In the pantheon of modern physics, few figures can match the quiet authority of Gerard ’t Hooft. The Dutch theoretical physicist, now a professor emeritus at Utrecht University in the Netherlands, has spent much of the past half-century reshaping our understanding of the fundamental forces that knit together reality. But ’t Hooft’s unassuming, soft-spoken manner belies his towering scientific stature, which is better revealed by the mathematical rigor and deep physical insights that define his work—and by the prodigious numbers of prestigious prizes he has accrued, which include a Nobel Prize, a Wolf Prize, a Franklin Medal, and many more.
His latest accolade, announced on April 5, is the most lucrative in all of science: a Special Breakthrough Prize in Fundamental Physics, worth $3 million, in recognition of ’t Hooft’s myriad contributions to physics across his long career.
His most celebrated discovery—and the one that earned him, along with his former Ph.D. thesis adviser, the late Martinus Veltman, the 1999 Nobel Prize in Physics—showed how to make sense of non-Abelian gauge theories, which are complex mathematical frameworks that describe how elementary particles interact. Together, ’t Hooft and Veltman demonstrated that these theories could be renormalized, meaning that intractable infinite quantities that cropped up in calculations could be tamed in a consistent and precise way. This feat would change the course of science history, laying the groundwork for the Standard Model, the reigning paradigm of particle physics.
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But beyond this, ’t Hooft has made many other breakthroughs, which are too numerous—and, in most cases, too technical—to thoroughly describe here. Among them, however, some of the most notable include his contributions to our understanding of the way that quarks are confined within protons and neutrons and the way that magnetic monopoles naturally emerge from the high-energy unification of fundamental forces, as well as the physics of black holes. In particular, his explorations of the latter area led to his proposal of the holographic principle in the 1990s. This is the notion that all the information within a three-dimensional volume of space can be encoded on a surrounding two-dimensional surface, akin to a hologram. The idea has since become central to many efforts to unify quantum mechanics with Einstein’s general theory of relativity in an all-encompassing theory of quantum gravity.
In a conversation with Scientific American, ’t Hooft spoke about his Breakthrough Prize, his optimism for the future of particle physics, his dissatisfaction with quantum mechanics, and the scientific and cultural effects that have arisen from some of his most provocative ideas.
[An edited transcript of the interview follows.]
It seems you’ve won practically all the big physics prizes at this point.
Some are still missing! But, yeah, I’ve won quite a few prizes. What worries me a little bit is that most of them were for the same thing. You get prize after prize for something that has already been recognized as such, whereas I’ve done other things in science that are not as well known—not by the general public, at least. But anyway, the Breakthrough Foundation has made a summary of my work for which they gave this prize, and that contains practically everything!
Yes, the foundation included it all! But given how many prizes you have won, does this one feel like just another notch on your belt? Has this all become routine for you, or is it still exciting?
I can assure you: nothing is routine. All these things are different. The climax really was the Nobel Prize itself, which is only granted to a very few people every year. And that’s something very special. But this one is also very special. It’s a big prize, literally speaking.
And as you mentioned, this one recognizes the full sweep of your scientific career rather than just one facet of it, such as your work in the 1970s with Veltman to explain the electroweak interaction that led to you both sharing the 1999 Nobel Prize in Physics. That work, of course, was fundamental to the subsequent formulation of the Standard Model of particle physics, now celebrated as the most well tested and successful scientific theory ever devised. But in some respects, the Standard Model has become notorious, too, because its many myriad experimental validations have contributed to a crisis in particle physics wherein progress has slowed down as researchers have seen no obvious path forward to further breakthroughs. Does this aspect of the Standard Model’s decades-long dominance worry you?
No, not at all. I think it is natural for science that we cannot always have an infinitely continuous stream of discoveries and new insights. There will be periods, like the one we are in now in particle physics, where things seem to be quieter. I just saw the news from CERN, for instance, that at the Large Hadron Collider, they’ve detected in new channels the absence of CP [charge parity] symmetry. And that’s fine. That’s not earth-shattering. Nowadays, we’re in a period where scientists in my field make many smaller discoveries that, in themselves, are very pleasing because they make our understanding more complete. But I think history shows it won’t be always like this.
A few centuries ago, when [James Clerk] Maxwell joined electricity and magnetism, and after that, when Max Planck made the first observations about energy being quantized, there were long periods in which very little seemed to be happening. In reality, of course, many things did happen in other fields, such as statistical physics and other fundamental branches of science. And both then and now, there’s been steady progress in those domains. Look at astronomy right now; the astronomers have their great moments all the time, and you can’t say there’s a dull moment at all! They’re discovering many new things in the universe as their telescopes become bigger and more accurate and as they use more and more fundamental scientific techniques to enhance their resolution. You can say much the same thing about biophysics or medicine, where discoveries are made nearly every day.
But in my field, you’re right, it seems to be that nothing is happening. I don’t agree with that, though. Things are happening, just at a more modest scale.
Are you optimistic, then, that this situation will change, and we’ll see a resurgence in big particle physics discoveries?
That’s a very good question because it looks as if there’s nothing we can do. If the situation proceeds in such a way that every new breakthrough requires a 10-fold, or even larger, increase in the machines’ size, power and costs, then clearly we won’t get much beyond where we are now. I cannot exclude such obstacles standing in the way of progress, but the history of science suggests, in such a case, progress will simply go in different directions. One may not only think of precision improvements but also [think of] totally different avenues of discovery such as cosmology and black hole physics.
But I would like to advise to the new generation of scientists: don’t worry about that, because the real reason why there’s nothing new coming is that everybody’s thinking the same way!
I’m a bit puzzled and disappointed about this. Many people continue to think the same way—and the way people now try to introduce new theories doesn’t seem to work as well. We have lots of new theories about quantum gravity, about statistical physics, about the universe and cosmology, but they’re not really “new” in their basic structure. People don’t seem to want to make the daring new steps that I think are really necessary. For instance, we see everybody sending their new ideas first to the [preprint server] arXiv.org and then to the journals to have it published. And in arXiv.org, you see thousands of papers coming every year, and none of them really has this great, bright, new, fine kind of insight that changes things. There are insights, of course, but not the ones that are needed to make a basic new breakthrough in our field.
I think we have to start thinking in a different way. And I have always had the attitude that I was thinking in a different way. And particularly in the 1970s, there was a very efficient way of making further progress: think differently from what your friends are doing, and then you find something new!
I think that is still true; however, I’m getting old now and am no longer getting brilliant new ideas every week. But in principle, there are ways—one could argue about quantum mechanics, about cosmology, about biology—that are not the conventional ways of looking at things. And to my mind, people thinking in such novel ways is not happening enough.
Could you give an example of the novelty or difference you’re referring to?
Sure. My way of thinking about the world, about physics, about the other disciplines related to physics is that everything should be much more logical, much more direct, much more “down to Earth.”
Many people who write papers on quantum mechanics like to keep some sense of mysticism about it, as if there’s something strange, something almost religious about the subject. I think that’s totally false. Quantum mechanics is based on a mathematical method used to describe very ordinary physical effects. I think the physical world itself is a very ordinary one that is completely classical. But in this completely classical world, there are still too many things that we don’t know today, there are “steps” we’re basically missing on our path to deeper understanding.
What sorts of steps?
I’m talking about steps that would exploit the fact that the whole world is very simple and straightforward. The trouble is, the world still appears complicated to us now, which is why we’re in this situation.
You already mentioned the Standard Model, this marvelous discovery from the previous century. It’s an instructive example because, basically, it’s very simple, but if you look at it deeper, you see there’s something very important missing. The Standard Model is based on quantum mechanics, and quantum mechanics tells you what happens when particles approach one another and scatter. But they can scatter in many different ways; they have a large number of choices of ways in which they scatter against each other, and the Standard Model doesn’t give any sound prediction there. It only gives you statistics. The Standard Model is a fantastic theory that handles the statistics of what things are doing. But the theory never tells you which choice nature makes; it only tells you that these different possibilities are there at a certain probability amplitude. That is the world as we know it. That’s how we know how to phrase the laws of nature. But it’s not the laws of nature themselves.
What’s missing is our understanding as to what it is that sometimes makes a particle go this way, sometimes that way. Well, you can easily argue particles can hit each other at a tiny distance. They don’t hit each other directly head-on but hit at some angle, and then they scatter away from some angle. That may be true. But what the theory today is not saying is what I should actually be looking at if two particles approach each other to predict how they’ll scatter ahead of time.
Imagine if you knew the way such interactions would go as precisely as you could know what will happen when two grand pianos hit each other. In principle, for the pianos, you could say exactly which wire will hit each other wire; you could predict exactly what happens when two grand pianos collide. Could it be the same with particles? In practice, such predictions for particles are considered to be too hard, and you turn to statistics, and you conclude that your piano particles can scatter in all directions, and that’s all there is to be said. Well, for looking at pianos, maybe you can say something more. If you know exactly where and at which angle they will hit each other, you can predict ahead of time how they will scatter. And that should be in our theories of the elementary particles as well—and it isn’t.
I’m saying we should start to think in these ways. And people refuse that because they think quantum mechanics is too beautiful to be wrong. Whereas I believe that quantum mechanics is not the right way of ultimately saying what basic laws objects obey when they hit each other.
Incidentally, while I was preparing for this interview, I found a conversation you had in 2013 with one of my predecessors here at Scientific American, George Musser. And one of the things you discussed was the work of physicist John Bell and its implications for the nature of reality. You said that you considered locality to “be an essential ingredient for any simple, ultimate law governing the universe.” It sounds like that’s still your view.
Very much, absolutely. I think, in fact, that you can understand and explain quantum mechanics very well if you only assume that the laws are local laws. Let us say what these particles do when they collide is determined by where they are, at that very spot when they hit each other. That is, what happens at other spots in the universe, in principle, should not matter. And if it does matter, then you have what we call “nonlocality.” But nonlocality would be a disaster for most solid scientific theories!
I don’t believe nonlocality is necessary. We don’t know exactly what to do when two particles collide because we don’t know whether particles look like grand pianos or like pure points. But, then again, they can’t be pure points because pure points can’t do anything. There’s something in there, and we should be able to write down all the laws on what’s in there, in these particles: How can they collide against each other? And why is it that they sometimes go this way and sometimes go that way? How can they exhibit spin?
We should be able to phrase such things as solid laws, and we are not even close to that. And this is why I think other breakthroughs should still be possible—many of them!—to help us get closer to this level of understanding for particles that we simply don’t have today, not even as something approximate.
You know, in my talks with theoretical physicists, I’ve noticed that the greater and more accomplished the individual is, the more likely they are to say, “The real challenge is not in answering old questions but rather in finding new, better questions for whatever problem you’re addressing.” I think that’s because there’s this temptation for optimism about what can be known—this feeling that by asking the “right” questions, meaningful answers must emerge. Do you really think the problem is that we’re not asking the right questions, or is it instead that we’ve been asking the right ones, and their answers are, against our hopes, simply beyond our reach?
What you just said, that the questions are beyond our reach, is exactly what people said a decade and a century and a millennium ago. And of course, that was the wrong answer each time. We can answer these questions, but to do so requires lots and lots of science. Before Maxwell, nobody understood how exactly electric and magnetic fields hang together, and they thought, “Oh, this is impossible to find out because it’s weird!” But then Maxwell said, no, you just need this one term, and then it all straightens out! And now we understand exactly what electric and magnetic interactions do. It’s simply not correct that you cannot answer such questions. No, you can, but you have to start from the beginning, like I said about quantum mechanics.
If you believe right from the beginning that quantum mechanics is a theory that only gives you statistical answers and never anything better than that, then I think you’re on the wrong track. And people refuse to drop the idea that quantum mechanics is some strange sort of supernatural feature of the particles that we will never understand. No! We will understand, but we need to step backward first, and that’s always my message in science in general: before you understand something, just take a few steps back. Maybe you have to make a big march back, all the way back to the beginning.
Just imagine: What would your basic laws possibly be if you didn’t have quantum mechanics? Answering that, of course, requires saying what quantum mechanics is.
Okay. So what is quantum mechanics?
Quantum mechanics is the possibility that you can consider superpositions of states. That’s really all there is to it. And I’d argue that superpositions of states are not real. If you look very carefully, things never superimpose. [Erwin] Schrödinger asked the right questions here: You know, take my cat, it can be dead; it can be alive. Can it be in a superposition? That’s nonsense!
And he was quite right. People shouldn’t continue to insist that a dead cat and a live cat superimpose. That’s complete nonsense—yet, at that level, it seems to be the only correct answer to say exactly where the particle is, what its velocity is, what its spin is, and so on. Whereas there must be different kinds of variables that evolve in time, such as integer-valued variables or discretely moving variables, to name just two possibilities. These would be variables in terms of which you can’t move a cat, you can’t say whether it’s dead or alive, unless you would make more nonlocal changes. There must be ways to describe all states for alive cats and for dead cats, but these states will mix with states that don’t describe cats at all.
Using superpositions is then just a trick that works at first but doesn’t get at the states we want to understand. We have to make that step backward.
Walk me through this for a moment. If superpositions are illusory in that they are purely mathematical concepts that have no basis in physical reality, how does that square with the ongoing success of quantum information science and quantum computing, where it seems as if superpositions are a real physical phenomenon that can be leveraged, for instance, to do things that can’t be done classically?
Well, I think quantum technology is just what you get if you assume the reality of superimposed systems. What do I mean by that? We know superpositions in the macroscopic world are nonsense. That’s clear. And I believe, in the microscopic world, it’s clearly nonsense, too, even though it may seem we have nothing besides superpositions to use for understanding atoms. And I think what people in quantum technology probably don’t realize is that they’re doing the very converse of what they think they are doing. They think they’re understanding quantum mechanics. Instead I think what they should be doing is trying to remove the quantum mechanics from the description, trying to use more fundamental degrees of freedom, like those discrete states I mentioned.
They’re not asking the right questions, and that failure to do so makes things look more and more complicated—more and more quantum-mechanical—whereas, in reality, it shouldn’t be interpreted that way.
Weren’t we just discussing the tendency of eminent theorists to talk about not asking the right questions?
Well, let me say that, yes, they do the right experiments. Yes, they try to make the right things. And yes, their quantum computers may be more powerful than anything else for certain applications because they understand “quantum mechanics”—by which I mean they understand how these microscopic systems actually act, in great detail, because this is something that actually came out of studying the quantum world. Yes, we know how small objects react and interact. But our problem is that, at present, we can only make statistical predictions. And as soon as a quantum computer gives you statistical distributions instead of correct answers, well, that’s the end of your “computer”; you can’t use it for most applications anymore.
For most things, you want to use a computer in such a way that you avoid making superpositions—because you want to get a sharp answer. For instance, you want to decipher a secret code or something like that. You want to have the exact answer: “This is what it means, not that!” And let’s not equate this answer to a superposition of those two possibilities—again, that’s nonsense.
What I’m saying is: we must unwind quantum mechanics, so to speak, as to see what happens underneath. And until the quantum technologists start doing that, I believe they won’t make really big progress. For instance, quantum computers always make errors, and their designers and operators try to correct them. And if you’re trying to correct these errors, what that means to me is: you want to go to more basic degrees of freedom that do not ever carry any error in them because they’re exact—they’re just classical. But to have this realization is apparently very difficult.
This is my feeling as to why we don’t make breakthroughs. We should think about things in a different manner.
It seems you’re saying we must live in a clockwork universe, one in which things must be purely deterministic at a very fundamental level, and thus there’s very little room for any sort of quasi-mystical speculation. And one consequence of that would seem to be the dissolution of mystery, to some degree. You mentioned earlier the stubborn persistence of an almost religious approach to quantum mechanics within the scientific community, not to mention in popular culture. Perhaps this attitude endures because, for so many people, it preserves something ineffable about all that we experience in the world rather than assuming everything can be known by filling in the right equations.
So if you do believe in this sort of clockwork universe, I wonder what you’d say its most mysterious aspect would be.
Well, there are still many mysteries that make our problem very, very difficult. And this deterministic universe we discuss is something that could only be fully understood by someone with a much bigger mind, a much bigger brain, than I have because they’ll have to consider all possibilities. And as soon as you make some wrong assumption, then you again get this quantum-mechanical situation in which things get to superimpose each other.
A simpler question is: Can you formulate quantum mechanics without a superposition principle? And my answer is yes. And in one of my last [preprint] papers on arXiv.org, I wrote a little simple model—too simple to be useful in a real world. But the model is just a clock, a clock that has a pendulum that moves in a very organized way, and that pendulum drives a wheel that shows the time, the hands that show the minutes and seconds. And because of this, I call it my grandfather’s clock model. And from the pendulum, you can derive what time the hand should show. And these hands are deterministic. They are just showing a time with infinite precision, say. And the pendulum is really a quantum pendulum; it can be quantized; we can write quantum equations for it.
I found the connection to the mathematics of this pendulum and the mathematics of this hand that shows the time. Keep in mind, the hand that shows the time is completely classical, and the pendulum is completely quantum-mechanical, but one is related to the other—it’s just one machine.
But I got very few reactions to this. I would have thought that people would say, “Oh, yes, of course. Now we understand how to continue!” But instead they’ve said, “Okay, right, ’t Hooft has another hot idea, another crazy idea. And he has many of those crazy ideas. Let him be happy with it; we’re going to do our own thing.” And that’s the most common reaction I’ve gotten.
I’d suspect the reasons for that reaction are, in some sense, not scientific and rather more “cultural,” right? I’m thinking of this in terms of the signal-to-noise ratio that exists for anyone trying to drink from the firehose of new preprint papers on arXiv.org and elsewhere. It can be very tough to know what to pay attention to and how to evaluate whatever does get one’s attention.
That leads me to one more question. I’m curious how you feel about the cultural impacts of your scientific contributions, in particular the holographic principle, which you first proposed in the early 1990s.
Arguably because of this idea, there are people—mostly nonscientists, I’d imagine—who truly believe that the cosmos is in fact within a black hole or that it’s all some simulation in a higher-dimensional computer. The idea being for this “simulation hypothesis” that perhaps nothing is “real” besides information itself, as everything else could just be a projection of patterns of 1’s and 0’s encoded on the outermost boundary of the observable universe. I wonder what you think about this phenomenon in which you put forth a provocative theoretical insight more than 30 years ago, and it has somehow led to the world’s richest man seriously suggesting on a popular podcast that “we are most likely” all just avatars in some cosmic-scale video game.
Well, I do have some reservations. Maybe I should have never talked about the holographic principle because, yes, some people are galloping away into nonsense, linking this idea with supernatural features and poorly defined dimensionality, all to sound very mysterious. And I have a big problem with that. I think you shouldn’t phrase the laws of nature in more complicated terms than strictly necessary. You should simplify as much as possible. Even Einstein once said something like this, that you have to simplify things as much as possible but not beyond reality, not beyond the truth. We should try not to be supernatural; if we, as scientists, only leave a wake of mysteries behind us, we’re not doing the right thing.
I am a bit worried that the holographic principle has only invited people to be more mysterious because I want the extreme opposite. I want people to try to be super rational. For me, even quantum mechanics is already too far away from reason. And you know, if you rephrase quantum mechanics to treat Hilbert space [a type of vector space that allows for infinite dimensions] as something used for practical purposes rather than Hilbert space being a fundamental property of nature, you don’t even need this sort of holography anymore! I wish more people understood that. We have to try to phrase things more precisely to avoid public misunderstandings wreaking havoc on science.
