Does Quantum Gravity Cast Doubt On Fate of Black Holes?


LIGO Black Hole



“The fate of black holes in a quantum theory of gravity is, in my view, the most important problem in theoretical physics,” said Jorge Pullin, the Horace Hearne professor of theoretical physics at LSU.

Black holes created by collapsing stars are mysterious objects with an outer edge called an event horizon, which traps everything including light. Einstein’s theory of general relativity predicted that once an object falls inside an event horizon, it ends up at the center of the black hole called a singularity where density apparently becomes infinite and it is completely crushed. At this point of singularity, gravitational attraction is infinite and all known laws of physics break down including Einstein’s theory.

Singularities in Doubt

Theoretical physicists have been questioning if singularities really exist through complex mathematical equations over the past several decades with little success. 

Astrophysicists at LSU and Penn State developed mathematical equations that go beyond Einstein’s theory of general relativity overcoming its key limitation–the central singularity of black holes.


Loop Quantum Gravity

Artist depiction of loop quantum gravity effects in a black hole is shown above. The bottom half of the image depicts the black hole which, according to general relativity, traps everything including light. Loop quantum gravity, a theory that extends Einstein’s general relativity using quantum mechanics, overcomes this tremendous pull and liberates everything shown in the top half of image, thus solving the fundamental problem of black hole singularity. (A. Corichi and J. P. Ruiz).

Theorectical physicist, Carl Rovelli, founder of the quantum gravity group of the Centre de Physique Théorique (CPT) of the Aix-Marseille University and author of  “Quantum Gravity” and “Reality Is Not What It Seems”, offers a brilliant update on the current state of our understanding of quantum gravity below:


Loop Quantum Gravity

Theoretical physicists developed a theory called loop quantum gravity in the 1990s that marries the laws of microscopic physics, or quantum mechanics, with gravity, which explains the dynamics of space and time. Abbay Ashtekar, Javier Olmedo and Parampreet Singh’s new equations describe black holes in loop quantum gravity and showed that black hole singularity does not exist.

“In Einstein’s theory, space-time is a fabric that can be divided as small as we want. This is essentially the cause of the singularity where the gravitational field becomes infinite. In loop quantum gravity, the fabric of space-time has a tile-like structure, which cannot be divided beyond the smallest tile. My colleagues and I have shown that this is the case inside black holes and therefore there is no singularity,” Singh said.

Instead of singularity, loop quantum gravity predicts a funnel to another branch of space-time.

“These tile-like units of geometry–called ‘quantum excitations’– which resolve the singularity problem are orders of magnitude smaller than we can detect with today’s technology, but we have precise mathematical equations that predict their behavior,” said Ashtekar, who is one of the founding fathers of loop quantum gravity.

“At LSU, we have been developing state-of-the-art computational techniques to extract physical consequences of these physical equations using supercomputers, bringing us closer to reliably test quantum gravity,” Singh said.

Failures of Einstein’s Theory

Einstein’s theory fails not only at the center of the black holes but also to explain how the universe was created from the Big Bang singularity. Therefore, a decade ago, Ashtekar, Singh and collaborators began to extend physics beyond the Big Bang and make new predictions using loop quantum gravity. Using the mathematical equations and computational techniques of loop quantum gravity, they showed that the Big Bang is replaced by the “Big Bounce.” But, the problem of overcoming black hole singularity is exceptionally complex.

The Black Hole Will Shrink and Disappear

Incorporation of quantum physics also demonstrates that matter and energy can actually escape the event horizon of a black hole. As virtual pairs of matter and antimatter particles are spontaneously created via quantum fluctuations just beyond the event horizon, Stephen Hawking calculated that some pairs are quickly annihilated while others become separated. One particle or photon falls into the event horizon and its partner escapes the gravitational field of the black hole, leading to a net decrease in the mass of the black hole. 

“Black Holes Will Shrink and Eventually Disappear”

“Hawking showed that black holes radiate. His calculation is approximate only: the black hole is static and does not feel the back-reaction of the emitted radiation,” wrote Jorge Pullin in an email to The Daily Galaxy. “It is expected that as the black hole radiates, it will shrink and possibly eventually disappear.”

“I think, Pullin added, “it is the most important problem in fundamental theoretical physics (it is important to stress “fundamental” here as there are many other very important problems in theoretical physics as a whole) because it involves general relativity, the quantum theory and thermodynamics in their most extreme regimes.”

Maxwell Moe, astrophysicist, NASA Einstein Fellow, University of Arizona via Louisiana State University and Jorge Pullin

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