Existential physics: answering life's biggest questions - with Sabine Hossenfelder

The speaker begins by expressing gratitude to the audience for attending despite the tube strike, humorously labeling them as the most dedicated audience. The speaker then introduces the concept of 'existential physics,' which pertains to aspects of physics that concern human existence, such as the nature of time, the beginning of the universe, and the structure of matter. The speaker acknowledges a shift from their usual focus on the limitations of physics to exploring the possibilities it presents, including its intersections with philosophy and religion.

The speaker delves into the implications of relativity on the concept of time. They explain that the finite speed of light means that what we perceive is always in the past, as it takes time for light to travel from an object to our eyes. The speaker uses the example of light flashes on a moving train to illustrate how the notion of 'now' is relative and varies for different observers, leading to the concept of the 'block universe,' where all moments exist equally. This challenges our intuitive perception of time, where the present is considered a unique and special moment.

The speaker discusses the existence of the past based on the principles of physics, particularly Einstein's theories. They argue that the past exists in the same way the present does, as nothing in physics suggests that the past ceases to exist. The speaker also explores the idea of information and its indestructibility, explaining that fundamental laws of physics work through differential equations that allow for the calculation of a system's state at any time, both forward and backward. This suggests that information is never truly destroyed, though it may become unusable due to scrambling.

The speaker addresses the complexities introduced by quantum mechanics, particularly the measurement problem and black hole evaporation. In quantum mechanics, the process of measurement appears to be irreversible, as it changes the wave function in a way that the initial state cannot be determined from the outcome. This challenges the idea that information survives in a fundamental sense. Furthermore, the speaker discusses Stephen Hawking's discovery that black holes emit radiation and eventually evaporate, leading to the black hole information loss paradox, which contradicts the time reversibility of quantum mechanics. The speaker suggests that a missing theory of quantum gravity might resolve this paradox.

The speaker explores the question of the universe's beginning, touching on the overlap between science, religion, and philosophy. They explain that modern cosmology, based on Einstein's general relativity, suggests the universe began with a big bang. However, the speaker points out that this model leads to a singularity where energy density becomes infinitely large, which is likely not physically real. The speaker acknowledges that while there are many theories about the universe's origin, none can be scientifically proven or disproven due to the lack of observational evidence. They conclude that the universe's beginning remains unknown and may forever be a mystery.

The speaker discusses the concept of the multiverse, where our universe is one of many, potentially infinite other universes. They mention various theories of the multiverse, such as eternal inflation, quantum fluctuations leading to multiple big bangs, and the many worlds interpretation of quantum mechanics. The speaker emphasizes that these other universes are disconnected from our own, making it impossible to confirm or refute their existence through observation. They argue that while these theories are mathematically consistent with our observations, they do not provide evidence for the existence of other universes, making them 'ascientific' or modern creation myths.

The speaker addresses the philosophical concept of panpsychism, which suggests that all matter, including fundamental particles, possesses some form of consciousness. They argue that if this proto-consciousness were physically real, it would need to align with the standard model of particle physics, which defines particles by quantum numbers with no additional degrees of freedom. The speaker concludes that particles cannot think because adding such properties would contradict observed outcomes in particle collisions. They suggest that proto-consciousness, if real, would have no physical properties and thus not explain the emergence of consciousness in complex systems like the brain.

In the concluding paragraph, the speaker reflects on the broader scope of physics beyond what is typically taught in schools. They express a desire to convey that physics is not just about basic principles like magnets or batteries but is a powerful tool for answering profound existential questions. The speaker summarizes their book's aim to explore these questions and encourages the audience to see physics as a discipline that intersects with deep philosophical and existential inquiries.