Peebles at Princeton in 2016
[Curator’s note: The following exhibit quotes and paraphrases extensively from material posted by the Nobel Prize Committee. See sources below.]
The Nobel Prize in Physics 2019 was awarded "for contributions to our understanding of the evolution of the universe and Earth’s place in the cosmos" with one half to James Peebles *62 "for theoretical discoveries in physical cosmology", the other half jointly to Michel Mayor and Didier Queloz "for the discovery of an exoplanet orbiting a solar-type star."
Fundamental questions about the universe’s structure and history have always fascinated human beings. James Peebles’ theoretical framework, developed since the mid-1960s, is the basis of our contemporary ideas about the universe. The cosmic background radiation is a remaining trace of the formation of the universe. Using his theoretical tools and calculations, James Peebles was able to interpret these traces from the infancy of the universe and discover new physical processes. The results showed us a universe in which just five per cent of its content is known matter. The rest, 95 per cent, is unknown dark matter and dark energy.
Early life and academic studies
Peebles in 1970
James Peebles was born in St. Boniface, near Winnipeg, Canada on April 25, 1935, where his father worked in the grain business. After attending the University of Manitoba, he continued his studies at Princeton University in the United States, receiving his doctorate there in 1962. He remained at Princeton University where he is now Albert Einstein Professor Emeritus of Science.
“I had not given thought to what to study at the university. But I knew I liked to build things such as model airplanes, and I had the impression that engineers built things, so I enrolled in engineering at the University of Manitoba (which I will term the U of M for short). I liked many of the classes, and learned a lot, some of lasting value. I was exposed to calculus in what might be termed an engineering point of view, which suited me, for as I said I don’t have a strong intuitive feeling for mathematics. I remember with pleasure engineering drawing with India ink on linen fabric. We don’t do that sort of thing anymore, at least not in my field, but the manner of visualization it required was valuable and has stayed with me.
And the courses allowed intervals of free time that I filled by playing hearts, the card game. I particularly liked the physics courses. I remember, in my second year in engineering, complaining to a friend from Glenlawn Collegiate, Dale Loveridge, that I was running out of physics courses to take. He replied that I could transfer to physics. To the best of my recollection that thought had not occurred to me. So why, if I was born to be a physicist, did it take someone else to make me realize that I ought to transfer to physics? I can only say that I tend to be vague about such things. Anyway, I made the change in my third year at the U of M and felt at home. I guess I could have made my way through life as a mediocre engineer, but Dale directed me to something for which I am far better suited.
I remember the day my closest friend among the students in physics at the U of M, John Moore, came to me saying, Jimmy you have to meet Alison. That was because her surname is the same as mine. Maybe we’re related, but her line of Peebles came from Ireland, mine from England, so the relation looks kind of distant. We liked each other, and our physics friends saw us married and shipped off to Princeton in 1958. Al has been my best friend since we met.
Peebles studies at Princeton
Graduate study at Princeton was Ken Standing’s idea. He was a professor of physics at the U of M and had been a graduate student in nuclear physics at Princeton ten years earlier. He formed the opinion that Princeton is the only place for me. I don’t imagine he could have seen how right he was. It was a real pleasure to talk to him the last time we met, in the Spring of 2016. But he had started to exhibit the symptoms of Parkinson’s Disease, which soon took him. Ken had a productive career in precision measurements of masses of macromolecules, which biophysicists value, and he loved to spend time in his cabin in the beautiful woods toward the eastern edge of Manitoba, in the Precambrian Shield. I owe a lot to Ken.
I entered Princeton with the intention of doing something fancy in particle physics. I wrote one paper on that subject, which I see has gathered five citations, one of them mine. I was saved from a dismal future in that direction through the help of two fellow graduate students, both also from the U of M. Bob Pollock was a year ahead of me, we were friends while both of us were there, and he and Jean, and Al and I, remained good friends. Pollock was a gifted experimentalist. Soon after I arrived at Princeton, I was approached by Professor Donald Hamilton, who wanted to discover whether I was another Pollock, and if so whether he could persuade me to join his experimental atomic beams group. A short conversation revealed that I am no Pollock, and we parted as friends. I did not know Bob Moore while at the U of M; he was a few years earlier than us. But Moore led me to Professor Robert Henry Dicke’s Gravity Research Group.
After war research on radar and other electronics, Bob Dicke spent a decade at the laboratory bench in Princeton on what might be termed quantum optics. But then he decided that the study of the physics of gravity was seriously neglected, and that the great advances in electronics during the war would allow many of the classical experiments in gravity physics to be done better and would allow new experimental probes into the nature of gravity. He quite abruptly changed his direction of research to the empirical study of gravity physics. The first twelve PhD dissertations he guided had nothing to do with gravity. The last of these is dated 1959. The next, dated 1961, is mine. The twenty-six dated after 1960 include only one that has nothing to do with gravity.
The abrupt switch of direction may seem bold. But at about the same time another member of the faculty, John Archibald Wheeler, decided to turn the direction of his research to the theoretical study of gravity. This cannot have been entirely coincidental, but the two had quite different philosophies. Wheeler accepted Einstein’s general theory of relativity and explored to great effect its consequences and ways to reconcile it with quantum physics. Dicke seemed to be almost personally offended by the scant empirical support for general relativity, and he enthusiastically explored questions that many had considered settled without empirical support at the precision possible then. Is the period of a mechanical oscillator, measured at rest relative to the oscillator, really independent of its motion relative to distant matter? Is the period defined by a spectral line quite independent of the atom’s motion? Are parameters of physics such as the strengths of the gravitational and electromagnetic interactions really independent of motion? Might these parameters be evolving as the universe expands?
I was impressed by what Wheeler was doing and enjoyed interacting with his many graduate students and postdocs, but I was not inclined to join his group. Bob Moore took me to the weekly evening meetings of Dicke’s Gravity Research Group. The group cannot have been much more than a year old when I arrived, in the autumn of 1958, but graduate students and postdocs had already started ambitious experiments, while others were looking into such arcane things to me as the dating of historical eclipses, for the purpose of checking the orbits of the moon around the earth and the earth around the sun. Dicke had some of his graduate students and postdocs working with him on a repetition of the Eötvös experiment that demonstrates that the gravitational acceleration of a free test particle depends very little if at all on its composition. Eötvös had to observe his balance from a distance, using a telescope. Dicke buried his balance and used his elegant feed-back techniques to monitor the electro-static force needed to hold the balance fixed. The measurements have since been done even better, but Dicke showed the way. We heard progress reports and discussions of these projects, and thoughts about what other things might be investigated. Some of Wheeler’s students, who were looking into the theoretical side of general relativity and quantum physics, sat in on the Gravity Group meetings. And Dicke brought occasional visitors. It was a fascinating tour of physics. The Gravity Group meetings showed me what I wanted to do and taught me a lot about how to do it.
Dicke directed me to the issue of whether the strength of the electromagnetic interaction, represented by the fine-structure constant (in the old-fashioned units I still use) a=e2/hc might be evolving as the universe expands. This led me to learn a lot of nuclear physics, because if a evolves then the decay rates of long-lived isotopes change, increasing or decreasing according to how a change in a changes relative energy levels. And that could mean the radioactive dating of minerals and meteorites based on the assumption of constant decay rates would produce inconsistent results from different isotopes. So I read a lot of geology, and learned fascinating things such as the great extinctions. And I cooked up a relativistic classical field theory that allowed a to evolve without serious violation of the Eötvös experiment. All of this went into my doctoral dissertation. My bounds on the possible rate of change of the value of a are modest compared to what has been done since by observations of the spectra of galaxies and quasars at redshifts well above unity. And I have never reexamined my theory of how a might evolve, to see if it truly makes sense. But this was an excellent learning experience.
Peebles thinking about the expanding universe
I learned the standard thinking about the expanding universe from the book, The Classical Theory of Fields. It is part of the marvelous series on theoretical physics by Landau and Lifshitz. The books in this series do not deal much with phenomenology. The closest I find in The Classical Theory of Fields is in a footnote (on page 332 in my edition, the 1951 translation from the 1948 Russian edition). It cautions that the validity of the assumption that the universe is close to homogeneous and isotropic in the large-scale average remains an open question. That was a very sensible remark. My other reference was Tolman’s Relativity, Thermodynamics and Cosmology, published in 1934. It too is thin on phenomenology. I was left with the early impression that the subject of cosmology was pretty much free of the empirical physics I enjoy applying, and I saw lots of room to go about creating some physics.
Dicke had suggested that the universe may have expanded from a hot dense early condition, leaving a remnant sea of thermal radiation that was cooled by the expansion. I saw that this would imply interesting thermonuclear production of light isotopes. Toward the end of 1964 I learned that I had been reinventing the wheel; George Gamow published most of my ideas in 1948. But he left room for more detailed analyses of the evolution of the isotope abundances. And I did hit on new ideas about how the sea of thermal radiation would affect the gravitational assembly of matter into galaxies and groups and clusters of galaxies. I discuss all that in my Nobel Lecture.
I gave a one-term graduate course on these ideas about physical cosmology in the fall of 1969, and John Wheeler insisted that I turn my lectures into a book. To that end he took notes that he gave me at the end of each lecture. The sight of that great physicist taking notes in his elegant hand so unnerved me that I promised to produce a book. I meant its title, Physical Cosmology, to indicate that I did not intend to get into the subtleties of what might be termed astronomical cosmology: evidence from stellar evolution ages and the extragalactic distance scale. I don’t think I thought of it at the time, but the title also helps distinguish this book from the bloodless approach in Tolman’s Relativity, Thermodynamics and Cosmology and Landau and Lifshitz’s The Classical Theory of Fields. I meant to explore the physical processes that are observed to operate, or we might imagine operate, in an expanding universe. At about the time of publication of my book, in 1971, Steve Weinberg published his book, Gravitation and Cosmology. It presents more complete theoretical considerations. Mine is more complete on the phenomenology and the physics that might show how the phenomenology all hangs together. The two books mark the start of the growth of physical cosmology from its near dormant condition in the early 1960s to a productive branch of physical science by the end of the 1960s.
Peebles today
Peebles is a Princeton University Professor Emeritus, Albert Einstein Professor of Science in the Department of Physics. Along with his notable books and articles, in recognition of his efforts, he received honorary degrees from the Universities of Chicago, Toronto, Manitoba, McMaster, Newcastle, and Louvain and earned numerous awards, including the Gold Medal of the Royal Astronomical Society and the Eddington Medal of the Royal Astronomical Society.
SOURCES:
• Source: James Peebles – Facts – 2019. NobelPrize.org. Nobel Prize Outreach AB 2022. Mon. 6 Jun 2022.
OTHER RESOURCES:
• PAW: Why Jim Peebles got the Nobel Prize.
• PAW: Peebles reflects on his path to the Nobel
• PAW: Peebles shares Nobel Prize Insights
• PAW: Peebles guided tour through cosmology
• PAW: The spirit of truth seeking
