Reflections on the information paradigm in quantum and gravitational physics

Abstract

Abstract. We reflect on the information paradigm in quantum and gravitational physics and on how it may assist us in approaching quantum gravity. We begin by arguing, using a reconstruction of its formalism, that quantum theory can be regarded as a universal framework governing an observer's acquisition of information from physical systems taken as information carriers. We continue by observing that the structure of spacetime is encoded in the communication relations among observers and more generally the information flow in spacetime. Combining these insights with an information-theoretic Machian view, we argue that the quantum architecture of spacetime can operationally be viewed as a locally finite network of degrees of freedom exchanging information. An advantage -and simultaneous limitation -of an informational perspective is its quasi-universality, i.e. quasi-independence of the precise physical incarnation of the underlying degrees of freedom. This suggests to exploit these informational insights to develop a largely microphysics independent top-down approach to quantum gravity to complement extant bottom-up approaches by closing the scale gap between the unknown Planck scale physics and the familiar physics of quantum (field) theory and general relativity systematically from two sides. While some ideas have been pronounced before in similar guise and others are speculative, the way they are strung together and justified is new and supports approaches attempting to derive emergent spacetime structures from correlations of quantum degrees of freedom. IntroductionThe basic discipline underlying the current information age, namely information theory, is not a physical theory (in contrast to thermodynamics ruling the age of steam engines). Nevertheless, the information paradigm is now also permeating (at least part of) physics, offering novel perspectives on old and new problems, specifically in quantum and gravitational physics, and thereby attaining direct physical relevance. After all, physics without information is not possible; a description of the world requires to gather information about it. The information paradigm in physics is by construction very operational. From an information-theoretic perspective, physical systems are information carriers which can be used to acquire, store and communicate information. This perspective is practically realised in the field of quantum information which takes quantum systems to perform all sorts of information-theoretic tasks with them.Here we are less interested in practical applications within the limits of quantum information; rather, we wish to ask how far the information paradigm, and specifically the picture of physical systems as information carriers, can possibly lead us in understanding the physical content of quantum theory and general relativity from a new angle and what it may suggest for the construction of a new physical theory incorporating both. So the questions we will ask (and partially address) are much more basic than typically investigated in...

A change of perspective: switching quantum reference frames via a perspective-neutral framework

“…Methodologically, we have combined tools and concepts from constrained systems, also inherently used in the relational clock changes of [22][23][24], with the operational approach to quantum reference frames recently put forward in [1]. In particular, as proposed in [42], we took recourse to a gravity inspired symmetry principle to formulate a perspective-neutral super structure that, so to speak, contains all perspectives at once and via which one can switch among the individual perspectives of the different classical or quantum reference systems. This extends the method of [22][23][24], equips it In the final quantum reference frame A, the marginals of the total Wigner function representing the reduced state of system B (on the left) and C (on the right) when A was initially in the first excited state and B in the ground state.…”

“…However, there will be no global operational states and only states relative to a frame (which will not include its own degrees of freedom) will admit an operational interpretation. This will be exploited in [4] to develop a novel take on the 'wave function of the universe' in quantum cosmology, as proposed in [42,54], and suggests a new conception of the relative states of relational quantum mechanics [55,56] and their interrelations.…”

Toolbox for reconstructing quantum theory from rules on information acquisition

“…While this work is more generally motivated by the effort to understand physics from an informational and operational perspective and highlights a partial interpretation of quantum theory, it clearly does not single out 'the right one' (see also the discussion in [94]). For example, by speaking exclusively about the information accessible to an observer, we are by default silent on the fate of hidden variables and on whether they could give rise to the assumptions and rules which we impose.…”

Switching Internal Times and a New Perspective on the ‘Wave Function of the Universe’

“…Accordingly, relative to any subsystem, one can assign a 'catalog of knowledge' about the rest of the universe but, without external observer or reference frame, there can then be no global, operationally meaningful 'catalog of knowledge' about the entire universe at once (see also related discussions in [83][84][85][86]). In the proposal of this article, the global 'wave function of the universe' indeed does not admit an immediate operational interpretation as an informational state, yet it links all the different relational reference system perspectives on the universe consistently [50], something that was missing, e.g., in the discussion of [51,73,74,79,80,[83][84][85].…”