Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: The Kramers-Kronig relations for light moving backwards in time   The recent discovery of light moving backwards in time, when it propagates in
a suitable dispersive medium, obliges us to reexamine the Kramers-Kronig
relations. In their usual form, they are dealing with usual light (moving
forward in time), and they are a consequence of causality. In a similar
fashion, we derive the appropriate form of these relations for light moving
backwards in time, starting, instead of causality, from a principle of
anti-causality, appropriate to such a light.

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The categories of this paper are: acc-phys
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The categories of this paper are: patt-sol
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The categories of this paper are: chem-ph
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The categories of this paper are: atom-ph
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The categories of this paper are: chao-dyn
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The categories of this paper are: mtrl-th
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The categories of this paper are: hep-th
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The categories of this paper are: solv-int
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The categories of this paper are: astro-ph
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The categories of this paper are: eess
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The categories of this paper are: nucl-th
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The categories of this paper are: stat
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The categories of this paper are: hep-lat
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The categories of this paper are: math
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The categories of this paper are: q-fin
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The categories of this paper are: quant-ph