Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: How is spontaneous symmetry breaking possible?   We pose and resolve a seeming paradox about spontaneous symmetry breaking in
the quantum theory of infinite systems. For a symmetry to be spontaneously
broken, it must not be implementable by a unitary operator. But Wigner's
theorem guarantees that every symmetry is implemented by a unitary operator
that preserves transition probabilities between pure states. We show how it is
possible for a unitary operator of this sort to connect the folia of unitarily
inequivalent representations. This result undermines interpretations of quantum
theory that hold unitary equivalence to be necessary for physical equivalence.

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The categories of this paper are: physics
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The categories of this paper are: comp-gas
32
The categories of this paper are: atom-ph
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The categories of this paper are: cs
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The categories of this paper are: chao-dyn
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The categories of this paper are: eess
26
The categories of this paper are: bayes-an
33
The categories of this paper are: plasm-ph
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The categories of this paper are: cond-mat
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The categories of this paper are: mtrl-th
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The categories of this paper are: hep-th
0
The categories of this paper are: hep-ph
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The categories of this paper are: nucl-th
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The categories of this paper are: nucl-ex
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The categories of this paper are: q-alg
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The categories of this paper are: chem-ph