The solution found by Belinski, Khalatnikov and Lifschitz, known as the BKL solution for their initials, described what could happen in a disorderly and more realistic situation in which black holes are formed from irregular objects. The result was not a uniform expansion of space and time inside, but a turbulent sea of ​​space and time that expands and compresses in multiple directions.

Thorne brought the article clandestinely to the United States and sent a copy by mail to Misner. Throne knew that he thought similarly. It turned out that Misner and the Soviet group had reached the same ideas independently, using similar and technical assumptions. Moreover, the BKL group “used it to solve the biggest problem without that time in mathematical relativity,” said Thorne, in relation to the existence of what is known as a “generic” uniqueness. Belinski, the last surviving member of the BKL trio, recently declared in an email that the vivid descriptions of Misner helped him visualize the chaotic situation near the singularities that both revealed.