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Curiel, Erik ORCID: 0000-0002-5812-3033 (January 2009): Singularities and Black Holes in Relativistic Spacetimes. In: Zalta, Edward (ed.) : The Stanford Encyclopedia of Philosophy. Fall 2012.
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A spacetime singularity is a breakdown in the geometrical structure of space and time. It is a topic of ongoing physical and philosophical research to clarify both the nature and significance of such pathologies. Because it is the fundamental geometry that is breaking down, spacetime singularities are often viewed as an end, or “edge,” of spacetime itself. However, numerous difficulties arise when one tries to make this notion more precise. Our current theory of spacetime, general relativity, not only allows for singularities, but tells us that they are unavoidable in some real-life circumstances. Thus we apparently need to understand the ontology of singularities if we are to grasp the nature of space and time in the actual universe. The possibility of singularities also carries potentially important implications for the issues of physical determinism and the scope of physical laws. Black holes are regions of spacetime from which nothing, not even light, can escape. A typical black hole is the result of the gravitational force becoming so strong that one would have to travel faster than light to escape its pull. Such black holes contain a spacetime singularity at their center; thus we cannot fully understand a black hole without also understanding the nature of singularities. However, black holes raise several additional conceptual issues. As purely gravitational entities, black holes are at the heart of many attempts to formulate a theory of quantum gravity. Although they are regions of spacetime, black holes are also thermodynamical entities, with a temperature and an entropy; however, it is far from clear what statistical physics underlies these thermodynamical facts. The evolution of black holes is also apparently in conflict with standard quantum evolution, for such evolution rules out the sort of increase in entropy that seems to be required when black holes are present. This has led to a debate over what fundamental physical principles are likely to be preserved in, or violated by, a full quantum theory of gravity.