W. M. Briggs, statistician to the stars! has another excellent post up today.
I left a rather lengthy comment there, which I'm copying here, where it really belongs.
The entire argument behind the MWI is just an outcome of the “woo woo” caused by the Copenhagen Interpretation (CI). This is the root cause of so much misunderstanding in basic physics, and yet it has persisted for nigh onto a century now. The CI muddies the waters at all levels of quantum physics, even to the idiocy behind modern “quantum computers”, which do nothing that wasn’t done by gears and hydraulics decades ago (except at much, much greater expense).
Uncertainty is in our minds, not in the universe. Particles (or waves) exist in one state. We do not know that state, we *cannot* know that state without direct and careful measurement, because particles are tiny and easily disturbed by nearly everything, and everything depends on absolutely everything else, ad infinitum. “Superposition” means a variety of states. Yes, the particle/wave could be in any number of states. And yet, when we measure it, it is always found to be in exactly one. That’s not because of some magical “observer measurement” event. It’s because it was always in one and only one state, but we just didn’t (couldn’t even in principle) know which it was until we measured.
A fundamental principle of physics, seldom written down or articulated, is that “everything except energy sums up to nothing”. When two electrons are “entangled” by having opposite spins, their total spins adds up to nothing. One electron spins right (up), the other left (down). There is no magic involved. Which one is which is completely indeterminate (in our minds) until we measure it. At which point, voila! We instantly know the spin of the other, no matter how far away it may be. So what? If I buy a pair of shoes, and have a friend secretly mail one to Japan, and only after it arrives there I open the box with one left shoe in it, I instantly know the right shoe is in Japan. No hocus, and certainly no pocus, involved.
Uncertainty is in the mind.
As to the uncertainty principle? It is very difficult to measure a wave when using less than half of the amplitude. The less of a wave you have to measure, the more imprecise the measurement must be. The law is actually an inequality involving the product of two uncertainties. This is a basic principle of wave geometry (and you thought high school trigonometry was useless?), and has nothing to do with the precision of measuring instruments. It simply is and must be.
Fun fact: h-bar is the amplitude of not just a photon, but *every* photon. This is where the term “quantum”, meaning quantity, comes from.
