Stellar structure and compact objects before 1940: Towards relativistic astrophysics

Since the mid-1920s, different strands of research used stars as "physics laboratories" for investigating the nature of matter under extreme densities and pressures, impossible to realize on Earth. To trace this process this paper is following the evolution of the concept of a dense core in stars, which was important both for an understanding of stellar evolution and as a testing ground for the fast-evolving field of nuclear physics. In spite of the divide between physicists and astrophysicists, some key actors working in the cross-fertilized soil of overlapping but different scientific cultures formulated models and tentative theories that gradually evolved into more realistic and structured astrophysical objects. These investigations culminated in the first contact with general relativity in 1939, when J. Robert Oppenheimer and his students George Volkoff and Hartland Snyder systematically applied the theory to the dense core of a collapsing neutron star. This pioneering application of Einstein's theory to an astrophysical compact object can be regarded as a milestone in the path eventually leading to the emergence of relativistic astrophysics in the early 1960s.Comment: 83 pages, 4 figures, submitted to the European Physical Journal

Bridging the representational gap in the dynamic systems approach to development

We describe the relationship between the dynamic systems approach to development and a recent approach to the dynamics of representational states - the dynamic field approach. Both approaches share an emphasis on the concepts of stability (attractor states), instability (especially bifurcations), soft-assembly and flexibility. But the dynamic field approach adds the concept of 'activation' to capture the strength with which behaviorally relevant information is specified. By explicitly linking these dynamic systems approaches, we allow for more direct comparisons between dynamic systems theory and connectionism. We note three current differences between these two approaches to development: (1) the notion of stability is central to how representational states are conceptualized in the dynamic field approach; (2) the dynamic field approach is more directly concerned with the sensorimotor origins of cognition; and (3) the dynamic approach is less advanced with regard to learning. We conclude that proponents of the two approaches can learn from the respective strengths of each approach. We suspect these differences will largely disappear in the next 20 years