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?
«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»
What a coincidence…
Yes, you were fortunate enough! But, how was the paper received?
Let us go back to the question of black holes and particle creation. When did you ﬁrst think about that?
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?
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].
«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?
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?
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?
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?
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»