Superluminal Propagation of Pulse as another Ultra-Fast Phenomenon, And its Application

Another kind of ultra-fast phenomenon, superluminal transmission of light pulse，was proposed, its velocity can faster than the infinite that was proved in theory and experiment. Based on two kinds of epistemology and world, both deeply effects on Science and technology and open new application of Laser discussed briefly. We also suggested add the content of two kinds of epistemology and worlds in Physics to clear the relationship between the Relativity and general Physics

Causal hierarchy in modified gravity

We investigate the causal hierarchy in various modified theories of gravity. In general relativity the standard causal hierarchy, (key elements of which are chronology, causality, strong causality, stable causality, and global hyperbolicity), is well-established. In modified theories of gravity there is typically considerable extra structure, (such as: multiple metrics, aether fields, modified dispersion relations, Hořava-like gravity, parabolic propagation, etcetera), requiring a reassessment and rephrasing of the usual causal hierarchy. We shall show that in this extended framework suitable causal hierarchies can indeed be established, and discuss the implications for the interplay between “superluminal” propagation and causality. The key distinguishing feature is whether the signal velocity is finite or infinite. Preserving even minimal notions of causality in the presence of infinite signal velocity requires the aether field to be both unique and hypersurface orthogonal, leading us to introduce the notion of global parabolicity.