Andrei Linde

Lightman:

One thing I was very interested in is how you got into cosmology. You started as a particle physicist, I guess, is that right?

Linde:

It was a long story. My teacher was [D.A.] Kirzhnits in the Lebedev Physical Institute. First I came to him as a student. It was at the end of the 60's, and, during this time, I studied with him a theory of weak interactions and some practical questions connected with cross-sections and some problems related to divergences in this theory. But when I finished my study as a student, I came to him as a post-graduate and he told me "Forget about everything. Now is a new age, the Weinberg-Salam model is renormalizable, and you should study it. The most interesting thing is that it is very similar to the theory of superconductivity." And he [Kirzhnits] understood that at high temperatures, symmetry breaking should disappear in the Weinberg-Salam model, similar to the disappearance of superconductivity at high temperatures.

Lightman:

So, phase transitions...

Linde:

Phase transitions, yes. Superconductivity occurs due to the existence of Cooper pairs, which form a kind of scalar field. And in the Weinberg-Salam model, symmetry breaking occurs due to the scalar Higgs field. At high temperature a metal becomes normal. And at high temperature, symmetry breaking disappears in the Weinberg-Salam model. So there are two similar types of theories. He had written the first paper about it.^[1] Then he explained to me what does all this means and we started working together.

Lightman:

What year was that? Weinberg-Salam was 1967, so this is like 1970?

Linde:

No, you see Weinberg-Salam was in 1967, but the rapid development of this theory starts in 1971-1972, because before that it was not clear that this theory is renormalizable. So, in 1972 we started this investigation. First I did not understand much, being very fresh in this field. Then I started working, and then we started publishing papers on this problem.^[2] Later, [Steven] Weinberg,^[3] [Louise] Dolan and [Roman] Jackiw,^[4] and we with Kirzhnits^[5] wrote some other papers about symmetry breaking. But symmetry restoration occurs only at extremely large temperatures and such temperatures can occur only in the early universe. That led me to the investigation of the early universe, and these studies have been made by myself. Kirzhnitz [was interested] in elementary particles and superconductivity and other things, and I was very interested in applications of this theory to cosmology. So I continued in studying what would be the cosmological implications of these things. So this was the beginning.

Lightman:

Yes, that's very interesting. So, before this [1972], I gather that you really hadn't thought very much about cosmology. Is that right? What thinking had you done about cosmology before this, going all the way back to your childhood?

Linde:

I think that this was the beginning of my thinking about cosmology seriously. Of course, I was interested in such things as cosmology, but when I was just finishing [at the] Institute, I told my mother "Oh, I am already old. It is too late for me to study gravity." [laughs]

Lightman:

Because you thought there were people already working in it?

Linde:

Yes, I studied elementary particle theory, and gravity was something quite different, and so I was afraid... Imagine that I am going to be a post-graduate, and I am now going to choose my own field of research. At that moment, to start studying new objects such as gravitational theory after weak interaction theory would be too complicated. So I didn't believe that I would do it. After that I went into cosmology, and somewhat to my surprise, I studied gravity.

Lightman:

When you talk about how it was too late to study gravity, you were probably thinking of quantum gravity?

Linde:

I was thinking about everything, and that includes general relativity theory, since actually this theory is rather complicated. It has many branches and there was a lot of material which had been worked out for many years. People have studied it, and quantum gravity is extremely complicated. I was just lucky that such beautiful things were at the surface so I could see them. You see, my mind is not very technical. I work best of all in those places where I can use my intuition.

Lightman:

That's very interesting. I'd like to start asking you questions about that. I've noticed from your technical papers and in your paper in Physics Today^[6] and your lectures that you describe things intuitively, with pictures and so forth. I know there have been certain physicists in the past who have used images and visualization and pictures more than other physicists. I think Einstein used a lot of visual images. All of his Gedanken experiments were based on mental images rather than on writing out equations. Even here [at Harvard] we make a joke in the physics department that Weinberg is very technical and [Sheldon] Glashow is very intuitive. So there do seem to be different styles of doing physics. One question that I've been very interested in, and some psychologists are interested in too, is how physicists use mental pictures. Maybe not exactly pictures but, for example, the way we say in quantum mechanics that sometimes things act as particles and sometimes as waves. I guess we're attempting to make a connection to our daily experience with the world. How do you use images in your work? Do you find images useful or harmful?

Linde:

Typically, I just use them. Of course, I use mathematics, certainly.

Lightman:

Of course.

Linde:

But first we usually have a rough idea of how it could work and why, and what is the purpose. Without understanding the purpose of what we are doing, you may try many different ways and you just solve equations without understanding why it is necessary. You see, how it was for me, for example, that I have discovered this new inflation scenario.^[7] I was discussing things with many people. The full story is rather complicated. Actually, I wouldn't make an emphasis on this anecdotal story, but [I will tell you anyway). About three years before this paper by [Alan] Guth,^[8] I had studied similar problems with one of my colleagues at the Lebedev Institute, and we understood that the vacuum can decay and reheat the universe. We understood that the universe could be exponentially expanding and bubbles would be colliding, and we were seeing that it would lead to great in homogeneities in the universe. As a result, this [possible process] is bad [doesn't agree with observations] so what is the reason to publish such garbage? But at that moment, we did not realize the main advantages of this [exponential expansion], that it could possibly solve the flatness and horizon problem.

Lightman:

The flatness problem wasn't even stated at that point. Maybe you can correct me, but my understanding is that the first statement of the flatness problem was by [James] Peebles and [Robert] Dicke in 1979.^[9] Is that wrong?

Linde:

Probably so. We have studied and made this research in 1978. My colleague, Gennady Chibisov, knew that this problem existed and he said to me [that] we would try to explain why the total entropy of the universe is so large [as a result of] this phase transition. The total number of photons, why is it so large? And I did not know the problem, and he [my colleague at the Lebedev) lived outside of Moscow, and he was not very rapid, and after all that [discussion) we did not write anything. But the reason was that we just did not understand the problem correctly. So this was the result. Actually, I have written about this in my review article in 1979,^[10] but our main purpose at that time was not to solve the flatness problem, but just to obtain this ratio of photons to protons. We had observed that it is possible to get this large ratio 10⁸ even if you start from a cold universe filled only with baryons, without photons. So you have this phase transition, you have these bubble collisions, you have reheating and you can obtain a large number. But we saw that these bubble collisions lead to large in homogeneities. I [had] just written about two pages on it in my review article and we [never wrote] a large article about this. We just forgot about it. In 1981, when Guth had written his paper, some people in Moscow also had written a similar paper. They [had] submitted their paper for publication in Physics Letters. But they were a bit too late, and [the editors] told them that already a paper by Guth is published. Among these people was [V.A.] Rubakov, who is now a well-known person.

Lightman:

Yes. Guth says himself that if he hadn't done what he did, other people would have done it. It [the inflationary universe model] was just in the air, like special relativity. Guth has a very modest view about what he did. I guess you've talked to Guth a lot at conferences. The flatness problem is very interesting to me. As you mentioned last night in your lecture,^[11] there are certain issues that we thought were metaphysical, and then suddenly we realize that maybe physics can give some answers to these questions. The photon to baryon problem, the particle-to-antiparticle problem, is things that people had been talking about for a long time...

Linde:

Excuse me one moment. I just didn't finish answering your question.

Lightman:

Oh, I'm sorry. Excuse me.

Linde:

The story goes that I easily understood what Guth was trying to do. But during one year, I did not understand how it could be done, since we have seen that the inhomogeneities are large [contradicting observations]. I was discussing with people, and I was worrying about it myself, and I really was physically ill at that moment. I don't know what was the reason maybe this was one of them, since it was very difficult to abandon this simple explanation of many different problems. I just [had] the feeling that it was impossible for God not to use such a good possibility to simplify his work, the creation of the universe. I had previously studied a theory of tunneling with bubble formation, and I had some experience with this [process] and the idea came to me from the solution of the corresponding equation by computer. So it was not pure imagination. After that I could imagine how these bubbles look when they appear. Then it became clear to me that these bubbles, in some theories, are not of that type which had been considered by Guth, not with thin walls, and it is not so that inside these bubbles you are already at the minimum of the potential. He assumed that inside the bubble the phase transition had already occurred.

Lightman:

But I thought that the solution was a classical solution, not a quantum solution. When you have a potential [of the kind you have drawn on the board], and the ball [representing the state of the system] is rolling, it's in the classical domain, isn't it, not a quantum domain?

Linde:

At high temperature, there is some small maximum here [Linde indicates a point on the potential energy curve he has drawn on the blackboard. Guth assumed the phase transition was completed in the middle of the bubble.] But what was new with this computation is that in some realistic theories, the whole picture looks something like that. So that this minimum [of the potential] is very, very near to the origin and bubbles come to this point, a very, very large distance from equilibrium. And this means that inflation inside the bubbles continues. This was something new and when I understood this...

Lightman:

Basically, you've spread out this wall [of the bubble] right? Instead of being a sharp concentration [of energy] you've spread it out.

Linde:

That's right. What had been assumed by Guth is that this wall [expands] with the speed of light, while here [inside the bubble], space is Minkowski space. But what occurs is that if the bubble is very, very smooth, then inside [the bubble] it is also De Sitter space. This was the first revelation. And another revelation was that it was quite possible to create particles, since this field rolled down [the potential hill], oscillated there, and produced particles.

Lightman:

But couldn't you produce particles in Guth's [model] too. [The quantum field] could oscillate back and forth [at the bottom of the potential well].

Linde:

Oh, you see, in Guth's picture, it was assumed that the field inside [the bubble] is already at the minimum [of the potential energy], so it's not oscillating. All the energy [release] comes from collisions with the walls. It is actually so. In those types of theories which he has considered, this is a true result. He has not made any mistake.

Lightman:

Why wouldn't there be zero point oscillations of the field [using Guth's form of the potential]?

Linde:

Oh, no. Zero point oscillations do not reheat anything. One cannot produce energy by zero point oscillations. So it was necessary for the classical field to oscillate near a minimum of the Higgs potential and here, you see, there were two ideas: First that you have exponential expansion inside the bubble, and second that you have another mechanism for reheating the universe.

Lightman:

But you recognized it through these pictures that you formed from the computer calculations?

Linde:

There were very many lines of progress. I was simultaneously discussing similar matters with Rubakov, since he had also analyzed this question in the theories I had studied. But what he did not know at that time was that the universe inside the bubble would be exponentially expanding. I was sitting in my bathroom and discussing this with him by the telephone, since all my children and my wife were already sleeping at the time. I was asking him some technical questions about what he knows about these things and then I asked him "But do you know how it works?" I talked to him about these thin bubbles, etc. all in the bathroom [in order] not to wake up my children. After the whole picture had crystallized, I was very excited. I came to my wife and I woke her up and I said: "It seems that. I know how the universe originated." It was, of course, very fantastic.

Lightman:

When was this?

Linde:

It was in the summer, maybe even in the late spring of 1981.

Lightman:

This was in your apartment in Moscow?

Linde:

Yes. But at that time I almost did not believe that anybody will easily believe me, since the picture is so strange and the results are so fantastic. Maybe something was incorrect about [my theory], so I was suffering. I was trying to clean the picture up. I was trying to find out my own errors. I was discussing it with different people, and not everyone believed me.

Lightman:

You imagined that our universe was inside one of these bubbles?

Linde:

I was thinking, for example, that maybe I have [made] some error when I am saying that the universe inside is also exponentially expanding. Maybe some gradient terms here, in the walls of the bubble, do not permit this bubble to expand exponentially inside itself. So I had serious doubts myself and I tried to improve the picture and to prove it to myself, and each day I became more and more confident, but still I understood that probably nobody will easily believe me. It was too fantastic.

Lightman:

At this time, what did you think stopped the expansion?

Linde:

Because of the curvature of the potential, the ball [representing the value of the Higgs field] rolls rapidly, then it oscillates and heats universe, and then inflation stops.

Lightman:

So you knew that the exponential expansion would stop inside the bubble?

Linde:

Yes, I knew it, and the whole picture had appeared at once. It was not necessary for me further to change essentially some of these features. Though, about half a year or maybe a year later, I somewhat improved something connected with the initial stages of the evolution of the bubble. There were some problems connected with the curvature of the universe, and this is temperature. So the first picture was rather naive. I understood that it is naive, and I was worrying whether the main results are correct. Then I tried to write first two large papers in order to prepare people to believe that what I am saying is based on some real calculations. I had written two large preprints about tunneling in high density matter, and I was about two or three months doing them. [But as a result of my new work, they would] be published two years later. I had no time to publish them. They were not so interesting and not so important. But then I [wrote] this short paper^[12] and went on vacation and [gave] it to secretaries to type, since I was absolutely confident that nobody will have the same idea. It was a strange idea at that time. Then I returned to Moscow and [the preprint] was still not quite ready [laughs.] In the beginning of October, there was a seminar, a quantum gravity seminar in Moscow to which [Stephen] Hawking came and some other people. At this time, my preprint was already made, and I was going to send it to Physics Letters. But when I gave a talk at the seminar, Steve gave me some objections, so I was trying to polish this paper more and more, and actually I sent it to publication only in the end of the month. That is why it appeared in January 1982, not in 1981, which of course would be better [laughs]. Now the total reaction was very interesting. It was a funny story. The next day after my talk at this seminar, Steve Hawking gave a seminar at the Astronomical Institute, the Sternberg Institute in Moscow. But this is a subtle point. I am not quite sure that it is better to publish this point. I will just tell you a story. The story goes [that] Steve came and gave a talk. He had a paper^[13] with him in which he had proven with two co-authors that it is impossible to improve Guth's scenario. Simultaneously, we received a large paper^[14] by Guth and Erick Weinberg in which they also proved [the same thing] — not simultaneously, [but] somewhat later.

Lightman:

When you say impossible to improve, what do you mean?

Linde:

They had studied the "old inflationary universe model," and they had tried to see any way to make the universe smooth.

Lightman:

Less inhomogeneous?

Linde:

Yes. And the statement was that it was impossible.

Lightman:

So they just weren't considering potentials of this type [used by you].

Linde:

Yes, they were not considering the possibility that exponential expansion can occur inside the bubbles. Everyone believed that inside the bubble should be empty space. And in many realistic theorists, this is true. There were very nice papers^[15] by Sidney Coleman from here [Harvard] discussing it, and in all known cases, inside the bubble there was empty space. So everyone was quite confident that this was the only solution. Now Steve came to this Institute and gave his talk. I came to this talk occasionally. I didn't know what it would be. Typically, one of [Hawking's] students [at that time] translates his English into ordinary English and then somebody translated it to Russian. Steve is absolutely impossible to understand. Now he speaks into a computer and it's much easier for him. But they were not quite prepared to give this lecture. Steve [would] say a few words, and the student [would] say one word, and he did not understand what Steve wanted him to say. People asked me to translate it. And the talk looks like that. Steve [would] say "ah, ah," then his student [would] say one word, and then I was speaking for five minutes, since I understood what he wanted to say. So I was making my talk. [Both laugh.] [Then] Steve said that there was a very good suggestion by Linde at the quantum gravity conference, and I translated it and the whole institute was there to hear. "But I believe that this suggestion is wrong," said Stephen, and I translated it. And during half of an hour, he explained to everyone, and I had to explain [in translation] to everyone why my idea is wrong. So after that, I stopped my translation and said that I have translated but I disagree [more laughter].

Lightman:

When was this talk being given?

Linde:

Probably it was about the 20th of October, 1981. So after that I told Steve, "Would you like to hear what I can answer you?" "Yes." And he with his [wheelchair] came to some room of the Sternberg Institute. We closed the door, and I told him something in a more detailed way about this scenario. He is sitting here about one hour and a half and saying [to] me the same words: "But you did not tell this before. But you did not tell this before." [Linde laughs.]

Lightman:

That's what he was saying to you.

Linde:

Yes, I was explaining to him the details.

Lightman:

You were beginning to convince him that you were right?

Linde:

Yes. And he was saying, "But you did not tell this before."

Lightman:

He was probably embarrassed that he had objected so strongly to your theory.

Linde:

No, I don't think so, but I gave him some new information. Then he invited me to his hotel and we continued. By the way, during this half of an hour, the whole Institute tried to locate where Hawking was, since he [had] just disappeared with his chair. After that, he invited me to his hotel. We discussed the thing there. After that, he invited me to Cambridge in the summer of 1982; some delegation from the Soviet Union [was going] there. It was the first symposium on inflation. Then [Hawking] came here to Philadelphia, Pennsylvania. And, this is a subtle point [that] I also wouldn't be very happy to make a big fuss about, but Hawking told me later that he discussed my scenario at the seminar in Pennsylvania where both [Paul] Steinhardt and Andreas Albrecht were working. Steve told me that he was not very happy about his [talking] about my work at this seminar, since soon after that a paper by Albrecht and Steinhardt appeared.^[16]

Lightman:

So you're not sure whether they got the idea from you or whether they already had the idea?

Linde:

You see, here the matters are very complicated in many respects. When many people are thinking about one thing for example, if I knew that the idea is good, then I would publish it half of a year before. But I was sure that I must polish this work, etc., etc. If somebody comes to me and says that somebody already has done it, then I am publishing this work immediately.

Lightman:

Of course.

Linde:

So it seems that something like that has occurred.

Lightman:

Yes.

Linde:

So in their [Albrecht's and Steinhardt's] paper, they have referenced my work as [having obtained] the same results...

Lightman:

Independently, yes.

Linde:

So, I don't know, and now it doesn't make much difference. But still the historical events were like that. Then Stephen came to Cambridge, and he made his work with [I.G.] Moss.^[17]

Lightman:

Let me go back to something that you had said earlier. You mentioned that when you first read Guth's paper, and you thought that there were so many good ideas in it.

Linde:

At first, not read Guth's paper. I first heard about Guth's paper. I didn't have it before me, but since I had discussed this point with Rubakov and his collaborators, who had suggested a similar idea, then everything was quite clear for me. Lev Okun' from ITEP called me and asked have I heard anything about Guth's paper for explaining the flatness of the universe. I told him that "No, I haven't heard about it, but I know what it's about." [Linde laughs.] And I told him how it works without seeing it. So, it doesn't matter. It's just how the things work.

Lightman:

Yes, well, when you were thinking about these ideas, you said that for a year's time you were frustrated, you were upset because you thought it [Guth's inflationary universe model] was a good idea, but it had some problems that needed to be solved. You said something like that it was impossible that God could have missed such a good possibility to simplify his creation.

Linde:

Yes.

Lightman:

Do you think that the universe was set up by some intelligence? I know last night^[18] you talked about the [possibility] of many different universes, all of [which] would have to obey some laws of physics.

Linde:

This is a much more complicated question than those which I can confidently...

Lightman:

Of course. I understand.

Linde:

Yes, but, as for myself, I cannot say that I am religious. But I also am not a straightforward materialist who believes that everything is just matter and nothing except it. In my opinion — but it is also very dangerous to write such things in the press — we have overlooked something very important.

Lightman:

Last night you were talking about the possible role of consciousness.

Linde:

But it is very complicated. Probably about such things it is better to be silent or to say a lot. Otherwise, you see, there are a lot of crazy people who are saying very similar things but without any responsibility. So it is dangerous to say a few words since, in that case no one will be [sure] whether I am speaking the same thing as some [crazy person]. Here I am just rather confident that we have lost something important in the way of development of Western science. And this [thing] lost is connected with our behavioristic approach to science, to everything. You see, we have a black box. We have some input and we have some output. What we are interested in is: What is the reaction of the black box to the input; what will be the output? Nobody is much interested in what occurs in the black box.

Lightman:

Is our mind the black box?

Linde:

Yes. I mean, for example, that typically people use the same word for our consciousness, for our feelings and for the processes which occur in our brains. There are some reactions in our brain and they say, "This is my anger." You see, they mix reaction and feeling. My red, my blue, my yellow, my bad and my good — they are my feelings, and they are real to me. They are more real for me than these boxes and this chalk and any things made from metal. Then, when I am trying to make a good philosophy out of this, I can convince myself that, for example, you are also existing — in some sense closer to my existence than ordinary matter, [which] is also existing, but in some other sense. Probably your recorder is also existing, etc. But it takes some intellectual work to prove it. I know for sure that I exist, that this is my red, etc. But after further investigation and the development of science, my understanding of my own existence, from which I have started, becomes looking like a secondary point. I investigate relations between different objects, which in some sense influence myself. I can study these relations scientifically. I can investigate them, and, as a result, very soon, all my study, all my science becomes the science connected with matter and with motion of matter and the reactions, and how will the computer react when I send such and such information to it, and how will a man react if I send such and such information to him. It becomes unimportant for me how this man feels at the moment, if I know all his reactions.

Lightman:

What's inside the black box?

Linde:

We often use the word "feels" just in the sense that it helps us describe his reactions. This, in my opinion, goes the wrong way, although it is very productive, is very useful.

Lightman:

This is the way of science.

Linde:

Yes.

Lightman:

Western science.

Linde:

Yes. But this way omits something important. I think that in the future development of people's knowledge — I hope that it will be science — this point will not be omitted. But it is a real problem, and this problem was not quite seriously considered by science.

Lightman:

Yes. It seems that there still are not many scientists who are willing to consider it. I know John Wheeler is. I'm familiar with his work. A lot of people regard him as being kind of crazy in this regard, although he is very respected for all his other work in nuclear physics and so forth. I think the world is not ready; the scientific community is not ready yet for this.

Linde:

Absolutely. I was greatly afraid to speak yesterday, since, any such things expressed before a whole large auditorium, in average, would mean that you are a little crazy. You are doing not physics and this is the worst thing you can do. You're doing not physics.

Lightman:

Do you think that the field of cosmology, more than other fields of physics, has opened up the possibility of the black box playing a role?

Linde:

I mean that we are already in this black box. We cannot say what will be with the universe when some input goes to the universe and output goes out. The universe is already in the black box, and we are living in the black box, and our mind is in this separate black box which reacts to the universe. So my interest in cosmology, to a great extent, is connected with my interest in consciousness. I consider cosmology as a model which can be to some extent studied exactly, without making some absolutely metaphysical and unverifiable conjectures. You see, there are several ways to study such things. One way is to go and make some yoga exercises. I don't know, I haven't tried [this way], although I understand that maybe it would be necessary sometimes. I am not in a hurry at present. Another possibility is to forget about this problem at all, and this is the way of most scientists. It just doesn't bother them.

Lightman:

If it's not physics, if it can't be quantified in a way that they know, they won't study it at all.

Linde:

Yes. And this situation is actually very similar to what occurs, for example, if you are not a physicist. [If] you are just a technician, you can forget about general relativity, and [you do] not think about space and time very carefully. You are just making computers and technology, etc. You cannot, you should not be bothered with such abstract notions. So many people live and do not know anything about the fact that our space and time is actually space-time, and that it is curved. Nobody is bothered. The same is with physics. This is a productive way to do good things, so I cannot say that everything is wrong. However, what is dangerous is to forget some initial points, and that is why philosophy is so important for me. If we are forgetting the starting points, then we are forgetting our roots. And then we will, sometimes, crash.

Lightman:

What philosophers have you read that you think have been very influential for you? You don't have to give me names of philosophers but just any people, any writers who have philosophical ideas that you think [have been influential]. For example, have you read anything by Hegel or Kant?

Linde:

Yes, of course, but...

Lightman:

Does that have any influence on your thinking?

Linde:

I would say that the main influence on what I am thinking — it may be strange, maybe not; in your country it may be just natural — the main influence is coming from Indian philosophy. I don't know why. When I read it, I see that it very strongly fits into some quite modern schemes which appear in cosmology. I have tried to describe some of these things yesterday, but without going into too much detail, since if we are making the propaganda of Indian philosophy, it's also a dangerous way. A lot of crazy people are beating themselves in the head, and reading mantras, etc. It is a dangerous way to make propaganda for them. But, actually, you see what they have done. Many centuries before Greek philosophy appears, there were clever people who were just sitting and thinking — a lot of schools of people with very deep and profound thinking. Even if they were using sometimes strange notions, strange words, even if we cannot sometimes understand exactly the meaning of what they were doing. You know this concept of archetypes?

Lightman:

Yes.

Linde:

So when I read what they were writing, I see a lot of interesting archetypes of thought. If you take something for granted, then you are trying to make some construction, and this construction may exist by itself, as [in] mathematics. Mathematics can be used in some other branch of physics or in biology. These archetypes of thought can exist independently of initial assumptions. Maybe [they] are more stable than initial assumptions. And when I read the things following the line of thought of some of the Indian philosophical schools, I see a lot of analogies with the thoughts which come to my mind.

Lightman:

Do you think that [your ideas for] chaotic inflation and the reproducing universes^[19]have a root in the Indian archetype of a universe that lives and then dies, expanding and contracting, for one day in the life of Brahma?

Linde:

It is more complicated. The model of the universe which could be directly inspired by the Indian philosophy is just the model of an oscillating universe, which was known for maybe 50 years, maybe more. The idea that the universe oscillates has a very long history. But, for me, this idea of an oscillating universe became more real after it was combined with my own thoughts about the possibility that memory of the universe can be washed out by quantum fluctuations of the metric. And these thoughts were also expressed in the Soviet Union by Academician [M.A.] Markov. He is an old, very interesting person, who studies gravity, who is the head of the nuclear department of our Academy of Science. Sometimes, he suggested a model of [an] eternal, oscillating universe, which at each cycle of its evolution forgets what occurred with it before.

Lightman:

It [each new oscillation] can come out with a different dimensionality of space, perhaps?

Linde:

Not that. Just that the "memory" is washed out at some minimum size at the end of contraction. The universe expands and contracts, and then its memory [is] washed out.

Lightman:

What about the entropy problem?

Linde:

Yes, entropy. When I heard [Markov's ideas] for the first time, I was thinking that maybe he is just an old man, with strange ideas. He has made some important work in the past, so he is very respected. So let's forgive him his crazy ideas at present. But my teacher Kirzhnits came and told me, "Be very careful. Markov sometimes says things which sound crazy, and later people understand that they are correct." So I was trying to think about it once again. First, it seemed to me that it is impossible to make any sense out of this idea. Then I tried to think about it in a more detailed way, and I suggested a possible realization of this idea under some hypothesis which may be quite correct, I don't know now. So all these combinations of ideas were together, and they were somewhere at the boundary between philosophy and physics. Thus, the idea of washing out memory of the universe came to me from Indian philosophy and from Academician Markov. A similar idea is contained in Wheeler's papers,^[20] which were written about 15 years ago.

Lightman:

Were you reading Indian philosophy before you worked on the inflationary model, or afterwards?

Linde:

About Indian philosophy?

Lightman:

When did you start reading Indian philosophy?

Linde:

Oh my God, if I will tell you, then this would be the end. I discovered this new inflationary universe scenario — please don't tell it to anybody — I have discovered this new inflationary scenario a week after I [had] some discussions and contact with [the] Hare Krishna group in Moscow (laughs).

Lightman:

But was that the first time that you started studying Indian philosophy?

Linde:

This was not the first time when I was studying [Indian philosophy].

Lightman:

Did you study philosophy in college, or when you were going to school earlier?

Linde:

No. To go backwards in time, my interests in physics were deeply related to my interests in philosophy. [[When I was a schoolboy, I understood that something is wrong with the general development of [epistemological] line of thought in the Soviet Union. I [was] trying to understand some problems connected with philosophy and the origin of our knowledge, etc.]] When some woman who was a professor of philosophy came to us to talk — I was in a rather good school.

Lightman:

Was this in Moscow?

Linde:

In Moscow, yes.

Lightman:

Was it a private school?

Linde:

No. It was just a public school for boys.

Lightman:

With very good teachers?

Linde:

With very good boys. I had a lot of discussions with them. It was a good school — almost all of [the boys] from our class later [went] to different Institutes. So it was a rather special class. So this woman professor of philosophy came to us, and we [were] trying to ask her questions connected with the origin of knowledge, etc.

Lightman:

Origin of knowledge?

Linde:

Yes. "Why do you believe that matter is primary to our perceptions?" She said so many strange things, and it was quite clear that she just cannot answer. It was also quite clear that nobody will listen [to] what we [were] thinking. You see, people, when they are growing up, believe the same beliefs they learn when they are young. Then I was speaking about these things [to a] very, very clever man, a professor of physics, who was also very skeptical. Discussions were possible only up to some level. After this level, logic would not help at all. It was something like a wall and nobody can cross the wall. The man surrounds himself by a wall, and he lives inside this safe place, and he does not want to go out. It was so strange an experience for me. I believed that clever people can explain [to] other clever people simple things.

Lightman:

But you were not asking simple questions. You were asking questions that Aristotle asked.

Linde:

But actually these questions are not very complicated if you think about them a little. What is complicated is to [take] consistent steps if you understand something, and not to stop if public opinion says you that you are going in the wrong direction. Public opinion has grown from the experience of our fathers, who were grown on the experience of their fathers, who were peasants. They had no need to think deeply about some problems. What is the first: mind, matter or something else? Also there was another thing. When I was studying and thinking about philosophy, I [had] created my own reason for what is life, and I suggested some ideas about consciousness and even suggested a very curious theory of telepathic contact.

Lightman:

This is as a schoolboy?

Linde:

Yes. But then it became clear for me that my simple theory of telepathic contact violates Lorentz invariance.

Lightman:

Because the signal's going faster than the speed of light?

Linde:

Something like that. Therefore, two things became clear to me; first, that my professors of physics cannot at present — at least those whom I know — cannot at present teach me real philosophy. I must educate myself in some way. Another thing is that even if I educate myself in philosophy, I will still not be guaranteed from making simple mistakes which are connected with my absence of understanding of physics. So if you would like to study philosophy, if you would like to study consciousness and mind, you first should be guaranteed from [making] simple errors and simple mistakes. That was one of the reasons why I started studying physics. I did not want to make simple mistakes. I wanted to have some basis for studying more fundamental things and not to become crazy.

Lightman:

As a schoolboy, did you have any exposure to Indian philosophy?

Linde:

No, at that time, I actually didn't know much about it.

Lightman:

Did you in college?

Linde:

No. At that time, I was mainly thinking about positivists such as Wittgenstein, although I cannot say that I know him very good. The ideas [which he has] expressed are rather dry, but they just show you a limited way on which you can be sure of your words, in using your words. Later my own relation to the theory of knowledge and to physics was somewhat changing. When I was a student, I tried ab initio to investigate most abstract concepts. One of my teachers in mathematics told somebody about me: "Oh, Linde is very good. He will be a very good physicist. He is a real formalist." This was a compliment in his eyes, that I am a real formalist. If you would say this at present to any of my colleagues, they would laugh, since in our department I am probably the least formal man of all of them. It proves that when you start formalizing everything without trying to understand in which direction you should make this formalization, you sometimes can waste all your life formalizing unnecessary, and maybe just incorrect or at least fruitless, concepts. We had, in the development of physics, such a time, when axiomatic field theory first came, and it was believed that everything can be axiomatized. Nobody believed in field theory since there was a conference in Kiev in 1954 or sometime like that where [L.D.] Landau made a speech and said that the Hamiltonian is dead, and we must bury it with all the honor it deserves. So after that, everyone was making axiomatic field theory. It was very good mathematics and it was very well developed, but it was a dry field. It was not something to live in. A lot of good results still are useful from this area, but most of them are related to theories which are not realized in [our world]. The main line of development now is was quite different.

Lightman:

This happens commonly in physics.

Linde:

Yes. Everyone believed that the field should satisfy several obvious axioms. Afterwards, it appears that almost all the fields we study now do not satisfy these "obvious" axioms. At the end of it came strings, which are not are not point-like objects. So [we have obtained] not so many useful results from this axiomization of the field sitting at a point. So gradually it became clear, maybe after the influence of my teacher Kirzhnits, that we should not be too formal. It is necessary to use formalism, but if you are just starting from formalism and ending with formalism, then you are in a dangerous position. Only a few people can combine both the intuition and ability to work in a formal way. A good example here is Ed Witten in the United States. He is very bright, of course. He can do very formal things and have a very good understanding as well.

Lightman:

He is an exception to the rule. I'm trying to think of a good way to ask you. Do you think that when we use words like "universe," we understand what those words mean?

Linde:

If you ask me, do we understand precisely? No, of course we do not. Different people use this word differently. For example, I was a referee of one book about the universe, in which a man used this word as freely as he wanted. He used the word metagalaxy and universe and something else for the same object and for different objects. There was a mixing of different meanings. For anyone [who] tries to understand it, at least, the universe is everything which exists. However, this may be not the last word in the interpretation of this subject since, first of all, the question [arises] whether we should include consciousness in the definition of everything which exists.

Lightman:

Yes.

Linde:

Another question is that it is possible to suggest models in which there are two different times and spaces not intersecting with each other. I will discuss this in my last lecture. In some sense, each of them can be described as a universe. So, it will be just as two universes. However, [the two universes] will affect each other globally. Sitting in one universe, I will never see another, and vice versa. No particles from one universe will ever hit me. So, in some sense, each one can be considered as a universe. However the whole will be more complicated. So there are different levels in which one can use this word. That is why I have introduced a concept of a. mini-universe. It is safer and more definite.

Lightman:

Yes. So two or more mini-universes just [refers to] things that are a distance 1/H or more a part from each other. If you look at the history of cosmology, [you find] a meeting of the British Association in 1931,^[21] right after Hubble's observations, when people began interpreting [those observations] in terms of an expanding universe. There was a meeting in which Arthur Eddington was present, and [Georges] Lemaitre was present, and James Jeans was present, and [Wilhelm] De Sitter was present. They all talked about the significance of an evolving universe, and some of them questioned the whole use of the words "universe" and "radius." Most of the people then were thinking in terms of closed universes, where radius had a well-defined meaning in the mathematical sense. Several of those people in their lectures and in their papers wondered whether we knew what we were talking about in the use of the words universe and radius, because, for the first time, there was some kind of quantitative way for thinking about the universe. [The universe] was no longer just objects like galaxies, but we had these mathematical equations that somehow had some relationship to the entire universe. I wonder whether we have made any progress since then in understanding what these words mean.

Linde:

The radius of the universe?

Lightman:

Yes.

Linde:

But there [is] not much discussion about this, since if the universe is closed, then this word [radius] can be used in an exactly definite way. If the universe is open, instead of the word radius, we use scale factor, and scale factor is an unambiguous term. [It is] just how [the universe] grows. So there is no problem. There is no special need to use this word in the context in which it is not exact. I may use scale factor, and even this is not sufficient if the universe is inhomogeneous.

Lightman:

Yes. Let me just ask one more question and I'll stop then. We're talking about using your intuition, and modern physics, I think, has made our intuition less and less applicable in some ways. With quantum theory and relativity, a lot of those results seem non-intuitive. Do you find it equally difficult to think about relativity and quantum theory or do you find one to seem more familiar to you than the other?

Linde:

More difficult than what?

Lightman:

Non-intuitive. Do you find them equally intuitive or equally non-intuitive? Do you find, for example, that relativity is easier for you to relate to, to have an intuitive understanding of than quantum theory? Or do you find them on the same level in trying to understand intuitively?

Linde:

I would answer you this way: Intuition in general relativity is obviously complicated, and it will remain complicated for many people, since it is another geometry, we are not accustomed to use it, and we must each time think anew. With quantum mechanics, the situation is somewhat different. When you know some basic principles, you can forget for a while about problems of interpretation, and you can forget about any difficulties. You are just working and compare your experiments with accelerator data, and it is just an ordinary technical job. However, if you are going back to the origins of quantum mechanics, and if you are thinking about the "many worlds interpretation of quantum mechanics," there are some points which are not clear for me even now. There are some points in which intuition is strongly diverging in many people, and the problem of interpretation of quantum mechanics, in my opinion, is not quite clear even at present. For example, many people just do not want to consider seriously the point of view that our world is split.

Lightman:

Split. Yes, in the many worlds interpretation.

Linde:

So it is a complicated thing, and also it leads us maybe to some boundary of our knowledge, to the problems which indeed may appear to be related with our consciousness; maybe not, I don't know. I for myself would like to study these questions in a more deep way. I probably will try to answer [them] in a more sure manner. But, for example, when Bryce DeWitt published in Physics Today a paper^[22] about the many worlds interpretation [of quantum mechanics], in 1970 or in 1972, there were about 20 or so letters to Physics Today. Some of them were published in Physics Today, and each author, from his own point of view, discusses the problem of the interpretation of quantum mechanics, and all of them agree that it is just crazy, this thing about many universes. Maybe it is crazy; maybe it is not crazy enough. I don't know. But, on the other hand, the Copenhagen interpretation, which is accepted by most other physicists, seems to suffer from internal inconsistencies. So, for me, the interpretation of general relativity at least is unambiguous.

Lightman:

I was thinking really of special relativity. I was thinking of just the lack of a division between space and time and the non-absolute nature of time. That's what I was referring to.

Linde:

I understand. For me, the understanding of [some aspects] of general relativity is difficult, but these difficulties can be overcome.

Lightman:

Do you mean technical difficulties?

Linde:

Not only technical. There are technical difficulties of understanding and difficulties of intuition, many difficulties. But if one sits down and works, then in a month or two, if he is capable of work, he can understand at the end. But with quantum mechanics, maybe the situation is slightly more difficult at present. And life becomes even more difficult if one wishes to combine general relativity and quantum mechanics. This is what we are trying to do at present.