779 research outputs found
Geometric phases in 2D and 3D polarized fields: geometrical, dynamical, and topological aspects
Geometric phases are a universal concept that underpins numerous phenomena
involving multi-component wave fields. These polarization-dependent phases are
inherent in interference effects, spin-orbit interaction phenomena, and
topological properties of vector wave fields. Geometric phases have been
thoroughly studied in two-component fields, such as two-level quantum systems
or paraxial optical waves. However, their description for fields with three or
more components, such as generic nonparaxial optical fields routinely used in
modern nano-optics, constitutes a nontrivial problem. Here we describe
geometric, dynamical, and total phases calculated along a closed spatial
contour in a multi-component complex field, with particular emphasis on 2D
(paraxial) and 3D (nonparaxial) optical fields. We present several equivalent
approaches: (i) an algebraic formalism, universal for any multi-component
field; (ii) a dynamical approach using the Coriolis coupling between the spin
angular momentum and reference-frame rotations; and (iii) a geometric
representation, which unifies the Pancharatnam-Berry phase for the 2D
polarization on the Poincar\'e sphere and the Majorana-sphere representation
for the 3D polarized fields. Most importantly, we reveal close connections
between geometric phases, angular-momentum properties of the field, and
topological properties of polarization singularities in 2D and 3D fields, such
as C-points and polarization M\"obius strips.Comment: 21 pages, 11 figures, to appear in Rep. Prog. Phy
Science, Art and Geometrical Imagination
From the geocentric, closed world model of Antiquity to the wraparound
universe models of relativistic cosmology, the parallel history of space
representations in science and art illustrates the fundamental role of
geometric imagination in innovative findings. Through the analysis of works of
various artists and scientists like Plato, Durer, Kepler, Escher, Grisey or the
present author, it is shown how the process of creation in science and in the
arts rests on aesthetical principles such as symmetry, regular polyhedra, laws
of harmonic proportion, tessellations, group theory, etc., as well as beauty,
conciseness and emotional approach of the world.Comment: 22 pages, 28 figures, invited talk at the IAU Symposium 260 "The Role
of Astronomy in Society and Culture", UNESCO, 19-23 January 2009, Paris,
Proceedings to be publishe
A deterministic detector for vector vortex states
Encoding information in high-dimensional degrees of freedom of photons has led to new avenues in various quantum protocols such as communication and information processing. Yet to fully benefit from the increase in dimension requires a deterministic detection system, e.g., to reduce dimension dependent photon loss in quantum key distribution. Recently, there has been a growing interest in using vector vortex modes, spatial modes of light with entangled degrees of freedom, as a basis for encoding information. However, there is at present no method to detect these non-separable states in a deterministic manner, negating the benefit of the larger state space. Here we present a method to deterministically detect single photon states in a four dimensional space spanned by vector vortex modes with entangled polarisation and orbital angular momentum degrees of freedom. We demonstrate our detection system with vector vortex modes from the |[Formula: see text]| = 1 and |[Formula: see text]| = 10 subspaces using classical and weak coherent states and find excellent detection fidelities for both pure and superposition vector states. This work opens the possibility to increase the dimensionality of the state-space used for encoding information while maintaining deterministic detection and will be invaluable for long distance classical and quantum communication
Quantum space-time: theory and phenomenology
Modern physics is based on two fundamental pillars: quantum mechanics and Einstein's general relativity. Even if, when taken separately, they can claim success in describing satisfactorily a plenty of physical phenomena, so far any attempt to make them compatible with each other failed. It is a central goal of theoretical physics to find a common approach to coherently merge quantum theory and general relativity. Such an effort is not only motivated by the conceptual necessity of completeness that imposes us to look for a unified theoretical framework that gives a consistent picture at all scales, but also by the existence of physical regimes we can not fully describe without a quantum theory of gravity, e.g. the first instants of early universe cosmology. This problem has remained open for more than eighty years now and keeps challenging physicists that, in the struggle to find a solution, have proposed a myriad of models, none of whom can claim full success. In fact, mainly due to the lack of experimental hints, the landscape of quantum gravity currently looks like a variegated compound of approaches that start from different conceptual premises and use different mathematical formalisms. In the majority of cases, it is not clear whether different models reach compatible predictions or even if they produce observable outcomes at all.
Given the impossibility to achieve a unique acknowledged theory, it is of pivotal importance to seek insightful connections between different approaches. Such a strategy may help identifying few promising hot spots that may catalyze forthcoming efforts in the quantum gravity research community. In particular, more synergy between top-down and bottom-up models is certainly needed. Besides shedding light on some formal aspects of the models and eventually giving further support to specific ideas, reducing the gap between full-fledged quantum gravity proposals and simpler models that try to capture at least some expected features might produce tangible progress in the field of quantum gravity phenomenology. Indeed, it is now well-established that some effects introduced genuinely at the Planck scale by heuristic models can be efficiently investigated in ongoing and forthcoming experiments. Moreover, the exciting era of multi-messenger and multi-wavelength has started where several satellites, telescopes, and new generation detectors are furnishing us with an incomparable amount of data to probe the structure of gravity on cosmological scales and in new regimes which had remained inaccessible. Finding ways to rigorously derive Planckian testable effects from quantum gravity theories is then needed to enter another phase of maturity of quantum gravity phenomenology, i.e. the passage from the search for Planck-scale signals to the falsification of actual theories. This thesis represents a small step in this direction.
To put this plan into action, we start recognizing that, despite the aforementioned heterogeneity, there is the common expectation that near the Planck scale our description of the spacetime as a smooth continuum, a picture shared by both general relativity and quantum mechanics, should break down and be replaced by some ``fuzzy" structure we generically refer to as \textit{quantum spacetime}. Again there are different ways to implement such an idea in different models, however we feel that the most relevant feature that characterizes spacetime quantization from a physical point of view is the associated departure from classical spacetime symmetries that most significantly encode spacetime's properties. In this regard, in the literature of the last three decades there has been much interest in the development of deformations of the Poincar\'e symmetries of special relativity, which most notably took the form of \textit{quantum groups} or \textit{Hopf algebras}, with the aim of modeling Planck scale physics. However, almost the totality of these studies is confined to the limit where gravitational effects are negligible, i.e. a sort of ``quantum Minkowski regime". With the objective to bridge quantum gravity and, in general, beyond general relativity theories with quantum or non-standard Minkowski spacetime models we here devote our attention to the symmetry content of general relativity, synthesized in the \textit{hypersurface deformation algebra}, and explore possible deformations caused by non-classical spacetime effects. Candidate modifications of the algebra of diffeomorphisms have been already obtained in some recent analyses, others will be derived in this thesis for the first time. We then translate modifications of the hypersurface deformation algebra into corresponding deviations from special relativistic symmetry with the main objective of looking for phenomenological opportunities. In particular, studying the Minkowski limit of deformed diffeomorphism algebras, we shall infer two much studied Planckian phenomena, namely \textit{modified dispersion relation} and \textit{the running of spacetime dimensions} with the probed scale.
In this thesis we focus in particular on four different paths toward the characterization of non-classical (to be meant in a general sense as non-standard) spacetime properties: \textit{noncommutative geometry}, \textit{loop quantum gravity}, \textit{multifractional geometry}, and \textit{non-Riemannian geometry}; only the second being widely recognized as a candidate full-fledged quantum gravity theory.
We first motivate why these two phenomenological Planck scale effects, i.e. dimensional reduction and modifications of particles' dispersion in vacuum, can be ascribed to spacetime fuzziness or quantization intended as an intrinsic obstruction to the measurability of spacetime distances below the Planck scale, an effect which can be deduced from the heuristic combination of general relativistc and quantum mechanical principles. Modified dispersion relation is derived rigorously in the framework of noncommutative geometries and we discuss two different noncommutative models which are of interest for this thesis: -Minkowski and -Minkowski. The phenomenon of dimensional flow is instead presented from the perspective of multifractional geometry. Within this framework we show how dimensional flow and spacetime fuzziness are deeply connected. We illustrate how the assumption of an anomalous scaling of the spacetime dimension in the ultraviolet and a slow change of the dimension in the infrared is enough to produce a scale-dependent deformation of the integration measure with also a fuzzy spacetime structure. We also compare the multifractional correction to lengths with the types of Planckian uncertainty for distance and time measurements. This may offer an explanation why dimensional flow is encountered in almost the totality of quantum gravity models.
We then introduce the (classical) hypersurface deformation algebra and constructively present two different ways of deriving it which we designate as representations of the algebra: the gravitational constraint representation, where the brackets are reproduced by the time and spatial diffeomorphism generators, and the Gaussian vector field representation, in which the algebra can be read off from the Lie bracket involving the components of a certain class of vector fields. Using this second realization, we study possible Drinfeld twists of space-time diffeomorphisms with Hopf-algebra techniques. We consider both deformed and twisted diffeomorphisms and compute the associated hypersurface deformation algebra.
We then turn our attention to recent loop-qauntum-gravity-inspired studies that have motivated a restricted class of modifications of the algebra of gravitational constraints. We discuss these new results in the light of the possibility to identify an effective quantum-spacetime picture of loop quantum gravity, applicable in the Minkowski regime, where the large-scale (coarse-grained) spacetime metric is flat. We show that these symmetry-algebra results are consistent with a description of spacetime given in terms of the -Minkowski noncommutative spacetime, whose relevance for the study of the quantum-gravity problem has already been proposed for independent reasons. We exploit this unexpected link to extract viable testable predictions out of loop quantum gravity models. These loop-qauntum-gravity-inspired corrections to spacetime symmetries are used to analyze both the consequences on particle propagation and on dimensional running. Adopting a different strategy, we also construct a set of three operators suitable for identifying coordinate-like quantities on a spin-network configuration on the kinematical Hilbert space. Computing their action on coherent coarse-grained states, we are able to study some relevant properties such us the spectra, which are discrete.
After that we scrutinize the symmetry structure of multifractional theories with either weighted or - derivatives. These theories have the property that the spacetime dimensions are anomalous since they change with the scale of observation. Despite their different mathematical formalisms, both noncommutative and multifractional geometries allow for the spacetime dimension to vary with the probed scale. For this reason, we compare their symmetries and prove that, despite the presence of many contact points claimed by precedent studies, they are are physically inequivalent, yet one can describe certain aspects of -Minkowski noncommutative geometry as a multifractional theory and vice versa. Turning gravity on, we calculate the algebras of gravitational first-class constraints in the multifractional theories with - and weighted derivatives and discuss their differences with respect to the deformed algebras of -Minkowski spacetime and of loop quantum gravity. Finally, with the aim of traducing multiscale formal properties into physical effects, we derive black hole solution in multifractional gravity theories and highlight new properties in the horizon structure as well as in the thermodynamical properties. Potential phenomenological signatures are underlined.
The fourth non-standard spacetime approach we consider is given by non-Riemannian geometries with non-metricity. Among other reasons to modify classical general relativity, one motivation is that modified Einstein-Hilbert action could provide either a better behaved theory in the ultraviolet, while Einstein's theory is not renormalizable, or encode effective corrections to classical gravity, which could be remnants of quantum effects at low energy scales. In this context it is often claimed that a relaxation of the Riemannian condition to incorporate geometric quantities such as torsion and non-metricity may allow to explore new physics associated with defects in a sort of ``spacetime microstructure". We show that non-metricity modifies particles' equations of motion. In particular, we find that
it produces observable effects in quantum fields in the form of 4-fermion contact interactions. The analysis we present is carried out in the framework of a wide class of theories of gravity in the metric-affine approach having a modified Lagrangian of the form . Finally, we compute the non-metric deformations of the hypersurface deformation algebra by using the Gaussian-vector-field method and make a qualitative comparison with loop quantum gravity results.
The final part of this thesis is dedicated to the search for quantum spacetime effects on the propagation of very-high energy particles in the form of in-vacuo dispersion, i.e. a linear correlation between time of observation and energy of particles. Motivated by some recent studies that exposed rather strong statistical evidence of in-vacuo-dispersion-like spectral lags for gamma-ray bursts in the energy range above 40 GeV, we analyze 7 gamma-ray burst events detected by Fermi-LAT in the period 2008-2016 by extending the window of the statistical analysis down to 5 GeV. Intriguingly, we find results that are consistent with what had been previously noticed at higher energies and, thus, could be of quantum-spacetime origin. Reduced samples of the data set based on different energy cuts are also considered with the objective to strengthen the results of the study. Besides the obvious interest of the feature we find, the main importance of our study stands in the fact that it represents one of the first analyses done over a collection of gamma-ray burst events. This paves the way to statistical analyses needed to produce more robust and reliable results despite huge uncertainties on the astrophysical mechanisms behind the formation, the emission and the propagation of photons produced in gamma-ray explosions
On elementary particles as representations of the Poincaré group
This thesis is concerned with the definition of elementary particles as irreducible projective unitary representations of the Poincaré group. During the contents of this work, we will introduce the relevant prerequisites and results. Concerning differential geometry, we will discuss smooth manifolds, Lie groups and Lie algebras. About quantum mechanics, we will introduce Hilbert spaces and the basic structures of quantum mechanics, together with Wigner’s theorem on symmetries. With respect to special relativity, we will present the Minkowski spacetime as an affine space an derive its group of automorphisms, the Poincaré group. We will finally talk about representations of Lie groups and define an elementary particle to be an irreducible projective representation of the Poincaré group
Stability and instability of expanding solutions to the Lorentzian constant-positive-mean-curvature flow
We study constant mean curvature Lorentzian hypersurfaces of
from the point of view of its Cauchy problem. We
completely classify the spherically symmetric solutions, which include among
them a manifold isometric to the de Sitter space of general relativity. We show
that the spherically symmetric solutions exhibit one of three (future)
asymptotic behaviours: (i) finite time collapse (ii) convergence to a time-like
cylinder isometric to some and (iii) infinite
expansion to the future converging asymptotically to a time translation of the
de Sitter solution. For class (iii) we examine the future stability properties
of the solutions under arbitrary (not necessarily spherically symmetric)
perturbations. We show that the usual notions of asymptotic stability and
modulational stability cannot apply, and connect this to the presence of
cosmological horizons in these class (iii) solutions. We can nevertheless show
the global existence and future stability for small perturbations of class
(iii) solutions under a notion of stability that naturally takes into account
the presence of cosmological horizons. The proof is based on the vector field
method, but requires additional geometric insight. In particular we introduce
two new tools: an inverse-Gauss-map gauge to deal with the problem of
cosmological horizon and a quasilinear generalisation of Brendle's Bel-Robinson
tensor to obtain natural energy quantities.Comment: Version 2: 60 pages, 1 figure. Changes mostly to fix typographical
errors, with the exception of Remark 1.2 and Section 9.1 which are new and
which explain the extrinsic geometry of the embedding in more detail in terms
of the stability result. Version 3: updated reference
Celestial holography: An asymptotic symmetry perspective
We review the role that infinite-dimensional symmetries arising at the
boundary of asymptotically flat spacetimes play in the context of the celestial
holography program. Once recast into the language of conformal field theory,
asymptotic symmetries provide key constraints on the sought-for celestial dual
to quantum gravity in flat spacetimes.Comment: Invited review for Physics Reports (preprint), 79 pages, 7 figure
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Low energy physics for the high energy physicist - Effective theories, holographic duality and all that
In this work we discuss the application of high energy theory methods to the study of condensed matter problems. We focus in particular on the effective field theory (EFT) approach and on the holographic duality. We show that, in certain contexts, both techniques present some relevant advantages with respect to more standard approaches. In particular, we will study holographic superfluids, and make explicit connection between the holographic picture and the EFT one. We also determine for the first time the gravity dual of a solid, and show that it undergoes a first order phase transition, a “holographic melting”. On a more phenomenological ground, we study the motion of vortex lines in a confined superfluid. Using a suitable EFT we successfully reproduce the experimental results, and perform a number of steps forward with respect to traditional methods. Finally, we also discuss possible exciting directions for the future of EFTs and condensed matter
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