Now, one can ask how this might compare to the present. What kind of critical opalescence marks the science of recent times? It seems to me fairly rare, but one place you might see it is in the collection of sciences that have grown up around computation. Here, ideas about the mind, about how computers function, and about science, codes, and mathematical physics all come together. Von Neumann thinks about the mind and its organs (memory, input-output, processing) as a way of designing a programmed computer. The programmed computer then becomes a model for the mind. The ideas of information, which are encoded into the development of computation, also become ways to understand language and communication more generally, and again feed back into devices. Information, entropy, and computation become metaphors for us at a much broader level. Such opalescent moments are not that common, surely rarer than whatever it is that we mean by scientific revolutions. They’re something else. No, points of critical opalescence in this sense point to science in times and places where we’re starting to think with and through machines at radically different scales, where we are flipping back and forth between abstraction and concreteness so intensively that they illuminate each other in fundamentally novel ways—in ways not captured by models of simple evaporation or condensation. When we see such opalescence, we should dig into them, and deeply, for they are transformative moments of our cultures.
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Thinking About the Universe on the Larger Scales
Raphael Bousso
Professor of theoretical physics, Berkeley
INTRODUCTION by Leonard Susskind
The parable of the blind men and the elephant is a perfect metaphor for the universe and for the physicists who try to grasp its larger shape. Each man feels a part of the elephant and tries to visualize its overall essence: “It’s a wall”; “It’s a rope”; “It’s a tree.” They almost come to blows. The universe, even more than the elephant, is too big for any one perspective, and most of us are busy squabbling about some small part.
Fortunately, now and then someone comes along who’s brave enough, bold enough, and with clear enough vision, to have a chance of seeing the bigger picture. Raphael Bousso is one of those few.
Thinking About the Universe on the Larger Scales
We can ask ourselves questions at different levels of generality—or profundity, if you will—and I guess as a scientist it’s important to strike a balance. We tend to not make much progress if we decide to work on the deepest, most far-reaching questions straight out. It’s good to have those as a compass, but it’s important to break things up in some way, and the way that I would break things up is to say, “The far-reaching questions are things like how do we unify all the laws of nature, how do you do quantum gravity, how do you understand how gravitation and quantum mechanics fit together, how does that fit in with all the other matter and forces that we know?” Those are really far-reaching and important questions.
Another far-reaching question is, “What does the universe look like on the largest scales?” How special is the part of the universe that we see? Are there other possibilities? Those questions are connected with each other, but in order to try to answer them we have to come up with specific models, with specific ways to think about these questions, with ways to break them down into pieces—and of course, most important, with ways to relate them to observation and experiment.
One important hint that came along on the theoretical side a long time ago was string theory, which wasn’t invented for this sort of deep-sounding questions. It was invented to understand something about the strong force, but then it took on its own life and became this amazing structure that could be explored and that started spitting out these answers to questions you hadn’t even thought of asking yet, such as quantum gravity. It started doing quantum gravity for you.
This is a controversial issue. There are other approaches to this problem of quantum gravity. I personally find the string theory by far the most well developed and the most promising, and so I find myself looking for hints about the answers to these kinds of questions that are outlined by using string theory, by exploring the properties of this theory, by asking it what it tells us about these questions.
Another hint that helps us break things up and lower the questions down to accessible levels is, of course, observational: What do we see when we look out the window? The one thing that’s really remarkable that we see, and it’s remarkable in the way that the question of why the sky is not bright at night is remarkable, is . . . It sounds stupid, but when you really think about it, that’s a profound question and it needs an explanation: “Why isn’t there a star everywhere you look?” A similar kind of question is, “Why is the universe so large?” It’s actually extremely remarkable that the universe is so large, from the viewpoint of fundamental physics. A lot of amazing things have to happen for the universe not to be incredibly small, and I can go into that. One of the things that has to happen is that the energy of empty space has to be very, very small for the universe to be large. And in fact, just by looking out the window and seeing that you can see a few miles out, it’s an experiment that already tells you that the energy of empty space is a ridiculously small number, 0.000 and then dozens of zeros and then a 1. Just by looking out the window, you learn that.
The funny thing is that when you calculate what the energy of empty space should be using theories you have available—really well-tested stuff that’s been tested in accelerators, like particle theory, the standard model, things that we know work—you use that to estimate the energy of empty space, and you can’t calculate it exactly on the dot. But you can calculate what the size of different contributions is, and they’re absolutely huge. They should be much larger than what you already know it can possibly be—again, not just by factor of 10 or 100 but by a factor of billions, of billions of billions of billions.
This requires an explanation. It’s only one of the things that has to go right in order for the universe to become as large as we see it, but it’s one of the most mysterious properties that turned out to be right for the universe to become large. It needs an explanation.
Funnily enough, because we knew that that number had to be so small—that is, the energy of empty space, the weight of empty space, had to be so small—it became the lore within at least a large part of the physics community that it was probably zero, for some unknown reason. And one day we’d wake up and discover why it’s exactly zero. But instead, one day in 1998 we woke up and discovered that it’s non-zero. One day we woke up in 1998 and discovered that cosmologists had done some experiments that looked at how fast the universe has been accelerating at different stages of its life, and discovered that the universe had started to accelerate its expansion—whereas we used to think that what it would do is explode at the Big Bang and then kind of get slower and slower in the way that galaxies expand away from one another. Instead, it’s like you went off the brakes and stepped on the gas pedal a few billion years ago. The universe is accelerating. That’s exactly what a universe does if the energy of empty space is non-zero and positive, and you could look at how fast its acceleration is happening and deduce the actual value of this number. In the last thirteen years a lot of independent observations have come together to corroborate this conclusion.
It’s still true that the main thing we needed to explain is why the cosmological constant, or the energy of empty space, isn’t huge. But now we also know that the explanation was definitely not going to be that for some symmetry reason that number is exactly zero. And so we needed an explanation that would tell us why that number is not huge but also not exactly zero.
The amazing thing is that string theory, which wasn’t invented for this purpose, managed to provide such an explanation, and in my mind this is the first serious contact between observation, experiment on the one side, and string theory on the other. It was always interesting to have a consistent theory of quantum gravity. It’s very hard to write that down in the first place, but it turned out that string the
ory has exactly the kind of ingredients that make it possible to explain why the energy of empty space has this bizarre, very small, but non-zero value.
I thought I was going to become a mathematician and then decided to study physics instead, at the last minute, because I realized that I actually cared about understanding nature, and not just some abstract—perhaps beautiful, but abstract—construct. I went to Cambridge, the one in England, for my PhD. I worked with Stephen Hawking on questions of quantum properties of black holes and how they might interplay with early-universe cosmology. I went on to Stanford for a postdoc. At Stanford, I talked a lot to Andrei Linde and Lenny Susskind, and we all felt it was time for string theory to have some sort of say about cosmology—that string theory had not really taught us enough about cosmology—and we started thinking in various different ways about how string theory might do that.
One idea that was floating around was called the holographic principle. This is an idea that deals with the question of how much information you need to describe a region of spacetime at the most fundamental level, and surprisingly the answer is not infinity. Even more surprising, the answer doesn’t grow with the volume of the region. As ’t Hooft and Susskind had first intuited, the answer is related to the area surrounding the region. But the idea didn’t really fully work, especially in cosmology. So one of the topics I worked on was trying to understand whether this idea of the holographic principle is really correct, whether it can be formulated in such a way that it makes sense in all imaginable spacetime regions, in cosmology, inside black holes, and not just in some harmless place where gravity is not important. That turned out to be true, and so that was very exciting.
Another topic I started thinking about was trying to understand the small but non-zero value of the cosmological constant, the energy of empty space, or, as people like to call it, dark energy. I worked on that subject with Joe Polchinski, at KITP, in Santa Barbara, and we realized that string theory offers a way of understanding this, and I would argue that that is the leading explanation currently of this mysterious problem. From Stanford, I went on to a postdoc at Santa Barbara, and then a number of small stops here and there, including one year at Harvard. In 2004, I joined the faculty at Berkeley, where I am now a professor in the Physics Department.
I don’t do experiments, in the sense that I would walk into a lab and start connecting wires to something. But it matters tremendously to me that the theory I work on is supposed to actually explain something about nature. The problem is that the more highly developed physics becomes, the more we start asking questions which for technological reasons are not in the realm of day-to-day experimental feedback. We can’t ask about quantum gravity and expect at the same time to be getting some analogue of the spectroscopic data that in the late 19th century fed the quest for quantum mechanics. And I think it’s a perfectly reasonable reaction to say, “Well, in that case I think the subject is too risky to work on.” But I think it’s also a reasonable reaction to say, “Well, but the question, it’s obviously a sensible one.” It’s clearly important to understand how to reconcile quantum mechanics and general relativity. They’re both great theories, but they totally contradict each other, and there are many reasons to believe that by understanding both we will learn profound things about how nature works. Now, it could be that we’re not smart enough to do this—in particular, without constant feedback from experiments—but we have been pessimistic at so many junctures in the past and we found a way around.
I don’t think we’re going to understand a lot about quantum gravity by building more particle accelerators. We’ll understand a lot of other things, even a few things about quantum gravity, but rather indirectly. But we’ll look elsewhere, we’ll look at cosmological experiments, we’ll use the universe to tell us about very high energies. We’ll come up with ideas that we can’t even dream about right now. I’m always in awe of the inventiveness of my experimental colleagues, and I don’t doubt that they will deliver for us eventually.
It has been said that it’s been a golden age for cosmology in the last fifteen years or so, and it’s true. I was very lucky with timing. When I was a graduate student, the COBE satellite was launched, and started flying and returning data, and that really marked the beginning of an era where cosmology was no longer the sort of subject where there were maybe one or two numbers to measure and people had uncertainties on, say, how fast the universe expands. They couldn’t even agree on how fast the galaxies are moving away from one another. And from this, we move to a data-rich age where you have unbelievably detailed information about how matter is distributed in the universe, how fast the universe is—not just expanding right now but the expansion history, how fast it was expanding at earlier times, and so on. Things were measured that seemed out of reach just a few years earlier, and so indeed it’s no longer possible to look down on cosmology as this sort of hand-waving subject where you can say almost anything and never be in conflict with the data. In fact, a lot of theories have gone down the road of being eliminated by data in the past fifteen years or so, and several more are probably going to go down that road pretty soon.
An example of a theory that has been ruled out is one of the ideas for how structure originally formed in the universe. Why isn’t the universe just some sort of homogeneous soup? Why are there clumps of galaxies here, empty spaces there, another galaxy here? How did that come about, and how did the particular way in which these objects are distributed come about? Why are they the size they are, why aren’t they larger or smaller, why isn’t there maybe just one galaxy which is really huge, and the rest, all we can see, empty? This clearly needs an explanation.
There were a number of different theories on the market. One of them was inflation. One of the nice things about it was that it was not originally invented for the purpose of explaining this, but it turned out to have something to say about this question. Then there are other theories that were also reasonably well motivated, such as cosmic strings—not the same thing as the string-theory strings but objects that we call topological defects. Basically, these are objects which are stringlike, and energy is sort of locked into them in a way that it can’t get out, because of the way that the universe cooled down as it was expanding very early on. And cosmic strings would lead to some sort of structure if you have the right kind of cosmic strings around, but it makes very different detailed predictions about what that structure looks like, what kind of imprints it leaves in the cosmic microwave background that satellites like COBE have now measured so well, and that Planck is currently measuring with incredible precision.
We already know enough about the cosmic microwave background that we can completely rule out the possibility that cosmic strings are responsible for structure formation. It’s, of course, possible that there are cosmic strings out there, but they would have to be of a type that has not had any impact on structure formation.
Inflation looks really good. It’s not like we have a smoking-gun confirmation of it, but it has passed so many tests—it could have been ruled out quite a few times by now—that it, I would say, is looking really interesting right now.
Inflation comes in many detailed varieties, but it does make a number of rather generic predictions, and unless you work very hard to avoid them, they come with pretty much every inflation model you grab off the shelf. One of those predictions is that the spatial geometry of the universe would be flat. It should be the kind of geometry you learn about in high school, as opposed to the weird kind of geometry that mathematicians study in university, and that has turned out to be the case. To within a percent precision, we now know that the universe is spatially flat. Inflation predicts a particular pattern of perturbations in the sky, and again, to the extent that we have the data—and we have very precise data by now—there was plenty of opportunity to rule out that prediction, but inflation still stands. So there are a number of general predictions that inflation makes which have held up very well, but we’re not yet at a point where we c
an say it’s this particular make and model of inflation that’s the right one and not this other one. We’re zooming in. Some types of inflation have been ruled out, large classes of models have been ruled out, but we haven’t zoomed in on the one right answer, and that might still take a while, I would expect.
I was saying that string theory has in a way surprised us by being able to solve a problem that other theories, including some that were invented for that purpose alone, had not been able to address—i.e., the problem of why empty space weighs so little, why there’s so little dark energy. The way string theory does this is similar to the way we can explain the enormous variety of what we see when we look at the chair, the table, and the sofa in this room. What are these things?
They’re basically a few basic ingredients—electrons, quarks, and photons. You’ve got five different particles, and you put them together, and now you’ve got lots and lots of these particle. There are very few fundamental ingredients, but you have many copies of them. You have many quarks, you have many electrons, and when you put them together you have a huge number of possibilities of what you can make. It’s just like with a big box of Legos: There are lots of different things you can build out of that. With a big box of quarks and electrons, you can build a table if you want, or you can build a chair if you want. It’s your choice. And strictly speaking, if I take one atom and I move it over here to a slightly different place on this chair, I’ve built a different object. These objects in technical lingo will be called solutions of a certain theory called the standard model. If I have a block of iron, I move an atom over there, it’s a different solution of the standard model.
The Universe_Leading Scientists Explore the Origin, Mysteries, and Future of the Cosmos Page 28