 Another fundamental question is the relativistic extension of the modal approach. Dickson and Clifton (1998) have shown that a large class of modal interpretations of ordinary quantum mechanics cannot be made Lorentz-invariant in a straightforward way (see also Myrvold 2002). With respect to the extension to algebraic quantum field theory (see Dieks 2002, Kitajima 2004), Clifton (2000) proposed a natural generalization of the non-relativistic modal scheme, but Earman and Ruetsche (2005) showed that it is not yet clear whether it will be able to deal with measurement situations and whether it is empirically adequate. The problems revealed by these investigations relate to the fact that modal interpretations employ the non-relativistic concept of a global state, of an extended system, at one instant. In a local field-theoretic context this becomes different, and this may help avoiding conflicts with relativity (Earman and Ruetsche 2005). Berkovitz and Hemmo (2005) and Hemmo and Berkovitz (2005) propose a different way out: they argue that perspectivalism can come to the rescue here (see also Berkovitz and Hemmo 2006). The idea that perspectivalism is essential for ensuring compatibility with special relativity is elaborated by Dieks (2021), who puts forward a relativistically covariant treatment of experiments of the EPR-type, based on PMI.