But how, some ask, can we accept as truly scientific a proposition that can never be demonstrated to be true? Many Worlders reply that the truth of their interpretation is there for all to see in the Schrödinger equation itself; the onus is instead on other interpretations of the theory, which are forced to add something to the equation to explain how it “breaks” to give a unique outcome.
In other words, while some object that the Many Worlds Interpretation is poor science because it is too profligate with universes, Many Worlders say that it is in fact the most parsimonious interpretation in terms of assumptions. The truth is that neither of these positions is a strong argument.
A more serious objection to Many Worlds Interpretation is that it doesn’t explain how probabilities enter into quantum mechanics via Born’s rule for extracting an expected outcome from a wavefunction. How can we say that there is a 50% probability of outcome A (which we can verify experimentally) if the truth is that all outcomes always happen (in some world or other) with 100% probability? Some Many Worlders say that these probabilities should be understood instead as the “weighting” of the branches in a quantum split – what Nolan calls the “intensity”. But that doesn’t in itself mean much. If outcomes A and B have probabilities of 75 and 25%, say, this doesn’t make the universe in which B happens any less real, absolute or robust than that for A.Philip Ball, “The many worlds fantasy” at IAI.TV
Our physics color commentator Rob Sheldon offers, “Very briefly, Philip Ball, a truly outstanding science journalist, argues that Everett’s Many World’s Interpretation (MWI) of QM is trying to solve a QM paradox that has been better solved by the decoherence interpretation (DI). I tend to agree. But the test of a theory is its predictions and MWI makes no predictions, whereas DI has made several successful predictions and has experimentalists working on several new angles. So in addition to prediction, one might say that MWI can’t be falsified, whereas DI can be falsified—which makes MWI an inferior theory to boot.”
Likely so. But the multiverse isn’t really about evidence or falsifiability. The theory is held in defiance of the demand for evidence and believed in such a way as to make falsifiability sound unCool. As Ball perceptively notes, “Even though most physicists dismiss or even deride it, it is often eagerly embraced by physics popularizers and their audiences.” Perhaps it is best described as a lifestyle choice.