 One final issue we might mention here is whether there is a conflict between the superposition principle and general relativity. Curiel claims that “[t]here exists no physical phenomenon well characterized by experiment that cannot be accurately described by one of the two theories, and no physical phenomenon that suggests that one of the two is correct to the detriment of the other’s accuracy” (2001, p. S432). However, Roger Penrose (2004, Chapter 30) has forcefully argued that the superposition principle can, in some circumstances, threaten the principle of general covariance, surely a core principle of general relativity! The idea is that if we prepare a lump of matter in a superposition of two position states (stationary in their ambient spacetime), χ and φ, a state Penrose labels a “Schrödinger’s Lump” state, then the superposition is represented by: |Ψ⟩ = w|χ⟩ + z|φ⟩. Penrose then shows that a stationary gravitational field does nothing to affect the fact that any superposition of the (stationary) position states χ and φ will also be stationary. But then introducing the gravitational field of the lump itself raises a problem. By themselves, the components of the superposition would not seem to raise problems, and we can simply think of the field around the location associated with the lump’s states individually as being nearly classical. Given the stationarity of the states χ and φ, there will be a distinct Killing vector (i.e. a metric preserving vector field) associated with each them. The problem then arises: what of superpositions of these lump states? Are they stationary? Since the Killing vector fields of the two component stationary states live on different spacetimes, with different structures, it seems we don’t have the invariant spatiotemporal structure needed to answer the question. To try and say that the spacetime is really the same (the obvious answer) would conflict with general covariance since then one would be supposing a robust notion of spacetime points which enables one to match up the two spacetimes. As we have seen above, Penrose’s proposed solution is to consider such superpositions as generating a kind of geometric instability which causes the collapse of the superposition.