Gerard ′t Hooft

 Name: ′t Hooft, Gerard

 Postal address: Spinoza Instituut,

 Leuvenlaan 4

 Postbus 80.195

 3508 TD
 Utrecht.

 Voorts verbonden aan:

 Institute for Theoretical
 Physics

 Universiteit Utrecht

 Leuvenlaan 4, 3584
 CC Utrecht

 Tel.: +31 30 253 5928

 Tel.  +31 30 253 1863

 Fax:  +31 30 253 5937

  e-mail: g.thooft@uu.nl

Warning: due to severe clogging of this email post box, I can no longer guarantee that received messages will be processed and/or answered.

Warning 2: Requests to be recognized as “friend” in social networks such as Facebook or LinkedIn are only considered if they are written with some personal note, not when they seem to come from robots.

secretary: no secretary at the moment.

For urgent matters contact Joost van Zee.

Discussing Magnetic Monopoles with Rembert Duine

Professor Theoretical Physics

Click here for lectures (notes, PowerPoint files)

Research interests:

Gauge theories in elementary particle physics.
This was the topic of the 1999 Nobel Prize. An idea was proposed by C.N. Yang and Robert Mills in 1954: they suggested that particles in the sub-atomic world might interact via fields that are similar to, but more general than electricity and magnetism. But, even though the interactions that had been registered in experiments showed some vague resemblance to the Yang-Mills equations, the details seemed to be all wrong. Attempts to perform accurate calculations were frustrated by infinite - hence meaningless - results. Together with my advisor then, and my co-Nobel-laureate now, M. Veltman, I found in 1970 how to renormalize the theory, and, more importantly, we identified the theories for which this works, and what conditions they must fulfil. One must, for instance, have a so-called Higgs-particle. These theories are now called gauge theories.

It was subsequently discovered that, indeed, the observed details of all known forces exactly agree with this picture. First it was found that the so-called weak force, in combination with the more familiar electro-magnetic one, is exactly described by a Yang-Mills theory. In 1973 it was concluded that also the strong force is a Yang-Mills theory. I was among the small number of people who were already convinced of this from early 1971. During the later 1970s, all pieces fell into place. Of all simple models describing the fundamental particles, one was standing out, the so-called ‘Standard Model’.
Gauge theories are the backbone of this Standard Model. But now it also became clear that this is much more than just a model: it is the Standard Theory. Great precision can be reached, though the practical difficulties in some sectors are still substantial, and it would be great if one could devise more powerful calculation techniques. Also, in spite of all its successes, the Standard Model, as it is formulated at present, shows deficiencies. It cannot be exactly right. Significant refinements are expected soon, since the new European machine, the Large Hadron Collider (LHC) is now fully operational.

In particular the Higgs particle has not yet been detected. The most uncertain parameter is its mass, which in principle can be anything between 100 and 1000 GeV, though precision checks obtained from numerous experiments suggest that the most likely mass value is between 114 and roughly 150 GeV. Using results first from Fermilab in the USA, and now the ones pouring in from LHC, the margin of possible mass values is rapidly being narrowed down, the most difficult one being just above 114 GeV. I am still confident that a Higgs particle, closely in line with the Standard Model, will be discovered, but the verdict it still out. In the mean time, my colleagues are prepared for encountering different situations. Modest modifications of the Standard Model replace the Higgs with a multitude of particles, which will be more difficult to detect. The possibility that no Higgs will be found at all is unexpected, but not impossible; this would require a more considerable replacement of the Standard Model, one that describes a much more complicated zoo of strongly interacting objects near the energy values around 1000 GeV (or 1 TeV). Before the end of 2012, much more will be known, and whatever the outcome, it will be exciting news.

Quantum gravity and black holes .
The predominant force controlling large scale events in the Universe is the gravitational one. The physical and the mathematical nature of this force were put in an entirely new perspective by Albert Einstein. He noted that gravitation is rooted in geometric properties of space and time themselves. The equations he wrote down for this force show a remarkable resemblance with the gauge forces that control the sub-nuclear world as described in the previous paragraph, but there is one essential difference: if we investigate how individual sub-atomic particles would affect one another gravitationally, we find that the infinities are much worse, and renormalization fails here. Under normal circumstances, the gravitational force between sub-atomic particles is so weak that these difficulties are insignificant, but at extremely tiny distance scales, of the order of 10^-33cm, this force will become strong. We are tempted to believe that, at these tiny distance scales, the fabric of space and time is affected by quantum mechanical phenomena, but exactly how this happens is still very mysterious. One approach to this problem is to ask: under which circumstance is the gravitational force as strong as it ever can be? The answer to this is clear: at the horizon of a black hole.

As I have been emphasizing for more than two decades now, the text book description of quantum gravity (where the Einstein-Hilbert action is quantized using standard procedures) shows flaws here that run deeper than that it generates infinities: it does not allow a description of a black hole as a single quantum object. This is a direct contradiction, a paradox, a problem shouting for a radical solution, saying that there is something we are not doing right. For a long time I was convinced that also superstring theory, in this respect is fundamentally faulty, but two developments forced me to be more cautious here. One: it is now possible to describe at least some members of the black hole family using string theory with multidimensional membranes, called D-branes, added to it. The objects thus obtained are purely quantum mechanical and agree with naive expectations so well that many of my colleagues are convinced that “string theory solves the problem”. But why does this happen? How does string theory resolve the paradox? Curiously, string theorists themselves do not quite understand this. But I think I might understand this now. String theory is just an instrument to do calculations in regions of a theory that are otherwise inaccessible.

Here come twist number two: I claim to have found how to put quantum gravity back in line so as to restore quantum mechanics for pure black holes. It does not happen automatically, you need a new symmetry. It is called local conformal invariance. This symmetry is often used in superstring and supergravity theories, but very often the symmetry is broken by what we call “anomalies”. These anomalies are often looked upon as a nuisance but a fact of life. I now claim that black holes only behave as required in a consistent theory of all conformal anomalies cancel out. This is a very restrictive condition, and, very surprisingly, this condition also affects the Standard Model itself. All particles are only allowed to interact with gravity and with each other in very special ways, as if local conformal invariance is spontaneously broken, but otherwise an exact symmetry. This leads to the predition that models exist where all unknown parameters of the Standard Model, such as the finestructure constant, the proton-electron mass ratio, and in fact all other such parameters are computable. Up till now these have been freely adjustable parameters of the theory, to be determined by experiment but they were not yet predicted by any theory.

I am not able to compute these numbers today because the high energy end of the elementary particle properties is not known. There is one firm prediction: all attempts to detect possible space and time dependence of the Standard Model parameters will give negative results.

The Hierarchy Problem.
An important problem can now be addressed: the hierarchy problem, which is the question why particle masses are 20 orders of magnitude smaller than the Planck mass, and the cosmological constant even more than 120 orders of magnitude. Could my theory explain this? I have been studying some intriguing ideas. Could the coefficients that relate to the cosmological constant and the mass terms be due to instantons? These are known for generating exponentially suppressed amplitudes. My present theory allows me to investigate such approaches. I do have a candidate gravitational instanton that could be the culprit here, but details do not yet work out right. At this moment, only one firm prediction stands out: constants of nature are truly constant. Attempts to observe space and/or time dependence will yield negative results. Because of this prediction I strongly support experimental searches for space-time dependence of natural constants, in particular the searches using the "frequency comb" for high precision comparisons between different spectral frequencies in atoms and molecules.

Fundamental aspects of quantum physics. I have deviating views on the physical interpretation of quantum theory, and its implications for Big Bang theories of the Universe. This topic has been expanded upon in recent publications, cf. "The mathematical basis for deterministic quantum mechanics"; "The Free-Will Postulate in Quantum Mechanics"; "Entangled quantum states in a local deterministic theory", arXiv:0908.3408; Hilbert space in deterministic theories, a reconsideration of the interpretation of quantum mechanics; how a wave function can collapse without violating Schrödinger's equation, and how to understand Born's rule, arXiv:1112.1811v2[quant-ph] (see publication list). In April (arXiv:1204.4926) and May 2012, new important papers will appear in the ArXiv.
    The findings reported in these papers are under attack by crackpots and serious colleagues alike. Indeed, my findings seem to be ruled out right away by the so-called Bell inequalities. However, those who say this haven't read carefully what I wrote. The Bell inequalities are to be addressed by making a clear distinction between states and dynamics. Even if the dynamical laws are deterministic, a system can be in a quantum state, that is, a state that features strong quantum mechanical entanglement at all times. These states may first form during the Big Bang, and continue to be like this ever since. Secondly, the states commonly used to describe atoms, particles, etc., do not describe what really goes on, but they are templates. Such templates can easily be seen to evolve exactly in accordance with a Schrödinger equation, and their relation to reality is exactly as in ordinary quantum mechanics (so that they can violate Bell's inequalities). The true state Nature is in can only approximately be identified after macroscopic measurements. After the dust settles, Nature will be found to be in exactly one state, without any superpositions, but you will have to find the basis in terms of which this is true - the cellular automaton basis, and it will be difficult to identify that.
    The next objection one often hears is that my theory sounds like requiring some gigantic non-locality or conspiracy in the initial state, so as to produce collapsing wave functions later on. But this is exactly why the deterministic underlying theory is needed: in terms of the deterministic ("hidden") variables, the initial state and the entire evolution are completely natural.
    Notions often under discussion are the "collapse of the wave function" and Born's probability law. These two important features of quantum mechanics follow in a completely natural fashion from my theory. In competing theories, they either have to be put in by hand, or worse, modifications of quantum mechanics are introduced with the aim of generating the required decoherence.

To me, Nature is a big jig-saw puzzle, and I see it as my task to try to fit pieces of it together. Click and cut the pieces you see here from the screen and see how they fit, or: read more about it in my book: ‘Bouwstenen van de Schepping’ (Prometheus/Bert Bakker, ISBN 90 351 1327 6) or its English version: 'In Search of the Ultimate Building Blocks', Cambridge Univ. Press, Paperback 9.95 pounds, 14.95 US dollars, ISBN 0 521 578833; hardback 27.95 pounds, 39.95 US dollars, ISBN 0 521 550831) . In ‘Planetenbiljart’, a personal view is described of the potentials of scientific and technological developments in the future. Which possibilities are there and are there things that will be impossible forever? Maybe you enjoy SF novels as much as I do, but don’t mistake them for predictions of the future.

Important message for autograph collectors: Some people are afflicted with the desire to collect photographs and autographs of Nobel Prize winners. I am very flattered when I receive such requests, but I think I have done my share. I am sold out now. No more photographs or autographs, with apologies.

Tijd in machten van Tien, door Gerard ’t Hooft en Stefan Vandoren, Uitgave Natuurwetenschap & Techniek, onderdeel van Uitgeverij Veen Magazines B.V., ligt nu in de boekwinkels.

"Time in Powers of Ten " is written in Dutch, but we certainly plan to produce a version in English as soon as possible. The idea is simple: Physicists and astrophysicists have produced various popularized descriptions of the enormously varying length scales in the Universe, in books and films. The Universe is some 50 billion light years across, that's nearly 10²⁹ cm. The smallest conceivable objects are superstrings, somewhere around 10^-32 cm. At every scale in powers of ten, our world looks different and very special. So now, we do the same thing with time. In fact, the variations in the time scales are even bigger. The time scale for super strings is 10^-43 seconds, and age of the Universe is less than 10¹⁸ seconds, but there are phenomena that outlast our Universe by gigantic factors. The lifetime of the most fundamental particle in our world, the proton, is expected to be something around 10⁴¹ seconds, and large black holes can last much longer than that. At nearly every time scale, things happen in our world, and we describe them, starting with just one second, then 10 seconds, 100 seconds and so on, until in the middle of the book, where we switch to the fastest time scales conceivable, until at the end we return to one second.

Appointment as Universiteitshoogleraar beginning July 1, 2011.

My (very tiny) stamp collection.

Birth of first grandson, Rowan B.A. van Deutekom, son of Ellen ′t Hooft and Roland van Deutekom, February 5, 2011. Some pictures of my family.

Important warning:
To my colleagues: Recently a PhD candidate used an email of mine, where I gave a brief reply to a request for advise, to fabricate that into a letter of reference. Whenever you receive a copy of an emailed letter of mine, do realize that emails are easy to modify.

January 29, 2011: Erice Prize 2009 ("Ettore Majorana Prize - Erice - Science for Peace"). Here is the citation.

Lomonosov Medal:
The Russian Academy of Sciences has awarded the Lomonosov Large Gold Medals to a Full Member of the Academy Spartak Beliayev of Russia, and to a Dutch Professor Gerardus t Hooft.