12 research outputs found
The Quantum Mechanical Arrows of Time
The familiar textbook quantum mechanics of laboratory measurements
incorporates a quantum mechanical arrow of time --- the direction in time in
which state vector reduction operates. This arrow is usually assumed to
coincide with the direction of the thermodynamic arrow of the quasiclassical
realm of everyday experience. But in the more general context of cosmology we
seek an explanation of all observed arrows, and the relations between them, in
terms of the conditions that specify our particular universe. This paper
investigates quantum mechanical and thermodynamic arrows in a time-neutral
formulation of quantum mechanics for a number of model cosmologies in fixed
background spacetimes. We find that a general universe may not have well
defined arrows of either kind. When arrows are emergent they need not point in
the same direction over the whole of spacetime. Rather they may be local,
pointing in different directions in different spacetime regions. Local arrows
can therefore be consistent with global time symmetry.Comment: 9 pages, 4 figures, revtex4, typos correcte
Expansion-based Removal of Semantic Partial Redundancies
We develop an expansion-based algorithm for semantic partial redundancy elimination (SPRE), which overcomes the central drawbacks of the state-of-the-art approaches, which leave the program structure invariant: they fail to eliminate all partial redundancies even for acyclic programs. Besides being optimal for acyclic programs, our algorithm is unique in eliminating all partial k-redundancies, a new class of redundancies characterized by the number k of loop iterations across which values have to be kept. These optimality results come at the price of an in the worst case exponential program growth. The new technique is thus tailored for optimizing the typically considerably small computational "hot" spots of a program. Here it is particularly promising because its structural simplicity supports extensions to uniformly capture further powerful optimisations like constant propagation or strength reduction in a mutually supportive way