King's College London

Professor Gerard ’t Hooft, Theoretical Physicist who won the 1999 Nobel Prize in Physics, delivered the 2025 Higgs Lecture last week.

The renowned theoretical physicist, a Professor Emeritus at Utrecht University in the Netherlands, is best known for his groundbreaking work in gauge theories and for proposing the holographic principle. He spent the last half-century contributing extensively to our understanding of fundamental forces, black holes, and quantum gravity.

A recipient of numerous prestigious awards, Professor ’t Hooft received the 1999 Nobel Prize in Physics alongside Martinus Veltman for their contributions to the quantum structure of electroweak interactions.

In his lecture at King’s entitled, the “quantum black hole with (almost) no equations”, he unravelled some of the mysteries of black hole behaviours using only a few equations.

Kicking off the lecture he paid homage to Professor Peter Higgs, renowned physicist and King’s alumnus whom the annual lecture series is named after. Professor ’t Hooft shared how Higgs discovery of the Higgs Boson particle in 1964, would eventually transform physics contributing to the development of the Standard Model of all Particles – the most complete description of the subatomic world in modern physics.

Whilst this model, he said, reconciled Einstein’s theory of Special Relativity, as well as quantum mechanics, it falls down when dealing with the gravitational force and therefore Einstein’s other theory: General Relativity.

Finding ways to tackle this problem has dominated physics over the last century. Professor ’t Hooft explained how his own contributions to tackling this mystery have focused on observing black holes, which “play a big role in our theoretical understanding of the fundamental forces”. The crux of his talk came down to whether black holes behave like ordinary matter or “totally out of this world”, and “if the laws of physics are found to hold even inside black holes, then here is where we would get the most credible laws of physics”.

In this quest, he encouraged the audience to be inspired by the young Einstein and how he would proceed, which he believed would be along the lines of “don't change the laws of physics, but ask exactly how should they be reformulated so that everything makes sense”.

Professor ’t Hooft went even further in outlining the need to revisit questions surrounding the behaviour of black holes, including the infinite energies at the event horizon, and that if “we search for those answers…and get them correct...then we’ll learn how to reconcile…one theory for all forces.”

For Professor ’t Hooft, this investigation has involved observing how particles enter and leave a black hole, following Stephen Hawking’s prediction that black holes not only absorb particles but also emit them. This has, coupled with a proven theory on gravity called the Shapiro effect, enabled him to calculate what in-going particles into a black hole do to out-going ones, “using only the laws of physics that we know”.

He described that if you apply this theory to black holes and carry out the calculations correctly, “there is another surprise waiting for us: if you carefully fold the solution open, you find two universes”.

In describing position and momentum in quantum mechanics, one needs to rely on what is called a Fourier transform. He explained that there is a modification of the usual transform, where the integral is done over the half-line, such that “the outgoing particle does depend on the ingoing particle.”

This modification led him to conclude that “one universe is the mirror image of another universe.”. He also reflected that by applying these argumentations, the black hole is not “fundamentally different from a brick, or from a fundamental particle, a molecule or a star.” Therefore, it is governed by familiar rules of nature.

Drawing his lecture to a close, he reflected “you get mysteries, you get question marks and you get remarks from your colleagues, if you look carefully at all the boundary conditions, all the mathematics, you put it all together and you solve them one on one, you see it all works out…this actually solves the information problem.”

He ended the lecture sharing how these quests in physics can be a "diabolical nightmare" or a "mathematical riddle" for some people, but for him they are a "magical bit of sunshine."

To complement the lecture, theoretical physicists at King’s organised a technical workshop featuring Professor ’t Hooft alongside other speakers, who presented their own work and highlighted which past works of his had contributed to their research.

The Higgs Lecture is delivered by an invited guest speaker every year, since the inaugural lecture in 2012 presented by Professor Peter Higgs. Previous speakers include Nobel Laureates Sir Roger Penrose and Professor David Gross, as well as renowned astrophysicist, Dame Jocelyn Bell Burnell and prominent mathematician Professor Caroline Series.