OBSERVERS IN QUANTUM GRAVITY

Transcription

1 OBSERVERS IN QUANTUM GRAVITY PROGRAM MONDAY Jan 22nd Flavio Mercati (chairman): welcome Jeremy Butterfield (**) COFFEE BREAK Flaminia Giacomini (*) LUNCH BREAK Michele Arzano (*) Angel Ballesteros (*) COFFEE BREAK Henrique Gomes (*) Časlav Brukner (*) TUESDAY Jan 23 rd Julian Barbour (**) Henrique Gomes (**) COFFEE BREAK Časlav Brukner (**) LUNCH BREAK Maximilian Lock (*) Markus Mueller (***) COFFEE BREAK Jose Manuel Carmona (***) Giovanni Amelino-Camelia (***) SESSION 1 (*) Quantum Reference frames, Quantum Causal Relations 2 (**) Many-world interpretation(s), Quantum Cosmology, Measurement Problem 3 (***) Emergence of Spacetime

2 Flavio Mercati (Sapienza University of Rome) Welcome and Introduction Jeremy Butterfield (Oxford University) Title - The Observer in Cosmology: joint work with Feraz Azhar Abstract: We review conceptual issues about the role of the observer, i.e. the various ways in which the process of observation contributes to the scope, and indeed content, of physical knowledge: especially in cosmology, both classical and quantum. We proceed in three stages. First, we discuss the role of the observer, independently of the details of physics, whether classical or quantum. Second, we discuss the role of the observer in classical physics, especially classical cosmology: i.e. in a cosmology for a classical (not quantum) world. Thus this second stage considers issues about selection effects. Third, we discuss the role of the observer in quantum physics in general: and especially in quantum cosmology. Here we focus attention on the research programme of Hartle, Hawking and Hertog, especially the relation of their proposed no-boundary state to inflation, and to selection effects. Flaminia Giacomini (Vienna University) Title - Quantum mechanics and the covariance of physical laws in quantum reference frames Abstract: In physics, every observation is made with respect to a frame of reference. Although reference frames are usually not considered as degrees of freedom, in all practical situations it is a physical system which constitutes a reference frame. Can a quantum system be considered as a reference frame and, if so, which description would it give of the world? The relational approach to physics suggests that all the features of a system - such as entanglement and superposition - are observer-dependent: what appears classical from our usual laboratory description might appear to be in a superposition, or entangled, from the point of view of such a quantum reference frame. In this work, we develop an operational framework for quantum theory to be applied within quantum reference frames. We find that, when reference frames are treated as quantum degrees of freedom, a more general transformation between reference frames has to be introduced. With this transformation we describe states, measurement, and dynamical evolution in different quantum reference frames, without appealing to an external, absolute reference frame. The transformation also leads to a generalisation of the notion of covariance of dynamical physical laws, which we explore in the case of `superposition of Galilean translations' and `superposition of Galilean boosts'. In addition, we consider the situation when the reference frame moves in a `superposition of accelerations', which leads us to extend the validity of the weak equivalence principle to quantum reference frames. Finally, this approach to quantum reference frames also has natural applications in defining the notion of the rest frame of a quantum system when it is in a superposition of momenta with respect to the laboratory frame of reference. Michele Arzano (University of Rome La Sapienza)

3 Title - Accelerated observers and horizons at the Planck scale Abstract: We extend the concept of accelerated horizons to the framework of deformed relativistic kinematics at the Planck scale. We show that the non-trivial effects due to symmetry deformation lead to a blurring of the horizon which manifests in a finite redshift for signals sent by an observer which approaches the horizon. Angel Ballesteros (Burgos University) Title - From quantum groups to noncommutative spacetimes and worldlines Abstract: The construction of noncommutative Lorentzian spacetimes and their associated noncommutative spaces of worldlines is reviewed. In the commutative case both spaces are constructed as homogeneous spaces of the (A)dS and Poincaré groups, and their noncommutative analogues arise as quantizations of the corresponding Poisson homogeneous spaces that are invariant under the appropriate (co)action from (A)dS or Poincaré quantum groups. In this construction, a new duality between reductive homogeneous spaces and coisotropic Lie bialgebras can be envisaged. In particular, the noncommutative spaces arising from the well-known kappa- (A)dS and Poincaré quantum groups are explicitly given, and the role of the cosmological constant is outlined. Moreover, a proposal for the description of quantum observers as coordinates on the non-commutative space of time-like worldlines is sketched. Monday 22 talk Henrique Gomes (Perimeter Institute for Theoretical Physics) Title - Observers, gauge symmetries, and best-matching. Abstract: One problem of gauge theories in general, but general relativity in particular, is determining a local basis of observables. E.g. a Wilson loop is not gauge-invariant within a region whose boundary intersects it. Recently, interest in defining boundary degrees of freedom which restore gauge-invariance for such types of observables has resurged. Donnelly and Freidel have related such degrees of freedom (edge-modes) to choices of local frames, which can in their turn be related to choices of local observers. In this talk, I will provide a relation between these modes, BRST ghosts, and a generalization of Barbour and Bertotti's 'best-matching'. Tuesday 23 talk Title - What is an observer in quantum geometrodynamics? Abstract: General relativity has a notorious problem: parametrizing its physical configuration space, even at an abstract level, is beyond our current knowledge. Replacing refoliation invariance with conformal transformations as a local gauge-symmetry allows one to parametrize the physical configuration space. Nonetheless, each point in this reduced configuration space contains non-local

4 information about the fields throughout physical space. It is essentially a space of relations. I will argue for a notion of "instantaneous states of observers" as consistent subsets of these relations. Such subsets define not points, but submanifolds in configuration space, corresponding to the configurations of the entire universe compatible with that instantaneous state of the observer. Relative quantum volumes of such regions define relative probabilities for different outcomes as perceived by observers. According to such outcomes, observers update their proposal for the wavefunction. Monday 22 talk Title - Bell s theorem for temporal order Caslav Brukner University of Vienna Abstract: In general relativity causal relations between any pair of events is uniquely determined by locally predefined variables the distribution of matter-energy degrees of freedom in the events past light-cone. Under the assumption of locally predefined causal order, agents performing freely chosen local operations on an initially local quantum state cannot violate Bell inequalities. However, superposition of massive objects can effectively lead to entanglement in the temporal order between groups of local operations, enabling the violation of the inequalities. This shows that temporal orders between events can be indefinite in non-classical space-times. Tuesday 23 talk Title - A no-go theorem for ``facts of the world'' Abstract: Wigner's friend is a thought experiment in which, from the point of view of Wigner, the friend, who measures a quantum system, is entangled with the state of the system. Both Wigner and the friend can in principle confirm their state assignment by performing an appropriate measurement. The result of each of these measurements as manifested trough a click in a detector or a specific position of the pointer -- can be seen as a fact. I will show that the view that holds the coexistence of the "facts of the world" common both for Wigner and the friend runs into the problem of the hidden variable program, for which I will derive a Bell-type theorem. A possible implication of theorem would be that facts are defined only relative to an observer. Still considering Wigner's friend thought experiment I will show how the friend can prove herself that she is in a superposition, and how in such a situation the application of standard quantum formalism seems to fail. Reference: arxiv: Julian Barbour (Oxford University) Title The End of Time and the Many-Instants interpretation of Quantum Mechanics Abstract: I will review temporal relationalism, and the timeless approach to physics, with particular attention to its implications for quantum mechanics. A variant of Everett s many-worlds interpretation of quantum mechanics emerges as a consequence of this point of view. I named

5 this approach the `many instants interpretation of quantum mechanics. I will describe its features, and comment on its implications in light of recent findings, by my collaborators and me, regarding the cosmological arrow of time and the nature of the Big Bang. Maximilian Lock (Imperial College London/University of Vienna) Title - A quantum version of the relativistic light-clock Abstract: The conflict between quantum theory and the theory of relativity is exemplified in their treatment of time. We examine the ways in which their conceptions differ, and consider a quantum version of the light-clock thought experiment commonly used to illustrate relativistic time dilation. This ``semiclassical'' clock model combines elements of both theories, and exhibits both quantitative and qualitative differences from the corresponding proper time of an ideal pointlike observer. Einstein begins his original formulation of relativity with a kind of operationalism, defining simultaneity by the ability of two separated clocks to synchronise. Our results show that a similarly operational definition of time, now accounting for quantum mechanics, must include extra considerations. Markus Mueller (University of Vienna) Title - Quantum theory and the structure of spacetime in the light of operationalism Abstract: Quantum theory and the structure of spacetime in the light of operationalism Some approaches to quantum gravity, e.g. the AdS/CFT correspondence or Sean Carroll s approach, promote the idea that spacetime can ultimately be reconstructed from the structure of (some underlying) quantum theory. In this talk, I sketch a research program that aims at studying aspects of this idea without committing to a particular model or ansatz. Namely, simple thought experiments of communicating observers, based only on what we currently know about physics, can shed light on the structural relation between quantum theory and spacetime in both directions. I will first present a simple thought experiment that shows how relativity of simultaneity constrains the structure of the quantum bit, without assuming the validity of quantum theory from the outset [1]. Then I will show that this kind of reasoning can be extended and reversed: if we assume the validity of quantum theory and the existence of certain types of universal measurement procedures (but make no assumptions about spacetime), then observers will automatically relate their descriptions of local laboratory physics by elements of the Lorentz group SO(3,1) [2]. Thus, taking the question of how observers operate in a quantum world seriously can help illuminate the relation between two seemingly separate aspects of physics. [1] A.J.P. Garner, M.P. Mueller, and O. C. O. Dahlsten, The complex and quaternionic quantum bits from relativity of simultaneity on an interferometer, Proc. R. Soc. A 473, (2017); arxiv: [2] P.A. Hoehn and M.P. Mueller, An operational approach to spacetime symmetries: Lorentz transformations from quantum communication, New J. Phys. 18, (2016); arxiv: Jose Manuel Carmona (University of Zaragoza)

6 Title - Observers and their notion of spacetime beyond special relativity Abstract: It is plausible that quantum gravity effects may lead us to a description of Nature beyond the framework of special relativity. In this case, either the relativity principle is broken or it is maintained. These two scenarios (a violation or a deformation of special relativity) are very different, both conceptually and phenomenologically. We discuss some of their implications on the description of events for different observers and the definition of a universal or observer-dependent notion of spacetime. Amelino Camelia (University of Rome La Sapienza) Title - Scale-dependent notions of Spacetime Abstract: I will explore the possibility, motivated by certain approaches to QG, that the kinematical notion at the basis of an observer's picture of spacetime might depend on the properties of the probes the observer has access to.