The Physics and Philosophy of Time - with Carlo Rovelli

The speaker begins by expressing their shock and fascination with the concept of time, particularly the idea of 'now' in modern physics. They introduce the common metaphor of time as a linear progression, with a distinct past, present, and unknown future. However, the speaker aims to challenge this traditional view, suggesting that our understanding of time is multifaceted and not as straightforward as it seems. They share their personal journey of exploring the mysteries of time, from their early years to their work as a physicist, emphasizing the need to rethink time in the context of quantum gravity.

The speaker addresses the misconception that time is a one-dimensional line with a clear direction from past to future. They argue that this view is not accurate, as the past is fixed and known, while the future is uncertain and untraceable. The speaker points out that our daily experiences and the functioning of clocks reinforce the idea of a uniform, linear time. However, they challenge this notion by discussing the relativity of time, explaining that time can vary depending on one's position and velocity, as demonstrated by the difference in time experienced by clocks at different altitudes.

The speaker delves into the concept of simultaneity and the challenges it poses to the idea of a universal 'now.' They explain that the time it takes for light to travel means that what we perceive as 'now' is actually a moment in the past. This is true on larger scales as well, such as when observing celestial bodies, where the light from these objects takes time to reach us, and thus we see them as they were in the past. The speaker suggests that the notion of 'now' is localized and dependent on our frame of reference, challenging the idea of a universal, objective 'now' in the universe.

The speaker argues that the concept of a single, universal time is an illusion. They explain that time is not a single entity but a combination of properties, such as its one-dimensionality and its orientation. The speaker uses the analogy of a mechanical engine to illustrate that removing parts one by one leads to the loss of the whole, suggesting that time, as we understand it, ceases to exist when its constituent properties are separated. They emphasize that the idea of a shared, linear timeline is inadequate and that our intuitive understanding of time is more complex and layered.

The speaker discusses the arrow of time, which is the distinction between past and future, and attributes it to the second law of thermodynamics, which states that entropy always increases over time. They explain that this law is statistical in nature and depends on the level of detail at which we observe a system. The speaker also explores the idea that the past appears ordered because of our interactions with the universe and how this order is tied to the entropy of the system. They suggest that the low entropy of the early universe may have set the stage for the temporal structure we observe today.

The speaker shifts focus to the neuroscience of time, suggesting that our perception of time is deeply rooted in the way our brains function. They argue that the brain exploits the entropy gradient to build memories of the past and anticipate the future, effectively acting as a time machine. The speaker emphasizes that our emotional experiences and motivations are intertwined with our sense of time, making it a deeply personal and emotional aspect of our existence. They conclude that understanding time requires considering not only the physical structure of the universe but also the specific workings of our brains and the emotional connections we have with the passage of time.