Photons with sub-Planckian Energy Cannot Efficiently Probe Space-Time Foam

Extra-galactic sources of photons have been used to constrain space-time quantum fluctuations in the Universe. In these proposals, the fundamental "fuzziness" of distance caused by space-time quantum fluctuations has been directly identified with fluctuations in optical paths. Phase-front corrugations deduced from these optical-path fluctuations are then applied to light from extra-galactic point sources, and used to constrain various models of quantum gravity. However, when a photon propagates in three spatial dimensions, it does not follow a specific ray, but rather samples a finite, three-dimensional region around that ray --- thereby averaging over space-time quantum fluctuations all through that region. We use a simple, random-walk type model to demonstrate that, once the appropriate wave optics is applied, the averaging of neighboring space-time fluctuations will cause much less distortion to the phase front. In our model, the extra suppression factor due to diffraction is the wave length in units of the Planck length, which is at least $10^{29}$ for astronomical observations.Comment: This is a revised version of arXiv:gr-qc/060509

Amplitudes in the Spin Foam Approach to Quantum Gravity

In this thesis, we study a Spin Foam model for 4D Riemannian quantum gravity, and propose a new way of imposing the simplicity constraints that uses the recently developed holomorphic representation. Rather than imposing the constraints on the boundary spin network, one can impose the constraints directly on the Spin Foam propagator. We find that the two approaches have the same leading asymptotic behaviour. This allows us to obtain a model that greatly simplifies calculations, but still has Regge Calculus as its semi-classical limit. Based on this newly developed model, we aim at answering the following questions that previously has never been properly addressed in the field: how to efficiently evaluate arbitrary Spin Foam amplitudes in 4D? Do we have residual diffeomorphism invariance of the model? What happens to the amplitudes under coarse graining? Can we learn the degree of divergence of an amplitude simply by its graphic properties? What type of geometry in the bulk has the dominant contribution to the partition function? Using the power of the holomorphic integration techniques, and with the introduction of new methods: the homogeneity map, the loop identity and a natural truncation scheme, for the first time we give the analytical expressions for the behaviour of the Spin Foam amplitudes under 4-dimensional Pachner moves. The model considered is not invariant under the 5--1 Pachner Move, as the configuration of five 4-simplices reduces to a single 4-simplex with an insertion of a nonlocal operator inside. Similar behaviour occurs also for the 4--2 move. The non-invariance under 5--1 move means that the vertex translation symmetry, the residual of diffeomorphism invariance for discrete gravity, is broken in this path integral formalism. We also developed a natural truncation scheme that captures the dominant contribution and preserves the geometrical structures, while at the same time efficiently reduces the complexity. We then push the result to be more general -- evaluating arbitrary amplitudes. We study the amplitudes on arbitrary connected 2-complexes and their degrees of divergence. First we derive a compact expression for a certain class of graphs, which allows us to write down the value of bulk amplitudes simply based on graph properties. We then generalize the result to arbitrary connected 2-complexes and extract a formula for the degree of divergence only in terms of combinatorial properties and topological invariants. By regulating the model, this result allows us to find the dominant contributions to the partition function, which gives us some valuable hints about the continuum limit. The distinct behaviors of the model in different regions of parameter space signal phase transitions. However, in the regime which is of physical interest for recovering diffeomorphism symmetry in the continuum limit, the most divergent contributions are from geometrically degenerate configurations. We finish with discussing possible resolutions, the physical implications for different scenarios of defining the continuum limit and the analytical insights we have gained into the behavior of Spin Foam amplitudes

Geometrical Expression for the Angular Resolution of a Network of Gravitational-Wave Detectors

We report for the first time general geometrical expressions for the angular resolution of an arbitrary network of interferometric gravitational-wave (GW) detectors when the arrival-time of a GW is unknown. We show explicitly elements that decide the angular resolution of a GW detector network. In particular, we show the dependence of the angular resolution on areas formed by projections of pairs of detectors and how they are weighted by sensitivities of individual detectors. Numerical simulations are used to demonstrate the capabilities of the current GW detector network. We confirm that the angular resolution is poor along the plane formed by current LIGO-Virgo detectors. A factor of a few to more than ten fold improvement of the angular resolution can be achieved if the proposed new GW detectors LCGT or AIGO are added to the network. We also discuss the implications of our results for the design of a GW detector network, optimal localization methods for a given network, and electromagnetic follow-up observations.Comment: 13 pages, for Phys. Rev.

Towards low-latency real-time detection of gravitational waves from compact binary coalescences in the era of advanced detectors

Electromagnetic (EM) follow-up observations of gravitational wave (GW) events will help shed light on the nature of the sources, and more can be learned if the EM follow-ups can start as soon as the GW event becomes observable. In this paper, we propose a computationally efficient time-domain algorithm capable of detecting gravitational waves (GWs) from coalescing binaries of compact objects with nearly zero time delay. In case when the signal is strong enough, our algorithm also has the flexibility to trigger EM observation before the merger. The key to the efficiency of our algorithm arises from the use of chains of so-called Infinite Impulse Response (IIR) filters, which filter time-series data recursively. Computational cost is further reduced by a template interpolation technique that requires filtering to be done only for a much coarser template bank than otherwise required to sufficiently recover optimal signal-to-noise ratio. Towards future detectors with sensitivity extending to lower frequencies, our algorithm's computational cost is shown to increase rather insignificantly compared to the conventional time-domain correlation method. Moreover, at latencies of less than hundreds to thousands of seconds, this method is expected to be computationally more efficient than the straightforward frequency-domain method.Comment: 19 pages, 6 figures, for PR

Summed Parallel Infinite Impulse Response (SPIIR) Filters For Low-Latency Gravitational Wave Detection

With the upgrade of current gravitational wave detectors, the first detection of gravitational wave signals is expected to occur in the next decade. Low-latency gravitational wave triggers will be necessary to make fast follow-up electromagnetic observations of events related to their source, e.g., prompt optical emission associated with short gamma-ray bursts. In this paper we present a new time-domain low-latency algorithm for identifying the presence of gravitational waves produced by compact binary coalescence events in noisy detector data. Our method calculates the signal to noise ratio from the summation of a bank of parallel infinite impulse response (IIR) filters. We show that our summed parallel infinite impulse response (SPIIR) method can retrieve the signal to noise ratio to greater than 99% of that produced from the optimal matched filter. We emphasise the benefits of the SPIIR method for advanced detectors, which will require larger template banks.Comment: 9 pages, 6 figures, for PR

Extracting Information about EMRIs using Time-Frequency Methods

Abstract. The inspirals of stellar-mass compact objects into supermassive black holes are some of the most exciting sources of gravitational waves for LISA. Detection of these sources using fully coherent matched filtering is computationally intractable, so alternative approaches are required. In [1], we proposed a detection method based on searching for significant deviation of power density from noise in a time-frequency spectrogram of the LISA data. The performance of the algorithm was assessed in [2] using Monte-Carlo simulations on several trial waveforms and approximations to the noise statistics. We found that typical extreme mass ratio inspirals (EMRIs) could be detected at distances of up to 1–3 Gpc, depending on the source parameters. In this paper, we first give an overview of our previous work in [1, 2], and discuss the performance of the method in a broad sense. We then introduce a decomposition method for LISA data that decodes LISA’s directional sensitivity. This decomposition method could be used to improve the detection efficiency, to extract the source waveform, and to help solve the source confusion problem. Our approach to constraining EMRI parameters using the output from the time-frequency method will be outlined

Electrical Resistivity Changes During Heating Experiments Unravel Heterogeneous Thermal‐Hydrological‐Mechanical Processes in Salt Formations

Rock salt is considered a suitable medium for the permanent disposal of heat-generating radioactive waste due to its isolation properties. However, excavation damage and heating induce complex and heterogeneous thermal-hydrological-mechanical (THM) processes across different zones. Quantifying this heterogeneity is crucial for accurate long-term performance assessment models, but traditional methods lack the necessary resolution. This study employs 4D electrical resistivity tomography (ERT) monitoring during controlled heating experiments in a salt formation to unravel the spatiotemporal dynamics of THM processes. Advanced time-lapse inversion and clustering analysis quantify subsurface properties and map the heterogeneity of THM dynamics. The ERT results can estimate subsurface properties and delineate the damaged and intact zones, enabling appropriate parameterization and representation of processes for long-term modeling. This approach may be used in further improving the predictive models and ensuring the safe long-term disposal of radioactive waste in rock salt

Light-activated ferroelectric transition in layer dependent Bi2O2Se films

Bi2O2Se has attracted intensive attention due to its potential in electronics, optoelectronics, as well as ferroelectric applications. Despite that, there have only been a handful of experimental studies based on ultrafast spectroscopy to elucidate the carrier dynamics in Bi2O2Se thin films, Different groups have reported various ultrafast timescales and associated mechanisms across films of different thicknesses. A comprehensive understanding in relation to thickness and fluence is still lacking. In this work, we have systematically explored the thickness-dependent Raman spectroscopy and ultrafast carrier dynamics in chemical vapor deposition (CVD)-grown Bi2O2Se thin films on mica substrate with thicknesses varying from 22.44 nm down to 4.62 nm at both low and high pump fluence regions. Combining the thickness dependence and fluence dependence of the slow decay time, we demonstrate a ferroelectric transition in the thinner (< 8 nm) Bi2O2Se films, influenced by substrate-induced compressive strain and non-equilibrium states. Moreover, this transition can be manifested under highly non-equilibrium states. Our results deepen the understanding of the interplay between the ferroelectric phase and semiconducting characteristics of Bi2O2Se thin films, providing a new route to manipulate the ferroelectric transition