Steven Weinberg


Interview, December 2001

Interview with Professor Steven Weinberg by Joanna Rose, science writer, 7 December 2001.

Professor Weinberg talks about how he became interested in cosmology, his path into physics (1:40), the ‘Golden Age’ of cosmology (6:51), his greatest contribution (10:00), ‘the final theory’ (12:38), and the absence of a cosmic plan for humankind (16:01).

Interview transcript

Welcome to Stockholm, and to this Nobel interview, Professor Steven Weinberg. I have talked to some of your colleagues here, and everybody says that you’re the one to blame, or the one that brought light to the community of physicists when you wrote your book on cosmology in the early 1970s, called Gravitation and Cosmology. So how did you get interested in cosmology?

Steven Weinberg: I don’t really think there is anyone who isn’t interested in cosmology. If you go out and night and look at the stars it’s inevitable that you wonder what all this is. For me, it was a fantastic discovery when I was a young professor, just beginning, that there was a mathematical theory that could be applied to the whole universe. It had been worked out in the 1920s and the 1930s, and the theory of the whole universe was something I had to learn about, so I taught courses at Berkeley on the subject. Gradually I learned enough so that I wanted to put it all together in a book of my own, looking at things in my own way.

You are, and you were then also, a professor in physics …

Steven Weinberg: Yes, I’ve always been a professor of physics at Berkeley and then MIT, Harvard, and now in Texas.

How come you came into this field, into physics?

Steven Weinberg: It started with chemistry. When I was young I had a cousin who had been given a chemistry set. This is a toy, with chemicals, and test tubes that you play with, and he lost interest in it. He went into professional boxing. Perhaps he should have stayed in science, but anyway, the chemistry set came down to me, and I loved it, especially that beautiful wooden box that it came in. I loved playing with chemicals, and learned a little chemistry, of course. You always learn a little bit when you play with these things. I learned that all chemicals behave the way they do because of atoms, and then I wanted to learn about atoms. That was difficult because there was apparently a mysterious theory called quantum mechanics that had been developed in the 1920s. I read popular books by people like George Gamow and James Jeans, and I got very excited, not because I began to understand it, but because it seemed incomprehensible. And I thought if someone …

And still is, I would say.

Steven Weinberg: If someone could understand this you would have a hold on nature, you would be in possession of knowledge that would allow you to understand the deepest workings of nature. I had to learn about this, and I then wanted to participate in the creation of this knowledge. At a certain point there was a particular event that happened to me that was important. I was in a public library in New York where I grew up and borrowing some books about history or novels or something, and I saw on a table a book called Heat. A book about heat, a prosaic subject and not perhaps very exciting to a young teenage person, but the book was open and there was a symbol in the book that looked like this, and I had no idea what this meant, but I knew it was a symbol that was used in advanced mathematics. I didn’t understand the mathematics but I recognised it, and it suddenly occurred to me that advanced mathematics is used to understand something as elementary as heat. It gave me a sense of the power of mathematics, to understand everything in the world. Later on, I found out that this is called an integral sign and I learned what it was, but at the time it was just a magic symbol for me, but it showed that by manipulating mathematical symbols you could say something about the real world. Not just atoms, but also something just ordinary like heat.

Yes, so you became a theoretician.

Steven Weinberg: That’s right, I was always a theoretical physicist. I always wanted to be theoretical physicist, although I started with a chemistry set, I’ve never been any good at experiments. These days you’re either a theorist or an experimentalist. I don’t think, at least in the kind of physics I’m interested in, no-one is both. I could never be an experimental physicist. I’m only good at theory. If that.

Yes. But coming back to cosmology, in the 1970s, as you say, it could fascinate a lot of people, but this was not the subject for a scientist.

Steven Weinberg: It had begun to be in about 1965. I think the great breakthrough was the discovery of a faint radiation that fills the universe. It’s called the three degree radiation, because it’s the kind of radiation that would be emitted by a body that was at a temperature of just three degrees above absolute zero. This was radiation left over from a time when the universe was a few hundred thousand years old, and it was the first tangible evidence that there was a time in the history of the universe when it was very different from now, when there were no stars or galaxies, when the universe was just filled with a cold soup of matter and a faint whisper of radiation. Not so, it’s cold now, the radiation, it was not cold then. At this time the temperature was about three thousand degrees. But this was a great breakthrough in 1965 and it was trying to understand the implications of this from the period from 1965 until 1972 when I wrote my book, that was my main preoccupation.

You have also written a popular science book on cosmology called The First Three Minutes, which made cosmology maybe more comprehensible for the general public. This was more than 20 years ago. What has changed since then?

Steven Weinberg: The field has grown so much. I think this is a golden age now for cosmology. There are observations of not only that there is a radio background, this three degree radiation, but there are faint ripples in it that give evidence of conditions when the universe was a few hundred thousand years old, and our knowledge is getting more and more detailed. Also, there is now much more evidence about how the universe is expanding. It seems that the expansion at first was speeding up, then slowing down and now it’s beginning to speed up again. We have a theory inflation that describes what happened at the very earliest times, which we didn’t have when I wrote my text book, or when I wrote The First Three Minutes. It’s been a very exciting time for cosmology. Much more exciting in the last decade at any rate than in my own field of elementary particle physics.

What would you regard as the most important observation or evidence for the big bang cosmology?

Steven Weinberg: Of course the expansion of the universe. We’ve had that evidence since 1930 or thereabouts. The fact that all the galaxies in the universe are rushing away from each other. This at times has been questioned as an interpretation of the observations, but I think it is more and more solid that the universe in this sense is expanding. Now we know much more about the rate of the expansion and how it’s speeding up and slowing down. That’s the most important evidence, but there is lots of other evidence. For example, the abundance of the elements. Most elements are produced in stars, and that doesn’t have so much to do with cosmology, but the lightest elements, about five or six isotopes of the lightest elements, were produced in the first three minutes and astrophysicists can calculate the abundance of these elements and compare it with what’s observed in the oldest stars. It agrees really marvellously well. That’s a real triumph, I think, of theoretical science.

There’re lots of observations in astronomy and astrophysics but your contribution was also to make people, elementary particle physicists, interested in cosmology. There was an input from …

Steven Weinberg: I hope so. Some people have told me that. Yes. But in the same way that I became interested in it, yes.

What do you consider the greatest contribution that you made to these theorists, or to science?

Steven Weinberg: It’s not in astrophysics or cosmology. I’ve written some papers in cosmology but they’re not of the first importance. My main work has been in the theory of elementary particles, and particularly in the unification of two of the forces of nature, the weak force which causes particles of one type to turn into particles of another type, and the electromagnetic force, which people are familiar with, which is responsible for electricity flowing through wires or for magnets attracting pieces of metal. It turns out that these are both aspects of the same underlying force which now has become called the electroweak force.

These were two of the four forces of nature?

Steven Weinberg: Yes. I’ve also worked on the third force, the strong force. In fact, my work on the electroweak force grew out of my work on the strong force which is the force that holds quarks together inside the particles inside the nucleus of the atom. In that work, I had developed certain mathematical ideas that go by the name of broken symmetry, and shown how the … Well, I had not originated the idea of broken symmetry but I showed how it could be used to understand features of the strong force. Then it occurred to me suddenly, in 1967, that similar mathematical ideas would apply to the weak force and would allow us to unify it with the electromagnetic force in a very satisfactory theory. Other people, of course, have worked on the strong force and the electroweak force, and out of the work of many physicists came in the 1970s a theory of all the forces of nature, except for gravitation, known as the standard model.

This is the one that you have been awarded the Nobel Prize for?

Steven Weinberg: The Nobel Prize came for the contribution to the electroweak force. Not for the strong force, where I was not, as far as the strong force is concerned I made contributions which I think were important, but not of the most important contributions.

I see. There is still this force lacking in the theory.

Steven Weinberg: Yes, this force. Gravity, that pulls us all down to the earth.

When this is put into the theory then you will get the theory of everything?

Steven Weinberg: I don’t like to use the word ‘theory of everything’ because when this is accomplished it will not solve the problems of understanding the mind or curing cancer or …

Not everything.

Steven Weinberg: … or even solving all the problems of physics. We still won’t understand physical problems like the way turbulent fluid behaves, but it will be the fundamental theory that underlies all other theories. I like to call it the final theory rather than the theory of everything because it will end a certain historical progression toward deeper and deeper theories. Now, I think that that will happen, as you say, when gravity is unified with the other forces, but perhaps not. Perhaps we will then discover new things that require further work. We don’t know what is the crucial problem. That’s something that sometimes isn’t understood. Sometimes people outside of physics think well, there is a problem and if you solve that problem then you will have the answer to everything. But very often you don’t know what is the right problem, what is the important problem, until you’re close to solving it.

I see.

Steven Weinberg: But right now it does look, as you say, that the crucial problem is to bring gravity together with the other forces.

You have no idea when this final theory can be here?

Steven Weinberg: Some time between tomorrow and the next century. It could be tomorrow.

Just all of a sudden?

Steven Weinberg: Some very bright undergraduate may come up with a theory tomorrow, and send it out by email, and it may turn out to be the theory, and we will all recognise it.

You will? This is my question …

Steven Weinberg: I think so, yes.

You will recognise it?

Steven Weinberg: I think so. It’s probably going to be a theory that deals with structures much smaller than the elementary particles that we know today, and we may not be able to study these structures experimentally, but if there really is a successful theory, one of the things it will do will be to explain the things we already know experimentally. We already know the masses and the electric charges of all the particles in the standard model. It’s about 18 numbers, they just have been taken from observation. If the final theory correctly accounts for these 18 numbers, if it tells us why one particle, the muon, is 210 times heavier than the electron, for instance, which is now just a fact that we observe in nature, a mysterious fact. If it explains this fact, and other similar facts, then we will identify that as the correct theory. As I say, it could happen tomorrow, but frankly I doubt it. I think it’s going to be the work of several decades, at least.

Coming back to cosmology. There is especially one statement in your book on cosmology, the popular one, The First Three Minutes, that was cited and also discussed very often. You wrote “the more the universe seems comprehensible, the more it also seems pointless”. Can you elaborate a little on that?

Steven Weinberg: That’s not the last sentence in the book. If you look at the book then there’s another paragraph that follows that, that explains what I meant, although perhaps I didn’t explain it very well. What I meant in that statement is that there is no point to be discovered in nature itself. There is no cosmic plan for us. We are not actors in a drama that has been written with us playing the starring role. There are laws, we are discovering those laws, but they’re impersonal, they’re cold. We are the result of billions of years of accidents that have led to us, governed by laws of nature that have no care for us. But then after saying that, I went on and said that if there is no point in nature, we can make a point for ourselves. We can find things to cherish that we value. We can love each other, we can create things that are beautiful, and also one of the things that some of us find to give point to our lives is to learn about nature. It’s not an entirely happy view of human life. I think it’s a tragic view, but that’s not new to physicists. A tragic view of life has been expressed by so many poets, that we are here without purpose, trying to identify something to care about. Even when we find the final laws of nature we won’t know why those are the correct laws of nature. But, although, for example, Shakespeare very often expresses a tragic view of life: “golden lads and girls all must, like chimney sweepers come to dust”. Our tragedy is a little different from his, from the heroes of Shakespeare’s plays. For Lear and Othello, the tragedy is in Shakespeare’s script, and what I like to say is that our tragedy is that there is no script.

Or we don’t know it.

Steven Weinberg: We don’t know it, and I could be wrong about this, of course. We’re not certain about anything. But the more and more we understand about nature, we find no sign of a script written for us, and we have to write the script ourselves. If we’re in the position of actors in a tragic drama, it’s a drama we’re improvising as we go along.

This is a really fascinating topic, but I’m afraid we have to conclude now. Thank you very much for this interview.

Steven Weinberg: Thank you.

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