Andrei Linde, a 47-year-old physicist from Moscow, started teaching at Stanford University in 1990. He lives there with his wife, Renata Kallosh (also a Stanford physics professor, specialising in superstrings and supergravity), and his two sons, Dmitri and Alex. In 1982, he began formulating a new theory of the universe - an improvement on the big-bang model. He uses computer simulations for much of his research, and he has recently suggested that our universe may be the result of a physicist-hacker's experiment.
An attractive, tidily dressed man, Linde speaks with a thick Russian accent and a colourfully in-verted syntax. His responses to the questions of mathematician Rudy Rucker have been smoothed by Rucker's interpolation and editing.
Wired: By now, most of us have become quite comfortable with the big-bang model of the universe, the notion that the universe was born as a cosmic explosion that gave birth to an ever-expanding ball of space. What's wrong with this idea?
Linde: There are a number of problems with the big-bang theory, but I'll start by mentioning two of a physical nature and two of a philosophical nature.If you work out the physical equations governing the big bang, they predict that such a universe would be very small, even though we can see that our universe is large. One way to gauge the size of a universe is to talk about how many elementary particles it has in it how many electrons, protons, neutrons and so on are present. When I look out of my window, the matter I see is made up of perhaps 1088 elementary particles, but a typical theoretical big-bang model envisions a universe with only 10 elementary particles in it! This is perhaps the most serious problem with the big-bang model. It gives a false prediction about the size of the universe. For a number of years, this mathematical flaw in the big-bang theory was not taken seriously by many scientists.
But even if a big-bang universe is of the proper size, the theory doesn't explain why different regions of the universe resemble each other. In a big-bang model, it could just as easily have happened that most of the galactical matter would wind up, say, in only one half of the sky, but we can observe that in our universe, the distribution of distant galaxies is uniform in every direction.
Then come the philosophical questions. What came before the big bang? How did everything appear from nothing? Another philosophical problem with the big bang is: why does it happen that our universe worked out to be the way it is? Why, for instance, do we have three dimensions of space and one dimension of time? The big-bang theory offers no satisfactory answers. We can begin to resolve the puzzles in the context of the theory of the self-reproducing, inflationary universe.
What is the theory of the inflationary universe?
There have been several versions of this theory. The first was proposed by the Soviet physicist Alexei Starobinsky, but it was rather complicated. Then a much simpler theory was put forth by Alan Guth, a physicist at MIT; we call his model old inflation now. Guth took the big-bang model and added the idea that in the beginning, the universe expanded rapidly faster even than the speed of light.By using the model of the universe rapidly expanding, you solve the problem of why it's so big, and why all the regions of the universe we can see resemble each other. The idea is that the visible part of the universe was inflated from some very small and homogeneous region, and this is why we see large-scale similarities.
However, it turned out that Guth's old inflation had a theoretical flaw that caused the universes of his scenario to become extremely homogeneous after the inflation stopped. I invented a new inflation theory which worked so-so until I realised that we could have inflation without the assumption that the universe began in a hot and dense state. Therefore, I dropped the idea of the big bang but kept the idea of inflation. In my model, inflation can start anywhere. This concept is called chaotic inflation.
What causes the inflation?
There are things called scalar fields. Scalar is a word physicists use simply to mean a number this as opposed to a vector, which means something like an arrow. If you have a scalar field, you have a certain number defined at each point of space. Air pressure can be thought of as a scalar field: there is a specific number that measures air pressure at each point. In the same way, temperature is a scalar field: at each point, you can measure the value of the temperature.Two scalar fields that are important for inflationary theories are sometimes called the inflaton field and the Higgs field. These fields fill the universe and show their presence by affecting the properties of elementary particles. You don't notice a constant scalar field any more than you notice a constant air pressure or a constant electric charge. When there is a large air pressure, you get wind; when there is a large electrical charge, you get sparks; and when there is a large scalar field called the inflaton field, you get an expansion of space. Quantum mechanics implies that the scalar fields undergo unpredictable fluctuations as a result of the uncertainty principle. If there is a place where the fluctuations make the inflaton field sufficiently large, then at this spot the universe begins expanding very rapidly, which creates so much space that we can safely live there.
What about the "self-reproducing" aspect of your model?
The fluctuations that increase the speed of inflation can happen over and over again - because of the essential fuzziness that the uncertainty principle of quantum mechanics introduces into the equation. This makes the universe self-reproducing; the universe actually replicates itself in all its forms.The standard big-bang theory depicts a homogeneous universe that looks like a single bubble. But if we take into account quantum effects, the self-reproducing inflationary universe is a bubble producing new bubbles producing new bubbles producing new bubbles and so on. This kind of repeatedly branching pattern is what mathematicians call a fractal. A fractal pattern is characterised by the property that the small bits of the pattern are exact replicas of the whole pattern. An oak tree, for example, is like a fractal in that a single branch of an oak resembles a scaled-down model of the entire tree. Another example of a fractal is a mountain range. If you chop off the top of a mountain and look at it closely, it resembles the whole mountain range; a single rock on the mountain resembles a whole mountain in itself.
So, we think of the self-reproducing inflationary universe as a fractal. The big bang works as a description of each particular bubble, but it cannot describe the growing fractal. In the fractal model, there is no real reason for the universe to stop growing indeed, it is likely to keep growing and blooming in new regions forever.
Can you help me visualise this fractal self-reproducing inflationary universe?
There are two kinds of pictures I like to use. In one, I draw something that looks like lots of separate bubbles connected to each other where they touch. It looks a little like the linked flotation bladders on seaweed. In the other picture - and I've done several computer simulations of this image - I think of space as a flat sheet. Our space is three-dimensional of course, but I represent it in this picture as a two-dimensional rubber sheet. Then I add a randomly fluctuating scalar field, an inflaton field, and I represent the regions where the inflaton field has a low numerical value by valleys, and the regions where the scalar field is large by peaks.The peaks are the places where inflation takes place; at these places, the universe will rapidly expand. I can't show the inflation in my picture, but I can represent it by putting new, secondary peaks on top of the first peaks, third-level peaks on top of those peaks, and so on. It is like a mountain range.
What is a little hard to grasp is that the two images represent the same thing. The peaks in one image correspond to the bubbles in the other. A peak that rises on top of a peak is like a bubble that newly swells out from the side of a pre-existing bubble.
Can we travel to the other bubbles of our fractal universe?
In the future, our sky will look much different - as the stars in our neighbourhood begin to die. Then we will see into the other parts of the universe, parts with different laws of physics. Can we use the energy in our cooled-off bubble? Can we fly to the other tips of the fractal? Can we go there and live comfortably? The theory of cosmic flights over vast distances suggests that even if you were to travel at the speed of light, you lose so much time that when you get to another part of the universe, it will be cold and empty there.You say that some of the different bubble universes have different laws of physics - how does that work?
We've talked about the inflaton field responsible for the universe's expansion. As I mentioned before, it seems that there may also be a second scalar field that creates different kinds of physics in different regions of the universe. This is the Higgs field. There is one overall law of physics for the whole universe, but the Higgs field makes for different realisations of this law. The principle is similar to that of water existing in different phases.What if I could somehow fly up to the edge of a region of the universe with different physics? How would it look?
Between the different regions of the universe, there are boundaries called domain walls. There is a tendency of the domain walls to smooth out. You might think of them as being a little like the boundaries of soap bubbles in a foam, with the regions of different physics the insides of those bubbles. The domain walls might be irregular to start with, but over time they straighten out. In addition, the regions will tend to shrink or expand, and this expansion is in fact very rapid - it is at a speed approaching the speed of light. This means that the domain walls will be moving in one direction or another with a speed approaching that of light.So, first of all, it would be difficult for you to reach a domain wall if it is moving away from you at any speed. And if it is moving toward you, it would be difficult to run from it because it will be moving very fast. In fact, if a wall moves toward you at the speed of light, then you first see it only at the moment it hits you. And then you would almost definitely die, since the physics on the other side of the wall would be different and unlikely to support your form of life. You will be exactly like a fish out of water.
But we don't need to worry too much; the distance from us to this next domain wall is typically estimated to be much greater than 10 billion light years, so we may live for now.
Might we say that the regions with different physics compete with each other?
The regions of the universe that grow faster contain more volume, so perhaps they contain more inhabitants. This looks like Darwinian fitness. Should we discriminate and say those with greater volume are winners? There is a lot of place for losers as well; everything that can exist tends to have room for its existence in the self-reproducing inflationary universe. We can think of a Darwinian process without hate and killing.How did the whole process begin?
Maybe the universe didn't have a beginning. There are some philosophical problems with the idea of the universe having a beginning. When the universe was just created, where were the laws of physics written? Where were the laws of physics written if there was no space and no time to write them? Maybe the universe was created without obeying any laws, but then I don't understand. Well, maybe the laws and the universe came into existence simultaneously. Quantum mechanics might say that our universe together with its physical laws appeared as a quantum fluctuation, but how were the laws of quantum mechanics written before creation?In one of your papers, you talk about relating the nature of our consciousness to our universe. What do you mean?
For me, the investigation of the universe is mainly a tool for understanding ourselves. The universe is our cosmic home. You may imagine you can learn something about your friend by looking at how his house is built. My final purpose is not to understand the universe, but to understand life.An example of this is the question of why we humans see time as passing. According to the branch of physics called quantum cosmology, the universe is best represented as a pattern called a wave function that does not depend on time. But then why do I see the universe evolving in time? The answer may be that as long as I am observing the universe, the universe breaks into two pieces: me and the rest of the universe. And it turns out that the wave function for each of these separate pieces does depend on time. But if I merge with the universe, my time stops.
You've suggested that it might be possible to create a universe in the laboratory by violently compressing matter, that 1 milligram of matter may initiate a self-reproducing universe. How would this work?
It would be hard. You have to do more than just compress the matter. But with high temperatures and quantum effects, there is a chance of creating a universe. Our estimates indicate that you would need a very good laboratory indeed. And it is not dangerous to try. This new universe would not hurt our universe; it would only expand within itself like bulging a bubble out from the side of our space.Can you imagine there being any kind of economic or spiritual gain from creating new universes? Might this lead to a Silicon Valley industry or to a cosmological cult?
The question is: is it interesting to create a universe? Would you have a profit or benefit? What would be the use? Suppose life in our universe is dying, and we make a small private universe we can jump into so we have a place to live. But it's not easy to jump between universes. When we create a universe, it is connected to our universe by a very narrow bridge of space - we can't jump through it, and the new universe will repel us because it is expanding.Well, maybe you can get energy from the new universe? No, you can't get energy because of the law of energy conservation. The new universe gets its energy internally, and the energy has to stay inside there. We can't get in, we can't use the energy, but maybe we can do like we do with our children: we teach them and we live on in them. Maybe we can give knowledge and information to the new little universe.
Would you be able to communicate? To send information to and from that universe you helped create?
It is not so easy to send information inside. Say I wrote a message on the surface of an inflationary universe. But then the letters expand so much that for billions of years to come, each race of people in the universe will be living in the corner of just one letter. They will never see the message. The only way I have found to send information is strange and unusual. If I create an inflationary universe with a small density, I can prepare the universe in a particular state that corresponds to different laws of physics, masses of particles, interactions, etc. I can imagine a binary code describing all possible laws of physics; this would be quite a long sequence. So, if I am preparing a universe in some peculiar state, I can send the message encoded in the laws of physics.Can I send a long message in this way? Let's think about our own universe. Let's imagine that someone made our universe as a message. If our universe is perfect, with all particles having equal masses and charges, then the laws of physics would really be trivial, and it would be a very short message. But our particle physics looks weird, and it has a lot of information. We get these strange numbers; there is no harmony when we try to make sense of it all. There is information instead of harmony. Perhaps it would be more precise to say the harmony is there, but it is very well hidden.
To send a long message, you must make a weird universe with complicated laws of physics. It is the only way to send information. The only people who can read this message are physicists. Since we see around us a rather weird universe, does it imply that our universe was created not by God, but by a physicist-hacker?
I don't entirely think of this possibility as a joke. Even if something seems counterintuitive, you must be honest and follow the thought line and not be influenced by the common point of view. If you agree with everything everybody else thinks, you never move.
Rudy Rucker (rucker@jupiter.sjsu.edu) has written a novel The Hacker and the Ants about near-future Silicon Valley. To e-mail Andrei Linde: linde@physics.stanford.edu