Professor Hartle is concerned with applications of Einstein's theory of space, time and gravity—general relativity—to cosmology. He is a member of the US National Academy of Sciences, a fellow of the American Academy of Arts and Sciences, and past Director of the Institute for Theoretical Physics (ITP) at Santa Barbara.

Talks by James Hartle

What is a No-Boundary Quantum State?

James Hartle University of California
Contemporary final theories consist of a theory of the universe’s dynamics (I) like the avatars of string theory together with a theory of its quantum state (Ψ) like the no-boundary wave function of the universe (NBWF). This talk is concerned with the definition of a no-boundary quantum state at the semiclassical level where its predictions can be straightforwardly derived and compared with observation. A semiclassical no-boundary wave function is defined by an ensemble regular saddle points. The ensemble is restricted by simple considerations of symmetry such as time neutrality.

Classical Spacetime and Quantum Black Holes

James Hartle University of California
A quantum system behaves classically when quantum probabilities are high for coarse-grained histories correlated in time by deterministic laws. That is as true for the flight of a tennis ball as for the behavior of spacetime geometry in gravitational collapse. Classical spacetime may be available only in patches of configuration space with quantum transitions between them. Global structures of general relativity. such as event horizons may not be available.

Eternal Inflation in the Light of Quantum Cosmology

James Hartle University of California
If the universe is a quantum mechanical system it has a quantum state. This state supplies a probabilistic measure for alternative histories of the universe. During eternal inflation these histories typically develop large inhomogeneities that lead to a mosaic structure on superhorizon scales consisting of homogeneous patches separated by inflating regions. As observers we do not see this structure directly. Rather our observations are confined to a small, nearly homogeneous region within our past light cone.

Eternal Inflation in the Light of Quantum Cosmology

James Hartle University of California
If the universe is a quantum mechanical system it has a quantum state. This state supplies a probabilistic measure for alternative histories of the universe. During eternal inflation these histories typically develop large inhomogeneities that lead to a mosaic structure on superhorizon scales consisting of homogeneous patches separated by inflating regions. As observers we do not see this structure directly. Rather our observations are confined to a small, nearly homogeneous region within our past light cone.

Quantum Mechanics with Extended Probabilities

James Hartle University of California
We present a new formulation of quantum mechanics for closed systems like the universe using an extension of familiar probability theory that incorporates negative probabilities. Probabilities must be positive for alternative histories that are the basis of settleable bets. However, quantum mechanics describes alternative histories are not the basis for settleable bets as in the two-slit experiment. These alternatives can be assigned extended probabilities that are sometimes negative. We will compare this with the decoherent (consistent) histories formulation of quantum theory.

Quasiclassical Realms and Copenhagen Quantum Theory in a Quantum Universe

James Hartle University of California
One of the most remarkable features of our quantum universe is the wide range of time, place, scale, and epoch on which the deterministic laws of classical physics apply to an excellent approximation. This talk reviews the origin of such a quasiclassical realm in a universe governed fundamentally by quantum mechanical laws characterized by indeterminacy and distributed probabilities. We stress the important roles in this origin played by classical spacetime, coarse-graining in terms of approximately conserved quantities, local equilibrium, and the initial quantum state of the universe.

Generalizing Quantum Mechanics for Quantum Spacetime

James Hartle University of California
Familiar textbook quantum mechanics assumes a fixed background spacetime to define states on spacelike surfaces and their unitary evolution between them. Quantum theory has been generalized as our conceptions of space and time have evolved. But quantum mechanics needs to be generalized further for quantum gravity where spacetime geometry is fluctuating and without definite value. This talk will review a fully four-dimensional, sum-over-histories, generalized quantum mechanics of cosmological spacetime geometry.