Science Is Not About Certainty

We teach our students: We say that we have some theories about science. Science is about hypothetico-deductive methods; we have observations, we have data, data require organizing into theories. So then we have theories. These theories are suggested or produced from the data somehow, then checked in terms of the data. Then time passes, we have more data, theories evolve, we throw away a theory, and we find another theory that’s better, a better understanding of the data, and so on and so forth.

This is the standard idea of how science works, which implies that science is about empirical content; the true, interesting, relevant content of science is its empirical content. Since theories change, the empirical content is the solid part of what science is.

Now, there’s something disturbing, for me, as a theoretical scientist, in all this. I feel that something is missing. Something of the story is missing. I’ve been asking myself, “What is this thing missing?” I’m not sure I have the answer, but I want to present some ideas on something else that science is.

This is particularly relevant today in science, and particularly in physics, because—if I’m allowed to be polemical—in my field, fundamental theoretical physics, for thirty years we have failed. There hasn’t been a major success in theoretical physics in the last few decades after the standard model, somehow. Of course there are ideas. These ideas might turn out to be right. Loop quantum gravity might turn out to be right, or not. String theory might turn out to be right, or not. But we don’t know, and for the moment Nature has not said yes, in any sense.

I suspect that this might be in part because of the wrong ideas we have about science, and because methodologically we’re doing something wrong—at least in theoretical physics, and perhaps also in other sciences. Let me tell you a story to explain what I mean. The story is an old story about my latest, greatest passion outside theoretical physics—an ancient scientist, or so I say even if often he’s called a philosopher: Anaximander. I’m fascinated by this character, Anaximander. I went into understanding what he did, and to me he’s a scientist. He did something that’s very typical of science and shows some aspect of what science is. What is the story with Anaximander? It’s the following, in brief:

Until Anaximander, all the civilizations of the planet— everybody around the world—thought the structure of the world was the sky over our heads and the earth under our feet. There’s an up and a down, heavy things fall from the up to the down, and that’s reality. Reality is oriented up and down; Heaven’s up and Earth is down. Then comes Anaximander and says, “No, it’s something else. The Earth is a finite body that floats in space, without falling, and the sky is not just over our head, it’s all around.”

How did he get this? Well, obviously, he looked at the sky. You see things going around—the stars, the heavens, the moon, the planets, everything moves around and keeps turning around us. It’s sort of reasonable to think that below us is nothing, so it seems simple to come to this conclusion. Except that nobody else came to this conclusion. In centuries and centuries of ancient civilizations, nobody got there. The Chinese didn’t get there until the 17th century, when Matteo Ricci and the Jesuits went to China and told them. In spite of centuries of the Imperial Astronomical Institute, which was studying the sky. The Indians learned this only when the Greeks arrived to tell them. In Africa, in America, in Australia—nobody else arrived at this simple realization that the sky is not just over our head, it’s also under our feet. Why?

Because obviously it’s easy to suggest that the Earth floats in nothing, but then you have to answer the question, Why doesn’t it fall? The genius of Anaximander was to answer this question. We know his answer—from Aristotle, from other people. He doesn’t answer this question, in fact: He questions this question. He asks, “Why should it fall?” Things fall toward the Earth. Why should the Earth itself fall? In other words, he realizes that the obvious generalization—from every heavy object falling to the Earth itself falling—might be wrong. He proposes an alternative, which is that objects fall toward the Earth, which means that the direction of falling changes around the Earth.

This means that up and down become notions relative to the Earth. Which is rather simple to figure out for us now: We’ve learned this idea. But if you think of the difficulty when we were children of understanding how people in Sydney could live upside-down, clearly this required changing something structural in our basic language in terms of which we understand the world. In other words, “up” and “down” meant something different before and after Anaximander’s revolution.

He understands something about reality essentially by changing something in the conceptual structure we use to grasp reality. In doing so, he isn’t making a theory; he understands something that, in some precise sense, is forever. It’s an uncovered truth, which to a large extent is a negative truth. He frees us from prejudice, a prejudice that was ingrained in our conceptual structure for thinking about space.

Why do I think this is interesting?  Because I think this is what happens at every major step, at least in physics; in fact, I think this is what happened at every step in physics, not necessarily major. When I give a thesis to students, most of the time the problem I give for a thesis is not solved. It’s not solved because the solution of the question, most of the time, is not in solving the question, it’s in questioning the question itself. It’s realizing that in the way the problem was formulated there was some implicit prejudice or assumption that should be dropped.   

If this is so, then the idea that we have data and theories and then we have a rational agent who constructs theories from the data using his rationality, his mind, his intelligence, his conceptual structure doesn’t make any sense, because what’s being challenged at every step is not the theory, it’s the conceptual structure used in constructing the theory and interpreting the data. In other words, it’s not by changing theories that we go ahead but by changing the way we think about the world.

The prototype of this way of thinking—the example that makes it clearer—is Einstein’s discovery of special relativity. On the one hand, there was Newtonian mechanics, which was extremely successful with its empirical content. On the other hand, there was Maxwell’s theory, with its empirical content, which was extremely successful, too. But there was a contradiction between the two.

If Einstein had gone to school to learn what science is, if he had read Kuhn, and the philosophers explaining what science is, if he was any one of my colleagues today who are looking for a solution of the big problem of physics today, what would he do? He would say, “OK, the empirical content is the strong part of the theory. The idea in classical mechanics that velocity is relative: forget about it. The Maxwell equations: forget about them. Because this is a volatile part of our knowledge. The theories themselves have to be changed, OK? What we keep solid is the data, and we modify the theory so that it makes sense coherently, and coherently with the data.”

That’s not at all what Einstein does. Einstein does the contrary. He takes the theories very seriously. He believes the theories. He says, “Look, classical mechanics is so successful that when it says that velocity is relative, we should take it seriously, and we should believe it. And the Maxwell equations are so successful that we should believe the Maxwell equations.” He has so much trust in the theory itself, in the qualitative content of the theory—that qualitative content that Kuhn says changes all the time, that we learned not to take too seriously—and he has so much in that that he’s ready to do what? To force coherence between the two theories by challenging something completely different, which is something that’s in our head, which is how we think about time.

He’s changing something in common sense—something about the elementary structure in terms of which we think of the world—on the basis of trust of the past results in physics. This is exactly the opposite of what’s done today in physics. If you read Physical Review today, it’s all about theories that challenge completely and deeply the content of previous theories, so that theories in which there’s no Lorentz invariance, which are not relativistic, which are not general covariant, quantum mechanics, might be wrong.…

Every physicist today is immediately ready to say, “OK, all of our past knowledge about the world is wrong. Let’s randomly pick some new idea.” I suspect that this is not a small component of the long-term lack of success of theoretical physics. You understand something new about the world either from new data or from thinking deeply on what we’ve already learned about the world. But thinking means also accepting what we’ve learned, challenging what we think, and knowing that in some of the things we think, there may be something to modify.

What, then, are the aspects of doing science that I think are undervalued and should come up front? First, science is about constructing visions of the world, about rearranging our conceptual structure, about creating new concepts which were not there before, and even more, about changing, challenging, the a priori that we have. It has nothing to do with the assembling of data and the ways of organizing the assembly of data. It has everything to do with the way we think, and with our mental vision of the world. Science is a process in which we keep exploring ways of thinking and keep changing our image of the world, our vision of the world, to find new visions that work a little bit better.

In doing that, what we’ve learned in the past is our main ingredient—especially the negative things we’ve learned. If we’ve learned that the Earth is not flat, there will be no theory in the future in which the Earth is flat. If we have learned that the Earth is not at the center of the universe, that’s forever. We’re not going to go back on this. If you’ve learned that simultaneity is relative, with Einstein, we’re not going back to absolute simultaneity, like many people think. Thus when an experiment measures neutrinos going faster than light, we should be suspicious and, of course, check to see whether there’s something very deep that’s happening. But it’s absurd when everybody jumps and says, “OK, Einstein was wrong,” just because a little anomaly indicates this. It never works like that in science.

The past knowledge is always with us, and it’s our main ingredient for understanding. The theoretical ideas that are based on “Let’s imagine that this may happen, because why not?” are not taking us anywhere.

I seem to be saying two things that contradict each other. On the one hand, we trust our past knowledge, and on the other hand, we are always ready to modify, in depth, part of our conceptual structure of the world. There’s no contradiction between the two; the idea of the contradiction comes from what I see as the deepest misunderstanding about science, which is the idea that science is about certainty. 

Science is not about certainty. Science is about finding the most reliable way of thinking at the present level of knowledge. Science is extremely reliable; it’s not certain. In fact, not only is it not certain, but it’s the lack of certainty that grounds it. Scientific ideas are credible not because they are sure but because they’re the ones that have survived all the possible past critiques, and they’re the most credible because they were put on the table for everybody’s criticism.

The very expression “scientifically proven” is a contradiction in terms. There’s nothing that is scientifically proven. The core of science is the deep awareness that we have wrong ideas, we have prejudices. We have ingrained prejudices. In our conceptual structure for grasping reality, there might be something not appropriate, something we may have to revise to understand better. So at any moment we have a vision of reality that is effective, it’s good, it’s the best we have found so far. It’s the most credible we have found so far; it’s mostly correct.

But, at the same time, it’s not taken as certain, and any element of it is a priori open for revision. Why do we have this continuous …? On the one hand, we have this brain, and it has evolved for millions of years. It has evolved for us, basically, for running across the savannah, for running after and eating deer and trying not to be eaten by lions. We have a brain tuned to meters and hours, which is not particularly well-tuned to think about atoms and galaxies. So we have to overcome that.  

At the same time, I think we have been selected for going out of the forest, perhaps going out of Africa, for being as smart as possible, as animals that escape lions. This continuing effort on our part to change our way of thinking, to readapt, is our nature. We’re not changing our mind outside of nature; it’s our natural history that continues to change us.

If I can make a final comment about this way of thinking about science, or two final comments: One is that science is not about the data. The empirical content of scientific theory is not what’s relevant. The data serve to suggest the theory, to confirm the theory, to disconfirm the theory, to prove the theory wrong. But these are the tools we use. What interests us is the content of the theory. What interests us is what the theory says about the world. General relativity says spacetime is curved. The data of general relativity are that the Mercury perihelion moves 43 degrees per century with respect to that computed with Newtonian mechanics.    

Who cares? Who cares about these details? If that were the content of general relativity, general relativity would be boring. General relativity is interesting not because of its data but because it tells us that as far as we know today, the best way of conceptualizing spacetime is as a curved object. It gives us a better way of grasping reality than Newtonian mechanics, because it tells us that there can be black holes, because it tells us there’s a Big Bang. This is the content of the scientific theory. All living beings on Earth have common ancestors. This is a content of the scientific theory, not the specific data used to check the theory.

So the focus of scientific thinking, I believe, should be on the content of the theory—the past theory, the previous theories—to try to see what they hold concretely and what they suggest to us for changing in our conceptual frame.      

The final consideration regards just one comment about this understanding of science, and the long conflict across the centuries between scientific thinking and religious thinking. It is often misunderstood. The question is, Why can't we live happily together and why can’t people pray to their gods and study the universe without this continual clash? This continual clash is a little unavoidable, for the opposite reason from the one often presented. It’s unavoidable not because science pretends to know the answers. It’s the other way around, because scientific thinking is a constant reminder to us that we don’t know the answers. In religious thinking, this is often unacceptable. What’s unacceptable is not a scientist who says, “I know…” but a scientist who says, “I don’t know, and how could you know?” Many religions, or some religions, or some ways of being religious, are based on the idea that there should be a truth that one can hold onto and not question. This way of thinking is naturally disturbed by a way of thinking based on continual revision, not just of theories but of the core ground of the way in which we think.       

So, to sum up, science is not about data; it’s not about the empirical content, about our vision of the world. It’s about overcoming our own ideas and continually going beyond common sense. Science is a continual challenging of common sense, and the core of science is not certainty, it’s continual uncertainty—I would even say, the joy of being aware that in everything we think, there are probably still an enormous amount of prejudices and mistakes, and trying to learn to look a little bit beyond, knowing that there’s always a larger point of view to be expected in the future.    

We’re very far from the final theory of the world, in my field, in physics—extremely far. Every hope of saying, “Well we’re almost there, we've solved all the problems” is nonsense. And we’re wrong when we discard the value of theories like quantum mechanics, general relativity—or special relativity, for that matter—and try something else randomly. On the basis of what we know, we should learn something more, and at the same time we should somehow take our vision for what it is—a vision that’s the best vision we have, but one we should continually evolve.

If science works, or in part works, in the way I’ve described, this is strongly tied to the kind of physics I do. The way I view the present situation in fundamental physics is that there are different problems: One is the problem of unification, of providing a theory of everything. The more specific problem, which is the problem in which I work, is quantum gravity. It’s a remarkable problem because of general relativity. Gravity is spacetime; that’s what we learned from Einstein. Doing quantum gravity means understanding what quantum spacetime is. And quantum spacetime requires some key change in the way we think about space and time.

Now, with respect to quantum gravity, there are two major research directions today, which are loops, the one in which I work, and strings. These are not just two different sets of equations; they are based on different philosophies of science, in a sense. The one in which I work is very much based on the philosophy I have just described, and that’s what has forced me to think about the philosophy of science.

Why? Because the idea is the following: The best of what we know about spacetime is what we know from general relativity. The best of what we know about mechanics is what we know from quantum mechanics. There seems to be a difficulty in attaching the two pieces of the puzzle together: They don’t fit well. But the difficulty might be in the way we face the problem. The best information we have about the world is still contained in these two theories, so let’s take quantum mechanics as seriously as possible, believe it as much as possible. Maybe enlarge it a little bit to make it general relativistic, or whatever. And let’s take general relativity as seriously as possible. General relativity has peculiar features, specific symmetries, specific characteristics. Let’s try to understand them deeply and see whether as they are, or maybe just a little bit enlarged, a little bit adapted, they can fit with quantum mechanics to give us a theory—even if the theory that comes out contradicts something in the way we think.  

That’s the way quantum gravity—the way of the loops, the way I work, and the way other people work—is being developed. This takes us in one specific direction of research, a set of equations, a way of putting up the theory. String theory has gone in the opposite direction. In a sense, it says, “Well, let’s not take general relativity too seriously as an indication of how the universe works.” Even quantum mechanics has been questioned, to some extent. “Let’s imagine that quantum mechanics has to be replaced by something different. Let’s try to guess something completely new” —some big theory out of which, somehow, the same empirical content of general relativity and quantum mechanics comes out in some limit.  

I’m distrustful of this huge ambition, because we don’t have the tools to guess this immense theory. String theory is a beautiful theory. It might work, but I suspect it’s not going to work. I suspect it’s not going to work because it’s not sufficiently grounded in everything we know so far about the world, and especially in what I perceive as the main physical content of general relativity.  

String theory’s big guesswork. Physics has never been guesswork; it’s been a way of unlearning how to think about something and learning about how to think a little bit differently by reading the novelty into the details of what we already know. Copernicus didn’t have any new data, any major new idea; he just took Ptolemy, the details of Ptolemy, and he read in the details of Ptolemy the fact that the equants, the epicycles, the deferents, were in certain proportions.  It was a way to look at the same construction from a slightly different perspective and discover that the Earth is not the center of the universe.

Einstein, as I said, took seriously both Maxwell’s theory and classical mechanics in order to get special relativity. Loop quantum gravity is an attempt to do the same thing: take general relativity seriously, take quantum mechanics seriously, and out of that, bring them together, even if this means a theory where there’s no time, no fundamental time, so that we have to rethink the world without basic time. The theory, on the one hand, is conservative because it’s based on what we know. But it’s totally radical, because it forces us to change something big in our way of thinking.

String theorists think differently. They say, “Well, let’s go out to infinity, where somehow the full covariance of general relativity is not there. There we know what time is, we know what space is, because we’re at asymptotic distances, at large distances. The theory is wilder, more different, newer, but in my opinion it’s more based on the old conceptual structure. It’s attached to the old conceptual structure and not attached to the novel content of the theories that have proven empirically successful. That’s how my way of reading science coincides with the specifics of the research work that I do—specifically, loop quantum gravity.

Of course, we don’t know. I want to be very clear. I think string theory is a great attempt to go ahead, by great people. My only polemical objection to string theory is when I hear—but I hear it less and less now—“Oh, we know the solution already; it’s string theory.” That’s certainly wrong, and false. What’s true is that it is  a good set of ideas; loop quantum gravity is another good set of ideas. We have to wait and see which one of these theories turns out to work and, ultimately, be empirically confirmed.    

This takes me to another point, which is, Should a scientist think about philosophy or not? It’s the fashion today to discard philosophy, to say now that we have science, we don’t need philosophy. I find this attitude naïve, for two reasons. One is historical. Just look back. Heisenberg would have never done quantum mechanics without being full of philosophy. Einstein would have never done relativity without having read all the philosophers and having a head full of philosophy. Galileo would never have done what he did without having a head full of Plato. Newton thought of himself as a philosopher and started by discussing this with Descartes and had strong philosophical ideas.

Even Maxwell, Boltzmann—all the major steps of science in the past were done by people who were very aware of methodological, fundamental, even metaphysical questions being posed. When Heisenberg does quantum mechanics, he is in a completely philosophical frame of mind. He says that in classical mechanics there’s something philosophically wrong, there’s not enough emphasis on empiricism. It is exactly this philosophical reading that allows him to construct that fantastically new physical theory, quantum mechanics.  

The divorce between this strict dialogue between philosophers and scientists is very recent, in the second half of the 20th century. It has worked because in the first half of the 20th century people were so smart. Einstein and Heisenberg and Dirac and company put together relativity and quantum theory and did all the conceptual work. The physics of the second half of the century has been, in a sense, a physics of application of the great ideas of the people of the ’30s—of the Einsteins and the Heisenbergs.

When you want to apply these ideas, when you do atomic physics, you need less conceptual thinking. But now we’re back to basics, in a sense. When we do quantum gravity, it's not just application. The scientists who say “I don't care about philosophy” —it’s not true that they don’t care about philosophy, because they have a philosophy. They’re using a philosophy of science. They’re applying a methodology. They have a head full of ideas about what philosophy they’re using; they’re just not aware of them and they take them for granted, as if this were obvious and clear, when it’s far from obvious and clear. They’re taking a position without knowing that there are many other possibilities around that might work much better and might be more interesting for them.

There is narrow-mindedness, if I may say so, in many of my colleagues who don’t want to learn what’s being said in the philosophy of science. There is also a narrow-mindedness in a lot of areas of philosophy and the humanities, whose proponents don’t want to learn about science—which is even more narrow-minded. Restricting our vision of reality today to just the core content of science or the core content of the humanities is being blind to the complexity of reality, which we can grasp from a number of points of view. The two points of view can teach each other and, I believe, enlarge each other.