Into the Impossible — Carlo Rovelli: Loop Quantum Gravity & The Order of Time

A guest post by Phillip Levin.


In this episode of Into the Impossible, Brian Keating sits down with theoretical physicist Carlo Rovelli, author of “The Order of Time” and a founder of Loop Quantum Gravity, which is a theory of quantum gravity that combines ideas of quantum mechanics and general relativity.

The discussion starts lightheartedly, with Carlo Rovelli briefly discussing his appearance in a Disney film before jumping into a discussion of science and physics.

Rovelli has focused much of his career on the concept of time; a relevant question then, as Keating wonders, is how does Rovelli choose and make time for the topics he is most interested in?

Keating and Rovelli consider the relationship between science, Physics, and religion. Is there a connection? Rovelli shares an anecdote on this very topic as discussed by physicists Georges Lemaître and Paul Dirac — in particular: is cosmology the most relevant of sciences to religion? This raises the question, how do we understand religion? What area of science is most closely related to religion?

The topic of changing one’s mind in the process of doing science is discussed at length throughout the episode. Rovelli, for example, reminds listeners of the importance of revising our ideas as we learn more about how the world and universe works. Are there cases of major physicists getting it wrong?

The two discuss Georges Lemaître’s work on an expanding universe. How did Edwin Hubble get credit for the idea, if Lemaître also worked on it earlier? Rovelli admires Lemaître’s character as it relates to doing science and pursuing truth.

Moving onto quantum gravity, Rovelli talks about Loop Quantum Gravity (LQG). What is it? How does it work? How is it different from other theories of quantum gravity, namely string theory? Rovelli suggests that despite talk that it’s not possible to test theories of quantum gravity, that such experiments are possible and exist. He also considers an important question related to doing science and physics: do we ever “rule out things” completely in science?

Keating has in the past expressed concern about how important or necessary a theory of quantum gravity. In previous episodes of Into the Impossible, he has asked other physicists, such as Leoard Susskind, about this. Here he poses the question to Rovelli, who explains why he believes we do need a theory of quantum gravity.

Rovelli discusses primordial black holes and their relationship to LQG. He then goes on to explain an interesting prediction of LQG as related to black holes and white holes. Does one become the other? In modern theoretical physics, black holes are understood in terms of holography; Rovelli weighs in on this idea. He believes that black holes contain a great deal of information — and that Quantum Field Theory predicts this. The topic of black hole information is further considered. In particular, Rovelli suggests that he thinks that talk of firewalls and black holes is probably nonsense, despite that it has become a very popular topic in modern theoretical physics research. Rovelli considers himself a general relativist, and like many others of that camp, he says the surface of a black hole is not a special place, so there are probably no firewalls there. Rovelli believes that much of current theoretical physics is “crazy” and not realistic. Keating and Rovelli move onto discuss some attempts to do more realistic calculations, such as Juan Maldacena’s work on human traversable wormholes.

Moving back to LQG, Keating asks Rovelli about doing experiments to support or falsify LQG. Rovelli talks about a possible connection between the Cosmic Microwave Background and LQG. He seems more confident, however, about black holes as an experimental subject matter for those interested in LQG. For example, primordial black holes may trap early-universe photons and release them later, which we could perhaps detect today. There is also the subject of dark matter. Rovelli refers to dark matter as a type of “powder.” If black holes evaporate and become white holes, perhaps these could be components of dark matter, he wonders.

On the subject of testing theories of physics, the two consider the topic of supersymmetry. Rovelli asks: “do people still believe in it?” Keating references previous discussions with physicists who haven’t given up on the idea, such as Cumrun Vafa, who he talked with about supersymmetry in a previous episode of Into the Impossible.

The COVID-19 pandemic has had a profound effect on people around the world, including scientists and physicists. Keating and Rovelli consider how the pandemic has affected life as a scientist. In particular, how has the pandemic affected Rovelli’s productivity? Some physicists, for example, claim they have been more productive because they’ve been able to turn down public appearances during the pandemic and focus on their science. Keating talks about Kip Thorne, of Caltech, coming on Into the Impossible as a future guest.

Rovelli weighs the pros and cons of certain experimental approaches in physics — particularly as related to areas of physics he’s interested in, such as quantum gravity. He thinks black holes are a fantastic experimental laboratory, but is less of a fan of particle colliders. He also points to other potential experiments, such as LIGO and using astronomy.

Rovelli talks about a possible quantum gravity experiment that could be doable in a laboratory setting. This approach involves a quantum entanglement test of quantum gravity. He thinks that it may be possible that such an experiment is capable of revealing whether quantum geometry exists. Keating asks if something like BICEP could have been fruitful for studying quantum gravity?

The word “quantum” may be mysterious to non-physicists. So what does it mean? Rovelli explains. He thinks of LQG as putting general relativity and quantum mechanics together in a compatible way. Keating asks why Rovelli isn’t more excited about the idea of holography, suggesting that such a notion could be relevant to LQG,which quantizes space-time itself. Isn’t this related to holography? Rovelli answers. He then goes on to explain how according to LQG, space-time is quantized and therefore not a continuous but discrete quantity.

A problem in quantum gravity has been the problem of ultraviolet divergences, but Rovelli claims that LQG “cures” this problem by discretizing space and time. Keating wonders, who would Rovelli rather update on the latest developments of LQG: Richard Feynman or John Wheeler (both of which worked on quantum gravity)? Rovelli explains his answer and goes on to say that LQG is very much based on Wheeler’s intuitions about quantum gravity. Keating notes that Wheeler really should be a more-talked-about and well-known physicist to the masses, given his work. Rovelli agrees and says that he does think that Feynman would have liked LQG.

What is spin-foam? Rovelli explains how LQG leads from this idea of spin networks to spin foam. How do these things describe space-time? Is there a link between the famous Einstein-Podolsky-Rosen (EPR) paper, quantum entanglement, and LQG? Rovelli weighs in.

Keating talks to Rovelli about his new project involving Galileo Galilei. Perhaps Rovelli can lend his voice as Galileo Galilei? Keating asks Rovelli, can we make progress by reconsidering data from previous experiments? Could such data falsify or support LQG or string theory? Rovelli considers these questions and more, including the degree to which falsification plays a role in physics. Both Keating and Rovelli provide examples of previous discoveries that could have actually been made earlier had we looked at the data in the right way. Could we be sitting on new discoveries, hidden in “old” data?



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Brian Keating

Brian Keating


Chancellor’s Distinguished Professor at UC San Diego. Host of The INTO THE IMPOSSIBLE Podcast Author of Losing the Nobel Prize.