Emergence Of A Classical World From Within Quantum Theory

The starting point of this dissertation is that a quantum state represents the observer's knowledge about the system of interest. As it has been pointed out several times by the opponents of this epistemic interpretation, it is difficult to reconcile this point of view with our common notion of “physical reality”, which exists independently of our monitoring, and can be discovered without disturbance. Indeed, if quantum theory is correct, it should apply to classical systems—including measurement devices—as well as to any other system. In this dissertation, we will study the quantum mechanisms responsible for our perception of the world and demonstrate how they lead to the emergence of an operational objective reality from within quantum theory: several observers gathering information through these mechanisms will arrive at a common consensus about the properties of the world. The two mechanisms we study in great detail are the redundant proliferation of information in the environment and the direct measurement of a macroscopic observable. An example of the first mechanism is the photon environment which provides us with our visual data about the world. Several independent observers learning about their surroundings in this indirect fashion will agree on their findings. An example of the second mechanism is our tactile information: when the tip of our finger touches an object, it interacts collectively with a very large number of molecules. Again, under realistic assumptions, this type of information acquisition will lead to a classical perception of the world