What's so confusing about quantum physics? Okay, just right off the bat, I want to make it clear that this is a stupid question. Check out this graph, from SMBC:

Ignore the 'political discourse' part and replace it with 'talking about quantum physics'. I'm somewhere near the left peak of that graph, and I need someone more knowledgable than me, someone down there in the trough, to knock me down a peg and tell me where I'm completely wrong. The thing is, I simultaneously believe I'm right, and also know I'm almost definitely not, so maybe you could find this fun.
Here's what I don't get. I know enough about quantum information and quantum computation to have seen the basic equations underlying decoherence theory; here's my understanding of what they mean. Assume you have a density operator describing a full system X that decomposes into two separable subsystems A and B. Assume that we only 'know' the density operator describing the subsystem A. Then, when a unitary operator acts on the full system X, it does not necessarily factorize into two independent unitary operators acting on A and B. It can appear 'from our perspective', the perspective of system A, that a non-unitary 'superoperator' is acting on our subsystem A. This superoperator could be calculated exactly if we knew the density operator for subsystem B, and the full unitary operation U on X. This superoperator is what we perceive to be 'quantum measurement', in that both have an identical mathematical description.
So now here's what I don't understand. If this is true, and my understanding is correct, there isn't really anything to argue about when it comes to the 'meaning' of quantum measurement or to interpretations of quantum mechanics. Decoherence theory implies that the only thing that really exists in the Universe is one giant wavefunction describing the Universe, just a bunch of complex numbers, and the fact that measurements seem to generate some kind of 'classical probability' is simply caused by the fact that we have only unidirectional knowledge about the Universe - when studying a subsystem A of the Universe, we necessarily don't have a precise description of the surroundings B.
So the Copenhagen interpretation, many-worlds, objective collapse - all pointless, if we just accept that only complex 'probability' amplitudes, more precisely known then as just 'amplitudes', are the only 'real' things.
This seems obvious to me. Why am I an idiot?
 A: *

*How does this not imply many worlds? Does your wavefunction-of-the-universe have literally zero amplitude for 'Hitler won world war 2' and 'My best friend turns into a pumpkin' and all the rest of it?

*But if there are many "worlds", then you need to explain 
a. exactly what sort of "part" of the wavefunction is a "world" (is a world a single global amplitude? a particular set of amplitudes?)
b. why the Born rule applies to what we see in our particular world - why we see highly specific regularities, when (at least by the definition of one-world-per-global-amplitude) the typical world is one of homogeneous randomness?
In other words, you would have to answer the questions that any advocate of many-worlds faces.


*Your concept of quantum mechanics comes from the nonrelativistic Schrödinger equation. When you get to relativistic quantum field theory, you have to deal with changes of reference frame, the path integral rules, and amplitudes appear only after you compute it. 

A: Here https://en.wikipedia.org/wiki/Interpretations_of_quantum_mechanics#Tabular_comparison is a nice overview of the (metaphysical?) assumptions of differing viewpoints. And here http://www.preposterousuniverse.com/blog/2013/01/17/the-most-embarrassing-graph-in-modern-physics/ is an overview of favourite interpretations (among supposed experts).
So, I guess my point would be that you're probably not getting yourself into trouble with your basic assumption. However, that assumption itself is not necessarily 'true' (whatever that means). It could be false, true, unknowable, and/or meaningless. (Hence the 'standard' advice: don't try to understand, just use it.)
Such advice aims to avoid questions about what is 'really' real, like https://en.wikipedia.org/wiki/Ontology. If certain or many or all interpretations are predicting the same things, then one may argue: who cares? (Perhaps people who try to match it with GR.)
A: I suppose Quantum Theory has been known your whole life, but for its early pioneers it differed radically from Laplace's "Clockwork Model" completely deterministic Universe.
While predictions of Quantum Theory are always accurate, repeating experiments with say radioactivity can give different results each time, again disagreeing with Classical concepts of Physics.
