Andrew Strominger: Black Holes, Quantum Gravity, and Theoretical Physics | Lex Fridman Podcast #359

The conversation touches on the ethical implications of scientific discovery, using nuclear weapons as an example of ideas with the power to destroy. Strominger distinguishes between the invention of art (Picasso) and the discovery of scientific principles (Oppenheimer), arguing that certain scientific advancements have an unstoppable momentum. He extends this to artificial intelligence, acknowledging the responsibility of scientists to consider the societal impact of their work and strive to make the world a better place.

Andrew Strominger introduces the concept of a black hole as a mirror, where photons can orbit and return, creating an infinite number of self-images. He then discusses the Harvard Black Hole Initiative, which brings together theoretical physicists, experimentalists, and philosophers to explore the nature of black holes. A black hole is theoretically defined as a region of spacetime from which light cannot escape.

Strominger explains that light carries energy and is subject to gravitational pull. If enough mass is concentrated in a small enough region, the escape velocity can exceed the speed of light, preventing light from escaping. He highlights the groundbreaking nature of Einstein's idea of a universal speed limit, the speed of light, which seemed counter-intuitive before him.

Despite his own equations predicting their existence, Einstein initially doubted the reality of black holes and gravitational waves. Strominger notes that even brilliant minds can be confused, and that scientific progress involves continuously refining understanding. He uses the analogy of coordinate transformations to explain how subtle issues can lead to initial errors in interpretation, even for Einstein.

Strominger discusses singularities found at the center of black holes, which indicate a breakdown in Einstein's equations. He argues that these singularities are not flaws but rather signals that a theory needs improvement, often by incorporating quantum mechanics. He emphasizes that all scientific theories are approximations and that the discovery of their limits (demise) is a crucial step for scientific progress.

The speaker highlights the 'wonderful' tension between quantum mechanics and general relativity as a driving force for discovery. He explains that contradictions, like the incompatibility between Maxwell's theory and Newton's laws of gravity, led to groundbreaking insights. The challenge now is to unify the standard model of particle physics with gravity.

Strominger describes the Standard Model as an incredibly accurate theory that unifies the electromagnetic, weak, and strong interactions. However, gravity cannot be integrated into this framework using the same principles. String theory is introduced as the leading candidate for reconciling quantum mechanics and general relativity, though its direct experimental verification remains elusive.

String theory proposes replacing particles with tiny loops of string, which resolves infinities encountered in quantum field theories and explains the quantum nature of gravity. Initially seen as a toy model, it evolved to accommodate complex phenomena, aligning with observed reality. Strominger views string theory as a "stepping stone" to a deeper understanding, predicting its continued relevance in the distant future.

The discussion shifts to his work with Cumrun Vafa on black holes, connecting string theory to the problem of information storage in black holes. Beckenstein and Hawking's work suggested that black holes store information proportional to their surface area, not volume—a concept known as the holographic principle. This principle suggests that all information in a volume of spacetime can be stored on its boundary.

Strominger explains the concept of flat, negatively (anti-de Sitter), and positively (de Sitter) curved spacetime. He discusses how astronomers believe the universe is expanding and will eventually be dominated by "dark energy," which is synonymous with a positive cosmological constant. The nature and smallness of dark energy remain a significant mystery in physics.

Einstein's theory suggests black holes destroy information, but string theory offers corrections. Strominger recounts the moment of truth for string theory when it successfully calculated the number of 'gigabytes' a black hole could store, matching theoretical predictions. He then introduces the concept of "soft hair" from his work with Stephen Hawking and Malcolm Perry, which refers to subtle imprints on a black hole's horizon that preserve information, challenging Hawking's original no-hair theorem.

Soft particles (photons and gravitons) have zero energy and infinite wavelength, meaning they are spread throughout the universe. Despite carrying no energy, they possess angular momentum and cannot be ignored without violating conservation laws. These soft particles rush to the edges of the universe, including the boundaries of black holes, carrying information that needs to be accounted for in physical descriptions.

Strominger shares his experience working with Stephen Hawking, emphasizing Hawking's passion for physics and his unwavering commitment despite personal challenges and external distractions. He highlights Hawking's willingness to challenge his own previous conclusions and the importance of disagreement in scientific progress.

Strominger expresses his love for mathematics as a tool for physics and ponders the interconnectedness of the two. He questions whether mathematics is discovered or invented, aligning with the view that math is discovered and deeply linked to the physical world. He avoids philosophical questions that lack definitive, verifiable answers, preferring those that drive scientific motion and understanding.

While hopeful for experimental verification of string theory, Strominger acknowledges the current lack of a clear path for direct measurement. He believes exploration should continue, even without guaranteed experimental outcomes, as unexpected discoveries can lead to breakthroughs. He draws parallels to historical scientific challenges, emphasizing that the inability to see the end is not a reason to abandon fundamental problems.

Strominger discusses the concept of a "final theory" of everything, debating with Stephen Weinberg about whether such a theory is attainable. He believes that string theory, in its current form, is not a final theory as it doesn't address fundamental questions like the number of dimensions or the origin of the universe. He highlights the Big Bang as a significant challenge, as it defies the principle of determinism and struggles with the concept of something arising from nothing.

Strominger suggests that time, like space, might be emergent rather than fundamental. He explains this concept by referring to holographic plates, where an extra spatial dimension emerges from a lower-dimensional boundary. He then introduces the "photon ring" around warped black holes, a concept developed with Dan Corpet and Alex Lupsasca, which highlights unique light-bending phenomena and offers new insights into black hole properties.

Strominger likens a black hole to a "hall of mirrors" due to its ability to bend light, creating multiple images of an observer. This phenomenon allows scientists to study the black hole's geometry and spin, independent of the surrounding swirling matter. He and his team discovered new mathematics describing the photon ring, suggesting it might be a crucial part of the black hole's holographic plate, extending beyond the event horizon.

Strominger reflects on the challenge of identifying solvable and interesting problems in physics, describing it as finding a "knife edge" opportunity. He emphasizes the role of intuition and thought experiments in making progress, even for complex concepts like quantum mechanics and general relativity. He discusses his personal growth in balancing youthful ambition with a more refined approach to problem-solving.

Strominger shares his early motivations for pursuing physics, from a desire for Nirvana to understanding universal principles, eventually settling on the most challenging yet potentially solvable problems. He finds satisfaction in contributing to mathematical truths and remains excited by new discoveries, emphasizing that the current era is one of the most thrilling times to be a theoretical physicist, despite the uncertainties.

Strominger considers the possibility of alien civilizations, acknowledging Fermi's Paradox. He finds the idea of diverse intelligent beings exploring physics fascinating. He and Fridman discuss the limitations of human perception and cognitive abilities, suggesting that much beauty and many profound physical phenomena might be invisible to us, hinting at the potential for future breakthroughs in understanding intelligence and the universe.