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> all you need to do to falsify the many worlds-interpretation is to do an experiment where the wave function is not under the Schrödinger equation.

Which, Carroll conveniently fails to note, happens every time we make a measurement on a system that is not in an eigenstate of the observable being measured. For example, every time an H polarization measurement is made on a V+ polarized qubit, which happens all the time in quantum computing, the result is either H+ or H-, with 50% probability of each; but before the measurement the qubit was in state V+. The time evolution from V+ to H+ or H- is not time evolution under the Schrodinger equation.

MWI proponents explain this away by saying there is another branch of the wave function where the result is the other one (for example, H- if we observed the result H+), so that the whole time evolution is still under the Schrodinger equation. (Note that "the whole time evolution" here has to include the entire universe, not just the qubit being measured.) But MWI proponents also have to say that this other branch of the wave function is in principle unobservable. In other words, the MWI itself says it is not falsifiable.

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There are objective collapse theories that lead to different physical predictions. Copenhagen is not actually a well defined theory because it does not define measurement.

If we measured objective collapse it would falsify MWI.

> There are objective collapse theories that lead to different physical predictions.

Yes, that's true. So far all of the ones whose predictions can be tested have failed the tests. But if one were to succeed, yes, that would falsify the MWI.

In a general, philosophical sense it probably is non-falsifiable. But... where does it end?

What if we took the measurement 5ns, 500ms, 5 seconds, 5 hours later, because of, you know, things. Billions of universes springing up every second does not seem plausible to me and it does sound like an escape hatch: we don't know why it happened that way, so let's just say there is another unobservable universe where it happened the other way round.

Which is nice, I don't have to think about it anymore. But... it kind of reminds me of the flying arrow or a turtle you can never reach. If you are thinking in the same problem space you will never reach the correct conclusion. So, what we really need is the different mode of thought (I wish I would know to suggest something ;) ), not an escape hatch.

> Billions of universes springing up every second does not seem plausible to me

Why? Do you have any reason for this, or is it just a gut feeling?

If it’s just a gut feeling, I’ll note that this was exactly the reaction most of the intelligencia had to Capurnicus and Galileo. Turns out the universe is a caster place than we previously knew.

Occam's razor?

Occam's razor cuts both ways :-)

Some people when they hear that MWI implies that there are many worlds and that the wave function branches they think that implies that MWI is "adding" something so Occam's razor should work against it.

OTOH what MWI postulates that the Schrodinger equation is all there is and the collapse of the wave function is observed because the observer is entangled with the system it measured.

In order to make an intuitive sense of what's going on, MWI invokes the "worlds" and the "branching" but that's just a way for us to grasp how it would feel if ourselves are part of the entangled system. But as many analogies whose purpose is to tickle the intuition it doesn't work for everybody and may be confusing the discourse more than helping it.

But the thing is: the everett interpretation is simpler and it requires less additional rules and mechanisms to explain what we observe. So Occam's razor should favor it.

Simplicity in terms of weighing theory cost isn't about rules, but about explanatory posits. MWI posits exponential growth in discernible state about which sufficient information must be carried by whatever grounds the wavefunction. But you don't get this multiplicity of state/information for free. This is a theory cost that is as profligate as one can imagine. It is magical thinking to imagine that the universe gets this for free.

I don't know where the expression comes from, but "the only numbers that need no justification are zero and infinity". Think of it this way, if you found a coordinate system within the universe that goes from 0,0 to 100,100, you'd have to ask "but why exactly 100?" whereas it would be less surprising if the system had no apparent limit and just could go to infinity on any axis.

The multiplicity of MWI is like that: just a maximally extended coordinate system.

A coordinate system is an abstraction, whether anything has some large coordinate is irrelevant to the coordinate system itself. But my understanding of MWI is that there is discernible state being described by the Schrodinger that is taken as "real" or "concrete" by MWI. If we considered the non-actual branches as merely abstract coordinates, then there would be no problem with profligate state and its associated theory cost. But then it wouldn't be many worlds, but rather some kind of collapse theory.

Wavefunction state is just as “real” in the Copenhagen interpretation. There cannot be any hidden variables, it is not merely a mathematical abstraction. You then must posit this additional process of collapse

Where the MWI takes additional theory burden is where it explains how the born rule translates to our lived experience - ie. why do we only experience one path of the wavefunction

I have no clue what you mean by “theory cost” but MWI is just describing existing evolution of the wave function.

I think the measurement problem is a much bigger ‘theory cost’ to Copenhagen than just assuming wavefunction mechanics continues even after measurements are made.

Theory cost just means some quantity by which we compare theories and judge one to be more parsimonious. A theoretical posit has some cost associated with it. MWI posits actual discernible states, each of which is a unique posit and thus a unique contribution to the theoretical cost of the whole. This state grows exponentially which implies an exponentially growing cost. This is as profligate as one can imagine. Copenhagen does not have an exponential cost as the non-collapse branches do not have a concrete reality under Copenhagen. The cost associated with the new collapse construct pales in comparison.

the occam's razor is an abstract principle, so it is hard to really quantify what counts as additional 'theory burden'. i simply don't agree that this "actual discernible states, each of which is a unique posit and thus a unique contribution to the theoretical cost of the whole" is an actual way of judging theory burden vs. just "the rules we've already discovered continue to apply at large-scale and chaotic systems" which i would say is very low theory burden

Copenhagen isn't even clear on if the states have a concrete reality or not - there certainly are no hidden variables, at least.

There's a certain amount of intuition and taste involved when assigning theoretical costs, but that's no reason not to do it. We have general principles with some rational justification for preferring certain features of theories over others. Simplicity in terms of "not multiplying entities without need" is one such principle.

A discernible state is an entity. More discernible states requires more entities or more discernible features of an entity. This is the only way to make the concept of discernible state intelligible. To deny this is just to engage in magical thinking. But each posit has a cost associated with it. We don't know what that cost is exactly (relative to other theories with different features). But we do know that an exponential growth in discernible states overshadows the theoretical cost of the whole regardless of how we quantify the cost of any individual posit. MWI is almost maximally profligate; any empirically adequate theory without this cost should be preferred.

to me, this is like saying germ theory of disease has higher theoretical burden than miasma theory of disease because it posits all of these individual viruses and bacteria around us everywhere.

"sure, we can observe bacteria and viruses in the lab individually - but when we move to the physical world disease spread is described by miasma which is simpler and thus this is the more parsimonious theory." there are fewer entities in miasma - but you still have to posit this additional thing (the miasma) on top of the already observed viruses and bacteria.

similarly, i don't agree that 'count of entities' is the correct criterion for what makes a theory more or less parsimonious/occam's razor preferred. it is about the number of rules even if there is a single rule that creates exponential entities. Copenhagen would also predict this 'exponential branching' in the system under observation - it just proposes that it collapses when measured by the apparatus: an additional rule that makes it less parsimonious than many worlds

It's hard to compare theories that don't have well-developed mechanisms. If we're naive theorists comparing miasma vs tiny organisms, but you have no sense of how those organisms operate, its functionally just a comparison between one miasma and many miasmas, i.e. one vs. many nebulous non-mechanistic effects. In that case one miasma would be preferable. But that's not a demerit to the concept of minimizing entities as a theoretical virtue.

>i don't agree that 'count of entities' is the correct criterion for what makes a theory more or less parsimonious/occam's razor preferred.

I mean, its literally in the original formulation of Occam's razor: "plurality should not be posited without necessity". But there are many ways to justify this. My preferred argument is that fewer entities means fewer resources with which to "bake in" the explananda into the theory. The more knobs to turn to generate your empirically adequate theory, the less likely your theory will capture reality.

>[Copenhagen] just proposes that it collapses when measured by the apparatus

Measurement isn't limited to an "apparatus". Collapse of superposition is a feature of large scale interactions. While there is branching, in practice it is bounded because interactions tend to cause collapse.

Well. They're not discernible states if they are in another world. That's the very definition of "another world".

From an outside viewer that hasn't been entangled with you, no branching happens and they see the overall state evolve according to the Schrodinger equation.

Imagine that w create very small machines that could perform some observations and record them into some internal log.

Now imagine creating a particle in some state and have that machine measure that state and record it internally. Provided that the particle+machine system is isolated from the environment (and us) you'd probably agree that the particle and machine are entangled and that the state of the "log" is not in a defined state until we measure it.

Now, that information processing machine is not a human but I think it may be useful to describe how the external world would look like from it's point of view. For example, if that machine performed several experiments on that test particle or other particles it would note in its log data that would confirm that it obeys the Born rule.

In that sense, it's "useful" to imagine how things look like from the "point" of view of that machine and it's possible outcomes in the many worlds, although these worlds might not really matter to us since we can describe the system that includes that machine as having been in a mixed state all along until we humans actually observe its "log"

The likelihood that people start to object to this though experiment gets higher and higher s as soon as one starts to imply that we humans ourselves are exactly like that machine.

For me, the observation that our ability to accept or reject this interpretation is so much tied to our intuitions about our own _self_ is a strong hint that actually favours the theory. We humans have been known to be trapped by our point of view

>Well. They're not discernible states if they are in another world. That's the very definition of "another world".

Discernible in principle. Either the states are distinguishable from within the system (e.g. two branches that disagree on the state of the world), from an outside observer, or a God's eye view. If none of this is true then there is no other world.

>Now imagine creating a particle in some state and have that machine measure that state and record it internally. Provided that the particle+machine system is isolated from the environment (and us) you'd probably agree that the particle and machine are entangled and that the state of the "log" is not in a defined state until we measure it.

Lets further imagine a large collection of these machines connected in series. Each machine can perform one of two experiments at any given trial, and which experiment they run depends on the most recent outcome of the prior machine in the series (e.g. the direction of a particle's spin). There are an exponential number of scenarios as the number of machines in series grows. If we imagine this system entangled until we observe it, and we expect that the definite state is consistent in terms of which particle the machine measured and the corresponding prior outcome in the series, then the entangled system just has to carry discernible state in proportion to the exponential state space of the system. If not, then the system isn't in an indefinite state until observed. The concerns about exponential theory cost remain. Unless the state space is bounded by collapse or some other means, the theoretical cost to MWI or any theory based on Schrodinger without additional posits is supreme.

>In that sense, it's "useful" to imagine how things look like from the "point" of view of that machine and it's possible outcomes in the many worlds, although these worlds might not really matter to us since we can describe the system that includes that machine as having been in a mixed state all along until we humans actually observe its "log"

There's a tension here that needs to be released somehow. On the one hand, you have an external observer determining the system is in a superposition of states. On the other hand, you have the perspective from within the system of everything being definite. You can't just carry both forward simultaneously without addressing the disagreement on the state of reality. Either there are really an unbounded number of branches, the branches are bounded by some mechanism, or there are no branches and the evolution of reality happens asynchronously.

I'm not sure what you're actually suggesting.

Are you saying that under this scenario of N such machines in series the MWI would make different predictions that just solving the Schrodinger equation?

No, I'm saying that under the assumption of no collapse, the scenario with N machines after a long period of entanglement requires that nature "stores" (has discernible state for) information proportional to e^N bits about this region of space. The prediction is the same, the question is what does it say about how nature is constructed and how does this relate to the credence we give the theory.

I think you just highlighted the fundamental difference between a bit and a qubit

Do you speak Ithkuil?

The sibling reply is correct, but I'll give a different answer connected to the analogy I gave that you are replying to. Occam's razor prefers simpler theories not smaller universes.

The idea that the universe consists of seemingly uncountable numbers of stars, each of which is another Sun and perhaps has planets of its own, is vastly more detail than the old heliocentric model of one Sun, a fixed number of planets, and stars being pinpricks in the tapestry surrounding the solar system. Nonetheless Occam's razor suggests that we accept the view that the stars are suns and we are just one planet among many billions upon billions, because although that universe requires vastly more detail to describe, it is governed by a simpler set of rules. Unifying heaven and earth under a single, simple set of physical laws, discovered by Newton, makes for a simpler scientific model, even if it introduces the possibility of the universe being vastly larger than we originally thought.

Similar situation with many-worlds. If MWI is right, then the "universe" if interpreted to include all reachable branches of the so-called multiverse, is vastly larger than we previously believed. But it is also simpler, in a strict Occam's razor sense, because it does away with the concept of collapse. Wave function collapse is an additional physical rule in the Copenhagen interpretation, whereas it's just an illusion and not an enumerated part of physical law in MWI.

Occam's razor is not just about simple rules. The original formulation is: "plurality should not be posited without necessity". Necessity here is empirical adequacy. A theory that posits an unbounded, exponential growth in states fails the simplicity test against an empirically adequate theory that posits bounded state.

The philosophy of science has advanced significantly since the 12th century, and scientists don’t generally use the original definition anymore.

I have never, ever heard it expressed in this statistical mechanics sense, and it doesn’t line up with most advances in physics (my field) which have been accompanied by increased number of states but fewer rules.

well, it would not be Billions of universes, it would basically be infinite number of universes popping up every second. Which while interesting, does not seem possible under any law of conservation of mass?

They're not popping up, the wave function simply branches. In Copenhagen you cut off the branches, in MWI you do not. Total energy in MWI is the weighted sum of the energy of all branches (weighted according to borns rule). If one could dismiss MWI this easily, there wouldn't be any serious proponents of it and we wouldn't have to have this discussion.

>Why? Do you have any reason for this, or is it just a gut feeling?

MWI says these universes are actual, concrete physical state, for which the universe must somehow differentiate by discernible information. But you don't get this multiplicity of state/information for free. This is a theory cost that is as profligate as one can imagine. It is magical thinking to imagine that the universe gets this for free.

Every quantum physics interpretation agrees that these universe-branches multiply for any isolated quantum system until a human makes a measurement of that system. They only disagree on what happens when and after a measurement is made.

Even the Copenhagen interpretation agrees that if you do the two-slit experiment with a single photon, that one photon will traverse all trajectories to reach the target screen.

>until a human makes a measurement of that system.

No one sane thinks human observation has anything to do with measurement in QM anymore.

Then I'll take this opportunity to ask someone who presumably considers themself sane: How do you determine what kind of interaction counts as a measurement?

Let me say that my previous comment was unfair. It's unfair to say that no one sane would consider quantum measurement related to consciousness. Rather its an extremely fringe position at this point.

It's certainly a deep mystery of nature and I don't claim to have an answer. Quantum erasure experiments strongly suggest to me that measurement has to do with information about the system that entails constraints on the evolution of the system. Nature seems to abhor inconsistency. If the state of the system is such that one can in principle derive constraints on its evolution up to this point, then nature ensures its evolution up to this point is consistent with that information. A measurement then is just any interaction with the property of imparting a constraint on the evolution of the system up to now.

But that seems circular; what determines whether an interaction imparts a constraint, or doesn't?

Let's say that I send a photon through a beam-splitter which will deflect its momentum either right or left. QM predicts that the photon at the output of the beamsplitter will be in a superposition of both trajectories, and we can prove this by adding mirrors that combine both possible output beams back together with another and generate an interference pattern. The interference pattern will be detected by a sensitive photodiode array, capable of detecting individual photons.

Would you say the interaction with the photodiode array counts as a measurement? And the interaction with the mirror does not? How does QM know whether a photon interaction with a bunch of atoms in a mirror coating, vs in a silicon diode, creates a constraint on the evolution of the system? Why is one interaction measurement-like in nature, but not the other? Is there some property of silicon diodes that make them immune to being put into quantum superpositions?

I would have said that quantum erasure experiments make by far the most sense if you consider measurement to never happen, and the result is merely a superposition of both outcomes. The photon is put into a superposition of both states when it reaches the beamsplitter, the detector screen is put into a superposition of both states when the superposition-photon reaches it, and so are you in turn when you observe the detector screen.

> Why is one interaction measurement-like in nature, but not the other?

The mirror reflects all the energy of the photon whereas the photodiode absorbs the energy and its state is altered by it. How nature knows the difference is a deep question. Presumably the rules governing nature ensure this consistency. What that looks like is anyone's guess.

>I would have said that quantum erasure experiments make by far the most sense if you consider measurement to never happen

I agree, but then this interpretation comes with the exponentially growing cost of actual branching. In my view, this rationally must be the interpretation of last resort, for reasons stated in the other threads.

Counterintuitively, the best sensors actually behave just like mirrors themselves, re-emitting a reflected photon rather than absorbing any of the energy of the incoming signal. This increases the signal-to-noise ratio, because the E-field magnitude at the sensor is doubled, as it combines the constructive interference of the incoming and reflected photon.

And as mentioned elsewhere, all QM theories do have to deal with exponentially increasing branching. A cost that apparently increases if we grind up photodiodes and turn them into mirrors. If wavefunction branching is a problem, how does QM get away with it when nobody is looking?

Deep questions usually have simple answers! The simplicity is what makes the answer difficult to accept.

> the best sensors actually behave just like mirrors themselves, re-emitting a reflected photon rather than absorbing any of the energy of the incoming signal.

Presumably some state is changed somewhere in the system to indicate a detection.

>If wavefunction branching is a problem, how does QM get away with it when nobody is looking?

Do we know that branching is unbounded when we're not looking? Is there a detectable difference in a large, presumably entangled system and its classical counterpart?

> Presumably some state is changed somewhere in the system to indicate a detection.

Yes, but "state change" happens all the time! When the photon bounces off the mirror, even if 100% of its energy is reflected, it imparts momentum to the mirror. When the photon propagates through free space, it "changes state". All state changes occur exactly in accordance with the time-evolution of the Schrodinger equation.

> Do we know that branching is unbounded when we're not looking?

This is a basic principle of QM, which would have pretty measurable macroscopic impact if it were invalid. "Branches" are not even finite in number, even for a single particle in free space; its position and momentum distributions are both continuous, with a minimum possible area guaranteed by the Heisenberg uncertainty principle, and even so much as precisely localizing its momentum already creates an infinity of equally-likely possible positions.

>Yes, but "state change" happens all the time! When the photon bounces off the mirror, even if 100% of its energy is reflected, it imparts momentum to the mirror.

It depends on what you consider the system. For a "measuring apparatus" to signal the outcome of a measurement, its internal state must change in some structured way. That is, the configuration of its atoms must change in a discernible way as to indicate one outcome over another. Perhaps the change in momentum is too diffuse or too noisy to result in an informative state change with implications for collapse. Perhaps one's context is relevant to whether a state change results in collapse (e.g. Wigner's friend). I'm more inclined to deny a universal definite reality than to accept many-worlds.

> "Branches" are not even finite in number, even for a single particle in free space

This only has problematic metaphysical implications if we assume the definite reality involves particles rather than waves or some indeterminate in-between. Branching due to "measurement" appears to be different owing to its problematic metaphysical implications. The question I'm asking is do we have experimental evidence of "very large" macroscopic entanglement?

> It depends on what you consider the system.

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...

>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’s not “billions of universes springing up”, it’s just simple wavefunction evolution according to the laws we already know.

“collapse” seems much more arbitrary, especially as we are able to construct larger and larger objects that are best described by wavefunction mechanics. it seems obvious we just become entangled with the experiment through decoherence. idk, it probably is non-falsifiable

I mean, obviously. If you let a system interact with the external world that system on its own will not follow the schrodinger equation.

Carroll is suggesting we try to observe this behavior from a sufficiently large isolated system.

> If you let a system interact with the external world that system on its own will not follow the schrodinger equation.

Not only that--even the combined system + what it is interacting with won't.

> Carroll is suggesting we try to observe this behavior from a sufficiently large isolated system.

But you can't, even with the entire universe, if measurements have single results. In the MWI, they don't; any measurement has all possible results. But we don't observe that, we observe measurements to have single results, so the MWI has to jump through hoops to explain that discrepancy away--and in doing so, makes itself unfalsifiable.

sure it’s unfalsifiable in that there is always an interpretation of copenhagen that will predict the same thing as MWI, but if we are able to construct a very large object that doesn’t show “collapse”, it seems vibe-wise that the same process is what is happening to us when we make a measurement, just more chaotically (aka decoherence)

> But we don't observe that, we observe measurements to have single results

What do you think it would feel like to observe a measurement to have multiple results simultaneously? And do you think MWI predicts any branch of the wavefunction would exist in which a human would be experiencing that sensation? It doesn't; it predicts a superposition of (human observes result A + human observes result B), with ~zero probability of (human observes both results A + B). So I'm not sure which hoops you think MWI needs to jump through to match our subjective experience.

> What do you think it would feel like to observe a measurement to have multiple results simultaneously?

I have no idea.

> do you think MWI predicts any branch of the wavefunction would exist in which a human would be experiencing that sensation?

No. As you say, the MWI claims that in each branch of the wave function, the human experiences a single result of a measurement.

However, that is completely different from the way QM normally treats individual branches of the wave function in an entangled superposition, which is what we are talking about--"measurement" in the MWI is just an interaction that entangles a "measured system" with a "measuring device" (and eventually with "the brain of a human who looks at the measuring device to read off its result") and results in an entangled superposition of all of those subsystems. Normally, in individual branches of an entangled superposition of multiple subsystems, each subsystem has no well-defined state at all. Only the full system, including all branches of the superposition, has a well-defined state.

That would imply that in the case of a "measurement", the measuring device in each individual branch, and the brain of the human that looks at it, would have no well-defined state at all. It would not imply that in each branch, the measuring device and the brain of the human that looks at it has a well-defined state that corresponds to a particular measurement result being recorded and observed.

In terms of your "feel like", the way QM normally treats individual branches of the wave function in an entangled superposition, it should not "feel like" anything in any individual branch. The human should only "feel like" something, if at all, in the full wave function, containing all the branches. And of course nobody has any idea what it would "feel like" in such a state--but it clearly would not "feel like" having observed a particular result of a measurement.

> I'm not sure which hoops you think MWI needs to jump through to match our subjective experience.

It needs to explain why, somehow, when a "measuring device" or a human observer is involved, the way QM treats individual branches of entangled superpositions drastically changes. See above.

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