Successes and Challenges in Quantum Gravity

Carlo Rovelli is well known as a popularizer of science. His short book, Seven Brief Lessons on Physics, was an international bestseller. I have known Carlo as a physicist ever since he used to visit my Physics Department colleague, Lee Smolin, at Yale, when I was a Professor there. Carlo and Lee were part of a small group of physicists pioneering an idea called ‘Loop Quantum Gravity’ as a way to try and unify General Relativity and Quantum Mechanics. Less well known among the public than its chief competitor, String Theory, and also less popular among physicists as a whole, Loop Quantum Gravity is nevertheless an equally serious attempt to address the vexing paradoxes associated with of quantizing General Relativity.

Black Holes are the place in physics where the various problems of quantum gravity become manifest. If Stephen Hawking was correct, and black holes do completely evaporate through quantum processes that result in the emission of thermal radiation, then it appears that the information about what fell into the black hole in the first place will be forever lost. But this violates a central feature of quantum mechanics, which preserves information. At the same time, the final state of classical black hole collapse involves a singularity of infinite density. Most physicists expect this singularity to be removed in a quantum theory of black holes.

Rovelli argues that near the singularity of a black hole quantum processes can change a black hole to be a ‘white hole’, the time reversed version of a black hole. While anything that falls into a black hole stays there, everything inside a white hole eventually reappears. If Carlo’s ideas were correct, they could go a long way toward potentially resolving black hole paradoxes.

It is a big ‘If” however, and I remain skeptical. Nevertheless I wanted to discuss these ideas with Carlo on this podcast for a variety of reasons. First, any such discussion will illuminate a lot about the physics of black holes. Secondly, I think it is useful for laypeople to listen to physicists debate and discuss ideas at the forefront, presenting challenges to each other, being willing to openly question, and doing all of this with a sense of mutual respect.

At the same time, because I share Carlo’s great interest in both popularizing science, as well as connecting science and culture, I was extremely interested in discussing his motivations and thoughts about these important areas, and I was not disappointed. I hope listeners will find our discussions about science, literature, and politics equally enlightening.

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