7.5 Effective spin-two particles

Related to the previous point is the possibility of having quantum systems with no pre-geometric notions whatsoever (i.e., a condensed-matter–like system) that still exhibit in their low-energy spectrum effective massless spin-two excitations. This precise question has been investigated in [262, 263, 693, 694Jump To The Next Citation Point] for abstract quantum systems based on the underlying notion of qubits. Although not fully conclusive, these works indicate the possible existence of systems exhibiting purely helicity ±2 excitations. One crucial ingredient in these constructions is the existence of a specific vacuum state with the characteristics of a string-net condensate.

These authors also show that it is not easy to have just helicity ±2 excitations – typically one would also generate helicity ±1 and massless scalar excitations. This is what would happen, for instance, in the emergent gravity scenario inspired in the phenomenology of 3He, which will be described below.

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