As readers of this blog know, late last spring I spoke at a cool conference in England called How the Light Gets In, where I hung out with all kinds of professional reality-ponderers. I've already posted Q&As with two fellow speakers I shared housing with: biologist Rupert Sheldrake, who urges scientists to take telepathy more seriously; and physicist George Ellis, who deplores the philosophical philistinism of some fellow physicists. Below is my Q&A with another housemate, physicist Carlo Rovelli of Aix-Marseille University and the Intitut Universitaire de France. I interviewed Rovelli by phone in the early 1990s when I was writing a story for Scientific American about loop quantum gravity, a quantum-mechanical version of gravity proposed by Rovelli, Lee Smolin and Abhay Ashtekar. (General relativity, Einstein's theory of gravity, is notoriously difficult to reconcile with quantum mechanics.) I was thrilled to meet Rovelli face to face, especially since he turned out to be, like Sheldrake and Ellis, a good as well as smart guy. Rovelli is the author of a leading textbook on quantum gravity and a biography about the ancient Greek philosopher Anaximander (discussed below). For more on Rovelli's views on physics and philosophy, see this 2012 conversation with him on Edge.org.

Horgan: Why did you become a physicist?

Rovelli: I was young in the sixties and seventies, and shared the dream of my generation: changing the world and make it more just and gentle. We lost. I did not know what to do next. I found physics, where, instead, revolutions succeed. I got in love with it. It has been a passion that hasn't ended.

Horgan: Has physics lived up to your expectations?

Rovelli: It has been much better than I expected. Infinite fun and enthusiasm. Investigating the secrets of the world. Thinking things that nobody else has thought before. Great adventures in thinking. Great companions of travel. Fantastic.

Horgan: What is loop quantum gravity?

Rovelli: It is, in my humble opinion, the best tentative theory of quantum gravity we have at present. We do not know if it is right. But we know there is a problem, and this is the best possible solution found so far for this problem.

Horgan: Is it still a viable contender as a unified theory?

Rovelli: "Unified theory" generally means a theory that unifies all forces and all fields: a "theory of everything." Loop quantum gravity doesn't have anything to do with this. [Horgan note: LQG doesn't include electromagnetism or the nuclear forces.] I think that for the moment we know nothing about a "unified theory" and the attempts to write it are by far premature and ill conceived. So, LQG is not a contender as a unified theory. Much more humbly, it is just a tentative solution for a simpler problem: describing the quantum aspects of gravity. Which is to say the quantum aspects of spacetime. This is hard enough. But it is a problem that we have chances to solve, because we have the ingredients.

Horgan: Do multiverse theories and quantum gravity theories deserve to be taken seriously if they cannot be falsified?

Rovelli: No.

Horgan: Do you ever think it's time for physicists to abandon the quest for a unified theory?

Rovelli: The "quest for a unified theory" is a misconception. Physicists never really searched for it. They stumbled upon string theory, which to some appeared as a possible unification of everything, and, for lack of imagination, put too much energy into strings. When the enthusiasm for strings begun to fade, many felt lost. Now that supersymmetry is not showing up where string theorists expected it, it is a disarray.

Horgan: Can physics—or science in general—ever completely solve the mystery of the universe?

Rovelli: What is the "mystery of the universe"? There isn't a "mystery of the universe." There is an ocean of things we do not know. Many of them we'll figure out, if we continue to be somewhat rational and do not kill one another first (which is well possible.) There will always be plenty of things that we will not understand, I think, but what do I know? In any case, we are very very very far from any complete comprehension of everything we would like to know.

Horgan: Can science attain absolute truth?

Rovelli: I have no idea what "absolute truth" means. I think that science is the attitude of those who find funny the people saying they know something is absolute truth. Science is the awareness that our knowledge is constantly uncertain. What I know is that there are plenty of things that science does not understand yet. And science is the best tool found so far for reaching reasonably reliable knowledge.

Horgan: What's your opinion of the recent philosophy-bashing by Stephen Hawking, Lawrence Krauss and Neil deGrasse Tyson?

Rovelli: Seriously: I think they are stupid in this. I have admiration for them in other things, but here they have gone really wrong. Look: Einstein, Heisenberg, Newton, Bohr.... and many many others of the greatest scientists of all times, much greater than the names you mention, of course, read philosophy, learned from philosophy, and could have never done the great science they did without the input they got from philosophy, as they claimed repeatedly. You see: the scientists that talk philosophy down are simply superficial: they have a philosophy (usually some ill-digested mixture of Popper and Kuhn) and think that this is the "true" philosophy, and do not realize that this has limitations.

Here is an example: theoretical physics has not done great in the last decades. Why? Well, one of the reasons, I think, is that it got trapped in a wrong philosophy: the idea that you can make progress by guessing new theory and disregarding the qualitative content of previous theories. This is the physics of the "why not?" Why not studying this theory, or the other? Why not another dimension, another field, another universe? Science has never advanced in this manner in the past. Science does not advance by guessing. It advances by new data or by a deep investigation of the content and the apparent contradictions of previous empirically successful theories. Quite remarkably, the best piece of physics done by the three people you mention is Hawking's black-hole radiation, which is exactly this. But most of current theoretical physics is not of this sort. Why? Largely because of the philosophical superficiality of the current bunch of scientists.

Horgan: You have written about the Greek thinker Anaximander. Who was he, and why do you find him interesting?

Rovelli: He is the guy who understood that the Earth is a stone that floats in the middle of the Sky without falling down. He understood that the Sky is not just above out head: it is also under our feet. It surrounds us in every direction. He is the only one in the history of our planet who understood this, and convinced everybody else that this is the case. In fact, he has done much more than this, but this is his greatest achievement. I find him immensely interesting because he represents one of the main steps in the development of scientific thinking. He is a giant.

Horgan: Do you agree with philosopher Thomas Nagel that science needs a new paradigm to account for the emergence of life and consciousness in the cosmos?

Rovelli: No. When we do not understand something, people are tempted to think that "some new paradigm" is needed, or a "great mystery" is there. Then we understand it, and all fog dissolves.

Horgan: Do you believe in God?

Rovelli: No. But perhaps I should qualify the answer, because like this it is bit too rude and simplistic. I do not understand what "to believe in God" means. The people that "believe in God" seem like Martians to me. I do not understand them. I suppose this means that I "do not believe in God". If the question is whether I think that there is a person who has created Heavens and Earth, and responds to our prayers, then definitely my answer is no, with much certainty.

If the question is whether I believe that "God" is a powerful something in the people, which causes a lot of disasters but also a lot of good, then of course I believe it. In fact, I am extremely curious about religion. I think that we should study what is religion much more than what is done. There is a sort of taboo in this, a sort of respect towards people who "believe in God", which makes it difficult to understand better.

I think that viewing the "belief in God" just as a bunch of silly superstitions is wrong. The "belief in God" is one form of human religious attitude, and human religious attitude is something very general and universal about our functioning. Something which is important for man, and we have not yet understood.

Horgan: Are science and religion compatible?

Rovelli: Of course yes: you can be great in solving Maxwell's equations and pray to God in the evening. But there is an unavoidable clash between science and certain religions, especially some forms of Christianity and Islam, those that pretend to be repositories of "absolute Truths." The problem is not that scientists think they know everything. It is the opposite: scientists know that there are things we simply do not know, and naturally question those who pretend to know. Many religious people are disturbed by this, and have difficulty in coping with it. The religious person says, "I know that God has created light saying, 'Fiat Lux.'" The scientist does not believe the story. The religious people feel threatened. And here the clash develops. But not all religions are like that. Many forms of Buddhism, for instance, have no difficulty with the continual critical attitude of science. Monotheistic religions, and in particular Islam and Christianity, are sometimes less intelligent.

I have an idea about the source of the conflict: there is beautiful research by anthropologists in Australia which shows that religious beliefs are often considered a-temporal but in reality change continuously and adapt to new conditions, new knowledge and so on. This was discovered by comparing religious beliefs held by native Australians studied by anthropologists in the thirties and, much later, in the seventies. So, in a natural situation, religious beliefs adapt to the change in man's culture and knowledge. The problem with Islam and Christianity is that many centuries ago somebody had the idea of writing down beliefs. So now some religious people are stuck with the culture and knowledge of centuries ago. They are fish trapped in a pond of old water.

Horgan: Have you ever accepted money from a military organization?

Rovelli: No. In my country military service was compulsory when I was a young man. I refused to join the army and was briefly detained for this.

Horgan: Do you think physicists—and scientists in general--have a moral responsibility to oppose militarism?

Rovelli: I think that we have a moral responsibility to oppose war as human beings, not as physicists or scientists. I think that the problem is that everybody "opposes the war" in words, but then many people are ready to make exceptions to serve their interests, defend their power and economical superiority. And these people hide this behind "feel good" words like "help people" or "fight terrorism". I found this morally disgusting. I wish people were less religious and more moral .

Horgan: In 2002 I bet physicist Michio Kaku $1,000 that by 2020 "no one will have won a Nobel Prize for work on superstring theory, membrane theory, or some other unified theory describing all the forces of nature.” Who do you think is going to win? [Horgan note: Lee Smolin was originally going to bet against me but backed out at the last minute, the big chicken.]

Rovelli: You. No doubt.

Postscript from Horgan: In the comments section, you can see that Carlo Rovelli posts responses to some early comments on his remarks. I'm copying his responses here for easier access.

A few answers from carlo rovelli:

- to “lakefield”: I actually loved these questions: finally general questions allowing ideas to be expressed, instead of the common boring technicalities.

- to “pupplesan”: I think what you do in your post about the “hard problem of consciousness is not characterising it, but rather anticipating a possible answer. Which might be right. Or not.

- to “Strangelove”: Yes, Archimedes and Eudoxus are also giants. Establishing ranking is not of much interest, I think. I had the misfortune that my US publisher chose the title “The First Scientist” for my book on Anaximander, raising a silly priority debate. I meant bringing attention to Anaximander’s wonderful achievements, not to open an empty discussions about who is greatest or the first! More on the substance of your post, I agree that the key is being testable, which is the main thing you say. But I think that the picture of science as “random guesses” then tested is not a good picture. The “guesses” that have a good chance of being later confirmed are those that have ground. Your example about the spherical Earth is very good: there had been a long sequence of observations, arguments and rational thinking (An example in Aristotle’s ‘On the Heavens’ is the round shade of the Earth on the Moon during lunar eclipses) that provided very strong plausibility to the idea. I think that a problem of today’s theoretical physics is the difficulty of appreciating the difference between a “random guess” and a “well motivated guess”.

- to “ZoranOstric”. You ask about a way to overcome this separation. I think a good starting point could be simply physicists stoping talking philosophy down. For centuries scientists were cultured people, who knew the main philosophical ideas of the past, knew well the history of science, and were very curious about the philosophers of their own times. Yes, I do think that knowing what people in analytical philosophy do now would be useful. There are many thinker in analytical philosophy nowadays that I have found very interesting for a physics (Butterfield, Price, Earman, Norton, Dorato, van Fraassen, Bitbol, Halvorson, Brown, Wuthrich, Myrvold, Ismael, Weinstein, Sauders… just to name a few at random). An example: people talking about “other worlds existing” could usefully read John Austin and David Lewis, at least to avoid using “exist” so superficially…

- to "cshbar", who asks: 'how [my] favored theory can be falsified?' Good question. Thanks. Answers in several steps:(i) In principle, Loop Quantum Gravity gives specific quantitative predictions, for instance regarding the possible values of the area of any surface, or cross section. Any measurement of a cross section that does not enter the set of the values predicted by theory falsifies the theory. (ii) These predictions are pretty general, and the situation is very different from string theory, which has zillions of vacua each with a different set of predictions, or multiuniverses, where it is not even very clear what a 'prediction' is going to be.(iii) In practice, it is difficult to test predictions, because the Planck length is small.(iv) But the claim of LQG is simple, and low key: it is not a theory we try to sell as the final theory of everything that everybody should buy; we present it is a tentative theory whose physical viability we ourselves we are far from sure about, until some empirical support comes in.(v) As for myself, predictions and testing the theory is what I am consider interesting and what I am working on. This is why I am trying to use the theory to compute the probability of black holes tunnelling into white holes, with the hope to see signals of this process in cosmic rays, and get hard testing. So, the short answer is: LQG is developed with the idea that what cannot be tested empirically is not good science.

Post-postscript: Biologist Jerry Coyne comments on the Q&A at "Why Evolution Is True," http://whyevolutionistrue.wordpress.com/2014/08/23/accommodationism-from-a-physicist/.