“Come with me inside a black hole”: A talk by physicist Carlo Rovelli

“They’re remarkable objects… I hope I can tell you something more about their strangeness and their beauty,” says physicist Carlo Rovelli of black holes on January 30th at Western University’s Conron Hall, for the 2025 Duncanson Lecture.

Behind him beams a photo of Sagittarius A* — a black hole 26 million kilometers wide in the center of our Milky Way galaxy, 26,000 light years from Earth (1 light year is about 9.4 x 10¹²  kilometers). The photo — the average of thousands of photos taken from a telescope — shows a hazy orange-yellow ring of fluctuating brightness circling a dark region. It is the first evidence that Sagittarius A* exists beyond theory. Quintillions of black holes are predicted to exist in our universe. 

Rovelli explains that the bright ring is an “optical effect” created by the black hole’s strong gravitational pull, which “curves” light into an orbit. These light particles are emitted by superheated gases swirling in space. Some of this gaseous material is swallowed into the black hole. But what happens to materials that enter a black hole, whose devouring center is a “region of infinite density” that breaks the physics theories that predicted it? 

“I need your attention and your imagination,” says Rovelli.  “Reality is quite strange and different than what we’re used to.”

He lists two “spectacular things” that occur as something approaches and enters a black hole: “time dilation” and the “distortion of space”.

“Time dilation” refers to the slowing of time near the surface of a black hole. “Distortion of space” refers to the gradual, infinite “shrinking” of space inside the black hole.

“When you’re inside, the space is enormous…The space inside is like a long, long tube… like a funnel. If we try to come out, we can’t,” says Rovelli. “The radius is shrinking, becoming smaller and smaller and smaller, and squeezing us inside indefinitely. This is called the ‘singularity of the black hole’. The singularity is not a place, it’s not down there at the tip of a funnel. It’s at a time.”

The singularity breaks Einstein’s theory of general relativity, which had predicted the existence of black holes. It implies “quantum effects” within a black hole — behaviour at the level of atoms — that are instead described by quantum theories. And so the very existence of black holes lies perched between two unreconciled theories of reality. How do physicists deal with this?

“If we want to know what happens before the complete squeezing, we have to take quantum mechanics into account. Understanding the quantum aspect of what happened in the squeezing… you need a quantum version of Einstein’s theory. For the moment, no one has been able to find a theory,” says Rovelli. “But we do have tentative theories, ideas. That’s why I’m so attracted by this object, because inside there is a phenomenon that requires the quantum theory of gravity to be understood… to bring contradictory theories together gives us a better understanding of reality. ”

Rovelli’s attempt at this unification is to further develop a theory called “loop quantum gravity”, which first emerged in the 1980s and currently captivates about 30 research groups across the world. Under this theory, the matter sucked into a black hole’s distorted space might leave through a “white hole”. Rovelli calls this process a “bounce of a black hole into a white hole.”

“Imagine you have a ball and you let it fall and it hits the ground. What happens? It bounces back. When it comes up, it’s very similar in its trajectory when going down, but sort of backwards in time,” says Rovelli. “Imagine the formation of the black hole and the shrinking of a black hole. What would happen if it went backwards?”

Rovelli says Einstein’s theory not only predicts black holes, but complementary objects that are “exactly a black hole projected backwards in time”. These are called “white holes”. Unlike black holes, which can be fallen into and not exited, white holes can be exited and not fallen into. Unlike black holes, which infinitely shrink, white holes gradually swell. 

“We are inside a black hole, we’ve fallen into the center, and then we go through this jump, and now we find ourselves still in the funnel,” Rovelli illustrates. “But now the funnel is not shrinking, it’s opening up. And so we come out. This is the main idea.”

Just like a bouncing ball, whose peak height will diminish with bounces, so too will a black hole shed energy as its funnel-like space steadily opens. As its surface shrinks with dissipated energy from “bouncing”, its mass can minimize to a fraction of a microgram. Rovelli says these shrunken black hole surfaces might be scattered in the universe as the mysterious, powdery “dark matter” observed by astronomers. 

“Could it be that this mysterious dark matter that astronomers see is just a lot of very small white holes? I don’t know.”

All information that falls into a black hole might leave from these white holes in the form of radiation. Such radiation would be methodologically very hard to detect. 

As a student, when Rovelli had taken a class on black holes far before their observation, his professor had said the predicted objects “likely didn’t exist.” 

In 2020, the Nobel prize in physics was awarded to Roger Penrose, Reinhard Genzel, and Andrea Ghez for their black holes research. Penrose had mathematically proved that black holes can form in 1964; Genzel and Ghez successfully photographed Sagittarius A*, releasing the first ever photo of a black hole in our galaxy. The only other black hole to have been photographed is M87* in 2017, embedded in the Messier 87 galaxy located about 55 million light years from Earth.

“We see exactly what was predicted,” says Rovelli. “The Nobel prize (was awarded) because something that was written years ago is being observed.” ♦

To listen to a recording of Carlo Rovelli’s talk, visit: https://www.youtube.com/watch?v=qiNDTVewyAs