The Universe_Leading Scientists Explore the Origin, Mysteries, and Future of the Cosmos

by John Brockman

  So I began to wonder whether we might be wrong about time emerging from law in quantum cosmology. I began to think maybe we should make quantum cosmology in some way in which time is fundamental and space may emerge from something more fundamental. So that’s one thing that happened.

  The second thing that happened is that the picture of laws evolving, of a collection of universes evolving on a landscape of laws, went in about 2003 from being very much kind of a one-person little obsession that I engaged in on the side to a big deal when a bunch of people in string theory came to the same conclusion. This was a result of work at Stanford, and the impetus for this was making string theory accommodate itself to the positive dark energy, or positive cosmological constant or vacuum energy.

  And this collaboration of people at Stanford discovered that they could make string theories that had positive vacuum energy but only at the cost of there being vast numbers of string theories. So, really, they got back to where Andy Strominger was in 1988. And all of a sudden—and Lenny Susskind here played a big role—all of a sudden there was a lot of talk about the “landscape of theories” and the “dynamics” and “change” on the landscape of theories, which were the words I had used. So I was sort of jolted into, “My god, if everybody is taking this seriously, I should think carefully about this.”

  The third thing that happened was that I started interacting with a philosopher, Roberto Mangabeira Unger, who had on his own been thinking about evolving laws for his own reasons. And he basically took me to task—this is about six or seven years ago—and said, “Look, you’ve been writing and thinking about laws evolving in time, but you haven’t thought deeply about what that means for our understanding of time. If laws can evolve in time, then time must be fundamental.” And I said “Yes,” but he said, “You haven’t thought deeply, you haven’t thought seriously about that.” And we began talking and working together as a result of that conversation.

  And so those three things together, about five or six years ago, made me go back and put together the idea that laws have to change in time if they’re to be explained—with my thinking about the nature of space and time, quantum mechanically, and so I started playing with the idea that maybe time has to be really fundamental in the context of quantum gravity.

  That thinking changed my work, and much of my work for the last year has been devoted to thinking about various ways, various hypotheses, about how laws can change in time. Thinking about the consequences for understanding the nature of time, and thinking about how to make theories and hypotheses that can be checked. The reason is that this stuff can get pretty speculative. I’m sure it does sound speculative—so to tie it down, I focus on hypotheses that are testable.

  Feynman once told me, “You’re going to have to do crazy things to think about quantum gravity. But whatever you do, think about nature. If you think about the properties of a mathematical equation, you’re doing mathematics and you’re not going to get back to nature. Whatever you do, have a question that an experiment could resolve at the front of your thinking.” So I always try to do that.

  Let me first mention that cosmological natural selection did make some predictions, and those predictions have so far stood up. Let me talk about some newer ideas.

  Let me give some more examples, because cosmological natural selection was a long time ago. Here’s one that I call the principle of precedence. And I think it’s kind of cute. And let me phrase it in the language of quantum mechanics, which is where it comes from. It comes from actually thinking about the foundations of quantum mechanics, which is another thing I try to think about occasionally. We take a quantum system, and quantum systems are always thought about, from my point of view, as small bits of the universe that we manipulate and prepare in states and experiment with and measure. We’re always doing something to a quantum system. I don’t believe anymore that there’s anything that goes under the name of “quantum cosmology.”

  Let’s say we have a quantum system—let’s say some ions in an ion trap, and we want to measure their quantum mechanical properties. So we prepare them in some initial state. We evolve them by transforming them, by interacting with them from the outside, for example, by applying magnetic fields or electric fields or probing them with various probes. And then we apply a measurement. And because it’s quantum mechanics, there’s no prediction for the definite outcome of the experiment; there are probabilities for different possible outcomes.

  Let’s consider a system that’s been studied many times. We have measured the statistical distribution of outcomes through some collection of past instances, where we’ve measured the system before. And if we do it now and measure the system again, we’re going to get one of those past outcomes that we saw before. If we do it many times, we’re going to get a statistical distribution, which is going to be the same distribution that we saw before. We’re confident that if we do it next year, or in a million years, or in a billion years, we’re going to get the same distribution as we got before. Why are we confident of that? We’re confident of that because we have a kind of metaphysical belief that there are laws of nature that are outside time, and those laws of nature are causing the outcome of the experiment to be what it is, and laws of nature don’t change in time, they’re outside of time. They act on the system now, they acted on the system in the same way in the past, they will act the same way in a year, or a million or a billion years, and so they’ll give the same outcome. So nature will repeat itself, and experiments will be repeatable because there are timeless laws of nature.

  But that’s a really weird idea, if you think about it, because it involves the kind of mystical and metaphysical notion of something that is not physical, something that is not part of the state of the world—something that is not changeable acting from outside the system to cause things to happen. And when I think about it, that is kind of a remnant of religion. It’s a remnant of the idea that God is outside the system, acting on it.

  So let’s try a different kind of hypothesis. What if, when you prepare the system, transform it, and then measure it, nature has a way of looking back and asking, “Have similar things been done in the past? If they have, let’s take one of those instances randomly and just repeat it.” That is, nature forms habits. Nature looks to see whether a similar thing happened in the past. And if there was, what if it takes that? If there are many, it picks randomly among them and presents you with that outcome. OK, well, that will give the same statistical distribution you saw in the past, by definition, because you’re sampling from the past. So there doesn’t have to be a law outside of time. The only law needs to be what I call the principle of precedence—that when you do an experiment, nature looks back and gives you what it did before.

  Now, you can say that that involves some weird metaphysical idea that nature has access to its past and is able to identify when a similar thing is being done, a similar measure being made. And that’s true, but it’s a different metaphysical idea from the idea of the law acting from the outside, and it has different consequences. Let’s play with this. This reproduces the predictions of standard quantum mechanics, so it reproduces the success of standard quantum mechanics without having to believe in the timeless law.

  Can you test it? As I said, I’m only interested in ideas that can be tested. Yes, you can test it, because people are working a lot with quantum technologies and they’re making systems that have no precedents. For example, in Waterloo, there’s the Institute for Quantum Computing, and there Ray Laflamme and David Cory and their colleagues are making systems that have never been made before. And so I talk to them and I say, “Maybe if you make a really novel system, then we’ll have no precedents. It won’t behave as you expect it to, because it won’t know what to do, so it will just give you some totally random outcome.” And they laugh, and I say, “Why is that funny?” And they say, “Well, the first time we do an experiment, of course we get a totally random outcome, because there are experimental-design issues and experime
ntal-error issues. We never get what we think we’re going to get, the first time we set up an experiment in the laboratory and we run it.” So I say to them, “Great! That’s fine. But eventually the thing settles down and starts giving repeatable results?” and they say, “Sure.” And I say, “Well, could you separate that process”—of settling down to definite results—“could you separate out the effect of having to make your experiment work from the effect of my hypothesis that nature is developing habits as it goes along?” And they say, “Maybe.” So we have a discussion going on, about whether this can really be tested.

  Now, it’s like any idea—it’s probably wrong but it’s testable, and that, to me, proves that it’s science.

  The current scene is very confusing. Very smart people have tried to advance theoretical physics in the last decades, and we’re in an embarrassing situation. The embarrassing situation is that theories that were already around in the middle ’70s for particle physics and the early ’80s for cosmology are being confirmed over and over again, and to greater and greater precision, by the current experiments. And this goes both for particle physics and cosmology.

  In particle physics, the LHC, the Large Hadron Collider, identified a new particle that is probably the Higgs—it looks like a standard-model Higgs—and there’s nothing else. There’s no evidence for supersymmetry, for extra dimensions, for new generations of quarks, for substructure—for a whole variety of ideas, some of which have been very popular and some of which have not been very popular but nonetheless have been on the table. None of these ideas that go beyond the standard model have been confirmed. In cosmology, the results of the Planck seem to be right in line with the simplest version of inflation. And this is a triumph for the standard model and a triumph for inflation.

  Paul Steinhardt has a very interesting argument that the results of Planck really shouldn’t be taken as confirming inflation. I have enormous respect, I have deep respect, for Paul, but the case is not closed. The case is not closed. And certainly, at a naïve level, it looks like a universe that’s exactly the one that the inflationary theorists told us about, and they should be proud of that. But the situation leaves a conundrum, because we have nothing that confirms anything that goes beyond those models.

  And I should say that there has been, in my field of quantum gravity, a lot of interest in the idea that certain astrophysical experiments would be able to see the breakdown of the structure of space and time that we have from general relativity, and give us evidence of quantum space and time in the propagation of light coming from faraway gamma-ray bursts, in the propagation of cosmic rays. We’ve been expecting for about a decade to see signals of quantum spacetime. And those are not there either, so far. So we’re also very frustrated.

  So my impression is that when—Let me just come back to how I quoted Feynman: When very smart people have been working under certain assumptions for a long time—and these ideas have been around for a lot longer than the ideas that Feynman was concerned with were—and we’re not succeeding in uncovering new phenomena and new explanations, new understanding for phenomena, it’s time to reassess the foundations of our thinking. It doesn’t mean that everybody should do that, but some people should do that. And I find myself doing that because of my own intellectual history, partly because of my work in quantum gravity, partly because of cosmological natural selection, partly because I have an inclination—because part of my education was in philosophy. Although my PhD is in physics, part of my undergraduate was in philosophy, and I’ve always had an interest in philosophy. And I’ve always had even more than that—an appreciation, a deep appreciation, for the history of thought about these fundamental questions. I find myself doing some of this reassessment. And we’ll see where it goes.

  The conclusions that I come to, I think they’re not subtle, they’re easy to list, are, first, that—and I was opening with them before—the method of physics with fixed laws which are given for all time, acting on fixed spaces of states which are given for all time, is self-limiting. The picture of atoms with timeless properties moving around in a void according to timeless laws, this is self-limiting. It’s the right thing to do when we’re discussing small parts of the universe, but it breaks down when you apply it to the whole universe or when your chain of explanation gets too deep.

  Let me give one reason it breaks down. We can use the language of reductionism. It’s very good advice, it has worked for hundreds of years, that if we want to understand the properties of some composite system, some material, we explain it in terms of the properties of its parts or the things it’s made from. That’s good common sense, and a lot of the success of science is due to applying that good commonsense advice.

  But what happens when you get to the things you think of as the elementary particles? They have properties, too. They have masses and charges, with various forces that they move about with. But they have no parts, we believe. Or if they do have parts, you’re just continuing to do this, and then you should be looking for the breakdown into the parts that experiment has not seen so far.

  Is there any other way to explain the properties of fundamental particles? Well, not by further reductionism. There has to be a new methodology. So that’s the first conclusion: that the methodology that works for physics and has worked for hundreds of years, there’s nothing wrong with it in the context in which it’s been applied successfully, but it breaks down. When you push to the limits of explanation, reductionism breaks down. It also breaks down when you push on the other end, to larger and larger systems, to the universe as a whole. I mentioned several reasons why it breaks down but there are others. Let me mention one: When we experiment with small parts of the universe, we do experiments over and over again. That’s part of the scientific method; you have to reproduce the results of an experiment, so you have to do it over and over again. And by doing that, you separate the effect of general laws from the effect of changing the initial conditions. You can start the experiment off in different ways and look for phenomena which are still general. These have to do with general laws. And so you can cleanly separate the role of initial conditions from the role of the general laws.

  When it comes to the universe as a whole, we can’t do that. There’s one universe, and it runs one time. We can’t set it up. We didn’t start it. And indeed, in working cosmology, in inflationary theory, there’s a big issue, because you can’t separate testing hypotheses about the laws from testing hypotheses about the initial conditions—because there was just one initial condition and we’re living in its wake. This is another way in which this general method breaks down. So we need a new methodology.

  A good place to look for that methodology is in the relational tradition, the tradition of Leibniz and Mach and Einstein, that space and time and properties of elementary particles are not intrinsic but have to do with relationships that develop dynamically in time. This is the second point.

  The third conclusion is that time therefore must be fundamental. Time must go all the way down. It must not be emergent, it must not be an approximate phenomenon, it must not be an illusion. These are the conclusions that I come to and that my work these days is based on.

  So how do I situate myself? There are two areas that my work impinges on most directly. One of them is quantum gravity and the other is cosmology. Let me discuss each of those in turn. In quantum gravity, there are several programs of research. The one I’m most identified with is loop quantum gravity. Loop quantum gravity is doing very well, and let me take a minute for that, because we haven’t talked about that.

  Loop quantum gravity is a conservative research program. It comes from applying quantum mechanics directly to a form of general relativity, with no additional hypotheses about extra dimensions or extra particles or extra degrees of freedom. The particular form of general relativity we use is very close to gauge theory. It’s very close to Yang-Mills theory. This was a form developed by Abhay Ashtekar, and before him, although we didn’t know about this at the t
ime, by Plebanski. This is now a big research program.

  We have every two years an international conference. This year I’m among the organizers. We’re doing it at Perimeter, and we already have—it’s very early, the conference is not until July—and we already have more than two hundred people registered to come. So this is not Carlo Rovelli and Abhay Ashtekar and me sitting around in Verona writing in our notebooks, the way it was in the late ’80s—which, by the way, was a great experience. It’s great to be an inventor of something, and it’s great to have a period like that.

  Loop quantum gravity gives us a microscopic picture of the structure of quantum geometry with the Planck scale, which is 20 orders of magnitude smaller than an atom. The key problem that loop quantum gravity has had to face is, How does the spacetime we see around us emerge from that quantum picture? How do the equations of general relativity emerge to describe the dynamics of that spacetime on a big, macroscopic scale? And there’s been a lot of progress to answering those questions in the last five or ten years. So it’s very healthy as a theoretical research program.

  However, there are two big frustrations with it. One of them is that it still doesn’t connect to experiment. I and others have been hoping we would be able to make measurements that would detect the quantum structure of the geometry of spacetime. Those are astrophysical experiments, and those experiments are not showing any sign of that quantum structure. And the other thing is that from my present point of view, loop quantum gravity is successful also when applied to small parts of the universe, but I no longer believe in taking equations of quantum gravity and applying them to the universe as a whole, because time disappears when you do that and I think that time is fundamental. But loop quantum gravity is healthy and is making the kind of incremental progress that healthy research programs make—which doesn’t mean it’s right, but it means that it’s solving the questions it has to solve to be real science.