The Paradoxes of Time Travel

The speaker begins by expressing gratitude and excitement for being at Linda Hall Library, highlighting its unique resources. They then delve into the topic of the day: extraterrestrial life and the intriguing concept of time travel, particularly traveling backward in time. The speaker confesses their dislike for apologetic openings but acknowledges the necessity due to the complex and theoretical nature of the topic. They set the stage for a discussion on the physics of time travel, its possibilities, and the unknowns associated with it, emphasizing the paradoxical nature of time travel and its portrayal in popular culture.

The speaker delves deeper into the concept of time, highlighting its fundamental mystery and the human fascination with it. They discuss the paradoxes that arise when considering time travel, such as the famous 'grandfather paradox', and the limitations it imposes on our understanding of free will. The speaker asserts that even if time travel were possible, it wouldn't allow changes to past events as we know them to have occurred. They also touch upon the asymmetry between the past and future, and how time travel challenges our conventional perceptions of these distinctions.

The speaker introduces quantum mechanics into the discussion, explaining how it complicates our understanding of determinism and opens up the possibility of multiple universes. They describe the concept of quantum superposition and the many-worlds interpretation, which suggests that every quantum event creates a split, leading to multiple timelines. This theoretical framework allows for the possibility of time travel without paradoxes, as any changes made would occur in a different universe's version of the past, leaving the original timeline unaffected.

The speaker discusses Isaac Newton's groundbreaking work in mechanics and its implications for understanding space and time. They explain the Newtonian view of the universe as a series of discrete moments, like frames in a film strip, and the concept of absolute space and time. The speaker also introduces the idea of space-time as a four-dimensional continuum, where events are located by their spatial position and temporal occurrence. They emphasize the importance of clocks in measuring and understanding the passage of time in this framework.

The speaker transitions to Albert Einstein's special theory of relativity, which challenges the Newtonian concept of absolute time. Einstein proposed that time is not the same for everyone, varying with one's velocity through space. The speaker uses the example of clocks on a spaceship and on Earth to illustrate how time dilation occurs, with the spaceship's clocks running slower due to their high velocity. They also introduce the concept of light cones, which represent the paths light takes in all directions, and explain how these light cones shape our understanding of space-time and the limits of causality.

The speaker continues with Einstein's general theory of relativity, which posits that space-time can be curved by mass and energy. They explain how gravity is the result of this curvature, using the analogy of a bowling ball deforming a trampoline to illustrate the concept. The speaker also discusses how gravity affects the passage of time, with time moving more slowly in stronger gravitational fields. They mention the practical application of this theory in the Global Positioning System (GPS), which relies on accurate timekeeping to function correctly.

The speaker explores the hypothetical possibility of time travel within the framework of general relativity, focusing on the concept of closed time-like curves. They introduce the idea of cosmic strings, hypothetical remnants from the Big Bang, which could, in theory, create the necessary conditions for time travel if they moved at certain velocities. The speaker discusses Richard Gott's proposal involving two parallel cosmic strings and the potential for time travel, while also acknowledging the practical and theoretical challenges associated with this idea.

The speaker discusses another theoretical method for time travel: wormholes. They explain that wormholes, which are shortcuts through space-time, could potentially be used to create a time machine if they could be kept stable and if one could travel through them without causing a collapse into a black hole. The speaker references Kip Thorne's work on wormholes and the challenges of turning them into functional time machines, including the issue of quantum fluctuations and the resulting instability.

The speaker concludes the discussion on time travel by summarizing the current state of theoretical understanding. They emphasize that while general relativity allows for the conceptualization of time travel, all attempts to model such scenarios result in the formation of black holes, which are incompatible with the idea of time travel. The speaker expresses a personal conviction that the laws of nature likely prohibit the construction of a time machine, but acknowledges that our understanding of the universe is still evolving and that future discoveries could potentially change this conclusion.