In a brief section of Karl Popper’s Quantum Theory and the Schism in Physics he discusses his attraction to the Many Worlds Interpretation of quantum physics as well as the reason for his rejection of it. Popper is actually quite pleased with Everett’s three-fold contribution to the field of quantum physics. Despite his attraction to the interpretation he rejects it based on the falsifiability of the symmetry behind the Schrödinger equation.
Popper’s model allows for a theory to be scientific prior to supported evidence. There is no positive case for purporting a theory under his model. There can only be a negative case to falsify it and as long as it may be potentially falsified it is scientific. Thus, a scientific theory could have no evidence or substantiated facts to provide good reasons for why it may be true. What makes this discussion of MWI interesting is that despite Popper’s attraction to MWI it’s not the attraction that makes it scientific, it’s his criterion of falsification.
In favor of MWI:
- The MWI is completely objective in its discussion of quantum mechanics.
- Everett removes the need and occasion to distinguish between ‘classical’ physical systems, like the measurement apparatus, and quantum mechanical systems, like elementary particles. All systems are quantum (including the universe as a whole).
- Everett shows that the collapse of the state vector, something originally thought to be outside of Schrödinger’s theory, can be shown to arise within the universal [Schrödinger] wave function.
- The Schrödinger equation is symmetrical with regard to a reversal of the direction of time whereas MWI is not.
Popper argues that a beam of particles traveling through a narrow slit can theoretically test this. Each particle of the scattering beam can be taken as an analogue of one of the world-splits; and the whole beam as an analogue of Everett’s reality—the many worlds which are not only man, but also a scattering in a random manner relative to each other. Popper suggests that we then invert the direction of time and when we do we see that many worlds of the past are a random scatter. This scatter is arranged in a manner that when they fuse they become correlated, even though there was no interaction between them before their fusion. This is what Popper believes to be the crux in falsifying Everett. Popper’s argument rests on Schrödinger’s dependence on reversing the direction of time. I am not able to comment on the validity of Schrödinger’s claim here, but I do find it philosophically problematic.
 Karl Popper, Quantum Theory and the Schism in Physics, Ed. W. W. Bartley, III (Totowa, NJ: Rowman and Littlefield, 1956, 1982), 89-95.