Fabiano F. Santos is a Brazilian theoretical physicist whose research focuses on modified gravity, holography, and black hole physics. He is based at UEMASUL in Imperatriz and earned his Ph.D. in Physics from the Federal University of Paraíba in 2020. Santos investigates how Horndeski scalar–tensor extensions of general relativity alter the behavior of black holes, braneworlds, and boundary conformal field theories. His research spans topics such as AdS/BCFT correspondence, holographic entanglement entropy, complexity, transport coefficients, and Lifshitz black branes. Widely cited, his publications explore quantum information, thermodynamics, and condensed-matter analogs within the framework of general relativity.

Scott Douglas Jacobsen: What is Horndeski gravity in plain language?

Fabiano F. Santos: Horndeski gravity is a theory of gravity that extends Einstein’s general relativity by incorporating additional terms into the equations, enabling more intricate interactions between gravity and matter. It’s often used to explore how gravity behaves in extreme conditions, like near black holes or in the early universe.

Jacobsen: What problem does your research try to solve?

Santos: The research aims to understand how gravity, quantum mechanics, and thermodynamics interact in extreme environments, like black holes or the early universe, and to explore how these insights can help explain the fundamental laws of nature.

Jacobsen: Why use black holes to study the physics of materials?

Santos: Black holes are like “natural laboratories” for testing extreme physics. Using a principle called holography, we can study how black holes behave and use that knowledge to model the behavior of materials, especially those with complex quantum properties, such as superconductors or strange metals.

Jacobsen: What is AdS/BCFT?

Santos: AdS/BCFT (Anti-de Sitter/Boundary Conformal Field Theory) is a framework in theoretical physics that connects gravity in a curved spacetime (AdS) to quantum systems on the boundary of that spacetime. It’s a tool for studying how quantum systems behave when they have boundaries or edges.

Jacobsen: What is the key new insight from your AdS/BCFT research?

Santos: The research demonstrates how boundaries in quantum systems can impact their overall behavior, offering new approaches to modeling edge effects in materials or quantum systems using gravity.

Jacobsen: What is the shear viscosity in these models?

Santos: Shear viscosity measures how easily a fluid flows when a force is applied to it. In these models, it’s calculated using holography and often reveals universal properties of quantum fluids, like the ratio of viscosity to entropy density.

Jacobsen: When does the KSS bound fail in your results?

Santos: The KSS bound, which sets a lower limit on the ratio of viscosity to entropy density, can fail in systems with strong quantum effects or in theories with modified gravity, like Horndeski gravity.

Jacobsen: What does a “probe string” measure physically?

Santos: A probe string is a tool in holography that measures how particles or forces behave in a quantum system, like how charges move in a material or how forces act between particles.

Jacobsen: What do Lifshitz spacetimes let you test?

Santos: Lifshitz spacetimes enable the study of systems where time and space behave differently, which is helpful for modeling materials with unusual quantum properties, such as those near quantum critical points.

Jacobsen: How do Horndeski terms change black hole thermodynamics?

Santos: Horndeski terms modify the equations governing black holes, leading to changes in their temperature, entropy, and how they radiate energy, which can reveal new physics beyond Einstein’s theory.

Jacobsen: What real-world signals could test predictions?

Santos: Signals like gravitational waves, black hole shadows, or unusual patterns in cosmic radiation could test predictions from these models. In materials, experiments on quantum systems might reveal similar effects.

Jacobsen: How does entanglement entropy help “see” inside black holes?

Santos: Entanglement entropy measures the amount of quantum information shared between different parts of a system. In black holes, it helps us understand how information is stored and processed, offering clues about their internal structure.

Jacobsen: What does “holographic complexity” measure?

Santos: Holographic complexity measures how difficult it is to reconstruct the quantum state of a system, such as a black hole, using the smallest possible set of instructions. It’s a way to quantify the “computational difficulty” of a system.

Jacobsen: How do your models produce ferromagnetism or paramagnetism?

Santos: By introducing specific fields or interactions in the holographic models, the system can mimic the behavior of magnetic materials, showing how spins align (ferromagnetism) or respond to external fields (paramagnetism).

Jacobsen: What is a geometric Josephson junction?

Santos: A geometric Josephson junction is a theoretical model in which two quantum systems are connected by a “bridge” in spacetime, allowing quantum effects such as tunneling to occur, similar to how real Josephson junctions function in superconductors.

Jacobsen: Why study domain walls and thick branes?

Santos: Domain walls and thick branes are structures that separate different regions in spacetime or materials. Studying them helps us understand phase transitions, like how materials change from one state to another (e.g., solid to liquid).

Jacobsen: Which result is most ready for experimental checks?

Santos: The predictions about shear viscosity and the KSS bound could be tested in experiments on quantum fluids or ultracold atoms, which mimic the conditions described by the models.

Jacobsen: What is the most complex technical challenge now?

Santos: The most challenging aspect is solving the complex equations in these models, especially when incorporating effects such as Horndeski terms or Lifshitz spacetimes, which necessitate advanced numerical techniques.

Jacobsen: Which collaboration most influenced this line of research?

Santos: Collaborations between string theorists, condensed matter physicists, and gravitational physicists have been the most influential, as they bring together expertise from different fields to tackle these problems.

Jacobsen: What is the one-sentence takeaway you give to non-physicists?

Santos: We utilize black holes and gravity as tools to understand the intricate and beautiful workings of the universe, from the tiniest particles to the largest cosmic structures.

Jacobsen: Thank you for the opportunity and your time, Fabiano.