by Lee Smolin
Another way to look at our present situation is that seers are compelled, by their desire for clarity, to grapple with the deepest problems in the foundations of physics. These include the foundations of quantum mechanics and the problems associated with space and time. Many papers and books have been written on the foundational problems of quantum mechanics during the last few decades, but, to my knowledge, not a single one is by a leading string theorist. Nor do I know of any paper by a string theorist that attempts to relate the issues faced by string theory to the older writings by physicists and philosophers on the big issues in the foundations of space, time, or quantum theory.
The leaders of the background-independent approaches to quantum gravity tend, by contrast, to be people whose scientific views were formed by lifelong reflection on the deep foundational issues. It is easy to list those whose thinking has led to papers and even books addressing foundational issues: Roger Penrose is perhaps the best known to the public, but one can name many others including John Baez, Louis Crane, Bryce DeWitt, Fay Dowker, Christopher Isham, Fotini Markopoulou, Carlo Rovelli, Rafael Sorkin, and Gerard ’t Hooft.
By contrast, I can think of no mainstream string theorist who has proposed an original idea about the foundations of quantum theory or the nature of time. String theorists tend to answer this charge with a dismissive response, to the effect that these questions are all solved. Occasionally they acknowledge that the problems are serious but quickly follow this admission with the claim that it’s too soon to try solving them. Often one hears that we should just continue to follow the development of string theory, because since string theory is right, it must contain the necessary solutions.
I have nothing against people who practice science as craft, whose work is based on the mastery of technique. This is what makes normal science so powerful. But it is a fantasy to imagine that foundational problems can be solved by technical problem solving within existing theories. It would be nice if this were the case—certainly, we would all have to think less, and thinking is really hard, even for those who feel compelled to do it. But deep, persistent problems are never solved by accident; they are solved only by people who are obsessed with them and set out to solve them directly. These are the seers, and this is why it is so crucial that academic science invite them in rather than exclude them.
Science has never been organized in a way that is friendly to seers; Einstein’s employment situation is hardly a lone example. But a hundred years ago, the academy was much smaller and much less professional, and well-trained outsiders were common. This was a legacy of the nineteenth century, when most of the people who did science were enthusiastic amateurs, either rich enough to not need to work or convincing enough that they could find a patron.
Fine, you might say. But who are the seers? They are by definition highly independent and self-motivated individuals who are so committed to science that they will do it even if they can’t make a living at it. There should be a few out there, even though our professionalized academy is unfriendly to them. Who are they and what have they managed to do to solve the big problems?
They are hiding in plain sight. They can be recognized by their rejection of assumptions that most of the rest of us believe in. Let me introduce you to a few of them.
I have a lot of trouble believing that special relativity is false; if it is, then there is a preferred state of rest and both the direction and speed of motion must be ultimately detectable. But there are a few theorists around who have no trouble with this concept. Ted Jacobson is a friend who collaborated with me on a paper on the quantum mechanics of loop quantum gravity. Together we found the first exact solutions to a key equation known as the Wheeler-DeWitt equation.5 But when loop quantum gravity surged ahead, Jacobson grew pessimistic. He didn’t think loop quantum gravity would work, and he also didn’t think it went deeply enough. After mulling this over, he began to question the relativity principle itself and to believe in the possibility of a preferred state of rest. He has spent years developing this idea. In chapters 13 and 14, I noted that if special relativity is wrong, experiment may soon tell us. Jacobson and his students at the University of Maryland are among the leaders of the search for an experimental test of special relativity.
Another seer who has questioned the whole framework of relativity is the cosmologist João Magueijo (see chapter 14). He had no choice, because he had invented, and fallen in love with, an idea that seemed to contradict it—which is that the speed of light might have been much faster in the early universe. The papers he wrote about this are just barely consistent—and they certainly make no sense unless one assumes that the relativity principle needs to be thrown out, or at least modified.
Then there are the wild guys of solid-state physics—accomplished physicists who made great careers explaining real things about the behavior of real stuff. I am referring to Robert Laughlin, who was awarded a Nobel Prize in 1998 for his contributions to “the discovery of a new form of quantum fluid with fractionally charged excitations,” Grigori Volovik, of the Landau Institute for Theoretical Physics, in Moscow, who explained the behavior of certain species of very cold liquid helium, and Xiao-Gang Wen. These men are master craftsmen and seers both. Having done perhaps the best and most consequential normal science of the last few decades, they decided to try their hands at the deep problems of quantum gravity, and they started with the idea that the relativity principle is false, that it is just an approximate, emergent phenomenon. The particle physicist James Bjorken is another such seer/craftsperson. That we know that protons and neutrons contain quarks is due in large part to his insights.
One of the great seers is Holger Bech Nielsen, of the Niels Bohr Institute. He was an inventor of string theory, and he has many other key discoveries to his credit. But for many years he has been isolated from the mainstream for advocating what he calls random dynamics. He believes that the most useful assumption we can make about the fundamental laws is that they are random. Everything we think of as intrinsically true, such as relativity and the principles of quantum mechanics, he thinks are just accidental facts that are emergent from a fundamental theory so beyond our imagining that we might as well assume that its laws are random. His models are the laws of thermodynamics, which used to be based on principles but now are understood as the most likely way that large numbers of atoms in random motion will behave. This may not be right, but Nielsen has come remarkably far in his antiunification program.
There is a very short list of string theorists who have made lasting contributions to science on the level of those devised by these gentlemen. So how do the string theorists—or for that matter, the loop theorists—respond to the insistent warnings of these accomplished physicists that perhaps we are all making a wrong assumption? We ignore them. Yes, really, flat out. To tell the truth, we laugh at them behind their backs, and sometimes as soon as they have left the room. Having done Nobel Prize–level physics—or even having won the prize itself—apparently doesn’t protect you when you question universally held assumptions such as the special and general theories of relativity. I was shocked when Laughlin told me that he was under pressure from his department and funding agency to keep doing normal science in the field he had been working in, rather than spending time on his new ideas about space, time, and gravity. If such a person, after all his accomplishments, including the Nobel, cannot be trusted to chase down his deepest ideas, what exactly is the meaning of academic freedom?
Fortunately for physics, we will soon know whether special relativity is true or not. Most of my friends expect that experimental observations will prove these great men fools. I hope the iconoclasts are wrong and that special relativity passes the test. But I cannot rid myself of the fear that perhaps we are the ones who are wrong and they are right.
So much for questioning relativity. What if quantum theory is wrong? This is the soft underbelly of the whole project of quantum gravity. If quantum theory is wrong, then trying to combine it with gravity will have been
a huge waste of time. Does anyone think this is the case?
Yes, and one is Gerard ’t Hooft. As a graduate student at the University of Utrecht, ’t Hooft proved, with an older collaborator, that quantum Yang-Mills theories were sensible, a discovery that made the whole standard model possible, and he has a well-deserved Nobel Prize for these efforts. That’s only one of his many fundamental discoveries about the standard model. But for the last decade he has been one of the boldest thinkers on foundational issues. His main idea is called the holographic principle. As he formulates it, there is no space. Everything that happens in a region we are used to thinking of as space can be represented as taking place on a surface surrounding that space. Furthermore, the description of the world that exists on that boundary is not quantum theory but a deterministic theory he believes will replace it.
Just before ’t Hooft formulated his principle, a similar idea was proposed by Louis Crane in the context of background-independent approaches to quantum gravity. He proposed that the right way to apply quantum theory to the universe is not to try to put the whole universe into one quantum system. This had been tried by Stephen Hawking, James Hartle, and others and found to face severe problems. Crane suggested instead that quantum mechanics is not a static description of a system but a record of information that one subsystem of the universe can have about another by virtue of their interaction. He then suggested that there is a quantum-mechanical description connected with every way of dividing the universe into two parts. The quantum states live not in one part or the other but on the boundary between them.6
Crane’s radical suggestion has since grown into a class of approaches to quantum theory that are called relational quantum theories, because they are based on the idea that quantum mechanics is a description of relationships between subsystems of the universe. This idea was developed by Carlo Rovelli, who showed it to be perfectly consistent with how we usually do quantum theory. In the context of quantum gravity, it resulted in a new approach to quantum cosmology, made by Fotini Markopoulou and her collaborators. Markopoulou emphasized that describing the exchange of information between different subsystems is the same as describing the causal structure that limits which systems can influence each other. She thus found that a universe can be described as a quantum computer, with a dynamically generated logic.7 The idea that the universe is a kind of quantum computer has also been promoted by Seth Lloyd of MIT, one of the visionaries of the field of quantum computation.8 From the two sides of their respective disciplines, Markopoulou and Lloyd have been leading a movement that uses ideas from quantum information theory to reconceptualize the universe, leading to the understanding of how elementary particles can emerge from quantum spacetime.
Gerard ’t Hooft’s idea of a world represented on its boundary should remind you of the Maldacena conjecture. Indeed, ’t Hooft’s ideas were in part an inspiration for Juan Maldacena, and some think the holographic principle will turn out to be one of the basic principles of string theory. This alone could have easily made him one of the leaders of the string theory community, had he been interested in such a role. But in the 1980s, ’t Hooft began to go his own way. He did this while he was in his prime and at a point when no one was technically stronger. Still, the minute he deviated from the mainstream, he was laughed at by his fellow particle physicists. He didn’t seem to care, or even to notice, but I’m sure it stung. Nevertheless, he doubted almost everything and forged his own path in fundamental physics. His core belief, developed over decades, is that quantum physics is wrong.
There is no more earnest or sincere person than ’t Hooft. One thing we in the field of quantum gravity love about him is that he is so often there. He comes to many of our meetings, and there you never see him in the halls, politicking with the other prominent attendees. Instead, he comes to every session, something only the young students do. He arrives first thing each morning, impeccably dressed in a three-piece suit (the rest of us are generally in jeans and T-shirts), and he sits in the front row all day and listens to the talks by every single student and postdoc. He doesn’t always comment, and he may even doze off for a minute or two, but the respect he shows by being there for each of his colleagues is impressive. When it’s his turn to speak, he stands up and unpretentiously presents his ideas and results. He knows that his is a lonely road, and I would not be surprised if he resents it. How does a person give up the mantle of leadership, so richly deserved, just because he can’t make sense of quantum mechanics? Imagine what that says about someone’s character.
Then there’s Roger Penrose. Simply put, there is no one who has contributed more to our understanding and use of the general theory of relativity, save Einstein himself, than Roger Penrose. He is one of the four or five most talented and deeply original thinkers I have met in any field. He has done great mathematics and great physics. Like ’t Hooft, much of his work in the last two decades is motivated by his conviction that quantum mechanics is wrong. And like ’t Hooft, he has a vision of what should replace it.
Penrose has been arguing for years that the incorporation of gravity into quantum theory makes that theory nonlinear. This leads to a resolution of the measurement problem, in that quantum-gravitational effects cause the quantum state to collapse dynamically. Penrose’s proposals are well described in his books, although they have not so far been implemented in a detailed theory. Nevertheless, he and others have been able to use them to make predictions for doable experiments, some of which are presently under development.
A few of us take Penrose’s arguments seriously; an even smaller number are convinced of their validity. But most string theorists—and certainly all mainstream string theorists—show no signs of listening at all. If even the most honored visionaries are not taken seriously once they begin to question basic assumptions, you can imagine how well people fare who are seers but not lucky enough to have made substantial contributions first.9
If several of the best living theoretical physicists feel compelled to question the basic assumptions of relativity and quantum theory, there must be others who come to this position from the beginning. There are indeed people who, early in their studies, began to think quantum theory must be wrong. They learned it, and they can carry out its arguments and calculations as well as anyone. But they don’t believe in it. What happens to them?
There are roughly two kinds of such people: the sincere ones and the insincere ones. I am one of those who never found a way to believe in quantum mechanics, but I am one of the insincere ones. That is, I understood early in my education that I could not have a decent career as an academic theoretical physicist if I focused on trying to make sense of quantum mechanics. So I decided to do something that the mainstream would understand and appreciate well enough so that I could pursue a normal career.
Luckily I found a way to investigate my doubts about the foundations by working on something as mainstream as quantum gravity. Since I didn’t believe in quantum mechanics in the first place, I was pretty sure that this effort had to fail, but I hoped that understanding the failure would provide clues as to what might replace quantum theory. A few years earlier, I would have had as little luck with a career based on quantum gravity as with one based on worrying about quantum theory being wrong. However, an easy opportunity opened up while I was a graduate student, which was to attack the problem of quantum gravity using recent methods developed to study the standard model. So I could pretend to be a normal-science kind of physicist and train as a particle physicist. I then took what I learned and applied it to quantum gravity. Since I was among the first to try this approach, and since I used tools that the leaders of the mainstream understood, this made a decent, if not stellar, career possible.
But I could never completely suppress the instinct to probe the foundations of my subject. I wrote a paper in 1982 titled “On the Relationship Between Quantum and Thermal Fluctuations” that, when I look at it now, seems unbelievable to me in its boldness.10 I asked a new question about how space, time, a
nd the quantum fit together—a question that opened up a whole new way to tackle the problem. Even now, after having written many influential papers, I think this one was my best. Every once in a while, I meet a student who is reading back into the foundations of the subject, or some loner who has been on the outskirts for decades, and they say, “Oh, you’re that Smolin! I never made the connection. I thought he must have died, or left physics.” Now at last, along with my colleagues at Perimeter, I am finally returning to work on the foundations of quantum mechanics.
What of the sincere people, who didn’t believe basic assumptions like relativity and quantum theory and didn’t have a malleable enough character to suppress their inclinations? They are a special breed, and they each have a story to tell.
Julian Barbour is known to many who follow science as the author of The End of Time, in which he argues that time is an illusion.11 He is an unusual physicist, who, since receiving his doctorate in 1968 from the University of Cologne, has never held an academic job. But he has been highly influential among the small group of people who think seriously about quantum gravity, for it was he who taught us what it means to make a background-independent theory.
As Barbour tells it, on a climbing trip during graduate school, he was seized by a vision that time might be an illusion. This led him to investigate the roots of our understanding of time, contained in the general theory of relativity. He realized that he could not make a conventional academic career worrying about the nature of time. He also realized that if he was going to work on that problem, he would have to concentrate on it fully, without being distracted by the pressures of a normal career in physics. So he bought an old farmhouse in a little village half an hour from Oxford, brought his new wife there, and settled down to think about time. It was ten years or so before he had something to report back to his colleagues. During this period, he and his wife had four children, and he worked part-time as a translator to support them. The translating took him no more than twenty hours a week, leaving him as much time for thinking as most academic scientists have after the responsibilities of teaching and administration are taken into account.