#468 – Janna Levin: Black Holes, Wormholes, Aliens, Paradoxes & Extra Dimensions

Janna Levin, a theoretical physicist and cosmologist, specializes in black holes, extra dimensions, and gravitational waves. She clarifies that black holes are not merely the end state of dead stars but fundamental phenomena shaped by Einstein's equations. Levin recounts the story of Carl Schwarzschild, who discovered the first exact solution to these equations during World War I, leading to the conceptualization of black holes. A key point is the event horizon, which defines a black hole as a boundary beyond which nothing can escape, emphasizing that it is an empty region of space-time rather than a dense object.

The implications of curved space-time on matter and energy behavior are explored, highlighting the one-way nature of black holes. While relativity predicts their existence, it does not explain their formation. Roger Penrose's work on gravitational collapse suggests that singularities are inevitable, but he believed quantum mechanics would ultimately provide a clearer understanding. The discussion underscores the necessity of integrating quantum mechanics when examining black holes, particularly near singularities, and reiterates that black holes are regions of "nothing," challenging traditional perceptions.

Levin explains that massive stars burn thermonuclear fuel until they exhaust it, leading to a supernova explosion that disperses elements essential for life. If the core exceeds twice the sun's mass, it collapses into a black hole. The term "black hole" was coined by John Wheeler in 1967, although the catastrophic end state of gravitational collapse was discussed in complex terms prior. As a star collapses, it can shrink to about 60 kilometers in diameter, creating an event horizon from which not even light can escape.

The conversation also delves into the ethical implications of physicists developing nuclear weapons, reflecting on the moral dilemmas faced by scientists. The migration of scientists to the U.S. from Europe due to oppressive regimes has significantly contributed to American scientific achievements. The importance of intellectual freedom and vibrant scientific debate is underscored, especially amid geopolitical tensions.

The experience of falling into a black hole is contrasted with that of an outside observer. The observer sees a spherical shadow, while the astronaut perceives a shift in space and time, with the singularity becoming a point in their future. The time taken to reach the singularity is brief, estimated in microseconds for stellar mass black holes. As the astronaut approaches the event horizon, their perception of time dilates relative to the observer, illustrating the extreme effects of time dilation.

The formation of black holes is discussed, noting that while most are formed from stars, only about 1% end their lives as black holes. Supermassive black holes, which can be billions of times the mass of the sun, likely formed early in the universe's history. The relationship between black holes and galaxies is explored, suggesting that supermassive black holes are present in the centers of galaxies and may have influenced their formation.

The concept of space-time is visualized as a four-dimensional construct, combining three spatial dimensions with time. The discussion acknowledges the challenges humans face in visualizing four dimensions due to our three-dimensional existence. Einstein's General Theory of Relativity aimed to provide a comprehensive understanding of the universe, illustrating how matter and energy influence the curvature of space and time.

The conversation highlights the competitive nature of academia, where rivalry among scientists can drive innovation and creativity. It emphasizes that the best scientists maintain a childlike curiosity, asking fundamental questions about complex topics like the origin of life and consciousness, which remain largely mysterious.

Black holes are characterized by their charge, mass, and spin, making them featureless and similar to fundamental particles. Theoretical physicists utilize black holes for thought experiments, linking concepts from general relativity and quantum mechanics. The discussion delves into the information paradox associated with black holes, particularly regarding the vacuum around the event horizon.

The ongoing debate, referred to as the "Black Hole Wars," centers on the conflict between general relativity and quantum mechanics, particularly regarding the fate of information as black holes evaporate. Proposed resolutions include the idea of information loss under extreme gravitational conditions and the Fuzzball theory from string theory, suggesting that black holes are complex, horizonless objects rather than singularities.

The conversation explores the concept of "soft hair," which challenges the classical no-hair theorem by suggesting that black holes may have subtle quantum features at their event horizons that can store information about what has fallen into them. The discussion introduces the conjecture "er = er," linking quantum entanglement to space-time geometry, proposing that entangled particles are connected by non-traversable wormholes.

The exploration of extra dimensions is proposed as a potential pathway to understanding dark energy, with theories suggesting they may be tightly rolled up and necessary for string theory's coherence. The notion of living on a three-dimensional membrane within a higher-dimensional universe raises the possibility of other intelligent alien civilizations existing on different membranes, leading to speculation about a multiverse with varying physical laws.

The conversation touches on humanity's precarious position, highlighting the risks of self-destruction and comparing humans to long-lived species like whales. The potential for self-destruction is seen as a feature of evolution that drives technological advancement and exploration. The discussion also addresses the absence of advanced alien life forms, suggesting that life can thrive without manipulating its environment.

The dialogue concludes with reflections on the initial spark of life on Earth and the lengthy journey to multicellularity, emphasizing the importance of energetics in understanding what constitutes life. The versatility of carbon is noted as a key factor in life's complexity, with the emergence of life requiring a level of complexity that supports diverse biological functions.