Ivette Fuentes

University of Vienna
Stefano Mancini
Universiai di Camerino
David Edward Bruschi
University of York
Quantum Observers in a Relativistic World

The notion of observers in Quantum Theory (QT) is inseparable from the notion of measurements, since observations collapse the state of the system into a state that depends on the choice of measurements made by a conscious observer. This is contrary to what occurs in classical physics, where observers use clocks and rods to make measurements and observations do not affect systems. Space and time are very different notions for quantum observers. Time is an absolute parameter and thus all clocks tick at the same rate. However, positions can be quantized and systems can occupy different positions at the same time. On the other hand, observers in General Relativity (GR) see time and space as one and the same thing. Time is relative and can flow at different rates in different places. Einstein believed that space and time were not real, in the sense that they depended on the state of motion of the observer. Understanding how quantum and relativistic effects occur jointly is an open question. A consistent description of clocks and rods, is at the heart of our difficulty to unify the theories.

We aim at deepening our understanding of the notion of observer in physics by investigating the nature of observers at the overlap of quantum theory and relativity. We will employ recently developed techniques from quantum information, quantum metrology and their interplay with relativity, to develop a notion of clocks and spacetime probes which will provide ultimate bounds on possible measurements of physically accessible quantities. This will help us unveil the fundamental limitations imposed by the laws of Nature on observations. We will then exploit the channel capacity formalism to describe physical processes at the overlap of relativity and quantum physics as channels, which can transmit information between different regions of spacetime. Understanding the transmission rates and the bounds on the channel capacities will provide further indications on fundamental limitations imposed by Nature on human endeavors to measure and probe the Universe. Finally, we will discuss the philosophical implications of the notion of "observing the observer" within our relativistic and quantum framework. At last, we will attempt to understand if the limitations that Nature imposes on measurements are indeed fundamental or they cannot be distinguished from the human interaction with the world and are therefore inextricably linked to human perception.

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