# What's so confusing about quantum physics? [closed]

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?

## closed as primarily opinion-based by ACuriousMind♦, CuriousOne, Norbert Schuch, Gert, Kyle KanosMar 6 '16 at 13:37

Many good questions generate some degree of opinion based on expert experience, but answers to this question will tend to be almost entirely based on opinions, rather than facts, references, or specific expertise. If this question can be reworded to fit the rules in the help center, please edit the question.

• "if we just accept... " I like you. That is exactly what makes me have the same feeling you're having. It's like "come on people... it's not like that we're not used to understand things described as mathematical entities. Just stop thinking like things are snooker balls". – cinico Mar 5 '16 at 18:12
• This question as posed will likely elicit primarily opinion-based answers, which makes it off-topic. Can you perhaps focus this down somehow? Perhaps by asking something specific about the physics concepts themselves. That said, "complex probability amplitudes" can't be "real". How can a mathematical entity be "real"? We use mathematics to describe reality; it's not the reality itself. And anyway, the question isn't really about accepting QM as an accurate (and precise) description about the universe; it's about making the connection between our everyday experience and what QM says. – march Mar 5 '16 at 18:20
• @march I believe that's exactly what he's asking/stressing: "QM is about describing, as Physics is in general. So what's so confusing about QM if we could simply accept that this is a better description of reality?" – cinico Mar 5 '16 at 18:46
• I don't think quantum physics is particularly "confusing" either. This isn't an objective question because people can't even agree on whether its premise is correct (which is because "confusing" is not an objective property of a theory). – ACuriousMind Mar 5 '16 at 18:51
• Don't worry, QM isn't actually as confusing as most professors and science popularizers make it out to be, and indeed there is no physics whatsoever behind quantum interpretations. It sounds like you are in a good situation philosophically. – user10851 Mar 5 '16 at 20:33

1. 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?

2. 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.

1. 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.
• Yes, this wavefunction-of-the-universe has literally zero amplitude for those events. When you forget about quantum measurement entirely, the evolution of a pure-state quantum system, like I'm assuming the Universe is, is purely deterministic. So while from our perspective the choice between events/measurements may seem random, it isn't "actually". Regarding nonrelativistic Schrödinger - that's the kind of information I'm asking for. I don't know enough about quantum physics or field theory to know how or whether that would change the import of what I'm saying. – Billy Smith Mar 6 '16 at 0:13
• "while from our perspective the choice between events/measurements may seem random, it isn't "actually"." You've heard of superposition? The Schrödinger equation is linear. The unobserved branches of the wavefunction do not cease to exist, if its evolution is linear. – Mitchell Porter Mar 6 '16 at 0:30
• Right, but if there is no quantum measurement, then there are no "unobserved branches" of the wavefunction, no? The notion doesn't make sense then. Only unitary evolution of a quantum state, as stipulated by the Schrödinger equation. – Billy Smith Mar 6 '16 at 0:44
• The Schrödinger equation generates superpositions. If an atom radioactively decays and emits a particle, the wavefunction for the emitted particle is an expanding spherical wavefront. If you take into account collision of the emitted particle with other atoms, you end up with an overall wavefunction consisting of mutually decohered histories of collision. Are you saying that only one of those histories is real? – Mitchell Porter Mar 6 '16 at 1:51
• No. I'm saying that the wavefunction itself should be considered what's real. The fact that we only observe one consistent history of collision is merely due to our perception; if we had the full wavefunction representing the rest of the universe, including our measurement apparatus, we could predict the wavefunction that would result from entangling the atom and the rest of the Universe in the process of measurement, and that would be all the answer we would need. – Billy Smith Mar 6 '16 at 2:24

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.)

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.