Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Karlhede's invariant and the black hole firewall proposal   The Karlhede invariant is formed from the contraction of the covariant
derivative of the Riemann tensor. It is a coordinate invariant that vanishes at
the Schwarzschild event horizon $r=2m$. The vanishing of the invariant allows
an observer to construct a local measuring device and use it to detect an event
horizon while falling into a black hole. Recent proposals postulate the
existence of a "firewall" at the event horizon that may incinerate an infalling
observer. These proposals face an apparent paradox if a freely falling observer
detects nothing special in the vicinity of the horizon. The behavior of
Karlhede's invariant raises the possibility that the event horizon is a real
physical membrane with measurable properties that are detectable by a freely
falling observer.

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The categories of this paper are: astro-ph
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The categories of this paper are: hep-ph
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The categories of this paper are: adap-org
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The categories of this paper are: comp-gas
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The categories of this paper are: physics
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The categories of this paper are: quant-ph
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The categories of this paper are: q-alg
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The categories of this paper are: cmp-lg
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The categories of this paper are: cs
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The categories of this paper are: hep-lat
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The categories of this paper are: plasm-ph
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The categories of this paper are: eess
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The categories of this paper are: solv-int
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The categories of this paper are: nucl-ex
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The categories of this paper are: hep-ex
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The categories of this paper are: supr-con