Causality Rules (Second Edition)

Causality: Cause and effect. In classical physics, an effect cannot occur before its cause. In Einstein's theory of special relativity, causality means that an effect cannot occur from a cause that is not in the back (past) light cone of that event. The books cover the useful physical relations inferred by unitarity and causality. A famous example is the Kramers–Kronig relation for the refractive index of a gas or dilute medium. For example, chapter 3 generalises the Kramers–Kronig relation to relativistic medium, such as CMB (photon gas). These relations are extensively also used in particle and nuclear physics. Especially useful are the so-called 'sum rules', such as the Gerasimov–Drell–Hearn (GDH) or the Baldin sum rule. The author notes that the first edition is too brief. In his teaching practice, he sees that it is challenging to use as a standalone text. He intends to improve the explanations of many topics that students found particularly challenging. The additional material will make the book more timely, self-contained, and logically complete.