'Holographic' relations between theories have become a main theme in quantum gravity research. These relations entail that a theory without gravity is equivalent to a gravitational theory with an extra spatial dimension. The idea of holography was first proposed in 1993 by Gerard 't Hooft on the basis of his studies of evaporating black holes. Soon afterwards the holographic 'AdS/CFT' duality was introduced, which since has been heavily studied in the string theory community and beyond. Recently, Erik Verlinde has proposed (...) that even Newton's law of gravitation can be related holographically to the thermodynamics of information on screens. We discuss inter-theoretical relations in these scenarios: what is the status of the holographic relation in them and in what sense is gravity, or spacetime, emergent? (shrink)

Why did Einstein tirelessly study unified field theory for more than 30 years? In this book, the author argues that Einstein believed he could find a unified theory of all of nature's forces by repeating the methods he used when he formulated general relativity. The book discusses Einstein's route to the general theory of relativity, focusing on the philosophical lessons that he learnt. It then addresses his quest for a unified theory for electromagnetism and gravity, discussing in detail his efforts (...) with Kaluza-Klein and, surprisingly, the theory of spinors. From these perspectives, Einstein's critical stance towards the quantum theory comes to stand in a new light. This book will be of interest to physicists, historians and philosophers of science. (shrink)

During World War II, Niels Bohr realized that the nature of war had changed irrevocably due to the introduction of the atomic bomb. This, in his opinion, meant that nation states had to be open about nuclear knowledge and negotiate toward peace. The bomb presented a threat, yet at the same time, an opportunity, as Bohr would argue in his characteristic way. It is not too difficult to point to the epistemological origin of Bohr’s argument: One easily identifies resonances with (...) his ideas on “complementarity” from quantum mechanics. According to Bohr’s doctrine of complementarity, a quantum mechanical object shows certain qualities depending on the experimental perspective from which it is studied; and these qualities may be mutually exclusive. However, they should in fact be looked upon as “complementary” properties that together make up the full picture of the object under investigation.Initially, Bohr could express his ideas to the highest circles of power. This would soon change, howe .. (shrink)