Goddamnit Sarah Conner!
Fate is predetermined by physics!
So, a new Copenhagen interpretation? 🤔
Sounds like he is reading too much into the observer effect and now has been reclassified himself as an astrologer.
life in the suck zone.
See, this is why I prefer the (terribly named) “Many Worlds” interpretation. Unlike the Copenhagen interpretation, it does not privilege measurement over other types of interactions between systems. That is, the wave function never collapses, it only seems to because you, as the observer, are part of the system.
The easy way to see this is to imagine that you put some other experimenter inside of a box. When they perform a measurement, from your perspective the wave function has not yet collapsed, but from the experimenter’s perspective the wave has collapsed. Essentially, it is as if the system in a box has branched so that there are multiple copies of the experimenter within, one who sees each possible measurement result, but because you are outside of it you could, in theory, reverse the measurement and unite the two branches. However, it is important to understand that the concept of branches is just a visualization; it is nothing inherent to the theory, and when things get even slightly more complicated than the situation I have described, they do not meaningfully exist at all.
(Also, if it seems implausible that a macroscopic system in a box could remain in a superposition of multiple states, you actually are not wrong! However, the reason is not theoretical but practical: any system inside the box will interact thermally with the box itself, so unless it is perfectly insulated you cannot help but interact with it and therefore measure it yourself. This keeps going until essentially the entire world cannot help but perform a measurement of your system. Preventing this tendency from screwing things up is one of the things that makes building quantum computers hard.)
The Many Worlds interpretation is rather unconvincing to me for many reasons.
|1| It claims it is “simpler” just by dropping the Born rule, but it is mathematically impossible to derive the Born rule from the Schrodinger equation alone. You must include some additional assumption to derive it, and so it ends up necessarily having to introduce an additional postulate at some point to derive the Born rule from. Its number of assumptions thus always equal that of any other interpretation but with additional mathematical complexity caused by the derivation.
|2| It claims to be “local” because there is no nonlocal wavefunction collapse. But the EPR paper already proves it’s mathematically impossible for something to match the predictions of quantum theory and be causally local if there are no hidden variables. This is obscured by the fact that MWI proponents like to claim the Born rule probabilities are a subjective illusion and not physically real, but illusions still have a physical cause that need to be physically explained, and any explanation you give must reproduce Born rule probabilities, and thus must violate causal locality. Some MWI proponents try to get around this by redefining locality in terms of relativistic locality, but even Copenhagen is local in that sense, so you end up with no benefits over Copenhagen if you accept that redefinition.
|3| It relies on belief that there exists an additional mathematical entity Ψ as opposed to just ψ, but there exists no mathematical definition or derivation of this entity. Even Everett agreed that all the little ψ we work with in quantum theory are relative states, but then he proposes that there exists an absolute universal Ψ, but to me this makes about as much sense as claiming there exists a universal velocity in Galilean relativity. There is no way to combine relative velocities to give you a universal velocity, they are just fundamentally relative. Similarly, wavefunctions in quantum mechanics are fundamentally relative. A universal wavefunction does not meaningfully exist.
|4| You describe MWI as kind of a copying of the world into different branches where different observers see different outcomes of the experiment, but that is not what MWI actually claims. MWI claims the Born rule is a subjective illusion and all that exists is the Schrodinger equation, but the Schrodinger equation never branches. If, for example, a photon hits a beam splitter with a 50% chance of passing through and a 50% chance of being reflected and you have a detector on either side, the Schrodinger equation will never evolve into a state that looks anything like it having past through or it having been reflected, nor will it ever evolve into a state that looks anything like it having past through and it having been reflected. The state it evolves into is entirely disconnected from the discrete states we actually observe except through the Born rule. Indeed, even those probabilities I gave you come from the Born rule.
This was something Einstein pointed out in relation to atomic decay, that no matter how long you evolve the Schrodinger equation, it never evolves into a state that looks anything like decay vs non-decay. You never get to a state that looks like either or, both, or neither. You end up with something entirely unrecognizable from what we would actually observe in an experiment, only connected back to the probabilities of decay vs non-decay by the Born rule. If the universe really is just the Schrodinger equation, you simply cannot say that it branches into two “worlds” where in one you see one outcome and in another you see a different outcome, because the Schrodinger equation never gives you that. You would have to claim that the entire world consists of a single evolving infinite-dimensional universal wavefunction that is nothing akin to anything we have ever observed before.
There is a good lecture below by Maudlin on this problem, that MWI presents a theory which has no connection to observable reality because nothing within the theory contains any observables.
Rovelli also comments on it:
The gigantic, universal ψ wave that contains all the possible worlds is like Hegel’s dark night in which all cows are black: it does not account, per se, for the phenomenological reality that we actually observe. In order to describe the phenomena that we observe, other mathematical elements are needed besides ψ: the individual variables, like X and P, that we use to describe the world. The Many Worlds interpretation does not explain them clearly. It is not enough to know the ψ wave and Schrödinger’s equation in order to define and use quantum theory: we need to specify an algebra of observables, otherwise we cannot calculate anything and there is no relation with the phenomena of our experience. The role of this algebra of observables, which is extremely clear in other interpretations, is not at all clear in the Many Worlds interpretation.
— Carlo Rovelli, “Helgoland: Making Sense of the Quantum Revolution”
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First, working in terms of decoherence is significantly simpler than worrying about whether something has been measured or not at every single step of the evolution of a system, because I have observed that when people do the latter they tend to get headaches contemplating the meaning of the “quantum eraser” when there is no need to. Second, you actually can observe Born’s rule in action by modeling the evolution of a system with an experimenter performing measurements and watching it emerge from the calculation.
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The only way that the two sides of the EPR pair know that they agree or disagree is by communicating with each other and comparing results, which can only happen through local interactions.
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I have no idea what you even mean by this. What makes the (terribly named) Many Worlds Interpretation nice is precisely that you can just treat everything as a wave function, with parts that might be entangled in ways you don’t know about (i.e., decoherence, modeled via density matrices).
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The fact that you are even making this claim is why I have trouble taking the rest of your comment seriously at all, because I specifically said, “However, it is important to understand that the concept of branches is just a visualization; it is nothing inherent to the theory, and when things get even slightly more complicated than the situation I have described, they do not meaningfully exist at all.”
- Not sure what this first point means. To describe decoherence you need something like density matrix notation or Liouville notation which is mathematically much more complicated. For example, a qubit’s state vector grows by 2^N, but if you represent it in Liouville notation then the vector grows by 4^N. It is far more mathematically complicated as a description, but I don’t really see why that matters anyways as it’s not like I reject such notation. Your second point also agrees with me. We know the Born rule is real because we can observe real outcomes on measurement devices, something which MWI denies exists and something you will go on to deny in your point #4
- This is also true in Copenhagen. Again, if that’s your criterion for locality then Copenhagen is also local.
- I think you should read Everett’s papers “‘Relative State’ Formulation of Quantum Mechanics” and “The Theory of the Universal Wave Function” to see the difference between wavefunctions defined in a relative sense vs a universal sense. You will encounter this with any paper on the topic. I’m a bit surprised you genuinely have never heard of the concept of the universal wavefunction yet are defending MWI?
- That quotation does not come one iota close to even having the air of giving the impression of loosely responding to what I wrote. You are not seriously engaging with what I wrote at all. You denying the physical existence of real-world discrete outcomes is exactly what I am criticizing, so just quoting yourself denying it is only confirming my point.
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A simpler way of stating my point is that entanglement is sufficient to understand measurement, and more importantly, what phenomena are “measurement-like” and which aren’t. Also, you missed my point regarding the Born rule. You can write down a mathematical model of an experimenter repeating an experiment and recording their measurements, turn the crank, and see the probabilities predicted by the Born rule fall out, without any experiment ever having taken place.
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I am confused, then, about what we are supposedly even arguing about here. (Are you sure you are even arguing with me, rather than someone else?)
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I did some searching and I think that what you are calling “relative states” is an older term for what we now call “entangled states”. Being entangled with another system implies (by definition) that there is a greater system containing you and the other system, and so on, which is how you end up with a universal system that contains everything. However, we do not actually believe that reality is dictated by quantum mechanics but by quantum field theory, which is manifestly built on top of special relativity and posits a single field for each kind of particle for the entire Universe, and describes the microscopic behavior so well that it is absurd. Of course, the next step is figuring out how to reconcile this with general relativity, but that isn’t something Copenhagen helps you out with either.
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First you criticize the way that I talked about branches, which I only mentioned briefly as a sort of crude visualization and explicitly called out as being such. Now you are claiming that I am “denying the physical existence of real-world discrete outcomes”?
- Entanglement is just a mathematical property of the theory. If it is sufficient to explain measurement then there is not anything particularly unique about MWI since you can employ this explanation within anything. You also say I missed your point by repeating exactly what I said.
- You’re the one giving this bullet point list as if you are debunking all of my points one-by-one. If you agree there is nothing especially “more local” about MWI than any other interpretation then why not just ignore that point and move on?
- A relative state is not an entangled state. Again you need to read the papers I linked. We are talking about observer-dependence in the sense of how the velocity of a train in Galilean relativity can be said to have a different value simultaneously for two different observers. I drew the direct comparison here in order to explain that in my first comment. This isn’t about special relativity or general relativity, but about “relativity” in a more abstract sense of things which are only meaningfully defined as a relational property between systems. The quantum state observer A assigns to a system can be different from the quantum state observer B assigns to the system (see the Wigner’s friend thought experiment). The quantum state in quantum mechanics is clearly relative in this sense, and to claim there is a universal quantum state requires an additional leap which is never mathematically justified.
- Please for the love of god just scroll up and read what I actually wrote in that first post and respond to it. Or don’t. You clearly seem to be entirely uninterested in a serious conversation. I assume you have an emotional attachment to MWI without even having read Everett’s papers and getting too defensive that you refuse to engage seriously in anything I say, so I am ending this conversation here. You don’t even know what a universal wavefunction is despite that being the title of Everett’s paper and are trying to lecture me about this subject without even reading a word I have written, claiming that the opinions of the cited academics here are “not even worth taken seriously.” This is just an enormous level of arrogance that isn’t worth engaging with.
I assume you have an emotional attachment to MWI without even having read Everett’s papers and getting too defensive that you refuse to engage seriously in anything I say, so I am ending this conversation here.
Uhh, okay. Like, you were the one who felt the need to go on the attack here, but if you need to stop for your mental health than so be it. 🙂
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Irrational probabilities makes MWI impractical unless you interpret the branching much like a continous graph (as a visualization, see phase of matter graphs) with an ever increasing number of dimensions. And yes continous branching is weird
Again, as I said in my comment, the branches in MWI are just a visualization of the very simplest possible case, not a literal description of reality. It is unfortunate (though understandable) that people have latched on to them as if they were the central idea of MWI.
My understanding might be a bit superficial, but I thought the whole point of the MWI was to make explicit the fact that states are relative? To me the rationale was that states are relative and if we simultaneously describe relative states and their observers we can translate the shrödinger+born-rule in a density-operator+partial-trace-rule and make the wave function collapse physical (aka unitary) through branching and decoherence, even though that’s mathematically tedious and in practice people will keep using projectors (1). States being relative means their physical reality is somewhat broken but locality is mostly saved (2), so then we postulate that they derive from a universal wave function to rehabilitate some form of physical realism (3). As to (4), isn’t it solved if you assume that Schrödinger’s equation is actually the less fundamental formalism since it’s only valid for systems that are unrealistically isolated?
MWI very specifically commits to the existence of a universal wavefunction. Everett’s original paper is literally titled “The Theory of the Universal Wavefunction.” If you instead only take relative states seriously, that position is much closer to relational quantum mechanics. In fact, Carlo Rovelli explicitly describes RQM as adopting Everett’s relative-state idea while rejecting the notion of a universal quantum state.
MWI claims there exists a universal quantum state, but quantum theory works perfectly well without this assumption if quantum states are taken to be fundamentally relative. Every quantum state is defined in relation to something else, which is made clear by the Wigner’s friend scenario where different observers legitimately assign different states to the same system. If states are fundamentally relative, then a “universal” quantum state makes about as much sense as a “universal velocity” in Galilean relativity.
You could arbitrarily choose a reference frame in Galilean relativity and declare it universal, but this requires an extra postulate, is unnecessary for the theory, and is completely arbitrary. Likewise, you could pick some observer’s perspective and call that the universal wavefunction, but there is no non-arbitrary reason to privilege it. That wavefunction would still be relative to that observer, just with special status assigned by fiat.
Worse, such a perspective could never truly be universal because it could not include itself. To do that you would need another external perspective, leading to infinite regress. You never obtain a quantum state that includes the entire universe. Any state you define is always relative to something within the universe, unless you define it relative to something outside of the universe, but at that point you are talking about God and not science.
The analogy to Galilean relativity actually is too kind. Galilean relativity relies on Euclidean space as a background, allowing an external viewpoint fixed to empty coordinates. Hilbert space is not a background space at all; it is always defined in terms of physical systems, what is known as a constructed space. You can transform perspectives in spacetime, but there is no transformation to a background perspective in Hilbert space because no such background exists. The closest that exists is a statistical transformation to different perspectives within Liouville space, but this only works for objects within the space; you cannot transform to the perspective of the background itself as it is not a background space.
One of the papers I linked also provides a no-go theorem as to why a universal quantum state cannot possibly exist in a way that would be consistent with relative perspectives. There are just so many conceptual and mathematical problems with a universal wavefunction. Even if you somehow resolve them all, your solution will be far more convoluted than just taking the relative states of quantum mechanics at face value. There is no need to “explain measurement” or introduce a many worlds or a universal wavefunction if you just accept the relative nature of the theory at face value and move on, rather than trying to escape it (for some reason).
But this is just one issue. The other elephant in the room is the fifth point that even if you construct a theory that is at least mathematically consistent, it still would contain no observables. MWI is a “theory” which lacks observables entirely.
Ah so I think I sort of conflated RQM and MWI because I thought it was all about Everett’s other paper “relative state formulation of qm”.
I thought on top of an ad hoc rehabilitation of physical realism, the universal state also did something for the consistency. Something like all the density operators may be expressed as partial traces of the operator describing the their systems’ union, in order for everything to be consistent, and the ‘largest’ operator describes the state of the universe or something.
I’ll check out your sources next insomnia, thanks
Depends upon what you mean by realism. If you just mean belief in a physical reality independent of a conscious observer, I am not really of the opinion you need MWI to have a philosophically realist perspective.
For some reason, everyone intuitively accepts the relativity of time and space in special relativity as an ontological feature of the world, but when it comes to the relativity of the quantum state, people’s brains explode and they start treating it like it has to do with “consciousness” or “subjectivity” or something and that if you accept it then you’re somehow denying the existence of objective reality. I have seen this kind of mentality throughout the literature and it has never made sense to me.
Even Eugene Wigner did this, when he proposed the “Wigner’s friend” thought experiment, he points out how two different observers can come to describe the same system differently, and then concludes that proves quantum mechanics is deeply connected to “consciousness.” But we have known that two observers can describe the same system differently since Galileo first introduced the concept of relativity back in 1632. There is no reason to take it as having anything to do with consciousness or subjectivity or anything like that.
(You can also treat the wavefunction nomologically as well, and then the nomological behavior you’d expect from particles would be relative, but the ontological-nomological distinction is maybe getting too much into the weeds of philosophy here.)
I am partial to the way the physicist Francois-Igor Pris puts it. Reality exists as independently of the conscious observer, but not independently from context. You have to specify the context in which you are making an ontological claim for it to have physical meaning. This context can be that of the perspective of a conscious observer, but nothing about the observer is intrinsic here, what is intrinsic is the context, and that is just one of many possible contexts an ontological claim can be made. Two observers can describe the same train to be traveling at different velocities, not because they are conscious observers, but because they are describing the same train from different contexts.
The philosopher Jocelyn Benoist and the physicist Francois-Igor Pris have argued that the natural world does have a kind of an inherent observer-observed divide but that these terms are misleading being “subject” tends to imply a human subject and “observer” tends to imply a conscious observer, and that a lot of the confusion is cleared up once you figure out how to describe this divide in a more neutral, non-anthropomorphic way, which they settle on talking about the “reality” and the “context.” The reality of the velocity of the train will be different in different contexts. You don’t have to invoke “observer-dependence” to describe relativity. Hence, you can indeed describe quantum theory as a theory of physical reality independent of the observer.
The analogy to Galilean relativity actually is too kind. Galilean relativity relies on Euclidean space as a background, allowing an external viewpoint fixed to empty coordinates. Hilbert space is not a background space at all; it is always defined in terms of physical systems, what is known as a constructed space. You can transform perspectives in spacetime, but there is no transformation to a background perspective in Hilbert space because no such background exists. The closest that exists is a statistical transformation to different perspectives within Liouville space, but this only works for objects within the space; you cannot transform to the perspective of the background itself as it is not a background space.
…which is why eventually you need to switch to the grown-up version of Quantum Mechanics, Quantum Field Theory, is defined in terms of relativistic fields with a single “universal” field for each flavor of particle.
For what it’s worth, you’ve done a fairly good job describing my own understanding of MWI quite succinctly.
So you’re saying there’s a world where Bernie Sanders won in 2016, trump died of covid without the presidential medical suite, americans have universal healthcare, rent control, net neutrality, and free tuition, citizen’s united was repealed, the US never pulled out of international treaties, russia never invaded Ukraine, the latest iteration of the Israel-Palestine conflict never kicked off, the Taliban never took back control of the Afghan government, the resurgence of white supremacy and militant nationalism never took off, criminal justice systems were reformed into data-driven, prevention-first, community-centric public safety models, social and mental health services are fully-funded and effective addiction treatment strategies implemented, reducing demand for the illicit drug market and financially starving out violent criminal syndicates, victimless crimes were decriminalized and regulated for harm-prevention and reduction, nations actually kept their commitments towards climate action and reduced warming to below the target of 2° (possibly even below 1.5°), financial oligarchy was stopped in its tracks, billionaires and corporations are taxed at a fair rate, world hunger has been abolished, and we’re all well on our way towards world peace and prosperity?
No.
Yes.
Yes, but it can be mathematically proven that this world was only made possible by the decimation of the population due to the tide pod challenge having been started two years earlier.
You mean like natural selection?
That’s a sacrifice I am willing to make.
Unlike the Copenhagen interpretation, it does not privilege measurement over other types of interactions between systems.
Hmm, you could say it instead privileges the subjective experience over other types of interaction. There’s no reason in principle why you couldn’t experience every “world” at the same time, in the same way a measurement could in principle return all possible results at the same time.
But you don’t. Somehow your experience of reality is above unitary time evolution, even though “you” aren’t.
I agree completely that that the Copenhagen interpretation makes an excellent phenomenological model in simple (albeit, very common!) settings. However, the problem is that it breaks down when you consider experiments such as the “quantum eraser” (mentioned in other comments here), which causes people to tie themselves into intellectual knots because they are thinking too hard about exactly what is going on with measurement; once one deprivileges measurement so that it becomes just another kind of interaction, the seeming paradoxes disappear.
Copenhagen interpretation doesn’t break down for quantum erasure. Upon measurement you collapse the total quantum state into a result where the two measurements are consistent, that’s simply what entanglement means.
The timing of experiments, and the choice of what to measure, are elements ultimately irrelevant to the above statement, as the quantum erasure experiment demonstrates.
To clarify my imprecise language, what “breaks down” is not its ability to give the correct answer, but the ability of the conceptual framework to give a clear explanation of what is going on, because it essentially defines measurement as “you know one when you see one”, which can lead to confusion.
(However, separately, I do feel the need to point out that “entanglement” is not at all a term that is related to measurement results per se, but rather to the state of a system before you measure it. In particular, if a system is entangled, you can (in principle) disentangle it by reversing whatever process you used to entangle it so that you no longer get correlations in the measurements.)
I don’t know, Many Worlds always led to more confusion than Copenhagen for me. But I suppose that’s a matter of taste since they’re equivalent.
As per the relationship between measurement and entanglement, from an empiricist viewpoint all quantum mechanical terms are related to measurement. If entanglement didn’t affect the outcome of measurements, it wouldn’t exist.
Indeed, you can disentangle an entangled system, which of course will change the outcome of measurements - that’s how you know it’s been disentangled.
I think to some extent we have been talking past each other. Very roughly speaking, I think that am more worried about what happens in the middle of an experiment, where you are more worried about what happens at the end. I actually completely agree with you that when a conscious being performs a measurement, then, from the perspective of that being, both interpretations of what happened when it performed the observation are equivalent. That is, the being has no way of telling them apart, and asking which interpretation is true at that point is, in my opinion, roughly along the same lines as asking whether the objective world exists.
(Just to be clear, it’s not my intent to get mystical here. I think of consciousness as essentially just being a way of processing information about the world, rather than positing the existence of souls.)
Interesting framing. But without measurements there isn’t really a need for different interpretations, is there? If that’s what you mean by “in the middle of an experiment”.
I will happily agree that before measurement, it’s very useful to think of the system as existing in many states at the same time.
I’m confused, bc in my armchair reading I’ve thought about why quantum computers try to store information rather than read it from the quantum wave state. Like if everything is connected, if you know about one thing you know about all things. I don’t even know if that makes sense 😂🤷🏻♀️
One of the things that a quantum computer needs to be able to do in order to function is to hold information at rest, no different from your classical computer. There are two things that make this tricky. First, the information is analog, rather than digital. Second, the environment likes to sneakily “measure” your data so that it decoheres and no longer behaves the way it should. Both kinds of problems are in practice dealt with by encoding the quantum information so that errors can be corrected.
If the word “decoheres” sounds really fancy, think about it this way: coherence versus decoherence is the difference between a rainbow and a grey cloud. In the former case the waves are able to interfere with each other in interesting ways, whereas in the latter case they scatter and do not interfere, producing boring results.
That’s awesome, I hope I’m understanding it enough to think it’s awesome anyways 😂 Thanks for writing that out 💜
they do not meaningfully exist at all.
Does anything, really?
If you are able to read this, then you exist as a conscious being. Everything else is just a model, which you experience as thought projected into your consciousness, just as you experience other senses.
If later you realize that you did have time, you will have already read and understood it
What does it mean?
Pretty sure some slopmaster asked an AI chatbot to make a catchy headline for the quantum mechanics we’ve known for years.
You have a quantum cat-in-a-box. The cat is in a superposition of being alive and dead. You make the “human choice” to open the box. Now the cat turns out to be dead, and it actually died 10 minutes before you open the box. So you can kind of interpret it as your choice affecting reality 10 minutes backwards.
Sounds like it’s just trying to mix up words to make normal cause and effect sound like spooky action at a distance.
Words are a bad medium for conveying truth about reality. Or anything, really.
SCIENTISTS PROVE SPEAKING WILL KILL YOU!
Quantum realm particles seem to know the future or interact with their past.
Also touching them effects measurement, fucking sophons
Yes!
Ahhh, I see, thank you!
Yet again, much smarter than your username implies!
It’s a ruse to let me call people dumbass’s and get away with it and I’m also a dumbass.
We are all dumbasses in one way or another
Yeah we are, ya dumbass.
I’m intelligent, but not nearly intelligent enough for whatever this is…
Is this just another way of talking about the Teleological framework of time (like the heptapods in Arrival)?
I think it has something to do with a philosophical argument I read almost 2 decades ago.
It went something along the lines of; we create new physics the more we study and understand it. Like a rendering fractal, the closer you look the more complex it becomes.
That reminds me of the bit from Hitchhiker’s Guide to the Galaxy where the moment anyone actually understands the universe the whole thing disappears and is replaced with something even bigger, weirder, and more confusing.
Mystical thinking is a great excuse for being too stupid to unravel the secrets of universal field theory like a dummy
Halfway to a great comment there but it fizzled out a bit
I’m way too high for this shit.
Or not high enough.
Damn I’m way too tired to dig into this now but it sounds worth a day of rabbit hole-ing
Professor, we’re not sending information back in time, we’re just rearranging the structure of the multiverse!
Life is literally so easy once you understand this one trick.
Is it retrocausality ?
