[PhilPhys] Warsaw Spacetime Colloquium: JB Manchak (28 May on Zoom)

[Antonio Vassallo]

(With apologies for cross-posting)

On Friday, 28 May, JB Manchak (University of California, Irvine) will give
a talk entitled “On the (In?)Stability of Spacetime Inextendibility”
(abstract below).

The meeting will take place online on Zoom (17:00-19:00 CEST). If you have
not registered yet, you can do so by sending a message to
antonio.vassallo at pw.edu.pl.

The Colloquium is organized by the Philosophy of Physics Group at the
International Center for Formal Ontology (Faculty of Administration and
Social Sciences, Warsaw University of Technology).

The recordings of the previous meetings are available on ICFO's YouTube
channel
<https://www.youtube.com/playlist?list=PLM-1yNCyvJJAfiq7LDFjfYc1I5OOxhJ1A>.

ABSTRACT
Within the context of general relativity, the “stability” of various
spacetime properties has been one important focus of study. It has been
argued that “in order to be physically significant, a property of
space-time ought to have some form of stability, that is to say, it should
be a property of ‘nearby’ space-times” (Hawking and Ellis 1973, p. 197).
Questions concerning the stability of spacetime properties are often made
precise using the so-called “C^k fine” topologies on any collection of
spacetimes with the same underlying manifold. (The property of “stable
causality” is often defined using the C^0 fine topology.) Here we review
what is known concerning the (in)stability of spacetime properties within
this framework. After considering some foundational results concerning
causal properties (Hawking 1969; Geroch 1970) and a fascinating drama
concerning geodesic (in)completeness (Beem et al. 1996), we focus on the
property of spacetime inextendibility about which very little is known.
Because inextendibility is defined relative to a background “possibility
space” in the form of a standard collection of spacetimes, one can
naturally consider variant definitions relative to other collections. (Some
formulations of the “cosmic censorship” conjecture rely on such variant
definitions of inextendibility.) We find that the stability of
“inextendibility” can be highly sensitive to the choice of definition —
even when attention is limited to definitions that are relative to
“physically reasonable” collections of spacetimes. Indeed, it is not yet
clear that there is a physically significant sense in which
“inextendibility” is a stable property. We close by drawing attention to
some precise open questions which could be explored to clarify the
situation.
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