Quantum mechanics and general relativity are incompatible, but nature is not incompatible with itself. Nature figures out some way to reconcile these ideas. String theory is the obvious case of somewhere where it has been heavily investigated, starting in the ’60s and ’70s and taking off to become very popular in the ’80s. Here we are in almost 2010 and it’s still going strong without having made any connection to experiments. You might want to say at some point, “Show me the money.” What have you actually learned from doing this? String theorists have learned a tremendous amount about string theory, but the question remains, Have we learned anything about nature? That’s still an open question.
One of the reasons string theory is so popular among people who have thought about it carefully is that it really does lead to new things. It really is fruitful. It’s not that you have to make some guess like, “Oh, maybe spacetime is discrete,” or maybe the universe is made of little molecules, or something like that, and then you say, “OK, what do you get from that?” By making this guess, that instead of particles there are little strings, you’re led to thinking, “If I put that into the framework of quantum mechanics I get ten dimensions.” Then, “Oh, it also needs to be supersymmetric. There are different kinds of particles that we actually observe in nature, and if we try to compactify those extra dimensions and hide them, we begin to get things that look like the standard model. We’re learning things that make us think we’re on the right track.”
In the 1990s there was a second superstring revolution that really convinced a lot of the skeptics that we were on the right track. There are still plenty of other skeptics who remain unconvinced. One of the things we learned is that different versions of string theory all come from the same underlying theory. Instead of there being many, many different versions of string theory, there’s probably only one correct underlying theory that shows up in different ways. What you might have thought of as different versions of the universe, different versions of the laws of physics, are really more like different phases of matter. For water, we have liquid water, we have ice, we have water vapor. Depending on the conditions that the water is in, it will manifest itself in different ways and it will have different densities, different speed of sound, things like that. String theory says that’s what spacetime can be like. Spacetime can find itself in different phases, like liquid water or frozen ice. In those different phases, the local laws of physics—the behavior around you—can look completely different. It can look dramatically different.
The most famous example was discovered by Juan Maldacena, a young string theorist who showed that you could have a theory in one version of which spacetime looked like gravity in five dimensions and in another version of which it looked like a four-dimensional theory without any gravity. There are different numbers of dimensions of space. In one version of the theory, there is gravity, and in another there is no gravity, but they’re really the same theory under it all. To say that string theory [is a theory] of gravity is already not quite the whole story. It’s a theory of some versions look like gravity, some versions you don’t have any gravity.
The reason why that’s so crucial is that there are a lot of philosophical problems that arise when you try to quantize gravity that don’t arise when you try to talk about ordinary theories of particle physics without gravity. For example, the nature of time. Does the universe have a beginning? Do space and time emerge, or are they there from the start? These are very good questions, to which, a priori, we don’t know the answer, but string theory has now given us a concrete, explicit playground, a toy example, where in principle all the answers are derivable. In practice, it might require a lot of effort to get there, but you can translate any question you have into a question in ordinary field theory without gravity. There is no beginning to time. Time and space are there, just as they are in ordinary particle physics.
We have learned a lot from string theory about what quantum gravity can be like. Whether or not it actually is, whether quantum gravity shows up in the real world, is still a little bit up for grabs. One of the problems is that it’s easy to say you have different phases and that’s interesting. The problem is that there are far too many phases. It’s not like you have ten or twelve different possibilities and you need to match the right one onto the universe. It’s like we have 101000 different possibilities, or even maybe an infinite number of possibilities. Then you say, “Well, anything goes.” You run into problems with falsifiability. How do you show that a theory is not right if you can get anything from it? My answer to that is, we just don’t know yet. But that doesn’t imply that we will never know.
The other thing is that we predict in string theory that there is a multiverse—that not only can you have different conditions in different places in the universe, but you will. If you combine ideas from string theory with ideas from inflation, you imagine that this universe we observe ourselves to be in is only a tiny little part of a much, much larger structure where things are very different. People say, “Can you even talk about that and still call yourself a scientist? You talk about all this stuff we can never observe.” The thing to keep in mind is that the multiverse is not a theory. The multiverse is a prediction of a theory. This theory that involves both string theory and inflation predicts that there should be regions outside what we can observe where conditions are very different. That’s a crucially important difference, because we can imagine testing the theory in other ways even if we can’t directly test the idea of the multiverse. The idea of the multiverse might change our expectations for why a certain thing we observe within our universe is a problem or not. It might say this issue about the small vacuum energy that we have isn’t a problem, because we’re just in one region of the universe that’s not representative for one reason or another.
Basically, the short version of this long story is that we’re on a long-term project here. We have very good ideas within string theory for reconciling quantum mechanics and gravity. We don’t know if it’s the right idea, but we’re making progress. The fact that we don’t yet know the answer—we can’t yet make a firm falsifiable prediction for the Large Hadron Collider or for gravitational-wave observatories or for cosmology—is not in any way evidence that string theory is not on the right track. We have to both push forward with the experiments, get our hands dirty, learn more about cosmology, dark matter, and dark energy, and also push forward with the theories. Develop them to a point where we really can match them up to some experiment we haven’t yet done.
We need some great ideas to be pushing forward from the condition we’re in right now into the future, and my personal expertise is on the theoretical side of things. It is very often the case that the actual progress comes from the experimental side of things—and not just the experimental side of things but from experiments you hadn’t anticipated were going to surprise you. There are these wonderful experiments people are doing—not only the big experiments with LIGO, detecting gravitational waves, and the LHC looking for new particles, but also smaller, table-top experiments, looking for small deviations from Newton’s law of gravity, looking for new forces of nature that are very weak, or new particles that are very hard to detect. I need to say that it would be very likely that one of these experiments in some unanticipated way jolts us out of our dogmatic slumber and give us some new ideas.
I have an opinion, which is slightly heterodox, about the standard ideas in cosmology. The inflationary universe scenario that Alan Guth really pioneered, people like Andre Linde and Paul Steinhardt really pushed very hard—this is a wonderful idea, which I suspect is right. I suspect that some part of the history of the universe is correctly explained by the idea of inflation, the idea that we start in this little tiny region that expanded and accelerated at this superfast rate. However, I think that the way most people, including the people who invented the idea, think about inflation is wrong. They’re too sanguine about the idea that inflation gets rid of all the problems that the early universe might have had.
There’s this feeling that inflation is like confession—that it wipes away all prior sins. I don’t think that’s right. We haven’t explained what needs to be explained until we take seriously the question of why inflation ever started in the first place. It’s actually a mistake, and something wrong on the part of many of the people who buy into inflation, that inflation doesn’t need to answer that question because once it starts it answers all the questions you have.
When I was in graduate school happily reading all these different papers and learning different things, some of the papers I read were by Roger Penrose, who was a skeptic about the prevailing conventional wisdom concerning the inflationary universe scenario. Penrose kept saying over and over again in very clear terms that inflation doesn’t answer the question we want answered because it doesn’t explain why the early universe had a low entropy. It says why the universe evolved in the way it did by positing that the universe started in an even lower entropy state than was conventionally assumed. It’s true that if you make that assumption, everything else follows, but there’s no reason, Penrose said, to make that assumption. I read those papers and I knew that there was something smart being said there, but I thought Penrose had missed the point and so I basically dismissed him.
Then I read papers by Huw Price, who is a philosopher in Australia and who made basically the same point. He said that cosmologists are completely fooling themselves about the entropy of the universe. They’re letting their models assume that the early universe had a low entropy, the late universe has a very high entropy. But there’s no such asymmetry built into the laws of physics. The laws of physics at a deep level treat the past and the future the same. But the universe doesn’t treat the past and the future the same. One way of thinking about it is, if you were out in space floating around, there would be no preferred notion of up or down, left or right. There is no preferred direction in space. Here on Earth, there’s a preferred notion of up or down because there’s the Earth beneath us. There is this dramatic physical object that creates a directionality to space, up versus down. Likewise, if you were in a completely empty universe, there would be no notion of past and future. There would be no difference between one direction of time or the other.
The reason we find a direction in time here in this room, or in the kitchen when you scramble an egg or mix milk into coffee, is not because we live in the physical vicinity of some important object but because we live in the aftermath of some influential event, and that event is the Big Bang. The Big Bang set all of the clocks in the world. When we get down to how we evolve, why we are born and then die, and never in the opposite order, why we remember what happened yesterday and we don’t remember what’s going to happen tomorrow, all these manifestations of the difference between the past and the future are coming from the same source. That source is the low entropy of the Big Bang.
This is something that was touched on way back in the 19th century, when the giants of thermodynamics like Boltzmann and Maxwell were trying to figure out how entropy works and how thermodynamics works. Boltzmann came up with a great definition of entropy, and he was able to show that if the entropy is low, it will go up. That’s good because that’s the second law of thermodynamics. But he was stuck on this question of why was the entropy low to begin with. He came up with all these ideas which are very reminiscent of the same kinds of ideas that cosmologists are talking about today. Boltzmann invented the idea of a multiverse, the anthropic principle, where things were different in some regions of the universe than in others and we lived in an unrepresentative part of it. But he never really quite settled on what he thought was the right answer, which makes perfect sense, because still today we don’t know what the right answer is. We know very well how to explain that I remember yesterday and not tomorrow, but only if we assume that we start the universe in a low-entropy state.
I like to say that observational cosmology is the cheapest possible science to go into. Every time you put milk in your coffee and watch it mix and realize that you can’t unmix that milk from your coffee, you’re learning something profound about the Big Bang—about conditions in the very, very early universe. This is just a giant clue the real universe has given to us to how the fundamental laws of physics work. We don’t yet know how to put that clue to work. We don’t know the answer to the whodunnit, who is the guilty party, why the universe is like that. But taking this question seriously is a huge step forward in trying to understand how the universe we see around us directly fits into a much bigger picture.
8
In the Matrix
Martin Rees
Former president, the Royal Society; emeritus professor of cosmology and astrophysics, University of Cambridge; master, Trinity College; author, From Here to Infinity
This is a really good time to be a cosmologist, because in the last few years some of the questions we’ve been addressing for decades have come into focus. For instance, we can now say what the main ingredients of the universe are: It’s made of 4-percent atoms, about 25-percent dark matter, and 71-percent mysterious dark energy latent in empty space. That’s settled a question we’ve wondered about certainly the entire thirty-five years I’ve been doing cosmology.
We also know the shape of space. The universe is “flat”—in the technical sense that the angles of even very large triangles add up to 180 degrees. This is an important result that we couldn’t have stated with confidence two years ago. So a certain phase in cosmology is now over.
But as in all of science, when you make an advance, you bring a new set of questions into focus. And there are really two quite separate sets of questions that we are now focusing on. One set of questions addresses the more environmental side of the subject; we’re trying to understand how, from an initial Big Bang nearly 14 billion years ago, the universe has transformed itself into the immensely complex cosmos we see around us, of stars and galaxies, et cetera; how, around some of those, stars and planets arose; and how, on at least one planet, around at least one star, a biological process got going and led to atoms assembling into creatures like ourselves, able to wonder about it all. That’s an unending quest—to understand how the simplicity led to complexity. To answer it requires ever more computer modeling, and data in all wavebands from ever more sensitive telescopes.
Another set of questions that come into focus are the following:
• Why is the universe expanding the way it is?
• Why does it have the rather arbitrary mix of ingredients?
• Why is it governed by the particular set of laws which seem to prevail in it, and which physicists study?
These are issues where we can now offer a rather surprising new perspective. The traditional idea has been that the laws of nature are somehow unique; they’re given, and are “there” in a Platonic sense independent of the universe which somehow originates and follows those laws.
I’ve been puzzled for a long time about why the laws of nature are set up in such a way that they allow complexity. That’s an enigma, because we can easily imagine laws of nature which weren’t all that different from the ones we observe but which would have led to a rather boring universe—laws which led to a universe containing dark matter and no atoms; laws where you perhaps had hydrogen atoms but nothing more complicated, and therefore no chemistry; laws where there was no gravity, or a universe where gravity was so strong that it crushed everything; or whose lifetime was so short that there was no time for evolution.
It always seemed to me a mystery why the universe was, as it were, “biophilic”—why it had laws that allowed this amount of complexity. To give an analogy from mathematics, think of the Mandelbrot Set; there’s a fairly simple formula, a simple recipe that you can write down, which describes this amazingly complicated pattern, with layer upon layer of structure. Now, you could also write down other rather similar-looking recipes, similar algorithms, which describe a rather boring pattern. What has always seemed to me a mystery is why the recipe, or code, that determined our uni
verse had these rich consequences, just as the algorithms of the Mandelbrot Set, rather than describing something rather boring in which nothing as complicated as us could exist.
For about twenty years I’ve suspected that the answer to this question is that perhaps our universe isn’t unique. Perhaps, even, the laws are not unique. Perhaps there were many Big Bangs, which expanded in different ways, governed by different laws, and we are just in the one that has the right conditions. This thought in some respect parallels the way our concept of planets and planetary systems has changed.
People used to wonder, Why is the Earth in this rather special orbit around this rather special star, which allows water to exist, or allows life to evolve? It looks somehow fine-tuned. We now perceive nothing remarkable in this, because we know that there are millions of stars with retinues of planets around them: Among that huge number there are bound to be some that have the conditions right for life. We just happen to live on one of that small subset. So there’s no mystery about the fine-tuned nature of the Earth’s orbit; it’s just that life evolved on one of millions of planets where things were right.
It now seems an attractive idea that our Big Bang is just one of many. Just as our Earth is a planet that happens to have the right conditions for life among the many, many planets that exist, so our universe, and our Big Bang, is the one out of many which happens to allow life to emerge, to allow complexity. This was originally just a conjecture, motivated by a wish to explain the apparent fine-tuning in our universe—and incidentally a way to undercut the so-called theological design argument, which said that there was something special about these laws.
The Universe_Leading Scientists Explore the Origin, Mysteries, and Future of the Cosmos Page 11