James B. Hartle is Research Professor and Professor of Physics Emeritus at the University of California, Santa Barbara. His scientific work is concerned with the application of Einstein's relativistic theory of gravitation --- general relativity --- to realistic astrophysical situations, especially cosmology. He has contributed usefully to the understanding of gravitational waves, relativistic stars, black holes, and the theory of the wave function of the universe. He is currently interested in the earliest moments of the big bang where the subjects of quantum mechanics, quantum gravity, and cosmology overlap. Much of his recent work is concerned with the generalizations of usual quantum mechanics that are necessary for cosmology and quantum gravity. He is a member of the US National Academy of Sciences, a fellow of the American Academy of Arts and Sciences, and is a founder and past director of the Institute for Theoretical Physics at Santa Barbara.
A central problem in modern cosmology is to find a simple and compelling theory of the initial condition of the universe that will predict testable correlations among observations today. The search for such a theory is carried out in the context of quantum cosmology because the physics of the early universe is likely to be quantum mechanical in an essential way. In particular, quantum fluctuations in the geometry of space and time are likely at a sufficiently early epoch and therefore theories of the initial condition must be framed within a quantum theory of gravity. Quantum cosmology hopes to find explanations of present features of the universe on a variety of scales. On the largest scales it attempts to explain the approximate homogeneity and isotropy of the matter and radiation distributions, the approximate flatness of the spatial geometry universe, and the origin of the density fluctuations that were the seeds of galaxies. On more familiar scales, the initial condition may explain the approximate validity of classical physics over most of the history of the universe and the homogeneity of the thermodynamic arrow of time. Finally, the initial condition may play an important role in determining features of the universe on very small scales such as the dimensionality of spacetime, its topological structure and the value of the cosmological constant.
The usual textbook Copenhagen frameworks of quantum mechanics must be generalized to apply to quantum cosmology for two reasons: First quantum mechanics must be generalized so it is applicable to closed systems, most generally the universe, that are not measured by anything outside. Characteristically the Copenhagen formulations assumed a possible division of the world into “observer” and “observed”, assumed that “measurements” are the primary focus of scientific statements and, in effect, posited the existence of an external “quasiclassical realm”. However, in a theory of the whole thing there can be no fundamental division into observer and observed. Measurements and observers cannot be fundamental notions in a theory that seeks to describe the early universe when neither existed. In a basic formulation of quantum mechanics there is no reason in general for there to be any variables that exhibit classical behavior in all circumstances. The decoherent (or consistent) histories formulation of quantum theory of closed systems supplies the necessary generalization.
A further generalization of usual quantum mechanics is needed for quantum gravity. That is because usual quantum mechanics relies in essential ways on a fixed, background spacetime geometry, for example to specify the time that plays a preferred role in the Schroedinger equation and in the reduction of the state vector. centrally into the formalism. But in quantum gravity, spacetime is not fixed. Rather it is a quantum dynamical variable, fluctuating and generally without definite value. A generalization of usual quantum theory that does not require a fixed spacetime geometry, but to which the usual theory is a good approximation in situations when the geometry is approximately fixed, is therefore needed for quantum gravity and quantum cosmology.
Selected Expository or Less Technical Publications:
"Excess Baggage'', in Elementary Particles and the Universe: Essays in Honor of Murray Gell-Mann, ed. by J. H. Schwarz, (Cambridge University Press, Cambridge, 1991), gr-qc/0508001
"The Quantum Mechanics of Closed Systems" in Directions in Relativity, Volume 1, (Essays in Honor of Charles W. Misner’s 60th Birthday) ed. by B.-L. Hu, M.P. Ryan, and C.V. Vishveshwara, Cambridge University Press, Cambridge (1993); gr-qc/9210006
"Quantum Cosmology: Problems for the 21st Century", in Physics in the 21st Century: Proceedings of the 11th Nishinomiya-Yukawa Symposium, Nishinomiya, Hyogo, Japan, ed. by K. Kikkawa, H. Kunitomo, and H. Ohtsubo, World Scientific, Singapore (1997); gr-qc/9701022
"Scientific Knowledge from the Perspective of Quantum Cosmology", in the Boundaries and Barriers: On the Limits of Scientific Knowledge (1995 Abisko Conference), ed. by J.L. Casti and A. Karlqvist, Addison-Wesley Publishing Co., Reading, MA (1996); gr-qc/9601046.
"The State of the Universe", in The Future of Theoretical Physics and Cosmology: Celebrating Stephen Hawking 60th Birthday, ed. by G.W. Gibbons, E.P.S. Shellard, and S.J. Ranken, (Cambridge University Press, Cambridge UK, 2003); gr-qc/0209046.
"Theories of Everything and Hawking's Wave Function of the Universe", in The Future of Theoretical Physics and Cosmology: Celebrating Stephen Hawking's 60th Birthday ed.by G.W.Gibbons, E.P.S.Shellard, and S.J.Ranken, Universe", in The Future of Theoretical Physics and Cosmology: Celebrating Stephen Hawking 60th Birthday, (Cambridge University Press, Cambridge UK, 2003); gr-qc/0209047.
"Anthropic Reasoning and Quantum Cosmology", in The New Cosmology: Proceedings of the Conference on Strings and Cosmology, Col lege Station, Texas, March 14-17, 2004, edited by R. Allen, D. Nanopoulos and C. Pope, AIPConference Proceedings, v. 743 (American Institute of Physics, Melville, NY, 2004), gr-qc/0406104; reprinted in Universe or Multiverse, ed. by B. Carr, (Cambridge University Press, Cambridge, 2007); gr-qc/0406104.
"The Physics of Now", Am. J. Phys., 73, 101-109 (2005), gr-qc/0403001.
"General Relativity in the Undergraduate Physics Curriculum",
Am. J. Phys., 74, 14-21 (2006), gr-qc/0506075.
"Generalizing Quantum Mechanics for Quantum Spacetime", in The Quantum Structure of Space and Time, ed. by. D.Gross, M. Henneaux, and A. Sevrin, (World Scientific, Singapore, 2007), gr-qc/0602013.
``Quantum Physics and Human Language'', J. Phys. A: Math. Theor., 40, 3101 (2007), quant-ph/0610131.
``The Quasiclassical Realms of this Quantum Universe'', arXiv:0806.3776