The world of Bousso’s new theorem still departs from our universe in notable ways. For mathematical convenience, he assumed that there’s an unlimited variety of particles—an unrealistic assumption that makes some physicists wonder whether this third layer matches reality (with its 17 or so known particles) any better than the second layer does. “We don’t have an infinite number of quantum fields,” said Edgar Shaghoulian, a physicist at the University of California, Santa Cruz.
Still, for some experts, Bousso’s work delivers a satisfying denouement to the Penrose and Wall singularity story, despite its unrealistic abundance of particles. It establishes that singularities can’t be avoided, even in space-times with mild reactions to quantum matter. “Just by adding small quantum corrections, you can’t prevent the singularity,” Penington said. Wall and Bousso’s work “answers that pretty definitively.”
The Real Singularity
But Bousso’s theorem still doesn’t guarantee that singularities must form in our universe.
Some physicists hold out hope that the dead ends do somehow go away. What seems like a singularity could actually connect to somewhere else. In the case of a black hole, perhaps those light rays end up in another universe.
And a lack of a Big Bang singularity might imply that our universe began with a “Big Bounce.” The idea is that a previous universe, as it collapsed under the pull of gravity, somehow dodged the formation of a singularity and instead bounced into a period of expansion. Physicists who are developing bounce theories often work in the second layer of the onion, using semiclassical physics that exploits negative-energy quantum effects to get around the singularity required by the Penrose and Hawking theorems. In light of the newer theorems, they will now need to swallow the uncomfortable truth that their theories violate the generalized second law as well.
One physicist pursuing bounces, Surjeet Rajendran of Johns Hopkins University, says he is undaunted. He points out that not even the generalized second law is gospel truth. Rejecting it would make singularities avoidable and continuations of space-time possible.
Singularity skeptics can also appeal to the theory at the core of the onion, where space-time behaves in truly quantum ways, such as taking on superpositions. There, nothing can be taken for granted. It becomes hard to define the concept of area, for instance, so it’s not clear what form the second law should take, and therefore the new theorems won’t hold.
Bousso and like-minded physicists, however, suspect that a highly quantum arena with no notion of area is tantamount to a dead-end for a light ray, and therefore that something Penrose would recognize as a singularity should persist in the core theory and in our universe. The beginning of the cosmos and the hearts of black holes would truly mark edges of the map where clocks can’t tick and space stops.
“Inside of black holes, I am positive there is some notion of singularity,” said Netta Engelhardt, a physicist at MIT who has worked with Wall.
In that case, the still-unknown fundamental theory of quantum gravity would not kill singularities but demystify them. This truer theory would allow physicists to ask questions and calculate meaningful answers, but the language of those questions and answers would change dramatically. Space-time quantities like position, curvature and duration might be useless for describing a singularity. There, where time ends, other quantities or concepts might have to take their place. “If you had to make me guess,” Penington said, “whatever quantum state describes the singularity itself does not have a notion of time.”
Original story reprinted with permission from Quanta Magazine, an editorially independent publication of the Simons Foundation whose mission is to enhance public understanding of science by covering research developments and trends in mathematics and the physical and life sciences.
