What's a black hole?

A black hole is conventionally considered as an astronomical object that consumes irreversibly any matter and any radiation entering its sphere of influence. Physically, a black hole is defined by the presence of a singularity, that is to say a region of space, delimited by a "horizon of events", in which the mass / energy density becomes infinite and the normally observed physics laws apply. However, as an article in the January issue of the journal Nature Astronomy demonstrates, a precise and agreed definition of this "singular" state is elusive. Its author, Dr. Erik Curiel of the Munich Center for Mathematical Philosophy at the LMU, sums up the problem in the following way: "The properties of black holes are studied in various disciplines of physics – in optical physics, in quantum physics and of course in astrophysics, but each of these specialties addresses the problem with its own specific set of theoretical concepts. "

Erik Curiel studied philosophy as well as theoretical physics at Harvard University and the University of Chicago. His current research project, funded by DFG, is focused on developing a clear philosophical description of some confusing aspects of modern physics. "Phenomena such as black holes belong to a domain that is inaccessible to observation and experience, so work based on the hypothesis of the existence of black holes implies a level of unusual speculation even in the field of theoretical physics. " However, it is this difficulty that makes the physical approach to the nature of black holes so interesting from a philosophical point of view. "The physical perspective on black holes is itself inextricably linked to philosophical questions about ontological, metaphysical, and methodological considerations," Curiel said.

"Surprising" and "revealing" information

During the preparation of his philosophical analysis of the concept of black holes for Nature Astronomy, the author spoke with physicists working in a wide range of research fields. During these conversations, he was given a completely different definition of the black hole. It is important to note, however, that each has been used consistently within the limits of the specialized discipline concerned. Curiel himself describes these discussions as "surprising" and "revealing".

For astrophysicist Avi Loeb, "a black hole is the ultimate prison: once you register, you can never go out." For its part, the theoretical physicist Domenico Giulini considers that it is "conceptually problematic to think that black holes are objects in space, things that can move and move".

Curiel's message is that the very diversity of black hole definitions is a positive sign, as it allows physicists to approach the phenomenon from a variety of angles. However, in order to make a productive contribution to this diversity of points of view, it will be important to raise awareness of the differences in emphasis between them.

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More information:
Erik Curiel. The many definitions of a black hole, Nature Astronomy (2018). DOI: 10.1038 / s41550-018-0602-1