Leonard Parker is Emeritus Distinguished Professor at the University of Wisconsin-Milwaukee. He was the founder of a gravitational research center at his university. The center now has his name: Leonard E. Parker Center for Gravitation, Cosmology and Astrophysics, in recognition of his notable scientiﬁc achievements in gravitation. Prof. Parker is the pioneer of the theory of quantized ﬁelds interacting with gravity. His breakthrough discovery in the early sixties, that the expansion of the universe can create particles out of the vacuum, opened a new ﬁeld in physics. This surprising result was stated and analyzed in detail in his impressive Ph.D. thesis (Harvard University, 1966) and subsequent papers in Physical Review Letters and Physical Review.
Around the same time, the cosmic microwave background was discovered, changing completely the view of cosmology and reinforcing the Big-Bang theory. In 1992, the COBE satellite detected, for the ﬁrst time, small ﬂuctuations in the average temperature of 2.7 Kelvin degrees in the cosmic microwave background. The eﬀect discovered by Prof. Parker is the mechanism driving primordial perturbations that seeded these tiny ﬂuctuations in temperature. It is also the source for the clumping of matter that gave rise to galaxies and the large scale structure of our universe. Recently, an international team using the BICEP2telescope at the South Pole announced1 the detection of signals of gravitational waves created in the very early universe. This news has greatly excited the physics community and also important mass media. If conﬁrmed, this will provide the ﬁrst direct detection of gravitational waves, also created by the mechanism discovered by Professor Parker.
Leonard Parker is a tireless worker. At 76 years of age he is still capable of traveling across the Atlantic to meet with friends and colleagues to discuss and chat about what brings all of them together: the passion for physics. Because of the celebration of the centennial, in 2015, of Albert Einstein’s theory of General Relativity, the University of Valencia hosted the Conference ERE2014 (Almost 100 Years after Einstein’s Revolution) during the ﬁrst week of September. Leonard Parker delivered a very nice talk and delighted the specialized audience with an overall view of the phenomena of gravitational particle creation.
Despite the fact of being one of the most outstanding physicists in the world, he is also a modest person, approachable and charming. Tall, slim, with blue eyes, he observes everything with curiosity and a desire to learn. This is just the penetrating look of an artist, and this is not a coincidence because Leonard Parker is an artist at heart. He has loved classical music and painting since he was young -he plays piano, mainly Mozart, collects XVII century Dutch paintings and has even tried doing sculpture. He is a humanist who found in physics and mathematics the same consistency, harmony and beauty found in music and art.
With smooth and exquisite manners he comes to the interview with his wife Gloria. Aware of his humanistic inclinations, the interview takes place in the cloisters of the University of Valencia, very close to the statue of Lluís Vives.
The birth of new fundamental ideas is often very difficult. In the early sixties, even the novel ideas of spontaneous symmetry breaking of Brout-Englert-Higgs-Guralnik-Hagen-Kibble [two of them now sharing a Nobel Prize, after the discovery of the Higgs boson at CERN] were initially received with skepticism. In current times the idea of gravitational particle creation seems very natural, but at the time, I guess, it was not. How did you experience the initial reactions to the phenomena of cosmological particle creation?
Let me start by recalling the context of the time. In 1962, at Harvard when I began my Ph.D. thesis, I wanted to work at the interface of general relativity and quantum field theory. I had the good fortune to learn quantum field theory and particle physics at Harvard from Wendell Furry, Roy Galuber, Sidney Coleman, Sheldon Glashow, and Julian Schwinger (the three former eventually became awarded with a Nobel Prize). I wanted to ﬁnd new consequences of the quantum ﬁeld theory of elementary particles in the context of Einstein’s theory of general relativity. At the time, I felt that quantizing the nonlinear gravitational ﬁeld itself was so diﬃcult that I would not be able to make signiﬁcant progress in trying to go beyond the deep work that had already been done in that area. Nevertheless, I felt that it would be valuable to study quantized elementary particle ﬁelds in the curved space-times that were solutions of the nonlinear Einstein gravitational ﬁeld equations. Luckily, Sidney Coleman agreed to be my thesis advisor on such a project, which was outside the main stream of the time. To investigate the creation of particles by the metric of an expanding universe in general relativity, it was ﬁrst necessary to extend the quantum ﬁeld theory of elementary particles from the well-established ﬂat Minkowski space-time of special relativity to the context of a classical general relativistic expanding universe. The formulation of the new theory allowed me to prove unambiguously that particles were created from the vacuum state as a consequence of the expansion. I completed my Ph.D. Thesis at Harvard in 1965 during a time when my advisor, Sidney Coleman was away in Europe for a couple of years.
«While doing my PhD thesis I thought that quantifying the same gravitational field was too difficult and that I probably would not be able to go any further than what had already been donet»
«It was very gratifying to me that the particle creation mechanism described in my thesis played such a central role in relating apparently disconnected areas of physics»
This delayed the publication of your surprising and fantastic result.
I sent a copy of part of my thesis to Professor Bryce S. DeWitt [father of quantum gravity at the time] in 1964, including the work on particle creation. He immediately invited me to give a colloquium at the Institute of Field Physics at the University of North Carolina and oﬀered me a postdoc position there following Peter Higgs in 1965. His famous paper on the Higgs boson was written while Higgs was a postdoc there.
What a coincidence…
Unfortunately we were short of funding and my job entailed a huge teaching load that prevented me from transferring the results of my thesis to scientific papers. My position as Instructor disappeared after two years and I was offered an Assistant Professor position at the University of Wisconsin in Milwaukee, which entailed a heavy load of classes. Fortunately, a snow storm left my wife and I stranded in New York. The record snow storm kept us indoors and made it impossible for us to ﬂy back to Milwaukee for two weeks. I used this time to write the first paper, which described the gravitational creation of particles in detail in 1969.
Yes, you were fortunate enough! But, how was the paper received?
I did not know it at the time, but my papers were well received, as I later found out. In 1969, my research proposal was funded by the National Science Foundation. Among the projects I suggested in my NSF proposal was to determine, using the methods of my thesis, the particle creation that would occur when matter collapsed to form a black hole. In 1971, I was invited to visit Princeton University for the academic year. I have very fond memories of working with the group of Professor John A. Wheeler, who became interested in my black hole particle creation project. While I was at Princeton, it became clear that my work had been well received and generated a great deal of interest, particularly in Europe and in the former Soviet Union (as described in the Sakharov Memoirs). Nevertheless, it took time for my work to stimulate much interest in the United States.
Let us go back to the question of black holes and particle creation. When did you ﬁrst think about that?
Before leaving Harvard, I suggested to my advisor that I believed it would be interesting to calculate the particle creation by the gravitational ﬁeld of matter collapsing to form a black hole. He suggested that I applied for a postdoc position. I did that but the reply said that no postdoc position was available. As I mentioned above I went to Princeton using my NSF funds. When I returned to my position at Wisconsin I was shown Stephen Hawking’s beautiful paper on that topic and immediately recognized that Hawking had solved the black hole particle creation problem in a beautiful and insightful way. To my regret, in his paper Hawking did not cite my earlier papers from 1968 and 1969 on particle creation in an expanding universe, in which I showed that the expansion of the universe caused creation operators to evolve into superpositions of creation and annihilation operators. This corresponds to the creation of particle-antiparticle pairs.
Therefore, the phenomena of gravitational particle creation has been acquiring diﬀerent faces. Using your formalism S. W. Hawking realized in 1974 that black holes also create particles, in a way deeply connected with thermodynamics. How did it inﬂuence the subsequent research in the ﬁeld?
Hawking’s beautiful result was very inﬂuential. It convinced everyone that the second law of thermodynamics was valid for systems that included black holes. This revealed a deep connection between thermodynamics and general relativity. It was very gratifying to me that the particle creation mechanism described in my thesis played such a central role in relating apparently disconnected areas of physics.
Soon after that you were involved in the study of graviton production. Were you surprised to see in your equations that the expanding universe was actually able to create gravitons? [Gravitons would be gravitational waves in general relativity, what photons are to electromagnetic waves].
Let me ﬁrst ﬁll in some interesting background relevant to your question. As I was working on my Ph.D. thesis at Harvard, after proving that scalar particles were created by the isotropically expanding universe, I also focused on studying if there was a particular scenario where this did not happen. Indeed, the expansion of the universe cannot directly create photons. At the time, and inﬂuenced by a work by Roger Penrose, I thought this implied that in the isotropically expanding universes that I was considering, there would be no creation of (massless) gravitons. However, this initial conclusion was too quick. Graviton wave equations were not like the kind of equations governing photons (this fact had already been proved by the Russian physicist Evgeny M. Lifshitz, as Leonid Grixtxuk had pointed out in 1975). With my results we could demonstrate that the isotropic expansion of the universe necessarily produced gravitons. In 1977 Lawrence Ford and I published the first systematic and rigorous study on the subject.
«If conﬁrmed, the detection of the signature of these gravitons certainly would be one of the most remarkable achievements of our time»
«Hawking convinced everyone that the second law of thermodynamics was valid for systems that included black holes»
Did you think that sophisticated experiments (like those of BICEP2 and the Planck satellite) may even detect the eﬀects of those gravitons?
I must admit that I was excited when I learned that the eﬀect of the created gravitational waves on the polarization of the CMB could actually be within the range of current instrumentation! It is now up to the observers to separate the eﬀect of background from the eﬀect of gravitational waves coming from the very early universe.
After the discovery of the «Higgs» particle, do you think it is fair to say that the graviton (associated with gravitational waves) has been converted into physics’ most-wanted particle?
Yes, indeed. Gravitational waves have been detected indirectly by the Hulse-Taylor binary pulsar system [made up of two neutron stars]. Although they have not yet been directly measured by LIGO or other gravitational wave detectors on Earth, it is very likely that they will be detected within a few years. Until recently, the detection of gravitons, the quanta that are carried by gravitational waves, seemed outside the range of observation, because their interaction with matter is exceedingly small. That has all been changed by the possibility that we are detecting gravitons created by the very rapid early expansion of the universe. Highly sensitive polarization of cosmic microwave measures can detect the signature produced by gravitons that were created in the earliest stages of the expanding universe. Only that source would be powerful enough to create measurable signatures. The BICEP2 collaboration has used such detectors. The Planck satellite has detected related polarization patterns that may have been produced by the eﬀect of space dust. Currently, the two groups are combining and analyzing their respective measurements in order to arrive at a conclusive result. If conﬁrmed, the detection of the signature of these gravitons certainly would be one of the most remarkable achievements of our time!
Obviously, you could have been professor at Harvard or at any other of the most coveted univer¬sities. Why did you decide to stay at the University of Wisconsin-Milwaukee?
I have always had a clear view on this. We had no intention of uprooting our children to push my scientiﬁc position. I may have been very naive, but I do not regret this decision.
A more personal question. You play piano and also collect paintings of the 17th century. How have your humanistic interests inﬂuenced your work and your view of the universe?
Even as a young boy I was greatly fascinated by art and music. I studied the drawings of artists such as Leonardo da Vinci and tried to copy them. I enjoyed making pencil and charcoal drawings, water colours, and oil paintings. As a young teenager, I bought some sculpture tools and some marble and tried to sculpt a block of marble. However, it was literally too hard for me. I took piano lessons when I was young. Music so beautiful. At about the age of twelve, I became greatly interested in science and started reading about atoms and nuclei with great interest. In high school, I became very interested in chemistry, physics, and biology, including the genetics of fruit ﬂies. My interest in science and physics were like my interests in art and music, in that I was greatly inﬂuenced by their depth and beauty. Even later, I never felt that I was doing physics as a vocation, but rather as an art of great depth and beauty. I still feel like an artist with regard to doing research and I continue to do it, as best I can, although I am retired.
1. Results published in 2014, in a paper of 25 pages in Physical Reviews Letters (112: 241101). (Back to text)
«My interest in science and physics were like my interests in art and music, in that I was greatly influenced by their depth and beauty»
Unfortunately we were short of funding and my job entailed a huge teaching load that prevented me from transferring the results of my thesis to scientific papers. My position as Instructor disappeared after two years and I was offered an Assistant Professor position at the University of Wisconsin in Milwaukee, which entailed a heavy load of classes. Fortunately, a snow storm left my wife and I stranded in New York. The record snow storm kept us indoors and made it impossible for us to ﬂy back to Milwaukee for two weeks. I used this time to write the first paper, which described the gravitational creation of particles in detail in 1969
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