REFLECTIONS ON SPACETIME - FOUNDATIONS, PHILOSOPHY AND HISTORY During the academic year 1992/93, an interdisciplinary research group constituted itself at the Zentrum fUr interdisziplinare Forschung (ZiF) in Bielefeld, Germany, under the title 'Semantical Aspects of Spacetime Theories', in which philosophers and physicists worked on topics in the interpretation and history of relativity theory. The present issue consists of contributions resulting from material presented and discussed in the group during the course of that year. The scope of the papers ranges from rather specialised issues arising from general relativity such as the problem of referential indeterminacy, to foundational questions regarding spacetime in the work of Carnap, Weyl and Hilbert. It is well known that the General Theory of Relativity (GTR) admits spacetime models which are 'exotic' in the sense that observers could travel into their own past. This poses a number of problems for the physical interpretation of GTR which are also relevant in the philosophy of spacetime. It is not enough to exclude these exotic models simply by stating that we live in a non-exotic universe, because it might be possible to "operate time machines" by actively changing the topology of the future part of spacetime. In his contribution, Earman first reviews the attempts of physicists to prove "chronology protection theorems" (CPTs) which exclude the operation of time machines under reasonable assumptions.

These documents do nothing less than bear witness to one of the most dramatic changes in the foundations of science. The book has three sections that cover general relativity, epistemological issues, and quantum mechanics. This fascinating work will be a vital text for historians and philosophers of physics, as well as researchers in related physical theories.

The papers in this volume were presented at the colloquium "Reduktion in der Wissenschaft: Struktur, Beispiele, philos- ophische PrObleme", held in Bielefeld, West Germany, July 18- 21, 1983. Altogether eighteen talks were delivered at the symposium, and all appear here with the exception of Professor Ehlers' address. In addition, we are pleased to be able to include three papers by invited participants (Kamiah, Ludwig, Scheibe) who were unable to attend the meeting. The meeting itself brought together a sizeable group of logicians, philosophers and working scientists to discuss and debate the theme of reduction, one that occupies a central place in contemporary philosophy of science. The participants and contributors succeeded in opening up new directions in reduction studies and presenting fresh case studies of re- duction from many different areas of scientific practice. Their efforts will greatly enhance our understanding of reduction and, consequently, our grasp of the complex process of scien- tific change and the unity and growth of scientific knowledge.

The papers in this volume were presented at the colloquium "Reduktion in der Wissenschaft: Struktur, Beispiele, philos- ophische PrObleme", held in Bielefeld, West Germany, July 18- 21, 1983. Altogether eighteen talks were delivered at the symposium, and all appear here with the exception of Professor Ehlers' address. In addition, we are pleased to be able to include three papers by invited participants (Kamiah, Ludwig, Scheibe) who were unable to attend the meeting. The meeting itself brought together a sizeable group of logicians, philosophers and working scientists to discuss and debate the theme of reduction, one that occupies a central place in contemporary philosophy of science. The participants and contributors succeeded in opening up new directions in reduction studies and presenting fresh case studies of re- duction from many different areas of scientific practice. Their efforts will greatly enhance our understanding of reduction and, consequently, our grasp of the complex process of scien- tific change and the unity and growth of scientific knowledge.

REFLECTIONS ON SPACETIME - FOUNDATIONS, PHILOSOPHY AND HISTORY During the academic year 1992/93, an interdisciplinary research group constituted itself at the Zentrum fUr interdisziplinare Forschung (ZiF) in Bielefeld, Germany, under the title 'Semantical Aspects of Spacetime Theories', in which philosophers and physicists worked on topics in the interpretation and history of relativity theory. The present issue consists of contributions resulting from material presented and discussed in the group during the course of that year. The scope of the papers ranges from rather specialised issues arising from general relativity such as the problem of referential indeterminacy, to foundational questions regarding spacetime in the work of Carnap, Weyl and Hilbert. It is well known that the General Theory of Relativity (GTR) admits spacetime models which are 'exotic' in the sense that observers could travel into their own past. This poses a number of problems for the physical interpretation of GTR which are also relevant in the philosophy of spacetime. It is not enough to exclude these exotic models simply by stating that we live in a non-exotic universe, because it might be possible to "operate time machines" by actively changing the topology of the future part of spacetime. In his contribution, Earman first reviews the attempts of physicists to prove "chronology protection theorems" (CPTs) which exclude the operation of time machines under reasonable assumptions.