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FQXI ARTICLE
February 6, 2023

The Spacetime Revolutionary
Carlo Rovelli describes how black holes may transition to "white holes," according to loop quantum gravity—a radical rewrite of fundamental physics.
by Colin Stuart
FQXi Awardees: Carlo Rovelli
December 13, 2016
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Carlo Rovelli
CTP Luminy, Marseille
You might call Carlo Rovelli a reluctant physicist. "I wasn’t one of those kids who was enamoured with science at an early age," he says. "I only decided to study physics after the exclusion of everything else."

Rovelli, now at the Centre for Theoretical Physics, in Luminy, Marseille, France, certainly didn’t think it would become his career—or that he might one day be known for co-founding a radical new theory to explain the origins of spacetime, loop quantum gravity. He is now investigating whether evidence for this model could be found in the form of "white holes," formed as black holes turn themselves inside out, spewing, rather than swallowing, matter.

Rovelli recounts that he only went to university in order to delay enrolling in Italy’s compulsory military service. So he freely concedes that he was far from a model student. Attending university in the Seventies he was swept up in political activism, a legacy from the halcyon days of the late Sixties. "I was more into trying to change the world than studying," he admits. But Rovelli soon realised he wasn’t getting vary far with his political revolution. It was only then that he started studying relativity—Einstein’s ideas on gravity that involve weaving space and time together into a four-dimensional fabric—and quantum mechanics—the theory governing the world of the very small—in more detail. He describes what came next as a flash. "It was incredibly beautiful," Rovelli says. "I fell in love with it."

One Brief Lesson on Physics-Writing

Carlo Rovelli discusses his international bestseller, "Seven Brief Lessons on Physics," with Colin Stuart.

LISTEN:

Go to full podcast

Immersing himself in physics, Rovelli’s studying habits changed. During a wander around the library at the University of Bologna he stumbled across a review article on quantum gravity—the quest to unite Einstein’s theory of gravity and quantum mechanics—by British physicist and FQXi member Christopher Isham. It would change everything. Rovelli was captivated by how the subject required us to completely change our views about space and time. "I thought wow, this is better than LSD. I want to do this!" he says. He then realised that physicists might be able to change the world even more than political revolutions do. "I think that Copernicus and Dirac and Einstein changed the world quite a lot," Rovelli says. "I wanted to be part of this common adventure."

Rovelli then went on to do a PhD, at the University of Padua, Italy, but unusually for a doctoral student he didn’t publish any papers, instead choosing to focus on mastering the different approaches to quantum gravity. Doctorate complete, he set out on his own, initially without being attached to a school or research group. Instead Rovelli upped sticks and traveled the world, visiting Isham in London as well as journeying to the US.

The network of interacting
quantum events can be
identified with the very
fabric that weaves spacetime.
- Carlo Rovelli
Decades later and Rovelli, now 60, is at the coalface of potential revolutions in physics. He is one of the founders of loop quantum gravity, which attempts to combine general relativity and quantum theory. Recently, he has been awarded an FQXi grant of $90,000 to dig deeper into the problem. In loop quantum gravity, events—"happenings of something somewhere at some time"—are more important than objects. As Rovelli puts it, "an object is just a particularly monotonous sequence of events." Take a wave on the ocean: you can watch it moving but there are no actual objects moving, no single water droplet travels with the wave. Same for humans: Most of the atoms in your body have changed since you were a kid; so, in a sense, you’re a wave of things going in and things coming out. "We’re not a thing," Rovelli explains. "There is a continuity in the process, not in the material."

So to crack the conundrum of quantum gravity, Rovelli argues, we shouldn’t be formulating our equations in terms of things, but in terms of events. In loop quantum gravity, spacetime itself emerges from fundamental discrete quantum events. Although this approach is followed by a large number of researchers worldwide, "the precise formulation of the theory is still discussed, and I’d like to make this all more precise," says Rovelli.

Black Hole to White Hole

To this end, Rovelli has recruited a postdoc who will spend a year with him in Marseille chiseling the equations. He then hopes to test his results by comparing them to a concrete physical situation describing the fate of black holes. In Einstein’s standard theory of gravity, black holes form when stars die, and their matter is crunched down into the black hole’s infinitely small and dense core. In Rovelli’s analysis, in terms of loop quantum gravity, this matter still gets compressed, but only to an extent. Rather than being squeezed into an infinitely tiny point, the compression hits a limit and then the matter bounces out again.


White Hole
Could the antitheses of black holes exist—and provide evidence of loop quantum gravity?
Credit: JPL/NASA
This could mean that black holes eventually transition into so-called white holes, which spit out all the matter and light that were once swallowed by the black hole. (Rovelli talks about this idea in more depth and about possible observable signatures of white holes on the FQXi podcast.)

"The network of interacting quantum events can be identified with the very fabric that weaves spacetime," says Rovelli. Analysing events it is easier to see how spacetime itself may turn inside out, transforming from a black hole to a white hole.

Black Hole Fireworks

Can black holes turn into white holes? Carlo Rovelli chats about the possibility with Zeeya Merali.

LISTEN:

Go to full podcast

Flavio Mercati, an FQXi member from the Perimeter Institute for Theoretical Physics, in Waterloo, Ontario, says Rovelli’s interpretation of quantum theory is hard to overlook. "Carlo has thought hard about the interpretation of quantum mechanics, and he has formulated one point of view which might well be the correct way to think about it," he says. "Whether it is or not, it is essential for any researcher discussing the foundations of quantum mechanics to discuss Carlo’s interpretation."

David Sloan, from the University of Oxford, UK, thinks Rovelli’s is a well-founded, if ambitious, project. "Carlo has met with remarkable success in the past, and is known for taking on hard conceptual issues," Sloan says. "One can never guarantee success in these things—that’s the nature of research—but I’m confident that he will make some significant headway."

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The foundation of a superluminal field would start with an experiment that would show that virtual photons are the origin of the invariance of the speed of light. The overused "tachyon field" is the best name I can come up with. It would like this.

c' refers to a finite superluminal speed of light.

Virtual photon (c) + tachyon particle = virtual photon (c').

A generated tachyon field will turn virtual photons into superluminal virtual photons. The experiment would require...

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