In quantum mechanics, measurement plays a fundamental role and to me, its role is usually described in a rather odd way. The Copenhagen interpretation states thus
When that device makes a measurement, the wave function of the systems is said to collapse, or irreversibly reduce to an eigenstate of the observable that is registered.
But I could not find any understanding of when is the observation happening. Does it happen when the event (such as the electron hitting the screen) happens, or when a person seeing the plate, (or when that person tells me!)
I have read about the multiverse explanation, a few explanations like the importance of consciousness etc. I understand that this is a very fundamental question, and my question is not what really happens. My question is what is wrong with the following interpretation:
The process of measurement is the development of some sort of coherence (I am afraid of using the word entanglement because I do not understand it) between the observed object and the observer. So, from the observer's perspective, the wavefunction has collapsed, without the wavefunction ever collapsing from the perspective of anyone who has not observed it.
This is not the same as the Copenhagen interpretation because there is no real collapse of the wavefunction, nor the multiverse interpretation because there is no real splitting of the universe. This interpretation also makes the "Wigner's friend" experiment intuitive to understand, and removes the distinction between the observer and the observed.
Added after the comment by Luke:
In fact, this interpretation defines measurement as the observer attaining coherence with the observed. Consciousness plays no part, and neither does the act of conducting an observation modify anything about the observed nor split the universe. Repeat measurements by the observer will yield the same result because of the coherence. Isn't this consistency the meaning of reality? This will look to the observer as a collapsed wavefunction. However, for any external agent (Mr. Wigner) who has not made the observation, there is no collapse.
I am more of a quantum mechanics enthusiast, reading mainly from popular science books on this subject. I would be glad if anyone helps me understand why this interpretation is wrong.
Answer
What you're describing is nothing more, and nothing less, than the everettian Many-Worlds Interpretation, when it is understood correctly and without any extraneous metaphysics.
At its core, MWI is just about taking quantum mechanics at face value, without any artificial lines to try and distinguish "classical" systems from quantum ones. As such, the observer is a quantum system made of atoms and molecules and other quantum bits and pieces, and if you have a particle in a state $|a\rangle_\mathrm{particle}$ and you measure it, then the observer will go from the state $|\mathrm{ready}\rangle_\mathrm{observer}$ to the state $|\mathrm{observed}\ a\rangle_\mathrm{observer}$, or in other words, the measurement implements the transformation $$ |a\rangle_\mathrm{particle}|\mathrm{ready}\rangle_\mathrm{observer} \mapsto |a\rangle_\mathrm{particle}|\mathrm{observed}\ a\rangle_\mathrm{observer}. $$ Similarly, if the particle starts in the state $|b\rangle_\mathrm{particle}$, then the measurement will implement the transformation $$ |b\rangle_\mathrm{particle}|\mathrm{ready}\rangle_\mathrm{observer} \mapsto |b\rangle_\mathrm{particle}|\mathrm{observed}\ b\rangle_\mathrm{observer}. $$ And now comes the weird quantum mechanical bit: if take QM at face value and without limits to its validity, then the linearity of the Schrödinger equation tells us that if we start in a superposition state like $\frac{1}{\sqrt{2}}(|a\rangle_\mathrm{p}+|b\rangle_\mathrm{p})$ then the state will evolve to the superposition of the individual outcomes, i.e. $$ \frac{|a\rangle_\mathrm{p}+|b\rangle_\mathrm{p}}{\sqrt{2}} |\mathrm{ready}\rangle_\mathrm{o} \mapsto \frac{ |a\rangle_\mathrm{p}|\mathrm{observed}\ a\rangle_\mathrm{o} + |b\rangle_\mathrm{p}|\mathrm{observed}\ b\rangle_\mathrm{o} }{\sqrt{2}} . $$ Now, I commend you for your restraint in using complicated concepts without fully understanding them, but in this instance there is no need to mince words: the particle has become entangled with the observer.
And, as part of the standard package when it comes with entanglement, neither the particle nor the observer can be assigned a (pure) quantum state on its own; instead, there is only one big universal wavefunction. This is what Everett's interpretation really says (starting from the very title of his PhD thesis, The Theory of the Universal Wave Function). The additional baggage involved in things like
because there is no real splitting of the universe
is just additional mumbo-jumbo added in by monkey brains that have a hard time understanding what it would be like to be a conscious quantum system that's entangled with the rest of the universe.
Oh, and another thing:
helps me understand why this interpretation is wrong
If a given framework is an interpretation of quantum mechanics, in the proper sense of the term (i.e. it doesn't conflict with QM, in which case calling it an 'interpretation' is a good bit of a misnomer), then it can't be "right" or "wrong". Because interpretations of QM have no bearing on observed phenomena, their valuation lies strictly outside of scientific considerations, and you must necessarily use other, shall we say, more metaphysical criteria.
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