arXiv subjects

Andrea Russo

Publications and source records attributed to Andrea Russo.

15 recordsLinked to original sources

arXiv2024

A quantum oscillator interacting with a classical oscillator

We study a quantum oscillator interacting and back-reacting on a classical oscillator. This can be done consistently provided the quantum system decoheres, while the backreaction has a stochastic component which causes the classical system to undergo diffusion. Nonetheless the state of the quantum oscillator can remain pure conditioned on the trajectory of the classical oscillator. We solve the system using the classical-quantum path integral formulation, and investigate slow moving regimes of either the classical or quantum oscillator. Lastly, we study the correlators of this classicalquantum setup. We are able to identify the free correlators of the theory and compute the full partition function perturbatively up to second order. This serves as a toy model for a number of other systems in which one system can be treated as effectively classical, such as a scalar quantum field interacting with another field undergoing decoherence, or a system emitting radiation, one of which is treated classically.

quant-ph↗

arXiv2024

Anomalous contribution to galactic rotation curves due to stochastic spacetime

We consider a proposed alternative to quantum gravity, in which the spacetime metric is treated as classical, even while matter fields remain quantum. Consistency of the theory necessarily requires that the metric evolve stochastically. Here, we show that this stochastic behaviour leads to a modification of general relativity at low accelerations. In the low acceleration regime, the variance in the acceleration produced by the gravitational field is high in comparison to that produced by the Newtonian potential, and can act as an entropic force, causing a deviation from Einstein's theory of general relativity. We show that in this "diffusion regime", the entropic force acts from a gravitational point of view, as if it were a contribution to the matter distribution. We compute modifications to the expectation value of the metric via the path integral formalism, and find an anomalous contribution which corresponds to a cosmological constant, anti-correlated with a contribution which has been used to fit galactic rotation curves without dark matter. We caution that a greater understanding of this effect is needed before conclusions can be drawn, most likely through numerical simulations, and provide a template for computing the deviation from general relativity which serves as an experimental signature of the Brownian motion of spacetime.

gr-qc↗

arXiv2024

Renormalisation of postquantum-classical gravity

One of the obstacles to reconciling quantum theory with general relativity, is constructing a theory which is both consistent with observation, and and gives finite answers at high energy, so that the theory holds at arbitrarily short distances. Quantum field theory achieves this through the process of renormalisation, but famously, perturbative quantum gravity fails to be renormalisable, even without coupling to matter. Recently, an alternative to quantum gravity has been proposed, in which the geometry of spacetime is taken to be classical rather than quantum, while still being coupled to quantum matter fields [1, 2]. This can be done consistently, provided the dynamics is fundamentally stochastic. Here, we find that the pure gravity theory is formally renormalisable. We do so via the path integral formulation by relating the classical-quantum action to that of quadratic gravity which is renormalisable. Because the action induces stochastic dynamics of space-time, rather than deterministic evolution of a quantum field, the classical-quantum theory is free of tachyons and negative norm ghosts. The key remaining question is whether the renormalisation prescription retains completely positive (CP) dynamics. This consideration appears to single out the scale invariant and asymptotically free theory. We give further evidence that the theory is CP, by showing that the two-point function of the scalar mode is positive. To support the use of precision accelerometers in testing the quantum nature of spacetime, we also compute the power spectral density of the acceleration. The results presented here have a number of implications for inflation, CMB data, and experiments to test the quantum nature of spacetime. They may also provide a way to compute probabilities in the regime of quantum gravity where spacetime can be treated as effectively classical.

hep-th↗

arXiv2024

Topics evolution through multilayer networks; Analysing 2M tweets from 2022 Qatar FIFA World Cup

In this study, we conducted a comprehensive data collection on the 2022 Qatar FIFA World Cup event and used a multilayer network approach to visualize the main topics, while considering their context and meaning relationships. We structured the data into layers that corresponded with the stages of the tournament and utilized Gephi software to generate the multilayer networks. Our visualizations displayed both the relationships between topics and words, showing the word-context relationship, as well as the dynamics and changes over time by layer of the most frequently discussed topics.

cs.SI↗

arXiv2024

Diffeomorphism invariant classical-quantum path integrals for Nordstrom gravity

When classical degrees of freedom and quantum degrees of freedom are consistently coupled, the former diffuse, while the latter undergo decoherence. Here, we construct a theory of quantum matter fields and Nordstrom gravity in which the space-time metric is treated classically. The dynamics is constructed via the classical-quantum path integral and is completely positive, trace preserving (CPTP), and respects the classical-quantum split. The weak field limit of the model matches the Newtonian limit of the full covariant path integral but it is easier to show that the theory is both diffeomorphism invariant, CPTP, and has the appropriate classical limit.

gr-qc↗

arXiv2023

Matter relative to quantum hypersurfaces

We explore the canonical description of a scalar field as a parameterized field theory on an extended phase space that includes additional embedding fields that characterize spacetime hypersurfaces $\mathsf{X}$ relative to which the scalar field is described. This theory is quantized via the Dirac prescription and physical states of the theory are used to define conditional wave functionals $|ψ_ϕ[\mathsf{X}]\rangle$ interpreted as the state of the field relative to the hypersurface $\mathsf{X}$, thereby extending the Page-Wootters formalism to quantum field theory. It is shown that this conditional wave functional satisfies the Tomonaga-Schwinger equation, thus demonstrating the formal equivalence between this extended Page-Wootters formalism and standard quantum field theory. We also construct relational Dirac observables and define a quantum deparameterization of the physical Hilbert space leading to a relational Heisenberg picture, which are both shown to be unitarily equivalent to the Page-Wootters formalism. Moreover, by treating hypersurfaces as quantum reference frames, we extend recently developed quantum frame transformations to changes between classical and nonclassical hypersurfaces. This allows us to exhibit the transformation properties of a quantum field under a larger class of transformations, which leads to a frame-dependent particle creation effect.

quant-ph↗

arXiv2023

The weak field limit of quantum matter back-reacting on classical spacetime

Consistent coupling of quantum and classical degrees of freedom exists so long as there is both diffusion of the classical degrees of freedom and decoherence of the quantum system. In this paper, we derive the Newtonian limit of such classical-quantum (CQ) theories of gravity. Our results are obtained both via the gauge fixing of the recently proposed path integral theory of CQ general relativity and via the CQ master equation approach. In each case, we find the same weak field dynamics. We find that the Newtonian potential diffuses by an amount lower bounded by the decoherence rate into mass eigenstates. We also present our results as an unravelled system of stochastic differential equations for the trajectory of the hybrid classical-quantum state and provide a series of kernels for constructing figures of merit, which can be used to rule out part of the parameter space of classical-quantum theories of gravity by experimentally testing it via the decoherence-diffusion trade-off. We compare and contrast the weak field limit to previous models of classical Newtonian gravity coupled to quantum systems. Here, we find that the Newtonian potential and quantum state change in lock-step, with the flow of time being stochastic.

gr-qc↗

arXiv2023

Network analysis on political election; populist vs social emergent behaviour

Social networks play an important role in people's daily socialization, particularly through social media platforms, which have become key channels for communication and information dissemination. The digital ecosystem does not only evolve communication on multi-network (like TV, social media, and online newspapers) but also provides the social researcher with useful data to explain social-complex dynamics. Our work focus on cultural dynamics-reactions that occurred during the 2020 Emilia-Romagna elections'' in Italy, where a stronghold culture felt in danger of losing against the strong populism and Euro-scepticism present in digital ecosystems. We would like to show how the interaction between parts of the society, during cultural and/or political shifting, can lead to or induce emerging behaviour from society, creating groups that react against or improve the status quo. We developed a word-entry network based on three different levels of participation: pro, con, and neutral. We have analyzed the tweets collected (as text) with the word embedding tools, to see, the most used words (which may suggest the main topics) and the most related words among the various groups. We show how a careful analysis of groups through networks, can give important information about the current event.

physics.soc-ph↗

arXiv2023

Quantify how space mission influence geopolitical dynamics? A security and social policy approach

We present a computational method to quantify the geopolitical impact of a space mission, based on the national budget and data logs of previous missions, and evidencing how even if some missions succeed, they can bring negative effects to the sponsored country. The objective of this research is to study how the success (or failure) of a space mission can bring an economical and political benefit (or loss) to a country. By retrieving various data, including sentiment from #hashtags related to the considered space missions, national budgets for space exploration, and the reliability of space launch systems, from social networks, public institutions, and online repositories, we propose an equation to evaluate the geopolitical importance of a space mission for a particular country or space agency. The geopolitical equation can be used by public institutions or private companies to estimate the potential impact of a space mission on public opinion and international relationships, which can be either positive or negative, as even successful missions may negatively affect international relationships and negotiations with some countries and their partners. Also we combine the ideology of classic social policy with a security and space mission point of view, to enlighten cultural, institutional, and political limits in public spending decisions.

physics.soc-ph↗

arXiv2023

Organised Firestorm as strategy for business cyber-attacks

Having a good reputation is paramount for most organisations and companies. In fact, having an optimal corporate image allows them to have better transaction relationships with various customers and partners. However, such reputation is hard to build and easy to destroy for all kind of business commercial activities (B2C, B2B, B2B2C, B2G). A misunderstanding during the communication process to the customers, or just a bad communication strategy, can lead to a disaster for the entire company. This is emphasised by the reaction of millions of people on social networks, which can be very detrimental for the corporate image if they react negatively to a certain event. This is called a firestorm. In this paper, I propose a well-organised strategy for firestorm attacks on organisations, also showing how an adversary can leverage them to obtain private information on the attacked firm. Standard business security procedures are not designed to operate against multi-domain attacks; therefore, I will show how it is possible to bypass the classic and advised security procedures by operating different kinds of attack. I also propose a different firestorm attack, targeting a specific business company network in an efficient way. Finally, I present defensive procedures to reduce the negative effect of firestorms on a company.

cs.CY↗

arXiv2022

Entropy-rate as prediction method for newspapers and information diffusion

This paper aims to show how some popular topics on social networks can be used to predict online newspaper views, related to the topics. Newspapers site and many social networks, become a good source of data to analyse and explain complex phenomena. Understanding the entropy of a topic, could help all organizations that need to share information like government, institution, newspaper or company, to expect an higher activity over their channels, and in some cases predict what the receiver expect from the senders or what is wrong about the communication. For some organization such political party, leaders, company and many others, the reputation and the communication are (for most of them) the key part of a more and complex huge system. To reach our goal, we use gathering tools and information theory to detect and analyse trends topic on social networks, with the purpose of proved a method that helps organization, newspapers to predict how many articles or communication they will have to do on a topic, and how much flow of views they will have in a given period, starting with the entropy-article ratio. Our work address the issue to explore in which entropy-rate, and through which dynamics, a suitable information diffusion performance is expected on social network and then on newspaper. We have identified some cross-cutting dynamics that, associated with the contexts, might explain how people discuss about a topic, can move on to argue and informs on newspapers sites.

physics.soc-ph↗

arXiv2022

Computing spacetime

Inspired by the universality of computation, we advocate for a principle of spacetime complexity, where gravity arises as a consequence of spacetime optimizing the computational cost of its own quantum dynamics. This principle is explicitly realized in the context of the Anti-de Sitter/Conformal Field Theory correspondence, where complexity is naturally understood in terms of state preparation via Euclidean path integrals, and Einstein's equations emerge from the laws of quantum complexity. We visualize spacetime complexity using Lorentzian threads which, conceptually, represent the operations needed to prepare a quantum state in a tensor network discretizing spacetime. Thus, spacetime itself evolves via optimized computation.

hep-th↗

arXiv2022

Sewing spacetime with Lorentzian threads: complexity and the emergence of time in quantum gravity

Holographic entanglement entropy was recently recast in terms of Riemannian flows or 'bit threads'. We consider the Lorentzian analog to reformulate the 'complexity=volume' conjecture using Lorentzian flows -- timelike vector fields whose minimum flux through a boundary subregion is equal to the volume of the homologous maximal bulk Cauchy slice. By the nesting of Lorentzian flows, holographic complexity is shown to obey a number of properties. Particularly, the rate of complexity is bounded below by conditional complexity, describing a multi-step optimization with intermediate and final target states. We provide multiple explicit geometric realizations of Lorentzian flows in AdS backgrounds, including their time-dependence and behavior near the singularity in a black hole interior. Conceptually, discretized flows are interpreted as Lorentzian threads or 'gatelines'. Upon selecting a reference state, complexity thence counts the minimum number of gatelines needed to prepare a target state described by a tensor network discretizing the maximal volume slice, matching its quantum information theoretic definition. We point out that suboptimal tensor networks are important to fully characterize the state, leading us to propose a refined notion of complexity as an ensemble average. The bulk symplectic potential provides a specific 'canonical' thread configuration characterizing perturbations around arbitrary CFT states. Consistency of this solution requires the bulk satisfy the linearized Einstein's equations, which are shown to be equivalent to the holographic first law of complexity, thereby advocating for a principle of 'spacetime complexity'. Lastly, we argue Lorentzian threads provide a notion of emergent time. This article is an expanded and detailed version of [arXiv:2105.12735], including several new results.

hep-th↗

arXiv2021

Lorentzian threads as 'gatelines' and holographic complexity

The continuous min flow-max cut principle is used to reformulate the 'complexity=volume' conjecture using Lorentzian flows -- divergenceless norm-bounded timelike vector fields whose minimum flux through a boundary subregion is equal to the volume of the homologous maximal bulk Cauchy slice. The nesting property is used to show the rate of complexity is bounded below by "conditional complexity", describing a multi-step optimization with intermediate and final target states. Conceptually, discretized Lorentzian flows are interpreted in terms of threads or gatelines such that complexity is equal to the minimum number of gatelines used to prepare a CFT state by an optimal tensor network (TN) discretizing the state. We propose a refined measure of complexity, capturing the role of suboptimal TNs, as an ensemble average. The bulk symplectic potential provides a 'canonical' thread configuration characterizing perturbations around arbitrary CFT states. Its consistency requires the bulk to obey linearized Einstein's equations, which are shown to be equivalent to the holographic first law of complexity, thereby advocating a notion of 'spacetime complexity'.

hep-th↗

arXiv2021

The Second Law of Quantum Complexity and the Entanglement Wormhole

This work is originally a Cambridge Part III essay paper. Quantum complexity arises as an alternative measure to the Fubini metric between two quantum states. Given two states and a set of allowed gates, it is defined as the least complex unitary operator capable of transforming one state into the other. Starting with K qubits evolving through a k-local Hamiltonian, it is possible to draw an analogy between the quantum system and an auxiliary classical system. Using the definition of complexity to define a metric for the classical system, it is possible to relate its entropy with the quantum complexity of the K qubits, defining the Second Law of Quantum Complexity. The law states that, if it is not already saturated, the quantum complexity of a system will increase with an overwhelming probability towards its maximum value. In the context of AdS/CFT duality and the ER=EPR conjecture, the growth of the volume of the Einstein Rosen bridge interior is proportional to the quantum complexity of the instantaneous state of the conformal field theory. Therefore, the interior of the wormhole connecting two entangled CFT will grow as a natural consequence of the complexification of the boundary state.

quant-ph↗