At the beginning of time and the center of every black hole lies a point of infinite density called a singularity. To explore these enigmas, we take what we know about space, time, gravity and quantum mechanics and apply it to a place where all of those things simply break down. There is, perhaps, nothing in the universe that challenges the imagination more. Physicists still believe that if they can come up with a coherent explanation for what actually happens in and around singularities, something revelatory will emerge, perhaps a new understanding of what space and time are made of.
In the late 1960s, some physicists speculated that singularities might be surrounded by a region of churning chaos, where space and time haphazardly grow and shrink. Charles Misner of the University of Maryland called it a “Mixmaster universe,” after what was then a popular line of kitchen appliances (opens a new tab). If an astronaut were to fall into a black hole, “one can imagine it mixing up the astronaut’s body parts in the way that a mixmaster or eggbeater mixes up the yolk and white of an egg,” Kip Thorne (opens a new tab), a Nobel Prize–winning physicist, later wrote.
Einstein’s general theory of relativity, which is used to describe the gravity of black holes, uses a single field equation to explain how space curves and matter moves. But that equation uses a mathematical shorthand called a tensor to hide 16 distinct, intertwined equations. Several scientists, including Misner, had devised useful simplifying assumptions to let them explore scenarios like the Mixmaster universe.
Without those assumptions, Einstein’s equation couldn’t be solved analytically, and even with them it was too complicated for the numerical simulations of the time. Like the appliance they were named after, these ideas fell out of style. These “dynamics are supposed to be a very general phenomenon in gravity,” said Gerben Oling (opens a new tab), a postdoctoral researcher at the University of Edinburgh. “But it’s something that fell off the map.”
In the last few years, physicists have been revisiting the chaos around singularities with new mathematical tools. Their goals are twofold. One hope is to show that approximations that Misner and others made are valid approximations of Einsteinian gravity. The other is to push closer to singularities in the hope that their extremes will help reconcile general relativity with quantum mechanics in a theory of quantum gravity, which has been a goal of physicists for over a century. As Sean Hartnoll (opens a new tab) of the University of Cambridge put it, “The time is ripe now for these ideas to be fully developed.”
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