Something Deeply Hidden | Sean Carroll | Talks at Google

Sean Carroll begins his talk by expressing his enthusiasm for discussing quantum mechanics, a topic he presumes is familiar to some in the audience, but also acknowledges the presence of complete non-experts. He questions the need for another book on quantum mechanics, given the existing plethora of literature, but asserts that his book is necessary because he disagrees with the common portrayal of quantum mechanics as inherently confusing and mysterious. Carroll criticizes the tendency of emphasizing the difficulty of understanding quantum mechanics, and instead, he advocates for the belief that it is indeed comprehensible. He contrasts his view with the famous physicist Richard Feynman's, who once stated that nobody truly understands quantum mechanics. Carroll then delves into the history and development of quantum mechanics, highlighting its critical role in understanding the fundamental workings of the universe, from the sun's shining to the functionality of transistors.

Carroll introduces the concept of the wave function, which he considers the most important yet mundanely named concept in physics. He explains that electrons are not just particles orbiting the nucleus of an atom but are also described by waves spread out around the atom. This wave-like behavior of electrons leads to the idea of discrete energy levels or orbitals, which contribute to the stability of matter. Carroll then discusses Erwin Schrodinger's contribution to quantum mechanics through the Schrodinger equation, which provides a mathematical framework for understanding how these electron waves behave. He emphasizes that despite the equation's complexity, it is fundamental to predicting quantum behaviors and is still considered correct. However, he points out that even with the Schrodinger equation, there are unresolved questions about the nature of quantum mechanics, particularly when it comes to the behavior of particles like electrons in different experimental setups.

Carroll critiques the Copenhagen interpretation of quantum mechanics, which posits that the act of measurement causes the wave function to collapse, thereby localizing the observed particle. He argues that this interpretation is unsatisfactory as a fundamental theory of nature because it relies on the observer's role and the ambiguous process of measurement. Carroll highlights two main issues with the Copenhagen interpretation: the reality problem, questioning whether the wave function truly represents reality, and the measurement problem, questioning what constitutes a measurement. He asserts that a well-defined theory of physics should provide clear answers to these questions, which the Copenhagen interpretation fails to do. Carroll then introduces alternative interpretations, suggesting that there are other, more comprehensive frameworks for understanding quantum mechanics.

Carroll presents Hugh Everett's interpretation of quantum mechanics, which posits that the wave function is the sole reality and that it evolves according to the Schrodinger equation without any collapse. This leads to the idea of the 'many-worlds' interpretation, where every possible outcome of a quantum event exists in a separate, non-interacting branch of the universe. Carroll explains that decoherence, a process understood in the 1970s, provides a mechanism for why we don't experience superpositions of reality: each branch of the wave function becomes effectively separate, leading to the perception of a single, unique reality. He argues that this interpretation naturally incorporates the phenomenon of entanglement, where the state of one particle is dependent on the state of another, and maintains that the Everett interpretation is the most falsifiable and straightforward approach to quantum mechanics.

Carroll addresses common objections to the many-worlds interpretation, such as the perceived ontological extravagance of positing an infinite number of universes. He counters that the potential for these worlds was always present in quantum mechanics and that the many-worlds view does not actually increase the dimensionality of Hilbert space, the space of all possible wave functions. He also tackles the question of testability, asserting that the many-worlds interpretation is as falsifiable as any other interpretation of quantum mechanics. Carroll then discusses the challenging questions that remain, such as the origin of probability in quantum mechanics and the connection between the quantum world and our classical perception of reality. He emphasizes the difficulty of deriving the classical world from quantum mechanics, especially within the many-worlds framework, and suggests that understanding this connection may be key to resolving issues in quantum gravity.

Carroll explores the idea of using quantum entanglement as a foundation for understanding the geometry of space itself. He explains that in quantum field theory, fields fill all of space, and particles are essentially vibrations in these fields. He suggests that the entanglement between different regions of space could be related to the geometry of space, with highly entangled regions being close together. Carroll proposes that by considering the entanglement structure of the universe's wave function, one might derive the geometry of space and its curvature, which would align with Einstein's general theory of relativity. While acknowledging that this is an ongoing research program and not yet proven, he expresses optimism that taking quantum mechanics seriously and considering space as an emergent property could be the key to understanding gravity and the nature of the universe.

In the concluding part of his talk, Carroll addresses the audience's curiosity about the practical engagement with quantum mechanics and theoretical physics. He asserts that the math required to understand foundational physics questions is not as daunting as one might think, and that it is accessible to those willing to learn. Carroll recommends books by David Albert and David Wallace for those interested in delving deeper into the subject. He also mentions his podcast, 'Mindscape', as a platform for discussing a variety of topics, including quantum mechanics, in an accessible manner. Carroll encourages the audience to pursue their interest in theoretical physics, emphasizing that progress is being made in understanding the nature of reality.