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Quantum Gravity, Black Hole Entropy and the Emergence of Spacetime.

Final Activity Report Summary - QUANTUMSPACE (Quantum Gravity, Black Hole Entropy and the Emergence of Spacetime)

The project was concerned with the two fundamental questions in theoretical physics, namely quantum gravity and quantum mechanics.

In quantum gravity the problem is to reconcile quantum mechanics and general relativity. The project proposed a completely new way to achieve this elusive goal. Instead of quantising the gravitational field the project proposed to find the gravitational field in the emergent physics of a quantum mechanical system that did not have a priori any gravitational degrees of freedom. In the course of the project we showed that this could indeed be achieved if one focused on an internal perspective. We showed that not only did special relativity arise in this way but also that Newtonian gravity emerged in the appropriate low velocity limit. We also gave a new formula for the gravitational mass of a large bound object. This view of gravity opened new avenues of research. One possible application was cosmology. We currently think that the universe underwent a period of exponential growth in its early history. The point of view explored here might enable us to replace this period of expansion with a phase transition instead.

In quantum mechanics, the question is how quantum mechanics and classical mechanics are related. How do we go from the quantum mechanical superposition principle to classical certainty? To solve this problem we proposed a new view of classical objects as large quantum mechanical systems. This removed the artificial split between the quantum world and the classical world that was introduced by the so called Copenhagen interpretation. It also allowed for the exploitation of special properties that only large systems have. One of these properties was the so called generalised rigidity. This was the presence of a property that requires a force to change it. We argued that it was these properties that we should identify as classical properties. A consequence of this identification was the natural presence of probability. The probability in quantum mechanics was thus not fundamental in our approach. It had the same origin that probability had in all other branches of physics, namely our lack of knowledge of the system.