Étienne Klein Que savons-nous du temps ?

The speaker begins with an introduction about his recent trip to Spitzbergen and the experience of continuous daylight affecting sleep patterns. He delves into the complexities of studying time and highlights two traps to avoid: the philosophical trap and the physicist's trap.

The speaker discusses obstacles in understanding time, including the lack of a definitive definition of time, language traps, and the polysemy of the concept of time. He emphasizes the challenges posed by language in capturing the essence of time.

The speaker explains the misconception of time passing and the confusion between the function of time and its essence. He addresses the issue of time passing and the misunderstanding surrounding the concept of time's passage.

The speaker explores how language shapes our perception of time through metaphors, emphasizing the limitations of language in expressing the complexities of time. He also examines the concept of time in different scientific fields and its impact on our understanding of time.

The relationship between causality and time is discussed, focusing on how causality influences our perception of time. The speaker highlights how causal relationships and temporal phenomena are intertwined, leading to a deeper exploration of the concept of time.

The emergence of time in phenomena is examined, highlighting the interplay between time and causal relationships. The discussion touches on the primacy of causality in shaping our perception of time and how time emerges from the world of phenomena.

Discusses the concept of causality and cyclical time, explaining how the effect can retroactively influence its cause, making the principle of causality inapplicable. Also mentions the importance of distinguishing between past and future in a cyclical time frame.

Explores the concept of eternal return as discussed by Nietzsche, highlighting the absurdity of the idea based on memory and repetition. Addresses the impossibility of experiencing the same event twice consciously.

Contrasts the linear time concept by Newton with cyclical time, emphasizing the chronology of cause and effect. Discusses how the linear time concept prohibits the effect from influencing its cause retroactively.

Touches on the discussions in the 19th century regarding the reversibility of phenomena and debates around explaining the arrow of time through mechanical terms.

Addresses misconceptions in representing time, highlighting the importance of not confusing irreversibility of time with irreversibility of phenomena. Discusses the implications of inverting the direction of time.

Explores the causal nature of time in quantum physics and its prohibition of time travel. Discusses the interplay between causal constraints and the potential for violations of causality.

Discusses Dirac's equation and its role in predicting the existence of antiparticles, specifically the positron. Highlights the significance of imposing causality as a constraint in the equation.

Anderson's discovery of the positron, the antiparticle predicted by Dirac's equation, generalized the concept of antimatter. The existence of antimatter is a concrete proof of the impossibility of time travel.

Potassium-40, a residue from stellar explosions 5 billion years ago, decayed to calcium-40 by emitting electrons and anti-neutrinos. This radioactive decay process demonstrates the constant emission of antineutrinos in the environment.

Neutrinos and antineutrinos were thought to be massless for a long time, but since 2002 it has been known they have weak non-zero masses. The continuous emission of antineutrinos during radioactive decay processes, like potassium transforming into calcium, reinforces the impossibility of time travel.

The discussion delves into the arrow of time concept and how it relates to the emission of particles like antineutrinos. The emission of particles constantly into space serves as evidence that time travel is unattainable, highlighting the impenetrable nature of the physical time dimension.

Theoretical perspectives on time, such as the block universe concept vs. presentism, are discussed. The implications of these views on the nature of time, space, and time-manipulation underscore the complexities of understanding temporal dimensions.