On Time

This text revolves around a new and unusual view on the most fundamental puzzle of physics. It focusses on the key aspect that makes the role of the time dimension fundamentally different: causality. It deals on the one hand with general relativity, and on the other hand with quantum theory. The implicit and intuitive way by which causality is usually taken for granted is just made explicit and less self-evident, shedding a new light on the gravity-quantum conflict. The case is made that gravity is a necessary condition for a causal universe. But upon turning to the "pure" unitary quantum physics explaining the nature of matter one is dealing with the strictly a-causal time expressed through the thermal quantum field theory machinery. When this a-causal microscopic and causal macroscopic world meet, one encounters the wavefunction collapse, that itself may be rooted in the quantum-gravity conflict. Modern ideas are discussed resting on eigenstate thermalization showing how this may lie eventually at the origin of irreversible thermodynamics, with its famous second law setting also a direction of time. The case is anchored in the sophisticated modern mathematical machinery of both general relativity and quantum physics which is normally barely disseminated beyond the theoretical physics floors. The book is unique in the regard that the consequences of this machinery - Riemannian geometry and Penrose diagrams, thermal quantum fields, quantum non-equilibrium and so forth -- are explained in an original, descriptive language conveying the conceptual consequences while avoiding mathematical technicalities.