There are amazing young people working in the subject, people who are technically brilliant, who are able to do things that just amaze me, and that’s a great pleasure. I’m partly in and partly not in that research community, because my interests in cosmology and my interest in the nature of time and other interests take me outside of it. But I still have many good friends there; I still go to the conferences. Some of what I do is in that context of loop quantum gravity, and I’m very happy to be a part of that community. But I’m not sitting at the center of it anymore, which is fine, because the people who are sitting at the center of it are better able to hold that position than I am.
String theory, which I’ve also worked on, is in part healthy as a research program and in part stuck. We no longer hear much from string theorists about what is the fundamental formulation of string theory, or M-theory, as we used to call it, which is the part I was most interested in and tried to work on. And we no longer hear—although I think many people still believe them—we no longer hear aggressive claims of string theory being a theory of everything.
There are two areas in which string theory is doing very well. One of them is mathematically. It’s beautiful mathematics and mathematical physics. And it also provides applications—through what’s called the Maldacena conjecture, or the AdS/CFT conjecture, to give the jargon—to ordinary systems: liquids, fluids, certain solid-state systems. The same methods can be used to in a new way illuminate some experimental phenomena. That has nothing to do with string theory as a unified theory, but it’s developing very nicely. Then there are other programs. There is causal dynamical triangulations, quantum gravity, causal sets—these are things worked on by handfuls of people, and they are part of the landscape of ideas.
In cosmology, inflation and the standard model of cosmology are doing very well observationally. But I think that Neil Turok and Paul Steinhardt have a very important point, which I agree with, which is that if you don’t address the singularity, the time before inflation—if inflation is true—when the universe becomes infinitely dense at a finite time in the past and general relativity stops working, you can’t address, really, the question of what chose the initial conditions. And also, to me, what chose the laws.
It seems to me a necessary hypothesis that the Big Bang was not the first moment of time but was an event—a transition, something like a phase transition, before which there was a universe that had possibly different properties and different laws. So the Big Bang becomes a phase transition, something like a black hole that formed in a previous universe. There would have been a singularity to the future of that formation of that black hole and instead that singularity is wiped out by quantum effects and, as we say, bounces. Whereas the star was collapsing and was going to just collapse to infinite density, quantum effects make it bounce back and start expanding again. And that makes a new region of space and time, which can be a new universe.
That’s one hypothesis about what the Big Bang was as a transition. Paul and Neil have a different hypothesis, which has to do with the whole universe, as a whole, going through a phase transition. The quantum-gravity people have a different hypothesis. According to this hypothesis, it’s as if the properties of space as we know it are like a frozen piece of ice, and when the universe goes through a Big Bang it’s like space melts and becomes liquid and rearranges its properties and then freezes again. The Big Bang was something like a big freeze following a temporary melting. It seems to me that it’s a necessary hypothesis to explain the initial conditions, because the one thing that inflation doesn’t do, which it claimed to do, was make the initial conditions of the universe probable or explain why the universe is so unusual in its early stages. And whatever the fate of Paul and Neil’s cyclic cosmology, their particular hypothesis, I think they’re right in their critique of inflation. Whether inflation is right or not, I think they’re right that there had to have been a phase transition replacing the Big Bang and therefore the explanations for things in the early universe will be pushed back to before the Big Bang.
And that, of course, intersects with my interest in quantum gravity, because quantum mechanics has to become important at those scales where the phase transition happened. And, indeed, over in the quantum gravity world we have models of quantum cosmology, so-called loop-quantum-cosmology models, developed by Martin Bojowald, Abhay Ashtekar, and many other people by now, which show this bounce happening—show that the singularities are always removed and replaced by bounces.
Cosmology has been very healthy because of the success of the standard model of cosmology. But we’re left with a question similar to the question the particle physicists are left with: the question of Why this peculiar universe? We’ve measured the properties of the universe very well. Whether inflation is true or not, it’s an improbable universe. Why this universe? Why not other universes that would be more typical, given what we understand about the laws?
This is the initial-conditions problem. One of my major points is that we can’t address those kinds of questions on the basis of the same methodology that has worked so far. We need a new methodology for physics—one in which laws evolve.
Is there a community of people thinking the same way? Yes and no. There aren’t very many people within either the cosmology community or the quantum gravity community. So, for example, Carlo Rovelli is my dear friend, and in some areas, like loop quantum gravity, we are very much in sync, but Carlo is still a believer in the fundamental timelessness of quantum gravity and quantum cosmology and I am not. Although we talk about it.
In the world of philosophy, what I’m doing is not new and not a surprise. I mentioned Roberto Unger. Our collaboration has been at times like what Picasso once said about his collaboration with Braque: It’s like being at times roped together on a mountain. And with Roberto, it’s been a wonderful adventure, to develop these ideas and to provoke each other. There’s also a philosophical context going back to the American pragmatist tradition, going back to Charles Sanders Peirce. In that context, none of the ideas I’m talking about are new or particularly surprising. So, how philosophers will react is unclear, but I’m in a context—the context of ideas I’m talking about, in which time is real and laws can change—these are issues that they’ve been talking about and discussing and debating and have positions on already for a century.
I hope to convince people, because the chain of thought that I’ve been through is not serendipity, it’s not where I planned to end up, and it’s not where I hope to end up. I don’t actually like being out on my own. I don’t actually enjoy controversy and conflict, unlike some other people we can mention. I feel like my job is to develop these ideas, to put them out there, and especially to develop them in a form in which they’re science and not philosophy. The philosophers can develop the philosophy.
And let me mention, if I may, another ramification of the idea that time is real as opposed to emergent or an illusion. The second law of thermodynamics is very well established and is, on a microscopic scale, clearly true. Disorder increases, entropy increases, most things we deal with in our everyday lives are irreversible. There are strong arrows of time. There’s a directionality of time: We can’t go backwards. We are born, we grow up, we get older, and we die. If I spill this Coke on the carpet, nothing we can do can make it go back into the cup. The birth of a child is irreversible. An unkind word said accidently to a friend is irreversible. Many things, most things, in life are irreversible. This is mostly codified by the second law of thermodynamics.
In the late 19th century, Boltzmann proposed successfully that thermodynamics was not fundamental, because matter was made of atoms. Instead, he proposed that the laws of thermodynamics could be explained as being emergent from the behavior of atoms, so that they are a consequence of the fundamental laws the atoms obey. So temperature is not a fundamental quantity, it’s the average energy caught up in the random motion of atoms, and so forth. And entropy is not a fundamental quantity, it’s a measure of the disorder or
the improbability or the probability of a configuration of atoms.
Boltzmann was right, but there was a paradox inherent in his reasoning which people at the time identified. It’s shocking to think that, but at the time, in the late 19th century, the atomic hypothesis was not wildly popular, and there was no consensus among physicists. So he had intellectual opponents. And they said to him, “You claim to have derived as emergent a theory that has a strong directionality of time, from the fundamental laws of motion of Newton. But the fundamental laws of motion of Newton are reversible in time.” If you take a picture, take a film of atoms moving about in a void, interacting according to Newton’s laws, and you run that film backwards, that’s something that also can happen, according to the laws. So there’s a kind of paradox, because Boltzmann could just as easily have used Newton’s laws to prove the anti-second law, to prove that entropy is always higher in the past and lower in the future.
And, indeed, the critics were right. The right way to resolve this puzzle was worked out by the Ehrenfests—Paul and Tatyana Ehrenfest, who were dear friends of Einstein around 1905, 1908, I think, or somewhere around then. And they understood that actually what Boltzmann had proved was symmetric in time. What he proved is that if you find a system with the entropy low at one time, it’s most likely to increase in the future, because disorder most likely increases when things move about randomly. But it’s also most likely that the entropy was higher in the past and that what you’re seeing is an accident—what he called a fluctuation. And so the question is not “What explains the second law?” but “What explains the conditions, the initial conditions?”
To explain the second law, you have to assume that the initial conditions are improvable, so the system starts out more ordered than it might be. And this was a great mystery to Boltzmann. He didn’t have the benefit of living long into the 20th century, so he imagined that the universe was governed by Newton’s law and was eternal. And he could only assume that we lived in the wake of a huge fluctuation where the universe was mostly in equilibrium—which is the state when entropy is maximal—and spent most of its time in equilibrium and just occasionally, due to a random fluctuation, got way out of equilibrium. And that formed the sun, and that was the cause of the world we were living in now. Now, that’s wrong. There’s no evidence for that.
So why is there such a strong arrow of time, if the laws of physics are fundamentally reversible in time? Well, Roger Penrose had an idea I think is very worth investigation, which is in two parts. One of them—this was in an essay in 1979—he argued, and I think correctly, that the only way to explain the arrow of time as we observe it in the universe is if the initial conditions of the Big Bang were very, very special and very, very improbable. And that’s a theme of my discussion, and that reoccurs here.
So, yet another sense in which to explain, within the present paradigm of reversible laws the tremendous irreversibility of phenomenon that we observe, you have to put all the weight on the cosmologists to explain why the initial conditions were so improbable. And, as a cosmologist I know remarked, that’s not a job the cosmologists signed up for. They have enough to do. They have enough problems of their own, let alone having to explain the whole irreversibility of nature and the second law. But that’s where the burden of proof is.
Now, Roger Penrose’s proposal was that maybe the fundamental laws are actually time-asymmetric and the time-symmetric laws are emergent and approximate. And so maybe all those histories where we take a movie of part of the universe and run it backwards couldn’t really be part of history of the real universe going all the way back to the Big Bang. Let me give an example of that.
When we look around, we see light coming from the past. I mean it’s more evident when we look out in telescopes; we see stars as they were in the past. We never see light coming to us from the future. We never see starlight coming to us from stars in the future. We never see supernova explosions in the future sending radiation back in time to us. But the laws that govern the propagation of light, Maxwell’s equations, are reversible in time, so it has solutions that involve light propagating from events in the future and propagating information and energy into the past for us to observe from the past. It has just as many solutions like that as it has solutions of the kind we use. So the law is symmetric in time, but to apply it to nature we throw away most of it, because we throw away any solutions where there’s any hint of anything propagating from the future to the past.
Roger would say, “Maybe the real theory that underlies Maxwell’s equations”—which for him, and I agree, would be the real quantum theory of gravity—“just propagates energy and information from the past into the future and doesn’t have this problem and this paradox.” So this then becomes a challenge. Can we make hypotheses about how the fundamental laws could really be asymmetric in time and irreversible in time and understand how the present laws become reversible? And with a colleague, Marina Cortes, I’ve been working on that.
So that’s another way in which these philosophical critiques that I think are necessary to understand why we’re stuck in fundamental physics and cosmology serve to motivate my work as a scientist. That work is then to be appreciated or not and evaluated on the basis of the usual criteria of science. That is, does it lead to new hypotheses that lead to new experiments to check them?
10
The Landscape
Leonard Susskind
Felix Bloch Professor of theoretical physics, Stanford University; coauthor (with Art Friedman), Quantum Mechanics: The Theoretical Minimum
INTRODUCTION by John Brockman
For some people, the universe is eternal. For me, it’s breaking news.
Back in 2003 I sat down to talk with Lenny Susskind, the discoverer of string theory. After he left, I realized I had become so caught up in his storytelling that I forgot to ask him, “What’s new in the universe?” So I sent him an email. Here’s his response:
The beginning of the 21st century is a watershed in modern science, a time that will forever change our understanding of the universe. Something is happening which is far more than the discovery of new facts or new equations. This is one of those rare moments when our entire outlook, our framework for thinking, and the whole epistemology of physics and cosmology are suddenly undergoing real upheaval. The narrow 20th-century view of a unique universe, about 10 billion years old and 10 billion light years across with a unique set of physical laws, is giving way to something far bigger and pregnant with new possibilities.
Gradually physicists and cosmologists are coming to see our ten billion light years as an infinitesimal pocket of a stupendous megaverse. At the same time theoretical physicists are proposing theories which demote our ordinary laws of nature to a tiny corner of a gigantic landscape of mathematical possibilities.
This landscape of possibilities is a mathematical space representing all of the possible environments that theory allows. Each possible environment has its own laws of physics, elementary particles and constants of nature. Some environments are similar to our own corner of the landscape but slightly different. They may have electrons, quarks and all the usual particles, but gravity might be a billion times stronger. Others have gravity like ours but electrons that are heavier than atomic nuclei. Others may resemble our world except for a violent repulsive force (called the cosmological constant) that tears apart atoms, molecules and even galaxies. Not even the dimensionality of space is sacred. Regions of the landscape describe worlds of 5,6 . . . 11 dimensions. The old 20th-century question, “What can you find in the universe?” is giving way to “What can you not find?”
The diversity of the landscape is paralleled by a corresponding diversity in ordinary space. Our best theory of cosmology, called inflationary cosmology, is leading us, sometimes unwillingly, to a concept of a megaverse, filled with what Alan Guth, the father of inflation, calls “pocket universes.” Some pockets are small and never get big. Others are big like ours but totally empty. And each lies in its own l
ittle valley of the landscape.
Man’s place in the universe is also being reexamined and challenged. A megaverse that diverse is unlikely to be able to support intelligent life in any but a tiny fraction of its expanse. Many of the questions that we are used to asking, such as “Why is a certain constant of nature one number instead of another?” will have very different answers than what physicists had hoped for. No unique value will be picked out by mathematical consistency, because the landscape permits an enormous variety of possible values. Instead the answer will be “Somewhere in the megaverse the constant is this number, and somewhere else it is that. And we live in one tiny pocket where the value of the constant is consistent with our kind of life. That’s it! There is no other answer to that question.”
The kind of answer that this or that is true because if it were not true there would be nobody to ask the question is called the anthropic principle. Most physicists hate the anthropic principle. It is said to represent surrender, a giving up of the noble quest for answers. But because of unprecedented new developments in physics, astronomy, and cosmology these same physicists are being forced to reevaluate their prejudices about anthropic reasoning. There are four principal developments driving this sea change. Two come from theoretical physics, and two are experimental or observational.
The Universe_Leading Scientists Explore the Origin, Mysteries, and Future of the Cosmos Page 15
