Table 4.
Interpretations of quantum theory
| (Q) | (S) | (C) | |
|---|---|---|---|
| Copenhagen | ✓ | ✓ | × |
| HV theory applied to subsystems | ✓ | ✓ | × |
| HV theory applied to entire universe | × | ✓ | ✓ |
| Many worlds | ? | × | ? |
| Collapse theories | × | ✓ | ✓ |
| Consistent histories | ✓ | ✓ | × |
| QBism | ✓ | ✓ | × |
| Relational quantum mechanics | ✓ | ✓ | × |
| CSM approach | × | ✓ | ✓ |
| ETH approach | × | ✓ | ✓ |
The proposed Gedankenexperiment can be employed to study the various interpretations of quantum theory. Theorem 1 implies that each of them must violate at least one of the Assumptions (Q), (C), and (S) (indicated by ×). For hidden variable (HV) theories, it is relevant whether agents who are using the theory apply its laws (e.g., the guiding equation in the case of Bohmian mechanics) to subsystems around them or to the universe as a whole.