Causality, Ensemble Invariance, and Black-Hole Phenomenology: Prof. Dr. İzzet Sakallı on the KSS Bound, Thermodynamic Topology, Hawking Radiation, and Quasinormal Modes (4)

Scott Douglas Jacobsen (Email: [email protected])

Publisher, In-Sight Publishing

Fort Langley, British Columbia, Canada

Received: October 25, 2025

Accepted: January 8, 2026

Published: March 22, 2026

Abstract

This interview with Prof. Dr. İzzet Sakallı explores several live questions in black-hole physics, holography, and quantum gravity phenomenology. Topics include whether thermal fluctuations threaten the Kovtun-Son-Starinets bound in quantum-corrected AdS black holes, how thermodynamic topology may be defined in a coordinate-independent and ensemble-stable way, and what unifies linear dilaton, Newman-Unti-Tamburino, and Bardeen geometries. The discussion also compares higher-order WKB methods and neural networks for extracting quasinormal modes, examines where the Hamilton-Jacobi tunneling method breaks down in Hawking-radiation derivations, and identifies the kind of falsifiable predictions needed to curb post-hoc parameter tuning in quantum-gravity phenomenology. Together, the interview presents a panoramic view of how gravity, thermodynamics, topology, and observational testing intersect at the frontier of contemporary theoretical physics.

Keywords

AdS black holes, black-hole thermodynamics, Hamilton-Jacobi tunneling, holography, KSS bound, quantum gravity phenomenology, quasinormal modes, thermodynamic topology, WKB method

Introduction

Black-hole physics remains one of the most fertile meeting grounds for gravitation, quantum theory, thermodynamics, and information theory. In recent decades, holography, quantum corrections, modified gravity, and computational methods have broadened the conceptual and technical landscape, making black holes not merely endpoints of gravitational collapse but laboratories for probing the structure of physical law itself.

In this interview, Prof. Dr. İzzet Sakallı discusses several of the most conceptually demanding issues at that frontier. The conversation ranges from the stability of the Kovtun-Son-Starinets bound under thermal fluctuations to coordinate-independent definitions of thermodynamic topology, from “hairy” black-hole geometries to the comparative strengths of semiclassical and machine-learning approaches for quasinormal mode extraction. The exchange concludes by asking what kinds of genuinely falsifiable predictions could move quantum-gravity phenomenology beyond flexible post-hoc fitting and toward more rigorous empirical discipline.

Main Text (Interview)

Title: Black-Hole Thermodynamics, Thermodynamic Topology, Quasinormal Modes, and Quantum-Gravity Phenomenology: Prof. Dr. İzzet Sakallı (4)

Interviewer: Scott Douglas Jacobsen

Interviewees: Prof. Dr. İzzet Sakallı

Professor Izzet Sakallı is a theoretical physicist at Eastern Mediterranean University whose research bridges quantum mechanics, general relativity, and observational astronomy. With over 180 publications exploring black hole thermodynamics, modified gravity theories, and quantum corrections to spacetime, his work sits at the exciting frontier where abstract mathematics meets observable reality. In this interview, he discusses the challenges of testing exotic gravity theories, the quest to observe quantum effects in astrophysical systems, and what the next generation of telescopes and gravitational wave detectors might reveal about the quantum nature of spacetime. 

Scott Douglas Jacobsen: Do thermal fluctuations genuinely threaten the Kovtun-Son-Starinets bound in quantum-corrected AdS black holes? 

Prof. Dr. ˙Izzet Sakallı: The Kovtun-Son-Starinets bound represents a proposed fundamental limit on how viscous any fluid can be relative to its entropy density. Viscosity measures how much a fluid resists flow—honey has high viscosity, water much less. The KSS bound suggests that the ratio of shear viscosity to entropy density cannot fall below a specific value involving Planck’s constant. If true, this would represent a universal constraint on all matter, from quark-gluon plasma to neutron star interiors. 

The bound emerges from AdS/CFT calculations. Black holes in AdS spacetime are extraordinarily good fluids—they flow with minimal viscosity. Computing their viscosity-to-entropy ratio via holography yields exactly the proposed bound. This isn’t coincidental; it reflects deep connections between gravity, thermodynamics, and hydrodynamics. 

But black holes, like all physical systems, experience thermal fluctuations. The horizon position fluctuates quantum mechanically. The area, and hence entropy, fluctuates. If we compute viscosity at one instant and entropy at another, might their ratio momentarily dip below the bound? 

The consensus among researchers is that thermal fluctuations do not genuinely threaten the bound, for several subtle reasons. First, the bound applies to ensemble-averaged, thermodynamic quantities, not to instantaneous microscopic configurations. Just as temperature represents average kinetic energy, not the energy of any individual molecule, the viscosity-to-entropy ratio reflects macroscopic, coarse-grained properties. Fluctuations around these averages don’t violate the bound any more than individual fast molecules violate temperature definitions. 

Second, causality protects the bound. Any configuration violating the bound would allow signals to propagate faster than light in the dual field theory. Such acausal behavior is forbidden by fundamental physics. Thermal fluctuations, no matter how large, cannot create causality violations because they’re constrained by the same underlying quantum field theory that enforces causality. 

Third, the fluctuation-dissipation theorem provides a deep connection between fluctuations and transport coefficients. This theorem ensures that fluctuations and viscosity vary in coordinated ways, preserving bounds even when individual quantities fluctuate substantially. If the entropy fluctuates upward, viscosity adjusts accordingly; if entropy dips, viscosity decreases proportionally. The ratio remains bounded. 

Fourth, quantum corrections modify the bound in controlled, calculable ways. Adding higher derivative terms to gravity, incorporating string theory corrections, or including additional fields shifts the bound’s value. It might become slightly larger or smaller, but a lower bound persists. The bound moves rather than disappears. 

In quantum-corrected AdS black holes—those including effects from Gauss-Bonnet gravity, dila ton fields, or other modifications—the situation becomes more intricate. These corrections alter both the entropy and the viscosity. Causality constraints in the modified theory ensure the bound adapts accordingly. Some theories predict the ratio slightly exceeds the standard bound; others might approach it from above. But violations remain absent. 

Experimental tests come from heavy-ion collisions creating quark-gluon plasma. Measurements consistently find viscosity-to-entropy ratios just slightly above the KSS bound—the lowest values ever measured for any substance. This near-saturation suggests the bound is indeed fundamental, and thermal fluctuations in these terrestrial experiments don’t cause violations. 

The key insight is distinguishing instantaneous fluctuations from thermodynamic properties. A single molecule in water might momentarily move faster than the sound speed in water, but that doesn’t violate the principle that sound waves propagate at a characteristic speed. Similarly, momentary excursions of viscosity or entropy don’t violate bounds on their thermodynamic ratio. 

Jacobsen: What is an operational, coordinate-independent definition of thermodynamic topology, and how should ensembles be chosen so invariants are well-posed? 

Sakallı: Thermodynamic topology represents a modern mathematical approach to understanding phase transitions through topological invariants—quantities that remain unchanged under continuous deformations. Rather than characterizing phases by specific values of temperature or pressure, topological methods classify them by global geometric properties that don’t depend on coordinate choices or units. 

The key idea is treating thermodynamic state space as a geometric manifold. Every possible equilibrium state—characterized by temperature, pressure, entropy, volume, and other thermodynamic variables—represents a point in this space. Thermodynamic potentials like Gibbs free energy or Helmholtz free energy define scalar fields on this manifold. Critical points, where derivatives of these potentials vanish, act as topological defects. 

An operational definition requires coordinate independence—the formulation shouldn’t depend on whether we describe states using temperature and pressure versus energy and volume. This is achieved using differential geometric objects. We can define a thermodynamic metric on the space of extensive variables, measuring ”distance” between thermodynamic states. The curvature of this metric provides coordinate-independent information about thermodynamic stability and fluctuations. 

Critical points are classified by their topological charge or winding number. Imagine walking a closed loop around a critical point in state space and tracking how the gradient vector of free energy rotates. The number of complete rotations—the winding number—is a topological invariant integer that classifies the critical point. Different types of phase transitions have different winding numbers. 

For black holes, we can construct thermodynamic state space using entropy, pressure (related 14 to cosmological constant), electric charge, and angular momentum as coordinates. The mass, expressed as a function of these variables, serves as the thermodynamic potential. Critical points where the temperature vanishes or diverges represent phase transitions. 

Ensemble selection is crucial for well-posed invariants. Different ensembles—microcanonical (fixed energy), canonical (fixed temperature), or grand canonical (fixed chemical potential)—describe different statistical situations. The key insight is that these are related by Legendre transformations, which are coordinate changes in a broader geometric structure called the thermodynamic phase space. This space combines position-like variables (extensive quantities) with momentum like variables (their conjugate intensive quantities). 

On this phase space, we can define a symplectic structure—a geometric object describing how variables are paired. This structure is ensemble-independent. Topological invariants computed using this structure remain consistent across different ensembles. A critical point in the canonical ensemble corresponds to a critical point in the grand canonical ensemble; they’re the same feature viewed in different coordinates. 

For well-posed invariants, the thermodynamic manifold should be compactified—made compact by appropriately treating infinities. Temperature ranging from zero to infinity can be mapped to a finite interval, allowing global topological analysis. The compactification must respect physical symmetries and boundary conditions. 

In AdS black holes, choosing the extended phase space—treating the cosmological constant as a thermodynamic variable—proves particularly natural. This choice reveals phase transitions analogous to Van der Waals fluids, with pressure-volume diagrams exhibiting critical points. The topological charges of these critical points sum to a conserved total determined by the manifold’s Euler characteristic. 

Quantum corrections introduce additional subtlety. As quantum effects modify thermodynamic potentials, critical points can appear, disappear, or merge. Each such event represents a topology-changing transition. Tracking these changes as quantum corrections increase provides a window into how quantum gravity reorganizes phase structure. 

The physical interpretation suggests that topological charges count something fundamental—perhaps distinct classes of microstates, or different ways the horizon can be organized. The conservation of total topological charge reflects deep consistency requirements of quantum gravity. 

Jacobsen: What phenomenologically unifies linear dilation, Newman-Unti-Tamburino, and Bardeen geometries? 

Sakallı: These three solutions—linear dilaton black holes from string theory, Newman-Unti-Tamburino spacetimes with gravitomagnetic charge, and Bardeen’s regular black holes—appear quite different at first glance. Yet they share profound phenomenological connections revealing general principles about black holes beyond Kerr-Newman. 

All three geometries possess additional structure beyond mass, charge, and angular momen tum—often called ”hair” in violation of the classical no-hair theorems. Linear dilaton solutions carry a scalar field that varies logarithmically with radius. NUT solutions possess gravitomag netic monopole charge, a topological feature without Newtonian analog. Bardeen black holes have magnetic charge manifesting as a regular core replacing the central singularity. This hair isn’t arbitrary; it emerges from extending general relativity or including additional fields. 

Their asymptotic structures differ from standard asymptotically flat spacetime. Linear dilaton spacetimes have a diverging scalar field at infinity. NUT geometries possess the Misner string—a pathological line at spatial infinity requiring careful boundary conditions. Bardeen solutions have modified falloff due to distributed magnetic charge. All require generalized asymptotic conditions, revealing that ”asymptotic flatness” is more subtle than introductory relativity suggests. 

Thermodynamically, all three exhibit modified Hawking temperature and entropy. The modifications follow a universal pattern: the standard Schwarzschild results get multiplied by functions of the hair parameter. The temperature of a dilaton black hole includes an exponential factor involving the dilaton field. NUT temperature includes corrections from gravitomagnetic charge. Bardeen temperature depends on the magnetic charge parameter. The entropy likewise deviates from the simple area formula, with corrections encoding the additional degrees of freedom associated with hair. 

These entropy modifications respect a generalized area law where an effective area—incorporating hair contributions—still determines entropy. This suggests the holographic principle, relating bulk information to boundary area, holds in generalized form. The additional fields contribute to the effective boundary. 

Regarding singularity structure, all three modify the standard Schwarzschild singularity differently. Linear dilaton solutions have singularities shielded or altered by the scalar field. NUT geometries have intricate singularity structure involving closed timelike curves requiring careful causal analysis. Bardeen solutions eliminate singularities entirely, replacing them with regular de Sitter-like cores. These varied approaches to singularity avoidance suggest multiple paths toward resolving gravitational singularities may exist in quantum gravity. 

Their geodesic structures—paths of freely falling particles and light rays—show similar modifications. Photon spheres where light can orbit shift from the standard Schwarzschild radius. Innermost stable circular orbits for massive particles shift comparably. Perihelion precession of nearly-circular orbits acquires additional contributions. These modifications, though arising from different physics, follow comparable patterns mathematically. 

All three connect to electromagnetic duality in intriguing ways. Bardeen emerges from nonlinear electrodynamics—generalizations of Maxwell’s equations. NUT charge represents the gravitational analog of magnetic monopoles. Dilaton couplings modify electromagnetic propagation. This hints at deep unity between electromagnetic and gravitational phenomena at fundamental levels. 

Symmetry-wise, each possesses enhanced symmetry algebras beyond standard Poincare invariance. Dilaton solutions have shift symmetries of the scalar field. NUT geometries have dual rotations mixing time and azimuthal angle. Bardeen solutions have scaling symmetries. These additional symmetries generate conserved charges beyond standard Komar mass and angular momentum. 

The unifying framework is an extended action including Einstein gravity plus matter fields or higher-derivative corrections. Different choices of potentials, coupling functions, and field content yield the three geometries. This suggests a landscape of black hole solutions, with Kerr Newman representing one island and dilaton, NUT, and Bardeen representing others. Mapping this landscape and understanding transitions between regions remains an active research area. 

Observationally, all three predict similar phenomenology: modified shadows compared to Schwarzschild, altered gravitational wave signals from inspiraling particles, changed accretion disk emission due to shifted ISCOs. Current constraints from EHT and LIGO place weak bounds on hair parame ters—typically limiting deviations to tens of percent of the black hole mass. Future observations will tighten these constraints or potentially reveal hair, revolutionizing our understanding of black hole uniqueness. 

Jacobsen: What are the comparative pitfalls of higher-order WKB versus neural networks for extracting Quasinormal Modes? 

Sakallı: Quasinormal modes—the characteristic oscillation frequencies of perturbed black holes—encode crucial information about spacetime structure. Computing them requires solving differential equations that generally lack closed-form solutions. Two modern approaches dominate: higher order WKB methods and neural network techniques. Each has strengths and weaknesses that researchers must navigate carefully. 

The WKB method, named for Wentzel, Kramers, and Brillouin, treats wave propagation in slowly varying media using semiclassical approximation. For QNM calculations, we apply WKB to the radial equation governing perturbations. Standard WKB gives zeroth-order frequencies; higher-order corrections systematically improve accuracy. Modern implementations extend to sixth or even thirteenth order, achieving extraordinary precision. 

The primary pitfall of higher-order WKB is its convergence properties. For highly damped modes—those decaying very rapidly—the WKB series can converge slowly or even diverge. The approximation assumes the potential varies slowly compared to wavelength, but highly damped modes have such short effective wavelengths that this breaks down. Researchers must carefully assess whether their WKB order is sufficient for target accuracy. 

Coordinate dependence presents another subtlety. WKB results technically depend on coordi nate choice used to define the radial coordinate. Different gauges—Schwarzschild, Eddington Finkelstein, Painlev´e-Gullstrand—yield formally different WKB expressions. For lower-order calculations, these differences can affect precision, though they vanish in principle for infinite order WKB. Practitioners must verify coordinate independence numerically. 

Boundary conditions require careful implementation. QNMs demand purely ingoing waves at the horizon and purely outgoing waves at infinity. Imposing these conditions in WKB involves matching solutions across turning points using connection formulas. Errors in these formulas propagate through higher orders, potentially degrading accuracy despite increased computational effort. 

Neural networks offer a radically different approach. We train networks to learn the relationship between black hole parameters and QNM frequencies using examples. Once trained, the network evaluates new cases nearly instantaneously—far faster than iterative WKB calculations. 

The major pitfall of neural networks is training data requirements. Networks learn from examples, so we need extensive, accurate training sets. Generating these sets typically requires running many WKB or numerical integration calculations anyway—potentially more work than just computing the specific cases we ultimately want. This ”cold start” problem limits neural network utility for truly novel black hole solutions. 

Extrapolation reliability poses another challenge. Neural networks interpolate well within their training domain but extrapolate poorly outside it. If we train on nearly-Schwarzschild black holes then apply the network to highly modified gravity, predictions may be wildly inaccurate without warning. Unlike WKB, which breaks down detectably when approximations fail, neural networks can confidently output nonsensical results for out-of-distribution inputs. 

Interpretability is limited. WKB calculations reveal how specific features of the spacetime potential affect QNM frequencies—physicists gain intuition about why certain modifications shift modes in particular directions. Neural networks are black boxes; they predict accurately but offer little insight into underlying physics. For research aiming to understand relationships between geometry and modes, this opacity is problematic. 

Overfitting is an ever-present danger. Networks can memorize training data rather than learning underlying patterns, performing excellently on training sets but poorly on test cases. Preventing overfitting requires careful regularization, cross-validation, and architecture choices—requiring expertise in machine learning beyond typical physicist training. 

Combining approaches offers promising paths forward. Use WKB to generate training data for neural networks, then deploy networks for fast exploration of parameter space. Use networks to identify interesting regimes, then apply higher-order WKB for rigorous verification. This hybrid strategy leverages each method’s strengths while mitigating weaknesses. 

For modified gravity theories, another consideration arises: equation complexity. WKB handles analytically tractable potentials well; extremely complicated potentials—common in higher derivative gravity—challenge even high-order WKB. Neural networks don’t care about analytical complexity; they work equally well for simple and complicated systems. This makes them attractive for theories where WKB applicability is questionable. 

Question 17: Where does the Hamilton-Jacobi tunneling method break down for Hawking radiation derivations? 

The Hamilton-Jacobi method provides an elegant semiclassical approach to deriving Hawking radiation, treating particle creation as quantum tunneling through the black hole horizon. A particle-antiparticle pair forms just inside the horizon; if the negative-energy antiparticle tunnels inward while the positive-energy particle escapes, the black hole loses mass and radiates. This picture, while intuitive, has important limitations that researchers must appreciate. 

The method works beautifully for static, spherically symmetric black holes—Schwarzschild being the canonical example. We write the particle action, apply Hamilton-Jacobi formalism to find classical trajectories, then quantize by imposing quantum penetration factors. The resulting thermal spectrum matches Hawking’s original quantum field theory calculation in curved spacetime. This concordance validates the tunneling picture as a useful computational tool. 

However, the method’s breakdown begins with rotating black holes. Kerr geometry allows particles with different angular momenta to tunnel with different probabilities. The straightforward Hamilton-Jacobi approach, treating radial motion independently, misses angular momentum correlations between particle-antiparticle pairs. More sophisticated treatments incorporating angular dependence become vastly more complicated, and it’s unclear whether they capture all relevant physics. 

For charged black holes, electromagnetic interactions introduce additional subtlety. The tunneling particle interacts with the black hole’s electric field during tunneling. Standard Hamilton Jacobi treats this as background, but quantum fluctuations of the electromagnetic field itself should contribute. Including these backreactions requires going beyond semiclassical approximation to full quantum electrodynamics in curved spacetime—precisely what Hamilton-Jacobi aims to avoid. 

Near extremality—when black holes approach maximum rotation for their mass or maximum charge—the surface gravity (related to temperature) approaches zero. Hamilton-Jacobi predicts vanishing emission, consistent with zero temperature. But quantum corrections become increasingly important as extremality approaches, and the semiclassical picture breaks down entirely. Some calculations suggest extremal black holes might emit radiation quantum mechanically even with classically zero temperature. Hamilton-Jacobi cannot capture this. 

Backreaction poses a fundamental limitation. As the black hole emits radiation, it loses mass, the horizon shrinks, and the geometry changes. Hamilton-Jacobi treats geometry as fixed—a probe particle tunneling through static spacetime. This is justified for large black holes emitting negligibly few particles, but becomes untenable for small black holes or long timescales. The method cannot self-consistently describe black hole evaporation from formation to complete disappearance. 

Information paradox considerations highlight deeper issues. Hamilton-Jacobi treats emission as random, independent particles—a thermal, maximum-entropy process. But Hawking radiation must ultimately carry information about the black hole’s formation to preserve quantum unitarity. This information transfer requires correlations between emitted particles across vast times and distances, impossible to capture in a local tunneling picture. 

Greybody factors—modifications to the spectrum from backscattering of radiation by the curved geometry outside the horizon—require separate calculation beyond Hamilton-Jacobi. The tunneling method gives emission at the horizon, but observable radiation reaching distant detectors differs due to scattering. Computing these factors demands solving wave equations in the full geometry, reintroducing much of the complexity Hamilton-Jacobi was meant to avoid. 

Trans-Planckian physics presents another concern. The tunneling picture involves wavelengths much smaller than the horizon size, potentially reaching Planck scales for high-frequency modes. At these scales, quantum gravity effects become important, and the classical geometry description underlying Hamilton-Jacobi becomes questionable. The method implicitly assumes space time remains classical arbitrarily close to the horizon—an assumption quantum gravity will likely violate. 

For analog systems—condensed matter systems mimicking black hole physics—the Hamilton Jacobi method’s applicability is even more limited. These systems have atomic discreteness, finite dispersion relations, and other features absent in general relativity. While they can exhibit Hawking-like radiation, the tunneling interpretation becomes strained when applied to systems without genuine event horizons. 

Despite these limitations, the Hamilton-Jacobi method remains valuable pedagogically and computationally. It provides intuitive pictures for Hawking radiation, yields correct temperature and spectrum for simple cases, and extends straightforwardly to many modified gravity theories. Recognizing its limitations helps researchers know when to trust its predictions and when more sophisticated approaches are needed. 

Question 18: What specific falsifiable predictions would curb post-hoc parameter tuning in quantum gravity phenomenology? 

Quantum gravity phenomenology faces a credibility challenge: theories often have enough free parameters that they can be adjusted post-hoc to match any observation. This parameter tuning prevents genuine falsification—if a prediction fails, we tweak parameters rather than abandoning the theory. Establishing specific, falsifiable predictions immune to such tuning is crucial for making progress. 

The gold standard would be parameter-free predictions—relationships between observables that don’t depend on unknown quantum gravity parameters. For instance, if a theory predicts a specific numerical relationship between the black hole shadow radius and quasinormal mode frequency, both measurable independently, this can be tested without tuning. Few theories make such definitive predictions, but seeking them should be a priority. 

Universal relations provide another approach. Even if individual quantities depend on parameters, ratios or combinations might be parameter-independent. The KSS bound on viscosity-to entropy ratio exemplifies this: it predicts a specific numerical value regardless of system details. Discovering similar universal relations involving multiple observables would provide robust tests. 

Null tests—observations specifically designed to yield zero if general relativity is correct and nonzero for alternatives—offer powerful falsification opportunities. Testing whether gravitational waves and light from the same event arrive simultaneously constrains graviton mass without needing to know its value a priori. A non-zero time delay would falsify massless gravitons regardless of other parameters. 

Multiple independent observations of the same system provide consistency checks resistant to tuning. If we measure a black hole’s mass from gravitational waves, from its shadow size, from orbital dynamics of nearby stars, and from X-ray spectroscopy, all four measurements should agree. Quantum gravity corrections affect these differently; consistent corrections across all channels constrain parameters far more tightly than single observations. 

Population-level predictions avoid tuning for individual objects. Rather than fitting parameters separately for each black hole, we predict how populations should distribute. For instance, quantum gravity might predict correlations between mass and spin in black hole populations, or specific cutoffs in the mass distribution. Such statistical predictions can’t be easily tuned away by adjusting parameters post-hoc. 

Time-dependent predictions are especially robust. If quantum gravity corrections grow with time—as some models suggest—long-baseline observations should reveal secular trends. Bi nary pulsar timing, monitored for decades, provides such long-baseline data. Any deviations accumulating systematically over forty years constrain time-dependent effects powerfully. 

Coincidence predictions—multiple effects occurring simultaneously at specific conditions—resist tuning. For example, if a theory predicts that at some critical black hole spin, both the shadow shape and the QNM spectrum exhibit specific anomalies, observing one without the other would falsify it despite parameter freedom. 

Selection rules—categorical predictions that certain processes cannot occur—provide binary tests. If quantum gravity forbids specific decay channels or transition types, observing them falsifies the theory definitively. No parameter tuning can resurrect a theory after an iron-clad selection rule is violated. 

For practical implementation, the community needs coordinated predictions across modified gravity theories. Rather than each theory making custom predictions incomparable to others, we should identify key observables and have each theory make specific predictions for them. This matrix of predictions versus theories would reveal which observations most powerfully discriminate alternatives. 

Multi-messenger observations—combining electromagnetic, gravitational wave, and potentially neutrino observations of the same event—provide cross-checks limiting tuning. The more independent channels we observe, the more constrained parameters become. Future observations of black hole mergers with electromagnetic counterparts will be particularly valuable. 

Blinding analysis protocols, borrowed from particle physics, could help. Observers can provide data with key features hidden until theorists commit to predictions, preventing unconscious bias toward expected results. This procedural protection complements structural protections against parameter tuning. 

Discussion

This interview presents black-hole physics as a domain in which thermodynamics, geometry, topology, and phenomenology are no longer separable intellectual provinces. The KSS bound, thermodynamic topology, non-standard black-hole geometries, quasinormal-mode extraction, Hawking-radiation derivations, and quantum-gravity testing all converge on a common question: how should one distinguish elegant mathematical possibility from physically robust structure?

A recurring theme is controlled modification rather than unrestricted breakdown. Thermal fluctuations do not abolish the viscosity bound but require more careful interpretation of ensemble-averaged quantities. Quantum corrections do not erase phase structure but reorganize it. Non-Kerr geometries do not abandon black-hole thermodynamics but generalize it. Computational methods do not remove analytic judgment but redistribute it between approximation schemes and data-driven inference. At the phenomenological level, the strongest demand is methodological discipline: predictions must be framed so that failure bites. In that sense, the conversation is as much about standards of inference as it is about black holes.

Methods

The interview was conducted via typed questions—with explicit consent—for review, and curation. This process complied with applicable data protection laws, including the California Consumer Privacy Act (CCPA), Canada’s Personal Information Protection and Electronic Documents Act (PIPEDA), and Europe’s General Data Protection Regulation (GDPR), i.e., recordings if any were stored securely, retained only as needed, and deleted upon request, as well in accordance with Federal Trade Commission (FTC) and Advertising Standards Canada guidelines.

Data Availability

No datasets were generated or analyzed during the current article. All interview content remains the intellectual property of the interviewer and interviewee.

References

This interview was conducted as part of a broader quantum cosmology book project. The responses reflect my current research perspective as of October 2025, informed by over 180 publications and ongoing collaborations with researchers worldwide. 

Selected References 

General Background and Foundational Works 

Hawking, S.W. (1974). Black hole explosions? Nature, 248(5443), 30-31. Bekenstein, J.D. (1973). Black holes and entropy. Physical Review D, 7(8), 2333. 

Bardeen, J.M., Carter, B., & Hawking, S.W. (1973). The four laws of black hole mechanics. Communications in Mathematical Physics, 31(2), 161-170. 

Modified Gravity and Quantum Corrections 

Sakallı, ˙I., & Sucu, E. (2025). Quantum tunneling and Aschenbach effect in nonlinear Einstein Power-Yang-Mills AdS black holes. Chinese Physics C, 49, 105101. 

Sakallı, ˙I., Sucu, E., & Dengiz, S. (2025). Quantum-Corrected Thermodynamics of Conformal Weyl Gravity Black Holes: GUP Effects and Phase Transitions. arXiv preprint 2508.00203. 

Al-Badawi, A., Ahmed, F., & Sakallı, ˙I. (2025). A Black Hole Solution in Kalb-Ramond Gravity with Quintessence Field: From Geodesic Dynamics to Thermal Criticality. arXiv preprint 2508.16693. 

Sakallı, ˙I., Sucu, E., & Sert, O. (2025). Quantum-corrected thermodynamics and plasma lensing in non-minimally coupled symmetric teleparallel black holes. Physical Review D, 50, 102063. 

Ahmed, F., Al-Badawi, A., & Sakallı, ˙I. (2025). Perturbations and Greybody Factors of AdS Black Holes with a Cloud of Strings Surrounded by Quintessence-like Field in NLED Scenario. arXiv preprint 2510.19862. 

Black Hole Thermodynamics and Phase Transitions 

Gashti, S.N., Sakallı, ˙I., & Pourhassan, B. (2025). Thermodynamic scalar curvature and topo logical classification in accelerating charged AdS black holes under rainbow gravity. Physics of the Dark Universe, 50, 102136. 

Sucu, E., & Sakallı, ˙I. (2025). AdS black holes in Einstein-Kalb-Ramond gravity: Quantum 26 corrections, phase transitions, and orbital dynamics. Nuclear Physics B, 1018, 117081. 

Sakallı, ˙I., Sucu, E., & Dengiz, S. (2025). Weak gravity conjecture in ModMax black holes: weak cosmic censorship and photon sphere analysis. European Physics C, 85, 1144. 

Pourhassan, B., & Sakallı, ˙I. (2025). Transport phenomena and KSS bound in quantum corrected AdS black holes. European Physics C, 85(4), 369. 

Gashti, S.N., Sakallı, ˙I., Pourhassan, B., & Baku, K.J. (2025). Thermodynamic topology, photon spheres, and evidence for weak gravity conjecture in charged black holes with perfect fluid within Rastall theory. Physics Letters B, 869, 139862. 

Quasinormal Modes and Spectroscopy 

Gashti, S.N., Afshar, M.A., Sakallı, ˙I., & Mazandaran, U. (2025). Weak gravity conjecture in ModMax black holes: weak cosmic censorship and photon sphere analysis. arXiv preprint 2504.11939. 

Sakallı, ˙I., & Kanzi, S. (2023). Superradiant (In)stability, Greybody Radiation, and Quasi normal Modes of Rotating Black Holes in Non-Linear Maxwell f(R) Gravity. Symmetry, 15, 873. 

Observational Tests and Gravitational Lensing 

Sucu, E., & Sakallı, ˙I. (2025). Probing Starobinsky-Bel-Robinson gravity: Gravitational lensing, thermodynamics, and orbital dynamics. Nuclear Physics B, 1018, 116982. 

Mangut, M., G¨ursel, H., & Sakallı, ˙I. (2025). Lorentz-symmetry violation in charged black-hole thermodynamics and gravitational lensing: effects of the Kalb-Ramond field. Chinese Physics C, 49, 065106. 

Ahmed, F., Al-Badawi, A., & Sakallı, ˙I. (2025). Geodesics Analysis, Perturbations and Deflec tion Angle of Photon Ray in Finslerian Bardeen-Like Black Hole with a GM Surrounded by a Quintessence Field. Annalen der Physik, e2500087. 

Generalized Uncertainty Principle and Quantum Effects 

Ahmed, F., Al-Badawi, A., & Sakallı, ˙I. (2025). Photon Deflection and Magnification in Kalb Ramond Black Holes with Topological String Configurations. arXiv preprint 2507.22673. 

Ahmed, F., & Sakallı, ˙I. (2025). Exploring geodesics, quantum fields and thermodynamics of Schwarzschild-AdS black hole with a global monopole in non-commutative geometry. Nuclear Physics B, 1017, 116951. 

Wormholes and Exotic Compact Objects 

Ahmed, F., Al-Badawi, A., & Sakallı, ˙I. (2025). Gravitational lensing phenomena of Ellis Bronnikov-Morris-Thorne wormhole with global monopole and cosmic string. Physics Letters B, 864, 139448. 

Ahmed, F., & Sakallı, ˙I. (2025). Dunkl black hole with phantom global monopoles: geodesic analysis, thermodynamics and shadow. European Physics C, 85, 660. 

Information Theory and Holography 

Pourhassan, B., Sakallı, ˙I., et al. (2022). Quantum Thermodynamics of an M2-Corrected Reissner-Nordstr¨om Black Hole. EPL, 144, 29001. 

Sakallı, ˙I., & Kanzi, S. (2022). Topical Review: greybody factors and quasinormal modes for black holes in various theories – fingerprints of invisibles. Turkish Journal of Physics, 46, 51-103. 

Modified Theories and String Theory 

Sakallı, İ., & Yörük, E. (2023). Modified Hawking radiation of Schwarzschild-like black hole in bumblebee gravity model. Physics Scripta, 98, 125307.

https://iopscience.iop.org/article/10.1088/1402-4896/ad09a1

Sucu, E., & Sakallı, ˙I. (2023). GUP-reinforced Hawking radiation in rotating linear dilaton black hole spacetime. Physics Scripta, 98, 105201. 

Event Horizon Telescope and Observational Cosmology 

Event Horizon Telescope Collaboration (2019). First M87 Event Horizon Telescope Results. I. The Shadow of the Supermassive Black Hole. The Astrophysical Journal Letters, 875(1), L1. 

Abbott, B.P., et al. (LIGO Scientific Collaboration and Virgo Collaboration) (2016). Observa tion of Gravitational Waves from a Binary Black Hole Merger. Physical Review Letters, 116(6), 061102. 

Recent Collaborative Works 

Tangphati, T., Sakallı, ˙I., Banerjee, A., & Pradhan, A. (2024). Behaviors of quark stars in the Rainbow Gravity framework. Physical Review D, 46, 101610. 

Banerjee, A., Sakallı, ˙I., Pradhan, A., & Dixit, A. (2024). Properties of interacting quark star in light of Rastall gravity. Classical and Quantum Gravity, 42, 025008. 

Sakallı, ˙I., Banerjee, A., Dayanandan, B., & Pradhan, A. (2025). Quark stars in f(R, T) gravity: mass-to-radius profiles and observational data. Chinese Physics C, 49, 015102. 

Gashti, S.N., Sakallı, ˙I., Pourhassan, B., & Baku, K.J. (2024). Thermodynamic topology and phase space analysis of AdS black holes through non-extensive entropy perspectives. European Physics C, 85, 305. 

Al-Badawi, A., & Sakallı, ˙I. (2025). The Static Charged Black Holes with Weyl Corrections. International Journal of Theoretical Physics, 64, 50. 

Textbooks and Reviews 

Carroll, S.M. (2004). Spacetime and Geometry: An Introduction to General Relativity. Addison Wesley. 

Wald, R.M. (1984). General Relativity. University of Chicago Press. 

Rovelli, C. (2004). Quantum Gravity. Cambridge University Press. 

Kiefer, C. (2012). Quantum Gravity (3rd ed.). Oxford University Press. Ashtekar, A., & Petkov, V. (Eds.) (2014). Springer Handbook of Spacetime. Springer. 

Note: This reference list includes representative works from Prof. Sakallı’s extensive publication record (181+ papers) and foundational works in the field. For a complete bibliography, please consult the INSPIRE-HEP database or Prof. Sakallı’s institutional profile.

Journal & Article Details

Publisher: In-Sight Publishing

Publisher Founding: March 1, 2014

Web Domain: http://www.in-sightpublishing.com

Location: Fort Langley, Township of Langley, British Columbia, Canada

Journal: In-Sight: Interviews

Journal Founding: August 2, 2012

Frequency: Four Times Per Year

Review Status: Non-Peer-Reviewed

Access: Electronic/Digital & Open Access

Fees: None (Free)

Volume Numbering: 14

Issue Numbering: 1

Section: A

Theme Type: Discipline

Theme Premise: Quantum Cosmology

Formal Sub-Theme: None.

Individual Publication Date: March 22, 2026

Issue Publication Date: April 1, 2026

Author(s): Scott Douglas Jacobsen

Word Count: 3,747

Image Credits: Izzet Sakallı

ISSN (International Standard Serial Number): 2369-6885

Acknowledgements

The author acknowledges Prof. Dr. İzzet Sakallı for his time, expertise, and valuable contributions. His thoughtful insights and detailed explanations have greatly enhanced the quality and depth of this work, providing a solid foundation for the discussion presented herein.

Author Contributions

S.D.J. conceived the subject matter, conducted the interview, transcribed and edited the conversation, and prepared the manuscript.

Competing Interests

The author declares no competing interests.

License & Copyright

In-Sight Publishing by Scott Douglas Jacobsen is licensed under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License.
© Scott Douglas Jacobsen and In-Sight Publishing 2012–Present.

Unauthorized use or duplication of material without express permission from Scott Douglas Jacobsen is strictly prohibited. Excerpts and links must use full credit to Scott Douglas Jacobsen and In-Sight Publishing with direction to the original content.

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Causality, Ensemble Invariance, and Black-Hole Perturbations: Prof. Dr. ˙Izzet Sakallı on the KSS Bound, Thermodynamic Topology, and Quasinormal-Mode Inference (3)

Scott Douglas Jacobsen (Email: [email protected])

Publisher, In-Sight Publishing

Fort Langley, British Columbia, Canada

Received: October 25, 2025

Accepted: January 8, 2026

Published: February 22, 2026

Abstract

This interview excerpt with Prof. Dr. ˙Izzet Sakallı synthesizes several live problems at the interface of black-hole physics, holography, thermodynamics, and computational method. First, it addresses whether thermal fluctuations can undermine the Kovtun–Son–Starinets (KSS) bound on the shear-viscosity-to-entropy-density ratio in quantum-corrected AdS black holes. Sakallı argues that the bound is formulated for ensemble-averaged thermodynamic quantities rather than instantaneous microstates, and that causality constraints and fluctuation–dissipation structure prevent genuine violations even when entropy and horizon data fluctuate. Quantum corrections are presented as shifting the numerical value of the bound in controlled ways rather than eliminating the existence of a lower bound. Second, the excerpt proposes an operational, coordinate-independent notion of “thermodynamic topology,” treating equilibrium state space as a manifold equipped with geometric structures (metrics, curvature, and topological invariants such as winding numbers) that classify critical points and phase transitions, with ensemble changes understood via Legendre transformations on a broader thermodynamic phase space. Third, Sakallı outlines a phenomenological unification of linear dilaton, Newman–Unti–Tamburino, and Bardeen geometries, emphasizing their shared “hair,” nonstandard asymptotics, modified thermodynamics, and comparable signatures in geodesic structure and observables such as shadows and inspiral waveforms. Finally, the excerpt compares higher-order WKB approaches and neural networks for extracting quasinormal modes, identifying the main methodological pitfalls—convergence, coordinate and boundary-condition sensitivity versus data dependence, extrapolation risk, and interpretability—and motivating hybrid strategies that combine physical transparency with computational speed.

Keywords

AdS/CFT, Asymptotics, Bardeen black holes, Boundary conditions, Black hole hair, Black hole shadows, Causality constraints, Compactification, Coordinate independence, Curvature (thermodynamic), Dilaton fields, Ensemble averaging, Euler characteristic, Extended phase space, Extrapolation risk, Fluctuation–dissipation theorem, Gauss–Bonnet gravity, Geodesics, Gibbs free energy, Grand canonical ensemble, Gravitational waves, Heavy-ion collisions, Helmholtz free energy, Higher-derivative corrections, Holography, Interpretability, ISCO (innermost stable circular orbit), KSS bound, Legendre transformations, Linear dilaton black holes, LIGO constraints, Machine learning, Microcanonical ensemble, Misner string, Neural networks, Newman–Unti–Tamburino (NUT) spacetime, Nonlinear electrodynamics, Overfitting, Phase transitions, Photon sphere, Pietist discipline, Quark–gluon plasma, Quasinormal modes, Quantum corrections, Shear viscosity, Symplectic structure, Thermal fluctuations, Thermodynamic manifold, Thermodynamic metric, Thermodynamic topology, Topological charge, Transport coefficients, Van der Waals analogy, Viscosity-to-entropy ratio, Winding number, WKB approximation

Introduction

Modern black-hole physics is no longer a single subject so much as a crowded intersection: holography links gravity to strongly coupled fluids; extended thermodynamics reframes spacetime constants as state variables; and observational astronomy increasingly constrains “beyond Kerr–Newman” possibilities. In this excerpt, Prof. Dr. ˙Izzet Sakallı engages several of these frontiers through a unifying methodological concern: which statements are invariant, operationally meaningful, and robust under corrections—whether those corrections are quantum, thermal, coordinate-based, or computational.

The discussion begins with the Kovtun–Son–Starinets bound, a conjectured lower limit on the ratio of shear viscosity to entropy density that emerges naturally from AdS/CFT calculations. Sakallı addresses a common worry in quantum-corrected AdS black holes: if entropy and horizon properties fluctuate, could momentary configurations appear to violate the bound? He frames the bound as a statement about coarse-grained thermodynamic quantities, protected by causality and by the structured relation between fluctuations and dissipation, and he argues that quantum corrections typically deform the bound’s value rather than abolish the existence of a bound.

The excerpt then shifts from bounds to classification, proposing an operational definition of thermodynamic topology that is explicitly coordinate-independent. Equilibrium state space is treated as a manifold where thermodynamic potentials define fields, critical points behave as topological defects, and invariants such as winding numbers provide a stable language for phase transitions. Because ensemble choice changes which variables are held fixed, Sakallı emphasizes a geometric formulation in which Legendre transformations are understood as coordinate changes within a broader phase space equipped with ensemble-independent structure, allowing invariants to remain well-posed across descriptions.

From there, the interview broadens to a phenomenological comparison of three nonstandard black-hole families—linear dilaton, NUT, and Bardeen—highlighting the shared presence of additional “hair,” altered asymptotics, and modified thermodynamics and geodesics, with implications for shadows, accretion signatures, and gravitational-wave observables. The excerpt closes with a methodological contrast between higher-order WKB techniques and neural networks for computing quasinormal modes, weighing physical interpretability and controlled approximation against speed, data dependence, and extrapolation risk, and motivating hybrid strategies that preserve both exploration and rigor.

Main Text (Interview)

Title: Causality, Ensemble Invariance, and Black-Hole Perturbations: Prof. Dr. ˙Izzet Sakallı on the KSS Bound, Thermodynamic Topology, and Quasinormal-Mode Inference (3)

Interviewer: Scott Douglas Jacobsen

Interviewees: Prof. Dr. İzzet Sakallı

Professor Izzet Sakallı is a theoretical physicist at Eastern Mediterranean University whose research bridges quantum mechanics, general relativity, and observational astronomy. With over 180 publications exploring black hole thermodynamics, modified gravity theories, and quantum corrections to spacetime, his work sits at the exciting frontier where abstract mathematics meets observable reality. In this interview, he discusses the challenges of testing exotic gravity theories, the quest to observe quantum effects in astrophysical systems, and what the next generation of telescopes and gravitational wave detectors might reveal about the quantum nature of spacetime. 

Scott Douglas Jacobsen: Which shadow features best test Lorentz-violating backgrounds once plasma and accretion-flow systematics are in cluded? 

Professor Izzet Sakallı: The Event Horizon Telescope’s stunning images of M87* and Sagittarius A* opened an unprece dented window for testing fundamental physics. These images show a bright ring of emission surrounding a dark shadow—the gravitational silhouette of the black hole. But extracting con straints on exotic physics requires carefully separating genuine signatures of modified spacetime from astrophysical contamination. 

Lorentz invariance—the principle that physical laws don’t depend on your orientation or ve locity—is a cornerstone of both special and general relativity. Various approaches to quantum gravity predict potential violations at high energies or in strong gravitational fields. These violations would modify how photons propagate, potentially changing the observed shadow. 

The challenge is that accretion disks—the hot, ionized material spiraling into black holes—are messy astrophysical environments. The plasma causes Faraday rotation of polarized light, in troduces frequency-dependent delays through dispersion, absorbs some wavelengths while trans mitting others, and Doppler shifts due to its orbital motion. All these effects must be modeled and subtracted to reveal underlying spacetime properties. 

The most robust tests use signatures that depend differently on fundamental physics versus astrophysical processes. Multi-wavelength shadow measurements provide one such discriminator. If Lorentz violation affects photon propagation, the shadow size should vary with observing frequency in a specific pattern—different from how plasma effects scale. Observing at 230 GHz, 345 GHz, and potentially infrared wavelengths with the next-generation Event Horizon Telescope would allow separating these effects. 

The photon ring—actually a series of nested subrings from photons completing multiple orbits before escaping—offers another powerful probe. The time delay between successive subrings de pends on geodesic motion in the spacetime geometry, largely insensitive to accretion flow details since it’s a geometric effect. Lorentz violation would modify these time delays in characteristic ways. 

Polarization patterns provide additional leverage. While plasma Faraday rotation certainly af fects polarization, Lorentz violation can break azimuthal symmetry in distinctive ways. Decom posing the polarization pattern into angular components helps separate astrophysical rotation (which affects all components similarly) from symmetry-breaking physics. 

Time variability studies offer yet another approach. Monitoring shadow features over days to months separates quasi-periodic variability from orbital motion, flares and turbulence in the accretion flow, and any secular trends from fundamental physics. If the shadow properties slowly drift in ways inconsistent with astrophysical explanations, that might signal Lorentz violation. 

Current EHT observations constrain deviations from general relativity at the ten to twenty percent level. This translates to saying that if Lorentz violation exists, its characteristic energy scale must be at least one or two percent of the Planck energy—still far below the Planck scale but better constraints than many other tests achieve. 

Future improvements will come from multiple directions. The next-generation EHT adds more telescopes and space-based stations, increasing angular resolution. Higher observing frequencies probe smaller scales. Broader bandwidth improves sensitivity. Perhaps most importantly, coordinated multi-messenger observations—combining radio interferometry with X-ray timing, optical monitoring, and potentially gravitational waves if the black hole is in a binary—would provide complementary information less susceptible to systematic uncertainties. 

Jacobsen: What are the limits of the Gauss-Bonnet approach for weak deflection in spacetimes with torsion? 

Sakallı: Einstein-dilaton-Gauss-Bonnet gravity represents an important modification of general relativ ity emerging from string theory. The Gauss-Bonnet term involves products of curvature quan tities in a specific combination that’s topologically interesting. In standard four-dimensional spacetime, this term is actually a topological invariant—it doesn’t affect the equations of motion by itself. But when coupled to a dilaton field (a scalar field from string theory), it produces genuine modifications to gravity. 

These modifications become significant in strong-field regions near black holes or in the early universe. For weak gravitational fields—like sunlight grazing the Sun—the corrections are typically small but calculable. We can expand solutions perturbatively, treating the Gauss Bonnet term as a small correction to general relativity, and calculate how it affects phenomena like light deflection. However, this approach encounters fundamental difficulties when spacetime possesses torsion.

Torsion represents a twisting of spacetime, a type of geometric structure absent in general relativity but present in some extended theories. Einstein-Cartan theory, for instance, allows spacetime to have both curvature and torsion, with torsion sourced by intrinsic spin of matter. 

The difficulty is that the Gauss-Bonnet term is defined for Riemannian geometry—geometry with curvature but no torsion. Generalizing it to spaces with torsion isn’t unique. Multiple inequivalent extensions exist: the Nieh-Yan form, modifications involving the Pontrjagin term, generalizations of the Holst term. These different extensions give different physics, and there’s no obvious principle selecting one over others. 

Furthermore, torsion couples to matter fields differently than curvature. For photons—massless spin-one particles—the interaction with torsion is subtle. Traditional geodesic equations must be reconsidered. The very notion of ”geodesic” becomes ambiguous because the connection determining parallel transport includes torsion contributions. 

The weak-field expansion faces technical breakdowns in torsion backgrounds. We typically expand the metric as small perturbations around flat spacetime. But when both curvature and torsion are present, the perturbative hierarchy becomes unclear. If torsion is as large as curvature perturbations, our expansion scheme breaks down. 

There are also boundary term subtleties. In formulating gravitational theories variationally, boundary terms matter for defining conserved quantities and obtaining correct equations of motion. The Gibbons-Hawking-York boundary term supplements the Einstein-Hilbert action in general relativity. With torsion, additional boundary terms are needed, and they affect asymptotic quantities crucial for calculating observable effects like deflection angles. 

Some torsion-Gauss-Bonnet couplings lead to pathologies: faster-than-light propagation, vio lations of causality, or violations of energy conditions guaranteeing stability. These signal the theory’s breakdown rather than providing viable alternatives to general relativity. 

Observationally, constraints on torsion in astrophysical contexts are weak. Particle physics experiments provide stronger bounds from spin-dependent effects. But astrophysical torsion contributions must satisfy stringent limits, roughly one part in a billion billion per centimeter from precision tests. 

For analyzing light deflection in torsion-rich spacetimes, alternative approaches work better than weak-field expansions. Full numerical ray-tracing through exact solutions avoids perturbative ambiguities. The Newman-Penrose formalism, adapted to non-Riemannian geometry, provides another rigorous framework. Modified Shapiro time delays offer complementary probes less sensitive to expansion scheme choices. 

Jacobsen: Which lensing signatures would rule out thin-shell wormholes for compact black-hole mimics? 

Sakallı: Wormholes—hypothetical tunnels connecting distant regions of spacetime—have captivated physicists and science fiction writers for decades. While the Schwarzschild solution allows math ematical wormholes, they’re unstable and not traversable. Maintaining a stable, traversable wormhole requires exotic matter violating energy conditions, specifically matter with negative energy density. 

Thin-shell wormholes represent a particular construction where exotic matter is confined to a narrow region around the wormhole throat, minimizing the total amount of energy-condition violating material needed. From a distance, such objects might mimic black holes—appearing as compact, massive objects with strong gravitational fields. Distinguishing them observationally is crucial, as finding a wormhole would revolutionize physics. 

Several observational signatures could definitively distinguish wormholes from black holes. The most dramatic is gravitational wave echoes. When a gravitational wave rings down after a black hole merger, the signal decays smoothly—the perturbations fall into the singularity. But if the object is a wormhole, perturbations can traverse the throat, reflect off the geometry on the far side, return, and emerge again. This creates periodic echoes in the gravitational wave signal, with the time delay determined by the wormhole’s size and geometry. 

Detecting echoes is challenging. The reflected signal is weaker than the initial ringdown, requir ing high signal-to-noise ratio observations. Current searches in LIGO/Virgo data have found no convincing echoes, placing limits on how common wormholes might be if they exist. Future detectors with better sensitivity, particularly Einstein Telescope and Cosmic Explorer, should significantly improve echo searches. 

The shadow morphology offers another discriminator. Black holes cast circular (or slightly elliptical for rotating ones) shadows. Wormholes can produce more exotic shadow shapes: non simply-connected topology (hole within hole), asymmetric brightness from different light paths traversing the throat, or chromatic effects if the throat has frequency-dependent properties. The Event Horizon Telescope’s resolution is reaching the point where such features might be detectable for nearby supermassive objects. 

Microlensing provides a third signature. When a compact object passes in front of a background star, its gravity acts as a lens, magnifying and distorting the star’s image. Black holes create characteristic magnification curves. Wormholes add complexity: light paths traversing the throat create additional magnification spikes, producing asymmetric light curves distinct from black hole lensing. Large microlensing surveys searching for dark matter or planets could potentially detect such signatures if wormholes exist in our galaxy. 

Time delay distributions between multiple images offer another probe. Gravitational lenses create multiple images of background objects arriving at different times. The distribution of these delays depends on the lens’s mass profile. Wormholes’ different internal geometry would create distinctive delay patterns. 

Future interferometric observations using nulling techniques could enhance faint photon ring structure while suppressing the bright accretion disk. Wormholes’ multiple light traversal paths would create interference patterns distinct from black holes. 

The theoretical challenge is that thin-shell wormholes face severe difficulties. Most solutions are dynamically unstable—perturbations cause them to collapse or expand. The exotic matter requirements are enormous, far exceeding what quantum field theory allows in normal circum stances. No known astrophysical process forms wormholes, unlike black holes which form from stellar collapse. 

Nevertheless, thoroughly testing whether astrophysical compact objects might be horizonless alternatives to black holes is scientifically important. General relativity so strongly predicts black holes that finding something else would indicate either a fundamental flaw in our under standing or exotic physics beyond the standard model. The observational tests described above will systematically close loopholes, either confirming black holes or revealing surprises. 

Jacobsen: Can information-theoretic diagnostics reveal quantum gravity corrections at current observational sensitivities? 

Sakallı: Information theory—the mathematical framework for quantifying information, uncertainty, and correlations—provides a novel lens for exploring quantum gravity. Traditional approaches focus on measuring physical quantities like masses, spins, and frequencies. Information-theoretic approaches ask: how much information do observations carry about the underlying physics? Can we detect quantum gravitational effects through information content rather than direct parameter measurements? 

The holographic principle suggests that information in any region of space is bounded by the region’s surface area rather than volume—as if the three-dimensional world is holographically encoded on a two-dimensional boundary. For black holes, this manifests as the Bekenstein Hawking entropy being proportional to horizon area. Quantum corrections modify this rela tionship, adding logarithmic and higher-order terms in the area. 

One information-theoretic approach analyzes the entropy of black hole shadows. The shadow pattern observed by telescopes carries information about the spacetime geometry. We can quantify this through Shannon entropy: treating the brightness distribution as a probability distribution and computing its entropy. Quantum corrections alter how photons propagate near the horizon, subtly changing the shadow structure and therefore its information content. 

The challenge is scale. For astrophysical black holes, Planck-scale quantum corrections are suppressed by the ratio of the Planck length to the black hole size—roughly 10 to the minus 78th power for a solar-mass black hole. Direct detection of such tiny effects is hopeless with foreseeable technology. 

However, indirect signatures might be more accessible. Rather than looking for tiny shifts in individual measurements, we can examine correlations and information flow. For gravitational wave signals, the mutual information between early inspiral and late ringdown phases quantifies how much information from the initial state survives to the final state. Quantum corrections introduce non-Markovian effects—memory effects where the system’s evolution depends on its history rather than just its current state. These memory effects could create measurable corre lation patterns. 

Fisher information quantifies how much information data carries about physical parameters. For Event Horizon Telescope observations, we can construct the Fisher information matrix describing precision limits on measuring black hole mass, spin, and potential quantum correction parameters. Current analysis suggests achieving roughly 10 to 30 percent precision on dimensionless quantum parameters—not sufficient to detect Planck-scale effects but potentially constraining if quantum gravity involves larger characteristic scales. 

Relative entropy (also called Kullback-Leibler divergence) measures how much observed distributions differ from general relativity predictions. Accumulating this measure across many observations provides a statistical test: are we seeing what general relativity predicts, or are systematic deviations emerging? 

Population statistics offer multiplicative power. While individual gravitational wave detections have limited precision, combining information from hundreds or thousands of events (expected in the next decade) increases statistical power. If quantum corrections produce consistent small biases across all events, population-level analysis might reveal them. 

Cross-correlation approaches combine complementary observations. Measuring a black hole’s mass from gravitational waves and independently from its shadow size provides a consistency test. If quantum corrections affect these measurements differently, inconsistencies would signal new physics even if neither measurement alone shows deviations. 

For primordial black holes—hypothetical small black holes from the early universe—information theoretic signatures might be more accessible. Their much smaller sizes enhance quantum corrections substantially. If primordial black holes exist and their evaporation products are detected, the information content in their emission spectra could constrain quantum gravity. 

Realistic assessment requires acknowledging limitations. Directly detecting Planck-scale quan tum gravity through astrophysical observations probably remains beyond reach. However, information-theoretic methods might reveal emergent quantum phenomena accumulating at astrophysical scales, modified information flow during black hole evaporation, or indirect con straints ruling out large classes of quantum gravity theories. These indirect routes may prove more fruitful than searching for minute corrections to individual measurements. 

Jacobsen: How do non-extensive entropy formalisms reshape Anti-de Sitter black hole phase diagrams? 

Sakallı: Non-extensive statistical mechanics, particularly Tsallis entropy, offers a fascinating general ization of conventional thermodynamics that has profound implications for black hole physics. The standard Boltzmann-Gibbs entropy assumes that systems are extensive—meaning the en tropy of two independent systems simply adds. But for systems with long-range interactions, like gravity, or systems with fractal structure, this additivity breaks down. Tsallis introduced a parameter, typically called q, that quantifies this non-extensivity. When q equals one, we recover standard thermodynamics; deviations from unity signal non-extensive behavior. 

For black holes in Anti-de Sitter spacetime—a universe with negative cosmological constant that curves like a saddle rather than a sphere—thermodynamics becomes particularly rich. AdS black holes can undergo phase transitions remarkably similar to everyday substances transitioning between solid, liquid, and gas phases. The famous Hawking-Page transition, where thermal radiation in empty AdS competes with forming a black hole, parallels water freezing into ice. 

When we replace standard entropy with Tsallis entropy, the phase diagram transforms dra matically. The temperature-entropy relationship changes because temperature, defined as the derivative of energy with respect to entropy, now involves the non-extensivity parameter. For astrophysical black holes with enormous entropy, even small deviations of q from unity can shift transition temperatures substantially. 

The heat capacity—which determines thermodynamic stability—becomes modified in intricate ways. Standard AdS black holes have a critical radius where heat capacity diverges, signaling a phase transition. With Tsallis entropy, this critical point shifts. Depending on whether q exceeds or falls below unity, the transition can occur at smaller or larger radii, or in extreme cases, disappear entirely or split into multiple transitions. 

Perhaps most intriguing is the emergence of reentrant phase transitions—phenomena where increasing temperature causes the system to cycle through the same phase multiple times. Imagine heating ice, which melts to water, but continuing to heat causes it to refreeze, then melt again at even higher temperatures. Such behavior, absent in standard thermodynamics, appears naturally when black hole entropy becomes non-extensive. This suggests the underlying quantum gravity degrees of freedom organizing the horizon might have complex, fractal-like structure. 

The Hawking-Page transition also shifts under non-extensive statistics. This transition repre sents a competition between entropy favoring thermal radiation spread throughout space and energy minimization favoring localized black holes. With modified entropy, the balance point changes. For q greater than one, transitions occur at lower temperatures; for q less than one, higher temperatures are needed. 

Through the AdS/CFT correspondence—the remarkable duality between gravity in AdS space time and quantum field theory on its boundary—modifications to bulk thermodynamics reflect in boundary physics. The confining-deconfining transition in strongly coupled gauge theories, relevant for understanding quark-gluon plasma in heavy-ion collisions, would exhibit modified behavior if the gravitational dual involves non-extensive entropy. 

The physical origin of non-extensivity in black holes remains debated. It might arise from quan tum fluctuations of the horizon, long-range gravitational correlations between horizon degrees of freedom, or fractal microstructure of spacetime near the Planck scale. Observationally, con straining the parameter q from astrophysical black holes is extremely challenging, but laboratory analogs using cold atoms or condensed matter systems might provide testable predictions. 

Jacobsen: What about the interpretation of Hawking-Page transitions under non-extensive entropy? 

Sakallı: The Hawking-Page transition represents one of the most elegant connections between quan tum field theory, gravity, and thermodynamics. In pure AdS spacetime, we can have thermal radiation at some temperature, or we can have a black hole at that temperature. Which con figuration has lower free energy depends on the temperature. At low temperatures, thermal radiation dominates; at high temperatures, black holes are favored. The transition between these phases occurs at a specific critical temperature. 

When entropy becomes non-extensive, this picture enriches considerably. The free energy—energy minus temperature times entropy—depends on how entropy scales. With Tsallis entropy, the relationship between physical temperature and thermodynamic temperature becomes modified by a function of the horizon area. This modifies the free energy comparison between phases. 

The latent heat—energy exchanged during the phase transition—changes dramatically. For standard black holes, the latent heat reflects the entropy jump when thermal radiation condenses into a black hole. With non-extensive entropy, this jump scales differently with black hole size. For large black holes and q greater than unity, the latent heat can become orders of magnitude larger than in standard thermodynamics, suggesting the transition involves reorganizing vastly more microscopic degrees of freedom. 

The order of the transition can even change. Standard Hawking-Page is first-order, with discon tinuous entropy at the transition point, like ice melting to water. But for certain special values of the non-extensivity parameter, the transition can become second-order, with continuous en tropy but divergent heat capacity, like the magnetic transition in iron when heated above its Curie temperature. Between these regimes lie tricritical points where the transition character changes. 

Through AdS/CFT, the Hawking-Page transition corresponds to confinement-deconfinement in the boundary gauge theory. At low temperatures, quarks and gluons are confined into hadrons—thermal AdS. At high temperatures, they form quark-gluon plasma—the black hole phase. Non-extensive modifications suggest the strongly coupled plasma might have non standard statistical properties, potentially observable in heavy-ion collisions at RHIC or LHC.

The interpretation becomes particularly interesting when considering the transition’s dynamical aspects. How quickly does thermal radiation condense into a black hole? How long does the mixed phase persist? Non-extensive statistics introduces memory effects—the system’s evolution depends on its history, not just its current state. This non-Markovian character might leave signatures in gravitational wave observations if black holes form dynamically through such transitions in the early universe. 

Multiple phase transitions can emerge in extended phase space where we vary not just tem perature but also pressure, charge, and angular momentum. The resulting phase diagrams can exhibit multiple critical points, isolated regions of stability, and complex connectivity between phases—far richer than standard thermodynamics allows. 

Discussion

Across its four themes, the excerpt advances a consistent thesis: the most reliable claims in gravitational physics are those formulated in terms of quantities that survive changes of description—ensemble choice, coordinate gauge, or computational representation—without losing physical meaning.

On the KSS bound, Sakallı’s position hinges on separating instantaneous microscopic variability from thermodynamic statements. The bound is treated as an assertion about ensemble-averaged transport and entropy rather than snapshot ratios of fluctuating horizon data. This move is not merely semantic; it aligns the bound with how viscosity and entropy are actually defined in statistical mechanics and hydrodynamics, and it clarifies why fluctuations should not be interpreted as counterexamples. The appeal to causality is also structurally important: in holography, apparent violations are not just “small corrections,” but would imply pathologies (such as superluminal signal propagation) in the dual field theory. The result is a picture in which quantum and higher-derivative effects shift the bound’s numerical value in controlled, model-dependent ways while preserving a lower-bound structure enforced by consistency conditions.

The segment on thermodynamic topology extends this concern for invariance into phase structure. By treating thermodynamic state space as a manifold and critical points as topological defects, the excerpt reframes phase transitions as objects that can be classified by invariants (winding number, topological charge) rather than by coordinate-dependent signatures alone. Ensemble dependence, often a practical nuisance in black-hole thermodynamics, is interpreted geometrically through Legendre transformations on a phase space with ensemble-independent structure. This provides a principled way to say when an invariant is “well-posed”: it must be defined on structures that do not change under reparameterization, and the manifold must be handled globally (including compactification) so that charges and conservation statements are mathematically meaningful.

The unification of linear dilaton, NUT, and Bardeen geometries functions as a case study in how departures from the Kerr–Newman template tend to recur in recognizable families. The shared features—additional hair, generalized asymptotics, modified temperature/entropy relations, and comparable shifts in photon spheres and ISCOs—support an interpretation in which “nonstandard” black holes are not ad hoc curiosities but coordinated outcomes of extended actions and additional fields. The excerpt’s observational remarks underline an important asymmetry: current data often constrain large deviations but still permit moderate hair, making the landscape of alternatives scientifically live rather than purely speculative.

Finally, the comparison of higher-order WKB and neural networks foregrounds a methodological dualism familiar across contemporary physics. WKB is valued for analytic control and interpretability but suffers from convergence and gauge subtleties and demands careful handling of boundary conditions. Neural networks promise speed and broad exploration but import risks that are epistemically different—dependence on training sets, unreliable extrapolation, and limited transparency. Sakallı’s proposed hybrid strategy is therefore not a compromise for its own sake; it is an explicit division of labor between exploration and verification, with physical interpretation and out-of-distribution caution serving as the safeguards that keep fast computation from becoming fast self-deception.

Methods

The interview was conducted via typed questions—with explicit consent—for review, and curation. This process complied with applicable data protection laws, including the California Consumer Privacy Act (CCPA), Canada’s Personal Information Protection and Electronic Documents Act (PIPEDA), and Europe’s General Data Protection Regulation (GDPR), i.e., recordings if any were stored securely, retained only as needed, and deleted upon request, as well in accordance with Federal Trade Commission (FTC) and Advertising Standards Canada guidelines.

Data Availability

No datasets were generated or analyzed during the current article. All interview content remains the intellectual property of the interviewer and interviewee.

References

This interview was conducted as part of a broader quantum cosmology book project. The responses reflect my current research perspective as of October 2025, informed by over 180 publications and ongoing collaborations with researchers worldwide. 

Selected References 

General Background and Foundational Works 

Hawking, S.W. (1974). Black hole explosions? Nature, 248(5443), 30-31. Bekenstein, J.D. (1973). Black holes and entropy. Physical Review D, 7(8), 2333. 

Bardeen, J.M., Carter, B., & Hawking, S.W. (1973). The four laws of black hole mechanics. Communications in Mathematical Physics, 31(2), 161-170. 

Modified Gravity and Quantum Corrections 

Sakallı, ˙I., & Sucu, E. (2025). Quantum tunneling and Aschenbach effect in nonlinear Einstein Power-Yang-Mills AdS black holes. Chinese Physics C, 49, 105101. 

Sakallı, ˙I., Sucu, E., & Dengiz, S. (2025). Quantum-Corrected Thermodynamics of Conformal Weyl Gravity Black Holes: GUP Effects and Phase Transitions. arXiv preprint 2508.00203. 

Al-Badawi, A., Ahmed, F., & Sakallı, ˙I. (2025). A Black Hole Solution in Kalb-Ramond Gravity with Quintessence Field: From Geodesic Dynamics to Thermal Criticality. arXiv preprint 2508.16693. 

Sakallı, ˙I., Sucu, E., & Sert, O. (2025). Quantum-corrected thermodynamics and plasma lensing in non-minimally coupled symmetric teleparallel black holes. Physical Review D, 50, 102063. 

Ahmed, F., Al-Badawi, A., & Sakallı, ˙I. (2025). Perturbations and Greybody Factors of AdS Black Holes with a Cloud of Strings Surrounded by Quintessence-like Field in NLED Scenario. arXiv preprint 2510.19862. 

Black Hole Thermodynamics and Phase Transitions 

Gashti, S.N., Sakallı, ˙I., & Pourhassan, B. (2025). Thermodynamic scalar curvature and topo logical classification in accelerating charged AdS black holes under rainbow gravity. Physics of the Dark Universe, 50, 102136. 

Sucu, E., & Sakallı, ˙I. (2025). AdS black holes in Einstein-Kalb-Ramond gravity: Quantum 26 corrections, phase transitions, and orbital dynamics. Nuclear Physics B, 1018, 117081. 

Sakallı, ˙I., Sucu, E., & Dengiz, S. (2025). Weak gravity conjecture in ModMax black holes: weak cosmic censorship and photon sphere analysis. European Physics C, 85, 1144. 

Pourhassan, B., & Sakallı, ˙I. (2025). Transport phenomena and KSS bound in quantum corrected AdS black holes. European Physics C, 85(4), 369. 

Gashti, S.N., Sakallı, ˙I., Pourhassan, B., & Baku, K.J. (2025). Thermodynamic topology, photon spheres, and evidence for weak gravity conjecture in charged black holes with perfect fluid within Rastall theory. Physics Letters B, 869, 139862. 

Quasinormal Modes and Spectroscopy 

Gashti, S.N., Afshar, M.A., Sakallı, ˙I., & Mazandaran, U. (2025). Weak gravity conjecture in ModMax black holes: weak cosmic censorship and photon sphere analysis. arXiv preprint 2504.11939. 

Sakallı, ˙I., & Kanzi, S. (2023). Superradiant (In)stability, Greybody Radiation, and Quasi normal Modes of Rotating Black Holes in Non-Linear Maxwell f(R) Gravity. Symmetry, 15, 873. 

Observational Tests and Gravitational Lensing 

Sucu, E., & Sakallı, ˙I. (2025). Probing Starobinsky-Bel-Robinson gravity: Gravitational lensing, thermodynamics, and orbital dynamics. Nuclear Physics B, 1018, 116982. 

Mangut, M., G¨ursel, H., & Sakallı, ˙I. (2025). Lorentz-symmetry violation in charged black-hole thermodynamics and gravitational lensing: effects of the Kalb-Ramond field. Chinese Physics C, 49, 065106. 

Ahmed, F., Al-Badawi, A., & Sakallı, ˙I. (2025). Geodesics Analysis, Perturbations and Deflec tion Angle of Photon Ray in Finslerian Bardeen-Like Black Hole with a GM Surrounded by a Quintessence Field. Annalen der Physik, e2500087. 

Generalized Uncertainty Principle and Quantum Effects 

Ahmed, F., Al-Badawi, A., & Sakallı, ˙I. (2025). Photon Deflection and Magnification in Kalb Ramond Black Holes with Topological String Configurations. arXiv preprint 2507.22673. 

Ahmed, F., & Sakallı, ˙I. (2025). Exploring geodesics, quantum fields and thermodynamics of Schwarzschild-AdS black hole with a global monopole in non-commutative geometry. Nuclear Physics B, 1017, 116951. 

Wormholes and Exotic Compact Objects 

Ahmed, F., Al-Badawi, A., & Sakallı, ˙I. (2025). Gravitational lensing phenomena of Ellis Bronnikov-Morris-Thorne wormhole with global monopole and cosmic string. Physics Letters B, 864, 139448. 

Ahmed, F., & Sakallı, ˙I. (2025). Dunkl black hole with phantom global monopoles: geodesic analysis, thermodynamics and shadow. European Physics C, 85, 660. 

Information Theory and Holography 

Pourhassan, B., Sakallı, ˙I., et al. (2022). Quantum Thermodynamics of an M2-Corrected Reissner-Nordstr¨om Black Hole. EPL, 144, 29001. 

Sakallı, ˙I., & Kanzi, S. (2022). Topical Review: greybody factors and quasinormal modes for black holes in various theories – fingerprints of invisibles. Turkish Journal of Physics, 46, 51-103. 

Modified Theories and String Theory 

Sakallı, İ., & Yörük, E. (2023). Modified Hawking radiation of Schwarzschild-like black hole in bumblebee gravity model. Physics Scripta, 98, 125307.

https://iopscience.iop.org/article/10.1088/1402-4896/ad09a1

Sucu, E., & Sakallı, ˙I. (2023). GUP-reinforced Hawking radiation in rotating linear dilaton black hole spacetime. Physics Scripta, 98, 105201. 

Event Horizon Telescope and Observational Cosmology 

Event Horizon Telescope Collaboration (2019). First M87 Event Horizon Telescope Results. I. The Shadow of the Supermassive Black Hole. The Astrophysical Journal Letters, 875(1), L1. 

Abbott, B.P., et al. (LIGO Scientific Collaboration and Virgo Collaboration) (2016). Observa tion of Gravitational Waves from a Binary Black Hole Merger. Physical Review Letters, 116(6), 061102. 

Recent Collaborative Works 

Tangphati, T., Sakallı, ˙I., Banerjee, A., & Pradhan, A. (2024). Behaviors of quark stars in the Rainbow Gravity framework. Physical Review D, 46, 101610. 

Banerjee, A., Sakallı, ˙I., Pradhan, A., & Dixit, A. (2024). Properties of interacting quark star in light of Rastall gravity. Classical and Quantum Gravity, 42, 025008. 

Sakallı, ˙I., Banerjee, A., Dayanandan, B., & Pradhan, A. (2025). Quark stars in f(R, T) gravity: mass-to-radius profiles and observational data. Chinese Physics C, 49, 015102. 

Gashti, S.N., Sakallı, ˙I., Pourhassan, B., & Baku, K.J. (2024). Thermodynamic topology and phase space analysis of AdS black holes through non-extensive entropy perspectives. European Physics C, 85, 305. 

Al-Badawi, A., & Sakallı, ˙I. (2025). The Static Charged Black Holes with Weyl Corrections. International Journal of Theoretical Physics, 64, 50. 

Textbooks and Reviews 

Carroll, S.M. (2004). Spacetime and Geometry: An Introduction to General Relativity. Addison Wesley. 

Wald, R.M. (1984). General Relativity. University of Chicago Press. 

Rovelli, C. (2004). Quantum Gravity. Cambridge University Press. 

Kiefer, C. (2012). Quantum Gravity (3rd ed.). Oxford University Press. Ashtekar, A., & Petkov, V. (Eds.) (2014). Springer Handbook of Spacetime. Springer. 

Note: This reference list includes representative works from Prof. Sakallı’s extensive publication record (181+ papers) and foundational works in the field. For a complete bibliography, please consult the INSPIRE-HEP database or Prof. Sakallı’s institutional profile.

Journal & Article Details

Publisher: In-Sight Publishing

Publisher Founding: March 1, 2014

Web Domain: http://www.in-sightpublishing.com

Location: Fort Langley, Township of Langley, British Columbia, Canada

Journal: In-Sight: Interviews

Journal Founding: August 2, 2012

Frequency: Four Times Per Year

Review Status: Non-Peer-Reviewed

Access: Electronic/Digital & Open Access

Fees: None (Free)

Volume Numbering: 14

Issue Numbering: 1

Section: A

Theme Type: Discipline

Theme Premise: Quantum Cosmology

Formal Sub-Theme: None.

Individual Publication Date: February 22, 2026

Issue Publication Date: April 1, 2026

Author(s): Scott Douglas Jacobsen

Word Count: 3,278

Image Credits: Izzet Sakallı

ISSN (International Standard Serial Number): 2369-6885

Acknowledgements

The author acknowledges Tor Arne Jørgensen for her time, expertise, and valuable contributions. Her thoughtful insights and detailed explanations have greatly enhanced the quality and depth of this work, providing a solid foundation for the discussion presented herein.

Author Contributions

S.D.J. conceived the subject matter, conducted the interview, transcribed and edited the conversation, and prepared the manuscript.

Competing Interests

The author declares no competing interests.

License & Copyright

In-Sight Publishing by Scott Douglas Jacobsen is licensed under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License.
© Scott Douglas Jacobsen and In-Sight Publishing 2012–Present.

Unauthorized use or duplication of material without express permission from Scott Douglas Jacobsen is strictly prohibited. Excerpts and links must use full credit to Scott Douglas Jacobsen and In-Sight Publishing with direction to the original content.

Supplementary Information

Below are various citation formats for Causality, Ensemble Invariance, and Black-Hole Perturbations: Prof. Dr. ˙Izzet Sakallı on the KSS Bound, Thermodynamic Topology, and Quasinormal-Mode Inference (3) (Scott Douglas Jacobsen, February 22, 2026).

American Medical Association (AMA 11th Edition)
Jacobsen SD. Causality, Ensemble Invariance, and Black-Hole Perturbations: Prof. Dr. ˙Izzet Sakallı on the KSS Bound, Thermodynamic Topology, and Quasinormal-Mode Inference (3). In-Sight: Interviews. 2026;14(1). Published February 22, 2026. http://www.in-sightpublishing.com/causality-ensemble-invariance-black-hole-perturbations-izzet-sakalli-kss-bound-thermodynamic-topology-quasinormal-mode-inference 

American Psychological Association (APA 7th Edition)
Jacobsen, S. D. (2026, February 22). Causality, ensemble invariance, and black-hole perturbations: Prof. Dr. ˙Izzet Sakallı on the KSS bound, thermodynamic topology, and quasinormal-mode inference (3). In-Sight: Interviews, 14(1). In-Sight Publishing. http://www.in-sightpublishing.com/causality-ensemble-invariance-black-hole-perturbations-izzet-sakalli-kss-bound-thermodynamic-topology-quasinormal-mode-inference 

Brazilian National Standards (ABNT)
JACOBSEN, Scott Douglas. Causality, Ensemble Invariance, and Black-Hole Perturbations: Prof. Dr. ˙Izzet Sakallı on the KSS Bound, Thermodynamic Topology, and Quasinormal-Mode Inference (3). In-Sight: Interviews, Fort Langley, v. 14, n. 1, 22 fev. 2026. Disponível em: http://www.in-sightpublishing.com/causality-ensemble-invariance-black-hole-perturbations-izzet-sakalli-kss-bound-thermodynamic-topology-quasinormal-mode-inference 

Chicago/Turabian, Author-Date (17th Edition)
Jacobsen, Scott Douglas. 2026. “Causality, Ensemble Invariance, and Black-Hole Perturbations: Prof. Dr. ˙Izzet Sakallı on the KSS Bound, Thermodynamic Topology, and Quasinormal-Mode Inference (3).” In-Sight: Interviews 14 (1). http://www.in-sightpublishing.com/causality-ensemble-invariance-black-hole-perturbations-izzet-sakalli-kss-bound-thermodynamic-topology-quasinormal-mode-inference. 

Chicago/Turabian, Notes & Bibliography (17th Edition)
Jacobsen, Scott Douglas. “Causality, Ensemble Invariance, and Black-Hole Perturbations: Prof. Dr. ˙Izzet Sakallı on the KSS Bound, Thermodynamic Topology, and Quasinormal-Mode Inference (3).” In-Sight: Interviews 14, no. 1 (February 22, 2026). http://www.in-sightpublishing.com/causality-ensemble-invariance-black-hole-perturbations-izzet-sakalli-kss-bound-thermodynamic-topology-quasinormal-mode-inference. 

Harvard
Jacobsen, S.D. (2026) ‘Causality, Ensemble Invariance, and Black-Hole Perturbations: Prof. Dr. ˙Izzet Sakallı on the KSS Bound, Thermodynamic Topology, and Quasinormal-Mode Inference (3)’, In-Sight: Interviews, 14(1), 22 February. Available at: http://www.in-sightpublishing.com/causality-ensemble-invariance-black-hole-perturbations-izzet-sakalli-kss-bound-thermodynamic-topology-quasinormal-mode-inference. 

Harvard (Australian)
Jacobsen, SD 2026, ‘Causality, Ensemble Invariance, and Black-Hole Perturbations: Prof. Dr. ˙Izzet Sakallı on the KSS Bound, Thermodynamic Topology, and Quasinormal-Mode Inference (3)’, In-Sight: Interviews, vol. 14, no. 1, 22 February, viewed 22 February 2026, http://www.in-sightpublishing.com/causality-ensemble-invariance-black-hole-perturbations-izzet-sakalli-kss-bound-thermodynamic-topology-quasinormal-mode-inference. 

Modern Language Association (MLA, 9th Edition)
Jacobsen, Scott Douglas. “Causality, Ensemble Invariance, and Black-Hole Perturbations: Prof. Dr. ˙Izzet Sakallı on the KSS Bound, Thermodynamic Topology, and Quasinormal-Mode Inference (3).” In-Sight: Interviews, vol. 14, no. 1, 2026, http://www.in-sightpublishing.com/causality-ensemble-invariance-black-hole-perturbations-izzet-sakalli-kss-bound-thermodynamic-topology-quasinormal-mode-inference. 

Vancouver/ICMJE
Jacobsen SD. Causality, Ensemble Invariance, and Black-Hole Perturbations: Prof. Dr. ˙Izzet Sakallı on the KSS Bound, Thermodynamic Topology, and Quasinormal-Mode Inference (3) [Internet]. 2026 Feb 22;14(1). Available from: http://www.in-sightpublishing.com/causality-ensemble-invariance-black-hole-perturbations-izzet-sakalli-kss-bound-thermodynamic-topology-quasinormal-mode-inference 

Note on Formatting

This document follows an adapted Nature research-article format tailored for an interview. Traditional sections such as Methods, Results, and Discussion are replaced with clearly defined parts: Abstract, Keywords, Introduction, Main Text (Interview), and a concluding Discussion, along with supplementary sections detailing Data Availability, References, and Author Contributions. This structure maintains scholarly rigor while effectively accommodating narrative content.

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