arXiv subjects

Sam Patrick

Publications and source records attributed to Sam Patrick.

At least 19 recordsLinked to original sources

arXiv2026

End state of the experimental black-hole bomb

Rotating black holes can amplify incident waves through superradiant scattering. When these waves are confined, repeated amplification gives rise to the black-hole bomb instability, whose nonlinear evolution remains poorly understood despite its central role in models of bosonic clouds around astrophysical black holes. Here, we reproduce the black-hole bomb mechanism in a laboratory setting using a gravity simulator based on a draining vortex in superfluid helium. Surface waves propagating on the superfluid interface experience an effective rotating spacetime and undergo repeated superradiant amplification within a cylindrical cavity. By tuning the temperature and flow parameters, we achieve exponential growth of a low-frequency resonant mode, followed by the arrest of the instability and the formation of a long-lived non-equilibrium steady state. Using spatially and temporally resolved measurements, we identify nonlinear frequency shifts, harmonic generation, and coherent three- and four-wave mixing that redistribute energy among interacting modes. This novel end state of the experimental black-hole bomb highlights the role of nonlinear wave interactions in quenching the runaway growth expected from linear theory and governing the system's late-time dynamics. Our results establish a laboratory framework for investigating the nonlinear evolution of black-hole bombs, with implications for analogous phenomena involving ultralight bosonic fields around rotating black holes.

gr-qc↗

arXiv2025

When is a sloshing vortex an analogue black hole bomb?

Draining vortices provide a powerful platform for simulating black hole phenomena in tabletop experiments. In realistic fluid systems confined within a finite container, low-frequency waves amplified by the vortex are reflected at the walls, rendering the system unstable. This process, known in the gravitational context as the black hole bomb, manifests as a sloshing motion of the free surface. The analogy, however, becomes more nuanced when a realistic vortex core with a non-singular vorticity distribution is considered. We investigate this by analysing a non-draining Rankine vortex in the shallow-water and inviscid limits. At low circulation, the sloshing corresponds to an instability of the vorticity field, whereas at high circulation where fluid is expelled from the vortex core, the destabilising mechanism coincides with that of the black hole bomb. Our variational framework distinguishes the energetic contributions of vorticity and irrotational perturbations, offering new insight into the rotating-polygons instability reported by, e.g. Jansson et al. (2006). From the analogue-gravity perspective, we identify hollow core vortices as an optimal regime for exploring black-hole-like instabilities in fluids.

physics.flu-dyn↗

arXiv2025

Rabi oscillations with close-range quantum vortex states

Quantum vortices separated through distances much larger than their core size interact via their long-range velocity field. At smaller separations, however, the influence of the core's compressibility strongly influences the vortex dynamics. Using the example of a compact ring of five vortices, it is shown that close-range effects lead to a new (slower) state of orbital motion which is not predicted by the usual long-range theory. This secondary state can be created starting from the usual orbital state by modulating the interaction strength to induce Rabi oscillations between the states.

cond-mat.quant-gas↗

arXiv2025

Black-hole spectroscopy from a giant quantum vortex

Black-hole spectroscopy aims to infer the fundamental properties of black holes by analysing the spectrum of gravitational waves emitted as they settle into equilibrium. These resonances, known as quasinormal modes (QNMs), decay rapidly, which limits the time-domain analysis of gravitational-wave data or numerical simulations to the longest-lived mode, except for a particularly loud event. Owing to the analogy between fields in curved spacetime and waves propagating in a flowing medium, QNMs can be equally excited in a laboratory. In these finite-sized systems, the QNM spectrum is expected to alter: compared to their counterparts in unbounded settings, the real frequencies of QNMs shift while their damping rates (imaginary frequencies) reduce, thereby enhancing their detectability. Here we show that multiple QNMs can be extracted from noise-driven interface waves surrounding a giant quantum vortex in superfluid helium-4, which emulates a spacetime geometry indicative of a rotating black hole. By resolving waves with different azimuthal periodicity, we find that both fundamental modes and their higher-frequency overtones are excited, and oscillate at frequencies given by the size of our system. Since similar effects may arise in astrophysical scenarios due to the interstellar medium or dark matter, gravity simulators now complement numerical and observational approaches to black-hole spectroscopy.

gr-qc↗

arXiv2024

Ultra slow-roll with a black hole

We investigate ultra slow-roll inflation with a seed black hole in a de Sitter background. By numerically tracking transitions from slow-roll to ultra slow-roll inflation, we find that quasi-normal mode solutions of the scalar field are excited following the decay of the slow-roll attractor, depending on the mass of the black hole. For small black holes, the picture is similar to standard inflation with the usual damping of the scalar field; with a large black hole, we find that the ringing modes dominate. It is believed that the transition to ultra slow-roll in the pure inflationary case enhances the peak of the primordial power spectrum, thereby increasing the likelihood of primordial black hole formation. We comment on how the novel ringing behaviour due to the seed black hole might impact on cosmological perturbations.

hep-th↗

arXiv2024

On the origin of quasinormal modes in semi-open systems

Astrophysical black holes are open systems which, when perturbed, radiate quasi-normal modes (QNMs) to infinity. By contrast, laboratory analogues are necessarily finite-sized, presenting a potential obstacle to exciting QNMs in experiments. We explore how the QNM spectrum of a toy-model black hole changes when enclosed by a partially reflecting wall with adjustable reflectivity. Our results reveal a continuous connection between the QNM spectra of open and finite-sized systems. Additionally, we demonstrate that QNMs in this setup are easily excited by incoherent background noise. This work opens new avenues for studying QNMs of black holes and compact objects in laboratory settings, where finite-size effects and noise are unavoidable.

gr-qc↗

arXiv2024

A primer on the analogue black hole bomb with capillary-gravity waves

Draining vortices with a free surface are frequently employed as rotating black hole simulators, both in theory and experiments. However, most theoretical work is restricted to the idealised regime, where wave dispersion and dissipation are neglected. We investigate the role of these effects on the analogue black hole bomb, an instability resulting from rotational superradiant amplification in confined systems. We reveal that the dispersion of deep water capillary-gravity waves significantly modifies the unstable mode eigenfrequencies, whereas viscosity only affects those with high frequencies. Furthermore, if the circulation is less than an order 1 multiple of the drain rate, superradiance does not occur and the vortex is stable. The instability is maximised in small systems with high flow velocities, provided there is sufficient space between the vortex and the outer boundary for the first excited state to lie inside the superradiant bandwidth. Implications for experiments on analogue black holes and free surface vortices are discussed.

gr-qc↗

arXiv2024

The sounds of science a symphony for many instruments and voices part II

Despite its amazing quantitative successes and contributions to revolutionary technologies, physics currently faces many unsolved mysteries ranging from the meaning of quantum mechanics to the nature of the dark energy that will determine the future of the Universe. It is clearly prohibitive for the general reader, and even the best informed physicists, to follow the vast number of technical papers published in the thousands of specialized journals. For this reason, we have asked the leading experts across many of the most important areas of physics to summarise their global assessment of some of the most important issues. In lieu of an extremely long abstract summarising the contents, we invite the reader to look at the section headings and their authors, and then to indulge in a feast of stimulating topics spanning the current frontiers of fundamental physics from The Future of Physics by William D Phillips and What characterises topological effects in physics? by Gerard t Hooft through the contributions of the widest imaginable range of world leaders in their respective areas. This paper is presented as a preface to exciting developments by senior and young scientists in the years that lie ahead, and a complement to the less authoritative popular accounts by journalists.

physics.soc-ph↗

arXiv2024

Quantum vortex stability in draining fluid flows

Quantum vortices with more than a single circulation quantum are usually unstable and decay into clusters of smaller vortices. One way to prevent the decay is to place the vortex at the centre of a convergent (draining) fluid flow, which tends to force vortices together. It is found that whilst the primary splitting instability is suppressed in this way (and completely quenched for strong enough flows) a secondary instability can emerge in circular trapping geometries. This behaviour is related to an instability of rotating black holes when superradiantly amplified waves are confined inside a reflective cavity. The end state of the secondary instability is dramatic, manifesting as a shock wave that propagates round the circular wall and nucleates many more vortices.

cond-mat.quant-gas↗

arXiv2023

Quasinormal Modes of Optical Solitons

Quasinormal modes (QNMs) are essential for understanding the stability and resonances of open systems, with increasing prominence in black hole physics. We present here the first study of QNMs of optical potentials. We show that solitons can support QNMs, deriving a soliton perturbation equation and giving exact analytical expressions for the QNMs of fiber solitons. We discuss the boundary conditions in this intrinsically dispersive system and identify novel signatures of dispersion. From here, we discover a new analogy with black holes and describe a regime in which the soliton is a robust black hole simulator for light-ring phenomena. Our results invite a range of applications, from the description of optical pulse propagation with QNMs to the use of state-of-the-art technology from fiber optics to address questions in black hole physics, such as QNM spectral instabilities and the role of nonlinearities in ringdown.

physics.optics↗

arXiv2023

Rotating Curved Spacetime Signatures from a Giant Quantum Vortex

Gravity simulators are laboratory systems where small excitations like sound or surface waves behave as fields propagating on a curved spacetime geometry. The analogy between gravity and fluids requires vanishing viscosity, a feature naturally realised in superfluids like liquid helium or cold atomic clouds. Such systems have been successful in verifying key predictions of quantum field theory in curved spacetime. In particular, quantum simulations of rotating curved spacetimes indicative of astrophysical black holes require the realisation of an extensive vortex flow in superfluid systems. Here we demonstrate that despite the inherent instability of multiply quantised vortices, a stationary giant quantum vortex can be stabilised in superfluid $^4$He. Its compact core carries thousands of circulation quanta, prevailing over current limitations in other physical systems such as magnons, atomic clouds and polaritons. We introduce a minimally invasive way to characterise the vortex flow by exploiting the interaction of micrometre-scale waves on the superfluid interface with the background velocity field. Intricate wave-vortex interactions, including the detection of bound states and distinctive analogue black hole ringdown signatures, have been observed. These results open new avenues to explore quantum-to-classical vortex transitions and utilise superfluid helium as a finite temperature quantum field theory simulator for rotating curved spacetimes.

gr-qc↗

arXiv2023

Stability of quantised vortices in two-component condensates

Multiply quantised vortices (MQVs) within single component Bose-Einstein condensates are unstable and decay rapidly. We show that MQVs can be stabilised by adding a small number of atoms of a second species to the vortex cores, and that these atoms remain in the vortex core as the system evolves. A consequence of the stabilisation is that nearby co-rotating vortices can orbit in the opposite sense to their individual rotations when enough of the second species is present. This has implications concerning the imaging of vortices, as well as quantum turbulence and vortex nucleation in two-component condensates, such as those involving mixtures of $^{87}$Rb and $^{133}$Cs.

cond-mat.quant-gas↗

arXiv2022

Imperfect draining vortex as analogue extreme compact object

Motivated by recent experimental progress, we study scalar wave propagation over an imperfect draining vortex, which can serve as an analogue for rotating and non-rotating extreme compact objects (ECOs). We encapsulate the absorbing properties of the analogue ECO by means of an effective boundary located around the analogue horizon. The presence of reflection at the effective boundary, characterised by a single parameter $\mathcal{K}$, allows for the existence of bound states located between the effective vortex core and the angular momentum barrier. The existence of these bound states leads to an enhanced absorption when the frequency of the incoming wave matches bound state frequencies, which result in Breit-Wigner type spectral lines in the absorption spectra. We also investigate the case of rotating analogue ECOs. In this scenario, some of the bound states undergo superradiant amplification and become unstable. In both the rotating and non-rotating case, we calculate numerically transmission/reflection spectra exhibiting the enhanced absorption/amplification. We complement our numerical study with WKB estimates as well as an extension of the P\"oschl-Teller toy model which we solve analytically. Our simple model exhibits distinctive properties which could be observed in future analogue gravity experiments. We further argue that the observation of the spectral lines could be a way to characterise the effective field theory at play in the vicinity of the vortex core.

gr-qc↗

arXiv2021

Quantum vortex instability and black hole superradiance

Vortices and black holes set the scene for many interesting dynamical processes in physics. Here, we study the dynamical instability of quantised vortices and rotational superradiance around rotating black holes, illustrating in the process that the same physics is at play in these two seemingly disparate phenomena. We also compare the instability of the vortex to the black hole bomb instability, which occurs for massive scalar fields in the Kerr spacetime. Taking inspiration from the analogy between black hole bomb modes and the hydrogen spectrum, the vortex instability is compared with nuclear resonances involved in $\alpha$-decay.

cond-mat.quant-gas↗

arXiv2021

Origin and evolution of the multiply-quantised vortex instability

We show that the dynamical instability of quantum vortices with more than a single quantum of angular momentum results from a superradiant bound state inside the vortex core. Our conclusion is supported by an analytic WKB calculation and numerical simulations of both linearised and fully non-linear equations of motion for a doubly-quantised vortex at the centre of a circular bucket trap. In the late stage of the instability, we reveal a striking novel behaviour of the system in the non-linear regime. Contrary to expectation, in the absence of dissipation the system never enters the regime of two well-separated phase defects described by Hamiltonian vortex dynamics. Instead, the separation between the two defects undergoes modulations which never exceed a few healing lengths, in which compressible kinetic energy and incompressible kinetic energy are exchanged. This suggests that, under the right conditions, pairs of vortices may be able to form meta-stable bound states.

cond-mat.quant-gas↗

arXiv2021

The sound-ring radiation of expanding vortex clusters

We investigate wave-vortex interaction emerging from an expanding compact vortex cluster in a two-dimensional Bose-Einstein condensate. We adapt techniques developed for compact gravitational objects to derive the characteristic modes of the wave-vortex interaction perturbatively around an effective vortex flow field. We demonstrate the existence of orbits or sound-rings, in analogy to gravitational light-rings, and compute the characteristic spectrum for the out-of-equilibrium vortex cluster. The spectrum obtained from numerical simulations of a stochastic Gross-Pitaevskii equation exhibiting an expanding vortex cluster is in excellent agreement with analytical predictions. Our findings are relevant for 2d-quantum turbulence, the semi-classical limit around fluid flows, and rotating compact objects exhibiting discrete circulation.

gr-qc↗

arXiv2021

Black hole evaporation in de Sitter space

We investigate the evaporation process of a Kerr-de Sitter black hole with the Unruh-Hawking-like vacuum state, which is a realistic vacuum state modelling the evaporation process of a black hole originating from gravitational collapse. We also compute the greybody factors for gravitons, photons, and conformal-coupling massless scalar particles by using the analytic solutions of the Teukolsky equation in the Kerr-de Sitter background. It turns out that the cosmological constant quenches the amplification factor and it approaches to zero towards the critical point where the Nariai and extremal limits merge together. We confirm that even near the critical point, the superradiance of gravitons is more significant than that of photons and scalar particles. Angular momentum is carried out by particles several times faster than the mass energy decreases. This means that a Kerr-de Sitter black hole rapidly spins down to a nearly Schwarzschild-de Sitter black hole before it completely evaporates. We also compute the time evolution of the Bekenstein-Hawking entropy. The total entropy of the Kerr-de Sitter black hole and cosmological horizon increases with time, which is consistent with the generalized second law of thermodynamics.

gr-qc↗

arXiv2020

Rotational superradiance with Bogoliubov dispersion

Rotational superradiance affects the dynamics of many rotating systems in nature, through either stimulated or spontaneous extraction of energy and angular momentum. By now, this process is well-studied in the relativistic setting, where systems are intrinsically dispersion-free. In many condensed matter systems, however, dispersion is an unavoidable aspect of the description for the short wavelength modes. For these systems, how might one expect superradiance to be modified? In this work, an answer to this question is provided using an illustrative example. The scattering of linear excitations of a Bose-Einstein condensate are studied in the presence of a rotating, draining vortex flow using the full Bogoliubov dispersion relation. It is shown that dispersion suppresses the extraction of energy and angular momentum, firstly, by decreasing the superradiant bandwidth, and secondly, by preventing high-angular momentum modes from superradiating.

gr-qc↗