AFAICT he's arguing that the Many world's interpretation should be renamed to the "Everything-Is-A-Quantum-Wave" interpretation. I've heard people suggest "Universal wave function" interpretation, which IMO is better than either.
I like Sean Carroll's suggestion: Austere Quantum Mechanics. Because many worlds is really just taking the state of quantum mechanics and accepting what we do know about it without added assumptions. There's a wave function, it obeys the Schrödinger equation.
Other interpretations always posit some extra stuff, like wave function collapse, or pilot waves. Many Worlds just takes the Schrödinger equation seriously on its own.
Therefore it is in a very real sense the most austere interpretation, even though it posits all those worlds.
I go back and forth myself on whether I buy the austere interpretation. Mainly I struggle with the open question of probabilities. Something about the combination of probability and philosophical argument - the two most hard to intuit things in the world - makes my head spin.
Though there are also so many compelling advantages, like providing an arrow in time in that branches of the wave function never re-merge.
I'm also fascinated by superdeterminism. I don't have the same philosophical issues with it that others have. I settled on the "useful fiction" view of free will a long time ago anyway. But I've yet to see any logic supporting the idea that the entire universe would, or even could conspire in this way. But I also used to think many worlds was inherently illogical, so I don't know.
Then there's the voice in me that says "it's just a collapse, it just has to be when there's enough moving parts", but again, can't think of a justification for it other than intuition.
And now there's a 4th shouting at me "why are you thinking about this, you're a programmer". I guess I'm just wired that way, I can't not ponder the things I don't understand, no matter how irrelevant they are to my life.
I find super determinism fascinating too. It's not really scientific because it's ultimately untestable. I just like the idea that the universe is whole, compete from end to end and time is just another axis through it. In my interpretation I'm not traveling through time but just exist in each moment of time thinking that I'm traveling through it because I remember the last moment. It's not a belief, just a thought.
> There's a wave function, it obeys the Schrödinger equation."
Or perhaps more precisely: "There is no wave function. The wave-function we talk about is just a mathematical recipe for computing the expected test-observations".
Functions are abstract things, they don't exist in the physical world. They only describe the physical world, at best.
So that really leaves the question why do our measurement results follow the predictions of the wave-function unanswered. I think we would like to come up with some theory of why the wave-function so closely predicts our observations.
How can you know there's not a Universal Quantum Turing Machine that has encoded precisely the functions we've discovered based on the test-observations we've conducted? And if there is, would the question "why" anything occurs be well defined? It does what it does because it was constructed to do so.
Earlier today I came across this on Scott Aaronson's site:
"Also, are the ultimate equations that govern the universe “real,” while tables and chairs are “unreal” (in the sense of being no more than fuzzy approximate descriptions of certain solutions to the equations)? Or are the tables and chairs “real,” while the equations are “unreal” (in the sense of being tools invented by humans to predict the behavior of tables and chairs and whatever else, while extraterrestrials might use other tools)? Which level of reality do you care about / want to load with positive affect, and which level do you want to denigrate?"
I think Scott Aarons' statement is misguided . It is not about what is "better" or "worse" . It is about the difference between concrete vs. abstract.
Abstract things are representations of many different concrete things, they are abstractions of concrete things. Abstractions are "real" but they exist on a different ontological level than concrete things. They only exist in our head, and therefore many say they don't "really" exist by which we commonly mean that they don't exist in the physical world.
There is clearly a big difference between what exists in the physical world, and what we SAY or write about it. The latter are descriptions, which are often abstractions. Like say "redness" is an abstraction of the common properties of all red objects.
> Therefore it is in a very real sense the most austere interpretation, even though it posits all those worlds.
I strongly disagree.
Wave function collapse is the best description for a randomized phenomena we can observe. That exact same phenomena exists in many worlds as well, just that many worlds invent whole different universes to try to get away from the fact that to us the event is random. So instead of saying "our universe is random" you say "Our universe is not random, we just observe a random part of the universe and can never observe the real whole part of it", that isn't solving anything at all, its just a more elaborate way to say "the universe is random".
But we observe a random phenomena and call that wave function collapses, we don't observe many worlds. Many worlds is outside of observation, it is an additional feature of the universe that isn't based on any observations.
I mean you're right of course, if this was easily experimentally testable we would've done so already. So we're left to choose between two theories: one in which the rules for macro systems and quantum systems are completely different and quantum systems collapse into a state consistent with the rules for macro systems when they interact with a macro system -- and one where there is only the one set of rules and no collapse.
That doesn't mean you should choose to believe one or the other, but it means that one model requires fewer mechanisms than the other.
> one in which the rules for macro systems and quantum systems are completely different and quantum systems collapse into a state consistent with the rules for macro systems when they interact with a macro system
That is wrong, quantum state collapse just makes the quantum wave take the shape of another quantum wave, nothing in it requires it to be related to macro state nonsense like pointwise particles etc. If that is the reason you don't like wavefunction collapse then you just misunderstood quantum mechanics basics, all wave function collapse is is that it is a rule for how quantum waves can transform, there is nothing about it that connects it to our macro world view.
> and one where there is only the one set of rules and no collapse.
MWI still has wave function collapse, the "we observe random part of the universe" part is the wave function collapse, that remains unexplained, why do we only see a random subset? You can't explain that. Many worlds make it harder to see that this part is unexplained, that makes it strictly worse as an interpretation in my opinion.
I was assuming you were arguing for something like the copenhagen interpretation. It now sounds like you're not, but I don't understand what interpretation you're referring to where the wave function takes the shape of another wave function. Do you mean that there is one universal wave function, but the "collapse" is one where the universal wave function discontinuously changes to reflect a probability distribution where (say) a particle's position has much less uncertainty? What is this interpretation called?
Anyways, that's still an additional mechanism.
> MWI still has wave function collapse, the "we observe random part of the universe" part is the wave function collapse, that remains unexplained, why do we only see a random subset? You can't explain that.
That's not wave function collapse. The wave function just keeps evolving as normal.
I don't know enough about this to explain how the MWI maps on to our subjective experiences, I could probably find some links but I'm sure you're as good at that as I am. I'm not arguing that MWI is a better interpretation here, just that it has fewer mechanisms.
> I was assuming you were arguing for something like the copenhagen interpretation. It sounds like you're not, but I don't understand what interpretation you're referring to where the wave function takes the shape of another wave function. Do you mean that there is one universal wave function, but the "collapse" is one where the universal wave function discontinuously changes to reflect a probability distribution where (say) a particle's position has much less uncertainty? What is this interpretation called?
Wave function collapse is an observed phenomena, this has no interpretation at all, its just taking what we observe and say "this is what we observe". I'm not sure why you call out "wave function collapse" as something strange or alien to quantum mechanics, it is one of the core parts.
In the basic quantum mechanic course every physics student takes nowhere do they say "quantum waves collapses to pointwise particle states", they say "quantum waves collapses to eigenstates of quantum waves". So, wave function collapse is just what we call the observed phenomena that when we measure a quantum wave it takes the shape of one of the eigenstates of the measurement. That is all it is, and that is true regardless if you have a MWI or a Copenhagen, or like me a "quantum waves behave weirdly during some interactions" interpretations of quantum mechanics. And I think my interpretation makes the least assumptions, we have quantum waves, and during some interactions they collapse to one of its eigenstates of that interaction. That is the purest form of quantum mechanics, no need to do more than that.
Edit: And trying to argue about more interpretations than that is like trying to invent some divine interpretation of Newtons laws of motions etc. Those are just descriptions of how things behaves.
Yes, the collapse of the wave function is ... when the wave function collapses from representing a superposition of multiple states to representing one state ("collapses to eigenstates of quantum waves"). It. Does. Not. Happen. With. MWI. The single universal wave function always just keeps evolving according to the normal rules.
In MWI the event where we stop observing some of its state is the collapse, it didn't disappear, you just made it harder to understand. So MWI doesn't make things clearer, it hides the complexity of quantum mechanics, you might like it better when you no longer see the complexity, but to me I want it front and center so that I don't miss it.
Edit: Or to make it clearer that it isn't magic, the event where some parts of the wave function suddenly stops interacting with the other parts of the wave function, that is a wave function collapse, and yes it does happen in MWI, you can't just change the laws of quantum mechanics so it has to work that way.
I did not say it made anything easier to understand. I did not say it made anything clearer. I did not say I like it better. Stop putting words into my mouth.
I said it has fewer mechanism. Which it does. Because it does not have the wave function collapse mechanism.
But the wave function collapse is there. Wave functions stops interacting with each other, that is a micro system phenomena, it requires nothing of our macro state magic, and we can observe it, it happens, and that is what we call "wave function collapse", and it happen in MWI.
In MWI, there is one single wave function. It is the single wave function which describes the state of "the multiverse". It is always in a superposition of eigenstates. It never collapses. That's literally the entire point of the MWI.
If you refuse to understand that, you have some reading to do. I don't believe further discussion is fruitful.
But that wave function still gets transformed similar ways as wave function collapses would. You can turn around a particle with wave function collapses, we have done that in reality, so no matter how you structure your MWI the wave function collapse is there. That giant waveform you are talking about thus has to react to wave function collapses, there is no way around it.
> But that wave function still gets transformed similar ways as wave function collapses would.
No it doesn't? After a wave function collapse, a particle's (or quantum system's) wave function is reduced from representing a superposition of eigenstates to representing one eigenstate. That does not happen in MWI. I don't understand why you find that so difficult to accept.
If a particle is in a superposition, and an observer observes the particle, MWI says that the observer also enters a superposition with one eigenstate for each of the particle eigenstates, following the normal "rules" of interaction/entanglement since the observer is also a quantum system. There is no collapse.
So if I use a series of quantum collapses to move a particle a meter forward, a state it wasn't originally in, how would that work in MWI? To do that you just make a long series of "measurements", each perturbation so small that the chance is basically 100% since the chance is squared with the overlap, how did that particle get moved according to MWI? How do you explain that without wavefunction collapse?
For each event you experience as a "collapse", you "branch" the wave function. You put a particle into a superposition between two eigenstates representing two positions, then you observe the particle, and as a result, you as the observer enter a superposition; you now have two eigenstates, one where you observed the particle being in the one location, and one where you observed the particle being in the other location. And since you and your measurement equipment and everything else macroscopic is highly entangled with the rest of the world, the whole world also enters the same superposition, one where the particle was in one place, and one where it was in the other. Repeat until one "branch" in the "tree" has observed the particle moving a meter.
But the particle moving a meter in this scenario isn't described by the Schrödinger equation or any other part of quantum mechanics. If you just measured the chance of it being there originally it would be 0% chance, it isn't there. The only thing moving the particle is the wave function collapse. I don't see how the theory can be both correct and ignore that the wavefunction collapse can move particles into states they weren't originally in.
You would need new rules for the wave function in order for MWI to work, one that handles the collapses. But if you do that I wouldn't call the theory "just standard quantum mechanics without collapses".
But you're influencing the particle between each step, right?
Is my understanding correct that you put the particle in a superposition, observe it a little bit further along, then put it in another superposition, observe it further along again, etc until it has moved a meter?
I don't see why you would expect there to be some probability to see the particle as having moved a whole meter after the first observation. Obviously you putting it in a superposition in between steps is an important factor. And each time you've first put it in a superposition and then observed it, you've created two branches of what we would classically call the universe.
The only thing which really changes between an interpretation with collapse and an interpretation without collapse, is that in one without collapse (i.e MWI), the universal wave function splits into two "branches" which can't interact after the split.
Sadly, I don't know much about the math though, so it's completely possible that I've missed some detail in your question and given a reply which doesn't address what you're getting at. If that's the case, I've probably reached my limit and you'd have to ask someone who has studied this way, way more than me.
> the universal wave function splits into two "branches" which can't interact after the split.
And how is that different from "wave function collapse"? How is that purer? To me that just looks like a wave function collapse with another name, and a whole slew of universes as extra garbage.
The difference is that it's something which already necessarily happens in quantum systems.
Say you have two isolated particles A and B, and A is in a superposition between eigenstates 0 and 1. If those two particles interact, we say that they have formed a quantum system in a superposition between "A is in state 0 and B interacted with A in state 0" and "A is in state 1 and B interacted with state 1", right?
MWI "simply" lets the same happen with observers. You have a particle in a superposition between states 0 and 1. You observe (i.e interact with) the particle, which puts the "you + particle" system in a superposition between "the particle was in state 0 and you observed the particle in state 0" and "the particle was in state 1 and you observed the particle in state 1".
There is no wave function collapse mechanism, what we describe as "wave function collapse" is nothing more than the particles in the macro world becoming entangled with the particle we're testing.
There is no state collapse in the MWI. This is a very basic fact about it. The whole point is that what we experience as a "collapse" in a lab experiment is not really happening.
> one in which the rules for macro systems and quantum systems are completely different and quantum systems collapse into a state consistent with the rules for macro systems when they interact with a macro system -- and one where there is only the one set of rules and no collapse
... and no clear relation with the states that we observe as it includes no rules defining macro systems?
The size at which the outside world starts leaking into the system (and vice versa) in short timespans is pretty well defined both theoretically and experimentally, especially because quantum decoherence is a major annoyance to address in quantum computing.
Huh? The universal wavefunction doesn't preclude you from modeling isolated systems with their own independent wavefunction. I.e. Psi(world) = Psi(system)*Psi(outside world).
The extent to which you can model a system as truly isolated depends on its size and quality of quantum isolation. This can be calculated theoretically as well as measured empirically.
If the universal wave function Psi(everything-in-this-universe-and-beyond) can be factorized as Psi(system)*Psi(everything else) then the state of the system is precisely defined.
Isn’t the universal wavefunction supossed to be able to accomodate distinct realities where that system may be in different states?
The question was whether you can describe the world with a “single set of rules” that doesn’t address at all the question of how to relate that “single-rule meta-world” with the world you try to describe.
We don't observe almost all real number either. We observe some rational numbers. All these infinite number of unobservable real numbers is just waste /s
The many worlds are not an additional feature of MWI compared to other interpretations. They all have them in the math. In pilot wave theories they are just claimed to not be real, yet they affect things. In objective collapse theories they disappear when measured.
Consider just the aspect of randomness alone. How can the universe implement it? It seems like a really awkward problem. [But the Everett interpretation gives it to us for free](https://www.preposterousuniverse.com/blog/2014/07/28/quantum...). It really seems like a killer argument to me.
That just deals with the random nature of wave function collapse, not the mechanical nature. Wave function collapse can be used to move around particles, or rotate them etc. A single measurement is just a normal random chance, but you can do a series of measurements over different basis you start to affect the system in non trivial ways, you can use that to control position or rotation or other states of particles, MWI doesn't explain that part at all.
A poster above posted something to explain this a bit:
I'm not really in the mood to play the game of figuring out what you're trying to say a second time today, sorry.
But you're wrong. Any serious researcher working on the foundations of quantum mechanics agrees that MWI is the most austere. That includes the ones who prefer a different interpretation. You can disagree with the whole field if you want. You do you, I guess.
Most quantum mechanics researchers just ignores waveform collapses entirely because you can do a lot without it. I don't think that they have anything significant to add to this discussion since MWI just has to do with wavefunction collapse., them being experts at wavefunctions when the function doesn't collapse doesn't make a difference.
Edit: To add, all quantum mechanics interpretations are equivalent when you ignore wavefunction collapse. So only experts on wavefunction collapses would have a credible say here. But there is very little research in that area, so there aren't many experts on it.
Yes, this is the emperor's new clothes of MWI, and it doesn't take any specialist knowledge to see it. Quantum mechanical theories produce probabilities for things happening. We can conduct repeated experiments and verify those probabilities against the results. If every outcome actually happens, there is no longer any place for those probabilities. It is meaningless to talk about the relative probabilities of events A and B happening if both A and B actually happen, deterministically.
These probabilities aren't just numbers produced by the theory, they are the predictive content of the theory, the very thing we're supposed to be interpreting, and MWI has no place for them.
The way this problem is often discussed is to talk about "recovering the Born probabilities", which frames the problem wrongly. It makes it sound like the problem is that MWI merely isn't producing some numbers we like. MWI proponents respond to this by going through absurd contortions involving things like decision theory as a way of causing the desired numbers to pop out of a calculation. But the problem isn't "recovering the probabilities". The problem isn't finding some way of performing a calculation that results in these particular numbers. The problem is that these probabilities have no physical meaning in MWI, and it's a catastrophic problem because those probabilities have a very real meaning in the actual world as we observe it.
It really is as simple as "how can all the things happen at once when we know some of the things are more likely than others?" And, like "why is the emperor naked?" it's a question that can literally be asked by a child.
An "interpretation" that renders the core predictive content of the theory meaningless is a bad interpretation and a waste of everyone's time.
I agree with a lot of what you have said, but I would strongly disagree with your conclusion.
>An "interpretation" that renders the core predictive content of the theory meaningless is a bad interpretation and a waste of everyone's time.
Given that the different interpretations do not make different predictions, the only relevant way to judge them is to ask the question "is X interpretation a useful way to think about the universe". This can be answered empirically, practitioners of both MWI and CI have both made important advances in quantum physics, therefore neither are a waste of anybody's time.
There is a reason why students generally get taught both, there is nothing wrong with having multiple ways to think about things.
In objective collapse theories, there is an "us" to do the observing, and there is a collapse event which samples randomly from the possible outcomes. Both the "us" and the sampling are additional things that we have to add to the theory.
In MWI, there is no "us" in an objective physical sense. "We" are on all the "branches" of the wavefunction, spread out in such a way that it isn't possible for any observation to see both events having happened.
We don't "observe a random part of the universe", we do observe all parts of it. It's just that "we" aren't classical in the sense that you are implying.
I like to call it the Universal Wavefunction, or Everettian theory. The splitting into many worlds is just a side effect.
That said, any discussion of this theory should mention the rather puzzling issue that gravitation is nonlinear, and thus it would be impossible for there to be a superposition of worlds in each of which general relativity works as observed.
> That said, any discussion of this theory should mention the rather puzzling issue that gravitation is nonlinear, and thus it would be impossible for there to be a superposition of worlds in each of which general relativity works as observed.
Are such superpositions not a thing in String Theory? How would gravity's nonlinearity preclude this? Quantum mechanics works fine with nonlinear systems like weird oscillators.
Re-reading my comment, I shouldn’t have said “impossible”. Yes, any theory that successfully quantizes gravitation will allow superpositions of gravitational worlds. String theory likely does this, though some will quibble with the phenomelogy as to whether it reproduces general relativity as we know it.
You don't need an interpretation to address them. The design of the measurement apparatus is what defines the basis in which the system is most easily expressed, which you don't see when doing abstract quantum information calculations but you do see when you are talking about magnetic fields and particles that move through them. For example in the stern-gerlach apparatus, the alignment of the magnet is what determines the measurement basis.
"most easily expressed"? How does the nature know that it has to behave according to the rules "most easily expressed" by humans? Mathematically, all bases are equivalent (which is exactly what the author reminds us about in the very first paragraph).
I don't think you really groked your parent's comment. Asking why we don't see |dead> + |alive> states is like asking why we don't measure states of definite Jx when measuring Jz -- you're simply measuring in the wrong basis.
So why do we measure only states of |dead> or |alive>? I don't know; maybe it's evolutionarily advantageous (those simultaneously dead and alive beings sound quite bothersome). Regardless, it's irrelevant -- evidently we are only capable of making measurements in the basis {|dead>, |alive>}, so naturally that's all we'll ever see. Of course, that restriction by itself doesn't make the |dead> + |alive> state less real, only unobservable when you're coupled with the system.
I admit I haven't really groked it, but the author of the article hasn't groked it either (why these 2 states are special - is the main question he's asking).
I'm not a physicist, so the following question might be naive.
Suppose there are 2 entangled particles - one goes to Andromeda, another - to Earth. Somebody on Andromeda (for whom the evolution has different "priorities", for we are talking about a totally different world) measures. They detect a glorph, so due to entanglement, we are guaranteed to detect a hibble when we measure. But we can't detect a hibble (by your theory). What gives?
For electron spin, a hibble detector would be a magnet rotated 90 degrees relative to an up detector. For photon polarization it would also be a filter rotated 45 degrees relative to an up-filter. Talking about it in the abstract conceals how easy it would be to realize the experiment.
So you agree with the theoretical possibility of somebody in Andromeda seeing a cat in the glorph state (dead & alive)? If not, then why can they (in theory) see the particle in the glorph state?
There's nothing theoretically impossible about building a detector that measured a cat's dead or alive state obliquely (in the "glorph basis"), and if you did it, the machine would look something like this: it would disassemble the cat without learning much about it, and then measure a lot of things that didn't tell you whether it was dead or alive.
Here is where the problem arises practically: The cat has an enormous number of particles, and (naturally) exists as a vector in an exponentially large number of dimensions. Observing whether it is alive or dead is a measurement along an axis that you get the equipment for by default. On the other hand, building a machine that can take arbitrary measurements of millions and millions of particles would naturally give it so much freedom within that giant space of possible axes that the smallest misalignment would take you away from the glorph basis.
If you accept things like space and time as a given, it is not so hard to see why. When you are measuring many-particle systems the easiest thing to do is measure each particle individually, with detectors that interact with small numbers of them. That privileges the basis that separates out particles over those that we would describe as entangled combinations. Carrying that over to the cat picture requires noting that most of our measurements of the cat, like reflecting photons off of it, interact strongly with individual particles in a way that separates them by spatial location.
This takes us back to my original point, which is that although the consequences of space seem mysterious in a perspective so abstract that there is no space, they no longer seem so mysterious when you include other facts about physics, like about particles and fields. I am aware that there are some attempts underway to figure out how space could come about as a consequence of quantum stuff, but even they add extra behavior on top of the postulates of QM in order to plant the seed, so to speak, that spacetime grows from.
However you look at it, this "special basis" we live in remains a mystery. One unorthodox way to address it is to speculate that our position/velocity in the Universe is special. It might be that by moving at a speed close to the speed of light, the astronaut could observe the glorphs, but he will turn into a glorph himself and cease to exist as a human before he can observe anything. I know, it sounds crazy and contradicts the principle of equivalence. But, as Matthew Cook pointed out, the principle of equivalence of all bases is broken is QM anyway. Please think about it. :-)
Not sure what you mean exactly? Nonlinearity in what? If you read an introductory textbook the first nontrivial quantum system you will study is the harmonic oscillator, which has potential V(x) ~ x^2.
Is there somewhere I can read/learn about it somewhere else? I'd rather not watch Hossenfelder's videos, she seems to live in a different branch of the wave function from me where e.g. superdeterminism is a reasonable thing to study.
I didn't study physics in university, so please forgive this naïve question. Would an appropriate interpretation of quantum physics simply be that we don't have an interpretation of quantum physics? We have mathematical equations and observations that confirm the math, but it seems that we don't have observations that give credibility to one interpretation over the others. Instead of speculating, can't we just agree that we don't know?
I'm not suggesting that we should do that -- I suspect that there's a lot that I'm missing. But I guess I don't understand why these speculative interpretations are so alluring if we can't confirm or invalidate them through experiments.
It's a valid concern to be sure, and one with a lot of interesting historical background. Foundations of quantum mechanics was a taboo subject in physics ever since the Bohr-Einstein debates, and still is to this day, in some circles.
And mosts physicists actually do this; they don't worry about intepretation of quantum mechanics in their work. They use the Schrödinger equation and the Born rule for their calculations and leave it at that. This is often referred to as "shut up and calculate".
But the measurement problem, addressed through these interpretations is a foundational problem, and the tricky thing with those is you have no idea what the implications might be on other foundational problems. An understanding of the measurement problem may very well have something to say about other open questions like the arrow of time or quantum gravity. It could even have implications for the scalability of quantum computers.
And I don't think it's fair to say that this question is empirically unanswerable. A number of possible interpretations have actually been ruled out by the bell inequality, for instance. And both theoretical and empirical research on this topic has seen a resurgence in recent decades.
On a purely sociological basis I feel inclined to think that the reason the measurement problem has seemed so incalcitrant is not because it's fundamentally unsolvable, but because physics as a field has denied itself from actually trying to solve it, and actively pushed out those who dared work on it.
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[ 2.4 ms ] story [ 128 ms ] threadOther interpretations always posit some extra stuff, like wave function collapse, or pilot waves. Many Worlds just takes the Schrödinger equation seriously on its own.
Therefore it is in a very real sense the most austere interpretation, even though it posits all those worlds.
I go back and forth myself on whether I buy the austere interpretation. Mainly I struggle with the open question of probabilities. Something about the combination of probability and philosophical argument - the two most hard to intuit things in the world - makes my head spin.
Though there are also so many compelling advantages, like providing an arrow in time in that branches of the wave function never re-merge.
I'm also fascinated by superdeterminism. I don't have the same philosophical issues with it that others have. I settled on the "useful fiction" view of free will a long time ago anyway. But I've yet to see any logic supporting the idea that the entire universe would, or even could conspire in this way. But I also used to think many worlds was inherently illogical, so I don't know.
Then there's the voice in me that says "it's just a collapse, it just has to be when there's enough moving parts", but again, can't think of a justification for it other than intuition.
And now there's a 4th shouting at me "why are you thinking about this, you're a programmer". I guess I'm just wired that way, I can't not ponder the things I don't understand, no matter how irrelevant they are to my life.
Man got to sit and wonder 'why, why, why?'
Tiger got to sleep, bird got to land;
Man got to tell himself he understand.
― Kurt Vonnegut, Cat's Cradle
Or perhaps more precisely: "There is no wave function. The wave-function we talk about is just a mathematical recipe for computing the expected test-observations".
Functions are abstract things, they don't exist in the physical world. They only describe the physical world, at best.
So that really leaves the question why do our measurement results follow the predictions of the wave-function unanswered. I think we would like to come up with some theory of why the wave-function so closely predicts our observations.
"Also, are the ultimate equations that govern the universe “real,” while tables and chairs are “unreal” (in the sense of being no more than fuzzy approximate descriptions of certain solutions to the equations)? Or are the tables and chairs “real,” while the equations are “unreal” (in the sense of being tools invented by humans to predict the behavior of tables and chairs and whatever else, while extraterrestrials might use other tools)? Which level of reality do you care about / want to load with positive affect, and which level do you want to denigrate?"
https://scottaaronson.blog/?p=3628
Abstract things are representations of many different concrete things, they are abstractions of concrete things. Abstractions are "real" but they exist on a different ontological level than concrete things. They only exist in our head, and therefore many say they don't "really" exist by which we commonly mean that they don't exist in the physical world.
There is clearly a big difference between what exists in the physical world, and what we SAY or write about it. The latter are descriptions, which are often abstractions. Like say "redness" is an abstraction of the common properties of all red objects.
I strongly disagree.
Wave function collapse is the best description for a randomized phenomena we can observe. That exact same phenomena exists in many worlds as well, just that many worlds invent whole different universes to try to get away from the fact that to us the event is random. So instead of saying "our universe is random" you say "Our universe is not random, we just observe a random part of the universe and can never observe the real whole part of it", that isn't solving anything at all, its just a more elaborate way to say "the universe is random".
That doesn't mean you should choose to believe one or the other, but it means that one model requires fewer mechanisms than the other.
At least that's my understanding.
That is wrong, quantum state collapse just makes the quantum wave take the shape of another quantum wave, nothing in it requires it to be related to macro state nonsense like pointwise particles etc. If that is the reason you don't like wavefunction collapse then you just misunderstood quantum mechanics basics, all wave function collapse is is that it is a rule for how quantum waves can transform, there is nothing about it that connects it to our macro world view.
> and one where there is only the one set of rules and no collapse.
MWI still has wave function collapse, the "we observe random part of the universe" part is the wave function collapse, that remains unexplained, why do we only see a random subset? You can't explain that. Many worlds make it harder to see that this part is unexplained, that makes it strictly worse as an interpretation in my opinion.
Anyways, that's still an additional mechanism.
> MWI still has wave function collapse, the "we observe random part of the universe" part is the wave function collapse, that remains unexplained, why do we only see a random subset? You can't explain that.
That's not wave function collapse. The wave function just keeps evolving as normal.
I don't know enough about this to explain how the MWI maps on to our subjective experiences, I could probably find some links but I'm sure you're as good at that as I am. I'm not arguing that MWI is a better interpretation here, just that it has fewer mechanisms.
Wave function collapse is an observed phenomena, this has no interpretation at all, its just taking what we observe and say "this is what we observe". I'm not sure why you call out "wave function collapse" as something strange or alien to quantum mechanics, it is one of the core parts.
In the basic quantum mechanic course every physics student takes nowhere do they say "quantum waves collapses to pointwise particle states", they say "quantum waves collapses to eigenstates of quantum waves". So, wave function collapse is just what we call the observed phenomena that when we measure a quantum wave it takes the shape of one of the eigenstates of the measurement. That is all it is, and that is true regardless if you have a MWI or a Copenhagen, or like me a "quantum waves behave weirdly during some interactions" interpretations of quantum mechanics. And I think my interpretation makes the least assumptions, we have quantum waves, and during some interactions they collapse to one of its eigenstates of that interaction. That is the purest form of quantum mechanics, no need to do more than that.
Edit: And trying to argue about more interpretations than that is like trying to invent some divine interpretation of Newtons laws of motions etc. Those are just descriptions of how things behaves.
Edit: Or to make it clearer that it isn't magic, the event where some parts of the wave function suddenly stops interacting with the other parts of the wave function, that is a wave function collapse, and yes it does happen in MWI, you can't just change the laws of quantum mechanics so it has to work that way.
I said it has fewer mechanism. Which it does. Because it does not have the wave function collapse mechanism.
If you refuse to understand that, you have some reading to do. I don't believe further discussion is fruitful.
No it doesn't? After a wave function collapse, a particle's (or quantum system's) wave function is reduced from representing a superposition of eigenstates to representing one eigenstate. That does not happen in MWI. I don't understand why you find that so difficult to accept.
If a particle is in a superposition, and an observer observes the particle, MWI says that the observer also enters a superposition with one eigenstate for each of the particle eigenstates, following the normal "rules" of interaction/entanglement since the observer is also a quantum system. There is no collapse.
So if I use a series of quantum collapses to move a particle a meter forward, a state it wasn't originally in, how would that work in MWI? To do that you just make a long series of "measurements", each perturbation so small that the chance is basically 100% since the chance is squared with the overlap, how did that particle get moved according to MWI? How do you explain that without wavefunction collapse?
You would need new rules for the wave function in order for MWI to work, one that handles the collapses. But if you do that I wouldn't call the theory "just standard quantum mechanics without collapses".
Is my understanding correct that you put the particle in a superposition, observe it a little bit further along, then put it in another superposition, observe it further along again, etc until it has moved a meter?
I don't see why you would expect there to be some probability to see the particle as having moved a whole meter after the first observation. Obviously you putting it in a superposition in between steps is an important factor. And each time you've first put it in a superposition and then observed it, you've created two branches of what we would classically call the universe.
The only thing which really changes between an interpretation with collapse and an interpretation without collapse, is that in one without collapse (i.e MWI), the universal wave function splits into two "branches" which can't interact after the split.
Sadly, I don't know much about the math though, so it's completely possible that I've missed some detail in your question and given a reply which doesn't address what you're getting at. If that's the case, I've probably reached my limit and you'd have to ask someone who has studied this way, way more than me.
And how is that different from "wave function collapse"? How is that purer? To me that just looks like a wave function collapse with another name, and a whole slew of universes as extra garbage.
Say you have two isolated particles A and B, and A is in a superposition between eigenstates 0 and 1. If those two particles interact, we say that they have formed a quantum system in a superposition between "A is in state 0 and B interacted with A in state 0" and "A is in state 1 and B interacted with state 1", right?
MWI "simply" lets the same happen with observers. You have a particle in a superposition between states 0 and 1. You observe (i.e interact with) the particle, which puts the "you + particle" system in a superposition between "the particle was in state 0 and you observed the particle in state 0" and "the particle was in state 1 and you observed the particle in state 1".
There is no wave function collapse mechanism, what we describe as "wave function collapse" is nothing more than the particles in the macro world becoming entangled with the particle we're testing.
... and no clear relation with the states that we observe as it includes no rules defining macro systems?
The extent to which you can model a system as truly isolated depends on its size and quality of quantum isolation. This can be calculated theoretically as well as measured empirically.
Isn’t the universal wavefunction supossed to be able to accomodate distinct realities where that system may be in different states?
A poster above posted something to explain this a bit:
http://www.paradise.caltech.edu/~cook/Workshop/Physics/QMQue...
But you're wrong. Any serious researcher working on the foundations of quantum mechanics agrees that MWI is the most austere. That includes the ones who prefer a different interpretation. You can disagree with the whole field if you want. You do you, I guess.
Edit: To add, all quantum mechanics interpretations are equivalent when you ignore wavefunction collapse. So only experts on wavefunction collapses would have a credible say here. But there is very little research in that area, so there aren't many experts on it.
These probabilities aren't just numbers produced by the theory, they are the predictive content of the theory, the very thing we're supposed to be interpreting, and MWI has no place for them.
The way this problem is often discussed is to talk about "recovering the Born probabilities", which frames the problem wrongly. It makes it sound like the problem is that MWI merely isn't producing some numbers we like. MWI proponents respond to this by going through absurd contortions involving things like decision theory as a way of causing the desired numbers to pop out of a calculation. But the problem isn't "recovering the probabilities". The problem isn't finding some way of performing a calculation that results in these particular numbers. The problem is that these probabilities have no physical meaning in MWI, and it's a catastrophic problem because those probabilities have a very real meaning in the actual world as we observe it.
It really is as simple as "how can all the things happen at once when we know some of the things are more likely than others?" And, like "why is the emperor naked?" it's a question that can literally be asked by a child.
An "interpretation" that renders the core predictive content of the theory meaningless is a bad interpretation and a waste of everyone's time.
>An "interpretation" that renders the core predictive content of the theory meaningless is a bad interpretation and a waste of everyone's time.
Given that the different interpretations do not make different predictions, the only relevant way to judge them is to ask the question "is X interpretation a useful way to think about the universe". This can be answered empirically, practitioners of both MWI and CI have both made important advances in quantum physics, therefore neither are a waste of anybody's time.
There is a reason why students generally get taught both, there is nothing wrong with having multiple ways to think about things.
In objective collapse theories, there is an "us" to do the observing, and there is a collapse event which samples randomly from the possible outcomes. Both the "us" and the sampling are additional things that we have to add to the theory.
In MWI, there is no "us" in an objective physical sense. "We" are on all the "branches" of the wavefunction, spread out in such a way that it isn't possible for any observation to see both events having happened.
We don't "observe a random part of the universe", we do observe all parts of it. It's just that "we" aren't classical in the sense that you are implying.
That said, any discussion of this theory should mention the rather puzzling issue that gravitation is nonlinear, and thus it would be impossible for there to be a superposition of worlds in each of which general relativity works as observed.
Are such superpositions not a thing in String Theory? How would gravity's nonlinearity preclude this? Quantum mechanics works fine with nonlinear systems like weird oscillators.
So why do we measure only states of |dead> or |alive>? I don't know; maybe it's evolutionarily advantageous (those simultaneously dead and alive beings sound quite bothersome). Regardless, it's irrelevant -- evidently we are only capable of making measurements in the basis {|dead>, |alive>}, so naturally that's all we'll ever see. Of course, that restriction by itself doesn't make the |dead> + |alive> state less real, only unobservable when you're coupled with the system.
Here is where the problem arises practically: The cat has an enormous number of particles, and (naturally) exists as a vector in an exponentially large number of dimensions. Observing whether it is alive or dead is a measurement along an axis that you get the equipment for by default. On the other hand, building a machine that can take arbitrary measurements of millions and millions of particles would naturally give it so much freedom within that giant space of possible axes that the smallest misalignment would take you away from the glorph basis.
The questions is why position is the preferred basis that ‘you get the equipment for by default’ - if that’s what you mean.
This takes us back to my original point, which is that although the consequences of space seem mysterious in a perspective so abstract that there is no space, they no longer seem so mysterious when you include other facts about physics, like about particles and fields. I am aware that there are some attempts underway to figure out how space could come about as a consequence of quantum stuff, but even they add extra behavior on top of the postulates of QM in order to plant the seed, so to speak, that spacetime grows from.
The point was made by Sabine Hossenfelder
I think Hossenfelder described it in the video the problematic in Chaos: The real problem with quantum mechanics
I'm not suggesting that we should do that -- I suspect that there's a lot that I'm missing. But I guess I don't understand why these speculative interpretations are so alluring if we can't confirm or invalidate them through experiments.
And mosts physicists actually do this; they don't worry about intepretation of quantum mechanics in their work. They use the Schrödinger equation and the Born rule for their calculations and leave it at that. This is often referred to as "shut up and calculate".
But the measurement problem, addressed through these interpretations is a foundational problem, and the tricky thing with those is you have no idea what the implications might be on other foundational problems. An understanding of the measurement problem may very well have something to say about other open questions like the arrow of time or quantum gravity. It could even have implications for the scalability of quantum computers.
And I don't think it's fair to say that this question is empirically unanswerable. A number of possible interpretations have actually been ruled out by the bell inequality, for instance. And both theoretical and empirical research on this topic has seen a resurgence in recent decades.
On a purely sociological basis I feel inclined to think that the reason the measurement problem has seemed so incalcitrant is not because it's fundamentally unsolvable, but because physics as a field has denied itself from actually trying to solve it, and actively pushed out those who dared work on it.