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Within the framework of either Euclidian (functional-integral) quantumgravity or canonical general relativity the signature of the manifold is apriori unconstrained. Furthermore, recent developments in the emergentspacetime programme have led to a physically feasible implementation ofsignature change events. This suggests that it is time to revisit the sometimescontroversial topic of signature change in general relativity. Specifically, weshall focus on the behaviour of a quantum field subjected to a manifoldcontaining regions of different signature. We emphasise that, regardless of theunderlying classical theory, there are severe problems associated with anyquantum field theory residing on a signature-changing background. (Such as theproduction of what is naively an infinite number of particles, with an infiniteenergy density.) From the viewpoint of quantum gravity phenomenology, wediscuss possible consequences of an effective Lorentz symmetry breaking scale.To more fully understand the physics of quantum fields exposed to finiteregions of Euclidean-signature (Riemannian) geometry, we show its similaritieswith the quantum barrier penetration problem, and the super-Hubble horizonmodes encountered in cosmology. Finally we raise the question as to whethersignature change transitions could be fully understood and dynamicallygenerated within (modified) classical general relativity, or whether theyrequire the knowledge of a full theory of quantum gravity.

Signature change events: A challenge for quantum gravity?