It certainly shouldn't! If wavefunction collapse happens for a photodiode but not for a mirror, that means that there should be some objective law of nature that says "if you arrange silicon atoms in a flat, polished plane, the wavefunction does not collapse, but if you dope them with phosphorus and nitrogen, and apply an electric field, it does collapse". Whether the wavefunction collapses or not shouldn't depend on what we consider the system to be.
> in a discernible way.
This is passing the buck. What makes a change discernible? You know there's only one place the bucks stops: The photon frees an electron in the photodiode, which creates a small flow of current; that current is wired to an amplifier, which flips a MOSFET gate, triggering the flow of charge into a capacitor, which results in a voltage that exceeds a comparator threshold, which triggers a software interrupt in a microprocessor, which increments a digital counter, which alters the light output in an LED screen, which a human observes and scribbles in their lab book.
At which point did the system change become "discernible", and change from a quantum wavefunction to a classical-but-randomly-behaved outcome? MWI provides an answer: when the observer became entangled with the system it was observing.
Nature doesn't even 'know' there's a photodiode there, or a human observing the screen. It's just a sea of fermions and bosons all doing their thing, with various positions and velocities. If some of them are arranged in a way that we call a "photodiode", that's only our orderly ontology imposed on the messiness of the natural world. The photodiode itself is merely a bubbling sea of quantum particles, just like its surroundings.
> The question I'm asking is do we have experimental evidence of "very large" macroscopic entanglement?
How large is "very large"? Is the Cosmic Microwave Background big enough? [1][2]
According to MWI, the classical-physics behavior we observe in macroscopic objects, and the unpredictable decay of nuclear isotopes, are exactly what we would expect from macroscopic entanglement. So all you have to do to quantum-entangle a cat is to literally put it in a box with a geiger counter and a uranium puck. But in order to prove something is quantum entangled by the standards of Copenhagen, you need to be able to create an interference pattern that an instrument can record, which necessarily requires that the instrument cannot itself be entangled with the system it's measuring. Otherwise it won't measure an interference pattern, it will measure (randomly but decidedly) one result or the other.
> I'm more inclined to deny a universal definite reality than to accept many-worlds.
What you apparently are inclined to deny is the notion that you could ever be put in a quantum superposition yourself, and that this would merely feel like observing one particular nondeterministic outcome out of a deterministic distribution of possible outcomes. Superpositions for thee, but not for me!
>Whether the wavefunction collapses or not shouldn't depend on what we consider the system to be.
It's not about consideration in terms of choice of what constitutes a system; the systems are defined by sensitivity to informative states. It's like a quantum analog to the light cone, the information boundary defines the system. But this information boundary is much more elaborate and dependent on the dynamics of the structures involved. In this case, the mirror imparts a different information dynamic than the diode and so has different implications for the information boundaries.
What the rules are that govern the interactions that integrate the informative states of two systems isn't obvious. Funnily enough, I've thought a lot about this problem in an entirely different context. Somewhat surprising that it comes up naturally in quantum mechanics. I don't have any fully worked out details, but I do have some intuition. The issue is related to supervenience of macro structures on micro details. The macro structures can carry information (i.e. have mutual information) about external systems; this is how two systems are informationally integrated. A measurement is when the macrostate of one system becomes correlated with the microstate of another system. The correlation is between macro and micro because for state to be useful requires it to interact in a specific way to result in a macrostructure correlation which can in principle be computed with. It's the information analog of free energy. Not all interactions produce state that one can compute with. Information integration requires precisely this kind of state. I suspect the exact nature of this kind of interaction can be characterized in principle.
>What you apparently are inclined to deny is the notion that you could ever be put in a quantum superposition yourself, and that this would merely feel like observing one particular nondeterministic outcome out of a deterministic distribution of possible outcomes. Superpositions for thee, but not for me!
I don't deny that I could, I just don't seem to be. And the interpretation that coheres with unobserved superposition I reject for external reasons. I don't have any qualms about the implications for personal identity from MWI (assuming that's what you're getting at).
It certainly shouldn't! If wavefunction collapse happens for a photodiode but not for a mirror, that means that there should be some objective law of nature that says "if you arrange silicon atoms in a flat, polished plane, the wavefunction does not collapse, but if you dope them with phosphorus and nitrogen, and apply an electric field, it does collapse". Whether the wavefunction collapses or not shouldn't depend on what we consider the system to be.
> in a discernible way.
This is passing the buck. What makes a change discernible? You know there's only one place the bucks stops: The photon frees an electron in the photodiode, which creates a small flow of current; that current is wired to an amplifier, which flips a MOSFET gate, triggering the flow of charge into a capacitor, which results in a voltage that exceeds a comparator threshold, which triggers a software interrupt in a microprocessor, which increments a digital counter, which alters the light output in an LED screen, which a human observes and scribbles in their lab book.
At which point did the system change become "discernible", and change from a quantum wavefunction to a classical-but-randomly-behaved outcome? MWI provides an answer: when the observer became entangled with the system it was observing.
Nature doesn't even 'know' there's a photodiode there, or a human observing the screen. It's just a sea of fermions and bosons all doing their thing, with various positions and velocities. If some of them are arranged in a way that we call a "photodiode", that's only our orderly ontology imposed on the messiness of the natural world. The photodiode itself is merely a bubbling sea of quantum particles, just like its surroundings.
> The question I'm asking is do we have experimental evidence of "very large" macroscopic entanglement?
How large is "very large"? Is the Cosmic Microwave Background big enough? [1][2]
According to MWI, the classical-physics behavior we observe in macroscopic objects, and the unpredictable decay of nuclear isotopes, are exactly what we would expect from macroscopic entanglement. So all you have to do to quantum-entangle a cat is to literally put it in a box with a geiger counter and a uranium puck. But in order to prove something is quantum entangled by the standards of Copenhagen, you need to be able to create an interference pattern that an instrument can record, which necessarily requires that the instrument cannot itself be entangled with the system it's measuring. Otherwise it won't measure an interference pattern, it will measure (randomly but decidedly) one result or the other.
> I'm more inclined to deny a universal definite reality than to accept many-worlds.
What you apparently are inclined to deny is the notion that you could ever be put in a quantum superposition yourself, and that this would merely feel like observing one particular nondeterministic outcome out of a deterministic distribution of possible outcomes. Superpositions for thee, but not for me!
[1] https://arxiv.org/pdf/2106.15100v1
[2] https://www.worldscientific.com/doi/abs/10.1142/S02182718110...