The art of the state

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JournalInternational Journal of Modern Physics D
DateAccepted/In press - 13 Mar 2018
DateE-pub ahead of print - 13 Mar 2018
DatePublished (current) - 20 Apr 2018
Number of pages27
Early online date13/03/18
Original languageEnglish


Quantum field theory on curved spacetimes lacks an obvious distinguished vacuum state. We review a recent no-go theorem that establishes the impossibility of finding a preferred state in each globally hyperbolic spacetime, subject to certain natural conditions. The result applies in particular to the free scalar field, but the proof is model-independent and therefore of wider applicability. In addition, we critically examine the recently proposed `SJ states', that are determined by the spacetime geometry alone, but which fail to be Hadamard in general. We describe a modified construction that can yield an infinite family of Hadamard states, and also explain recent results that motivate the Hadamard condition without direct reference to ultra-high energies or ultra-short distance structure.

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