Prof. Gerard ’t Hooft is a theoretical physicist whose career stretches more than five decades. Accordingly, his work spans several subfields of theoretical physics: the foundations of the Standard Model of particle physics, quantum field theory, gravitation, and the conceptual interpretation of quantum mechanics. His research is driven by a central goal to understand whether the apparent randomness and complexity of nature can be derived from deeper, more fundamental principles.
Although he is formally an emeritus professor, ’t Hooft is still connected to Utrecht University, and his work continues to have a far-reaching impact on the field of particle physics. For each of his research directions, we give a brief description of his contributions and a connection to relevant research currently ongoing at Utrecht University.
To me, Nature is a big jig-saw puzzle, and I see it as my task to try to fit pieces of it together.”
Particle physics and the Standard Model
A central achievement of modern theoretical physics is the formulation of the Standard Model, which describes the fundamental particles and their interactions. Much of this framework is based on gauge theories, a class of mathematical structures describing forces through symmetry principles.
Gauge theories are quantised field theories that can be characterised in terms of the mathematical Lie symmetries on which they are based, and the spectrum of particles with spin one that ensues, while also allowing them to interact directly with a limited number of particles with spin zero and particles with spin one half. Other particle types cannot interact directly with these basic building blocks.
In the 1970s, ’t Hooft and Martinus Veltman showed how such theories could be made mathematically consistent and predictive, work for which they were later rewarded the 1999 Nobel Prize in Physics. This work laid the foundation for precise calculations in particle physics and helped establish the Standard Model as the ‘standard theory’ of subatomic physics.
Today, research at Utrecht University’s Institute for Theoretical Physics continues to build on this foundation. The institute’s work on elementary particles and field theory explores extensions of the Standard Model, improved computational techniques, and connections to cosmology and high-energy experiments such as those at CERN.
Quantum gravity, black holes, and the structure of spacetime
One of the major open problems in physics is how to unify quantum mechanics with Einstein’s theory of gravity. Black holes play a central role in this challenge, as they represent situations where both theories must be applied simultaneously, a research problem that so far has remained unsolved, and even paradoxical, in ’t Hooft’s own words.
’t Hooft has long argued that black holes are not just astrophysical objects, but also theoretical laboratories for quantum gravity. Contrary to many string theorists, he is not convinced that black holes should lose information. His work explores how information, spacetime geometry, and quantum states are related at the deepest level.
At Utrecht University, these questions are actively studied at the Institute for Gravitational and Subatomic Physics, where research teams investigate black hole physics, gravitational waves, and the holographic principle. This includes work on how spacetime and quantum information might emerge from more fundamental principles.
Foundations of quantum mechanics and physical principles
A distinctive aspect of ’t Hooft’s work is his interest in the conceptual foundations of quantum mechanics. He has explored the possibility that quantum theory may emerge from a deeper, deterministic level of physical description, rather than being fundamentally probabilistic. One of his research aims is to write models that reproduce quantum behavior such that they can run on a classical computer.
In this approach, the familiar quantum description would arise from underlying physical states governed by deterministic evolution, while quantum superposition would reflect incomplete information about those states. This viewpoint challenges standard assumptions in the interpretation of quantum theory and continues to stimulate discussion about the meaning of quantum measurements and probability. Finding a deterministic explanation of quantum mechanics could even have consequences for our ideas about free will and predestination.
Understanding the nature of physical constants
Another long-standing motivation in ’t Hooft’s work is the search for explanations of the constants and numerical values appearing in physics, such as particle masses and coupling strengths. Instead of treating these as independent values, he is interested in exploring whether these numbers might be derived from deeper theoretical constraints.
This theme connects naturally to ongoing work in the Institute for Theoretical Physics, where research groups study extensions of the Standard Model, cosmology, and high-energy theory, aiming to understand how the observed Universe may arise from more fundamental laws.