Why the Many-Worlds Interpretation Has Many Problems

The wave function is a mathematical expression that defines all possible observable states of a quantum system, such as the various possible locations of a particle. Up until a measurement is made and the wave function collapses (whatever that means), there is no reason to attribute any greater a degree of reality to any of the possible states than to any other. It’s not that the quantum system is actually in one or other of these states but we don’t know which; we can confidently say that it is not in any one of these states, but is properly described by the wave function itself, which in some sense “permits” them all as observational outcomes. Where, then, do they all go, bar one, when the wave function collapses?

Consistent with random number interpretation…

When we make a measurement, we really do get just one result out of the many that quantum mechanics offers. Wave function collapse seemed to be demanded in order to connect quantum theory to reality.

As it evolves, some of these superpositions break down, making certain realities distinct and isolated from one another. In this sense, worlds are not exactly “created” by measurements; they are just separated. This is why we shouldn’t, strictly speaking, talk of the “splitting” of worlds (even though Everett did), as though two have been produced from one. Rather, we should speak of the unraveling of two realities that were previously just possible futures of a single reality.

But if you pull a number out of a bag you don’t argue that pulling a different number happened in different universes…. (same as splitting the bag with Bell’s theorem)

DeWitt argued that the alternative outcomes of the measurement must exist in a parallel reality: another world. You measure the path of an electron, and in this world it seems to go this way, but in another world it went that way.

Measurement is just decoherence…

In this view, splitting is not an abrupt event. It evolves through decoherence and is only complete when decoherence has removed all possibility of interference between universes.

Roland Omnès says the idea that every little quantum “measurement” spawns a world “gives an undue importance to the little differences generated by quantum events, as if each of them were vital to the universe.” This, he says, is contrary to what we generally learn from physics: that most of the fine details make no difference at all to what happens at larger scales.