### 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, 694] 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 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 excitations – typically one would
also generate helicity and massless scalar excitations. This is what would happen, for instance, in
the emergent gravity scenario inspired in the phenomenology of ^{3}He, which will be described
below.