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.

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.

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.

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.

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

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

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.

"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

>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.

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

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

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

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.

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?

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.

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.

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.

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.

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

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.

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.

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.

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.

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?

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.

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 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.

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.

[1] https://arxiv.org/pdf/2106.15100v1

[2] https://www.worldscientific.com/doi/abs/10.1142/S02182718110...

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).

“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

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

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.

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.

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.

If we measured objective collapse it would falsify MWI.

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.

Claiming that many-worlds met its burden of proof (compared to Copenhagen) by something other than Copenhagen not being proven is completely missing the point. If you want people to believe many-worlds rather than Copenhagen, you have to disprove Copenhagen. If you don't, then people will continue to believe Copenhagen.

> You're saying: it just is, deal with it. That's unsatisfactory. Copenhagen proponents never seem to have to defend their stance, and you're telling me they don't have to because they're the majority.

I'm saying that Copenhagen is the de facto default, no matter what you think should be. If you want that to change, you have to change peoples' minds. These being scientists, you typically need some evidence to make them change their minds - they're not going to do it just because you think they should.

Without evidence (impossible in principle, if Copenhagen and many-worlds make the same predictions), you can argue that one shouldn't have a firm stance on this, since both interpretations give the same results, and there is no experimental way of discerning one from the other. (This looks a lot like "shut up and calculate".) Or you can argue that the philosophical position of many-worlds is more "parsimonious" or better in some other sense, but that's philosophy, and that doesn't persuade scientists all that well.

TL;DR: If you want people to move from where they are to where they aren't, give them a reason. If you don't, don't be surprised when they don't move.

That is correct. Physics inherits that from culture, which runs on-top of reality, and that is how reality works.

Personally, I agree with you, but reality is consensus based. Deal with it (develop a better story, or meme - "Shut up and calculate" seems effective despite it being in no way whatsoever a proof of anything...try and come up with something catchy like that).

Bohm's implicate and explicate order is an interesting relevant theory.

Now that we're in the age of AI, perhaps things will change.

Because it drops straight out of the schroedinger equation, which is something you were already believing.

Do you consider believing in distant galaxies, rather than a novel type of astronomical object, to be more or less parsimonious? It means multiplying the size of the universe by millions; nevertheless most of us accept it.

Might as well quote Sean Carroll again, he's an expert and I am not:

"The Schrödinger equation tells us that there's a wave function that evolves over time and then you can ask how I can divide that wave function into a set of decohered non-interacting worlds and those are the worlds that happen. It is nowhere close to saying everything happens. It is what is predicted by the Schrödinger equation That is what happens."

https://www.preposterousuniverse.com/podcast/2024/05/06/ama-...

Interestingly, people seem less bothered by the idea that the universe might be infinite in size even though that has a similar feature that a lot of things happen. So it just seems to me that the discomfort is people worrying about the idea that there are copies of themselves out there somewhere when they really shouldn't. At least no more than if in an infinite universe there would also be infinite repetition and infinite identical "yous". This is starting to sound like a Douglas Adams bit now :D

Counting all daily QM-influenced events everybody experiences (a cosmic ray either passed through me or decayed before it reached me, a radon atom either decayed in the air right next to me or not when I went to my cellar etc.), I think it is fair to say there are nearly infinite copies of you in various multiverse branches (of course, some who collected lots of radioactive decay events are already dead from cancer by now).

Arguably this would be limited by the reporting accuracy of your equipment, but perhaps the thought experiment can be tweaked to get around that...

MWI has always suffered with issues around probability. Why should we assign probabilities to branches, and why do those probabilities coincide with the Born rule. My belief is that it won't be widely accepted until this is resolved. There are lots of (typically complex) attempts to derive the Born rule using decision theory or the like. There's no consensus that any of these attempts are compelling, or likely to succeed in the near future.

[0] https://arxiv.org/pdf/gr-qc/9703089