Why is Time a One-Way Street?

Jeffrey West introduces Leonard Susskind, a distinguished theoretical physicist and faculty member at the Santa Fe Institute. Susskind is known for his foundational contributions to physics, particularly string theory, which posits that fundamental objects in nature are not point particles but vibrating strings. The introduction also mentions Susskind's public dispute with Stephen Hawking over the role of information in black holes, which Susskind eventually won, and his popular science writing, including a book on the subject of black holes and information.

The speaker reflects on his career, mentioning his first job as a plumber, named Leonardo by his father who admired Leonardo da Vinci. He discusses his education at City College and Cornell, his time at Yeshiva University, and his move to Stanford. The speaker laments the loss of traditional lecture styles, where physicists would use chalk and gestures to explain complex ideas, criticizing the reliance on technology in modern presentations. He also humorously notes his age and accomplishments, drawing a parallel to Kurt Vonnegut's freedom in expression due to his age and status.

The speaker delves into the concept of time, questioning why the past is different from the future. He discusses the second law of thermodynamics, which states that entropy always increases, and contrasts it with the symmetric nature of other physical laws, such as those of Newtonian mechanics and Einstein's theory of relativity. The speaker explores the idea that the asymmetry of time is not inherent in the fundamental laws of physics but emerges from statistical probabilities, as suggested by Ludwig Boltzmann.

The speaker explains that Newtonian mechanics and quantum mechanics are symmetric with respect to time, meaning they do not distinguish between the past and the future. He uses the solar system as an example to illustrate that the direction of planetary motion (clockwise or counterclockwise) cannot be determined from a static image, implying that the fundamental laws of physics allow for time symmetry. The speaker also introduces Boltzmann's idea that entropy almost always increases, highlighting the probabilistic nature of the second law of thermodynamics.

The speaker discusses Boltzmann's thought experiment of a universe in a box, starting with all molecules in one corner, leading to the formation of structures and galaxies as they disperse. He explains Boltzmann's idea of 'recurrences,' where given enough time, the molecules could accidentally reassemble in the corner, suggesting that the universe could go through cycles of structure and uniformity. The speaker also introduces the concept of a 'Boltzmann Brain,' a hypothetical brain formed by random fluctuations in a universe, which challenges the idea of a one-shot universe.

The speaker introduces the multiverse theory, where spacetime is rapidly expanding and can give rise to fluctuations that form 'bubbles' with different physical properties. These bubbles can grow and spawn new universes, leading to a potentially infinite number of universes, each with its own laws and constants. The speaker presents a mathematical model, a Markov process, to describe the probability distribution of these universes and their evolution over time.

The speaker contemplates the future of cosmology, particularly the implications of the multiverse theory for understanding the arrow of time. He suggests that the multiverse could provide a framework for time to have a one-way direction, akin to a flow from a source to a sink, creating vortices that have a clear temporal progression. The speaker acknowledges that this theory may not be the final answer and invites further exploration and discussion.

The speaker concludes the formal part of his talk and opens the floor for questions from the audience. He discusses the possibility of bubbles in the multiverse theory colliding and potentially leaving observable imprints on the cosmic microwave background. The speaker also addresses the issue of the speed of light and its constancy, as well as the potential for different physical constants in other universes within the multiverse.

The speaker suggests resources for those interested in further exploration of cosmology, including online courses and forthcoming books that will accompany the courses. He emphasizes the importance of continued learning and engagement with the complex questions posed by modern cosmology.

The speaker discusses the concept of dimensions, particularly in the context of string theory and the possibility of additional dimensions at very small scales. He also addresses the constancy of the speed of light and other dimensionless constants, such as the fine-structure constant, and the lack of evidence for their change over time within our observable universe.