The art of the state

Understand

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.