The only difference in quantum physics is that there are actually two parallel universes: One in which you took out the red marble & one in which you took the blue one. You don't know what universe you're in until you look at the marble, but still it doesn't help you to transmit a message to your friend.
I don't think it is helpful to talk about multiple universes, that makes a strong implication towards a many world interpretation. It is better to say that the difference is that in the classical case the decision who gets which marble happens when one of the marbles is taken out of the pouch while in the case of entanglement we do not really know when the decision happens but it does provably work differently than in the classical case. It might be that the decision is never truly made, that both outcomes happen in two parallel worlds, it might be that the decision is only made when one party inspects their marble, it might be that it happens at the same time as in the classical example, ...we don't know.
> that makes a strong implication towards a many world interpretation
You say that like it's a shortcoming. :)
There are many who take the (very reasonable) position that the many worlds interpretation is the most epistemologically parsimonious one. Contrary to some misunderstandings of it, it doesn't "add" extra worlds; it removes the concept of "wave function collapse", and leaves all the other known laws of quantum mechanics completely unchanged. The "worlds" arise naturally as more and more particles in the environment become entangled with the measured system, and "wave function collapse" turns out to be the predicted observation of an observer who is themselves made out of quantum states.
The only difference between many worlds and the "standard" Copenhagen interpretation is that Copenhagen adds that, at some point, the entanglement process stops, and a bunch of states in the wave function disappear. And it doesn't specify how, or why, or how to calculate when it will happen. Those that advocate for many worlds would point out that this extra epistemological burden is questionable, given that the correct prediction is made without it.
My understanding is that "Wave function collapse" is an artifact of one of the many possible ways of describing quantum mechanics mathematically. There's really nothing to remove, is there?
Sure, but the Copenhagen interpretation is basically rejected as absurdist and is unnecessary for the exact same reason. It's also trying to give a physical explanation for an artifact of one of the many mathematical representations of quantum mechanics. Schrödinger's Cat is a reductio ad-absurditum to disprove the Copenhagen interpretation!
Contrary to some misunderstandings of it, it doesn't "add" extra worlds; it removes the concept of "wave function collapse", and leaves all the other known laws of quantum mechanics completely unchanged.
Yes, it gets rid of the collapse postulate, but no, it actually introduces many worlds. You can wiggle a bit around, claim that prior to the wave function collapse there are also many worlds in Copenhagen or whatnot, but in the end many worlds makes a metaphysical claim that two cats exist, one dead, one alive while Copenhagen claims only one cat exists in the end.
No, this is the misunderstanding that I'm talking about.
The extra "worlds" follow directly and exclusively from the existence of the various basis states in a wave function, and the laws of entanglement. No other postulates are needed.
Before the measurement/entanglement, the system and environment are independent, and can be written (|0> + |1>) ⊗ (|0> + |1>). After the entanglement, the wave function of the universe can no longer be factored that way, and the system and environment are in a joint state of |00> + |11>. The |00> and the |11> are the multiple "worlds", they show up— in both interpretations— whether you want them to be there or not.
Copenhagen doesn't want them to be there, so it says that one of the |00> or |11> goes away... at some point... because [waves hands and mumbles]. Many worlds merely declines to do this, and that is legitimately the only difference between the two.
The extra "worlds" follow directly and exclusively from the existence of the various basis states in a wave function, and the laws of entanglement. No other postulates are needed.
The many worlds are in the entangled state but then the collapse postulate reduces them to one world. If you remove the collapse postulate you put them back in. And sure, the collapse postulate is an awful solution breaking unitary evolution and you have every right to reject it, but that does not change the fact that many world introduces - or at least not removes - additional worlds that are not there in Copenhagen.
The distinction between "adding" and "removing" a postulate is an important, non-arbitrary one.
The "worlds" are there in both theories; Copenhagen adds a new phenomenon (non-unitary evolution) which makes some of them disappear at unspecified times. The "worlds" are direct consequences of suppositions shared with Copenhagen.
Many worlds has N postulates, Copenhagen has no fewer than N+1. One theory is a strict subset of the other's premises. It is not at all accurate to say that many worlds is the one that "introduces" suppositions.
I think it is not as simple as counting the number of postulates. The ultimate arbiter is physical reality and that decides whether removing or adding postulates is what brings you into agreement. The fact that the quantum state in some equation before wave function collapse looks like many worlds say nothing whether you should take this at face value or whether you are missing an additional postulate that gives you only one world.
If you argue that the collapse postulate is stupid because it is non-unitary and therefore in conflict with experimental evidence, then sure, I totally agree with this. But just because it is an additional postulate it does not per se make many worlds the better theory. Relativity without the constant speed of light is also a theory with one fewer postulate but it of course in much worse agreement with reality.
The problem with the interpretations is that we are currently unable to distinguish them experimentally which unfortunately adds much more personal preferences to the discussion then there should be.
Copenhagen sort of agrees with observation by introducing non-locality, which disagrees with observation of locality from other observations, which means Copenhagen disagrees with observation.
>But just because it is an additional postulate it does not per se make many worlds the better theory.
The postulate of collapse contradicts the postulate of Schrodinger equation and makes the system of postulates contradictory. By removing the contradiction MWI is strictly better, this in turn also removes non-locality and achieves a strictly better agreement with observation, like special theory of relativity.
> Yes, it gets rid of the collapse postulate, but no, it actually introduces many worlds. [...] but in the end many worlds makes a metaphysical claim that two cats exist, [...] while Copenhagen claims only one cat exists in the end.
If you are saying that many worlds makes a greater number of claims than Copenhagen, that's incorrect, as explained above. The claims made by many worlds are a strict subset of Copenhagen.
If you are claiming that many words "introduces" the worlds but Copenhagen does not, that's also incorrect, because the worlds (it seems we agree) are also there in Copenhagen. If they weren't, there would be nothing to "collapse" in the first place!
If you're not saying one of those things, then I'm not sure what that paragraph is trying to say.
> But just because it is an additional postulate it does not per se make many worlds the better theory
It absolutely does, given that both agree equally well with observation.
Each additional assumption in a theory is an opportunity to be wrong. Therefore, given two theories which are in agreement with observation, the theory with fewer unchecked assumptions has a higher probability of being right.
A theory which disagrees with observation (like your altered relativity example) has zero percent chance of being right, so those kinds of examples aren't applicable.
This is just a somewhat more rigorous way of explaining why Occam's Razor is so effective.
For example, take [the dragon in Sagan's garage][1]. We have two models of reality: A garage containing (a) a dragon, who is (b) invisible, (c) dodges touch, (d) floats in air, (e) gives off no heat (f) etc, etc. Or we have a world/garage where none of those things are true.
Both models agree with observation— any measurement we make will not contradict either "empty garage" or "undetectable dragon". And yet one of them is a better theory. How do we know which one is which? The Undetectable Dragon Theory has far more unchecked assumptions (a...f) than the Empty Garage theory (everything in Undetectable Dragon, minus (a...f)).
Same thing for (forgive the extreme example) conspiracy theories. Moon landing hoaxers' ideas agree with observation— they just pile on a mountain of unchecked suppositions in order to avoid contradictions. If we're holding ourselves to good standards of belief, we pick the world model without all those extra unchecked assumptions. This becomes crucially important when there's disagreement about which theory is the Invisible Dragon theory.
Of course Copenhagen isn't nearly as bad as either of those, but the point is to call attention to the epistemological weight of each assumption we add, and what strategy we use for picking between two theories that are not (yet) contradicted by data. Copenhagen is doing more epistemological lifting, and so if we want to be good skeptics and efficient world model-builders, we should require its unchecked assumptions to be checked before we prefer it over other, more parsimonious theories which also agree with the data.
> which unfortunately adds much more personal preferences to the discussion
I quite disagree. The question is of "which strategy to use for picking models of reality which are most likely to be right". There are objectively good and bad strategies for doing that, in the same way that there are good and bad strategies for designing an airplane, or winning at chess, or proving a theorem. Of course no strategy guarantees success, and a "good" strategy might occasionally perform worse than a "bad" one— But without advance access to the solution, we don't know what those exceptions are, so our best bet is to go with the strategy which performs best a priori. In this case, for the sake of avoiding accidental belief in Dragons, the number of unchecked assumptions is centrally important!
I've noticed that some people are simply allergic to the MWI view, likely because of a religious background. Strangely, my physics classes were about 80% fundamental, bible-carrying Christians, as were many of the "Fathers of Quantum Mechanics"!
Many mathematically and physically theories that are perfectly reasonable are rejected by such people out of hand because it doesn't mesh well with their preconceptions of "The Earth is Special", "I have a unique soul that is me", "Jesus came to us, here, specifically", etc...
I know I'll probably get voted down for this, but these are the literal arguments that I was given once I pressed some of my fellow students hard enough on why they reject MWI.
It's not because they investigated the logic of the situation, like you have. They just "feel" like MWI makes them less special and unique in Creation.
The problem with interjecting an implication that many worlds is true (regardless of how persuasive the arguments in its favor are) into a layperson's explanation of why quantum entanglement doesn't permit FTL communication are threefold:
1. It doesn't have anything to do with the topic at hand. It is, at best, a distraction.
2. It is impossible to design an experiment which is capable of distinguishing between a universe where many worlds is the correct interpretation of quantum mechanics or a universe where Copenhagen is the correct interpretation. Arguing about Copenhagen vs many worlds is literally no different than arguing about Jesus vs Mohamed. We simply do not have the tools to arrive at the truth of the matter.
3. You guarantee that everybody is going to stop discussing entanglement+FTL communication and will start arguing about interpretations of quantum mechanics. I'm not sure if anyone's started talking about De Broglie–Bohm theory yet, but it'll... oops I just did.
I say this as someone for whom many worlds feels truthier.
> It doesn't have anything to do with the topic at hand.
To the contrary, it provides an intuitive model that produces correct predictions. That is inherently valuable and extremely relevant.
> Arguing about Copenhagen vs many worlds is literally no different than arguing about Jesus vs Mohamed.
Strongly disagree— See further discussion[1] under my original post for support.
> You guarantee that everybody is going to stop discussing entanglement+FTL communication and will start arguing about interpretations of quantum mechanics
Uh. No. It definitely -- definitely -- doesn't do that. Many worlds, for all its many virtues, gives us "There's another dimension where you -- you -- are Batman!" Because that's what the phrase "many worlds" means to most people.
> > Arguing about Copenhagen vs many worlds is literally no different than arguing about Jesus vs Mohamed.
> Strongly disagree— See further discussion[1] under my original post for support.
So in that discussion, you made the argument:
> Both models agree with observation— any measurement we make will not contradict either "empty garage" or "undetectable dragon".
...which is literally the argument people make when they're arguing Jesus vs Mohamed. It's just that nobody agrees on which holy being is the empty garage and which one is the undetectable dragon.
Or, in the transactional interpretation, the other guy's marble sends a signal from the future back to your marble to change its color when you look at it.
I don't think it is helpful to talk about multiple universes, that makes a strong implication towards a many world interpretation.
You're correct but the point is "many worlds" or Copenhagen interpretation having no implications, they each describe the same mathematical/experimental results. They're just "ways to think about the results". They matter as much as whether you label the axes of a graph x and y or A and B. So any theory that "requires" many worlds is inherently not looking using the standard interpretation of many worlds and quantum mechanics.
Even if they have no observable differences, they still make different metaphysical claims. If I see Schrödinger's cat alive, many world claims that there actually also exists a dead cat while Copenhagen claims that there is only one alive cat. You may argue that the differences are irrelevant for all practical purposes but you don't get to claim that there are no differences between different interpretations.
You may argue that the differences are irrelevant for all practical purposes but you don't get to claim that there are no differences between different interpretations.
It depends what one considers "significant differences". If I go from caring about the practical implications of an interpretation to some other implications, I could make all sorts of distinctions. Explanation X might be written in French and explanation Y might be written in Spanish. Even if one is a translation of the other, you could say they're different in various ways. Or maybe one explanation contains swear words and makes the reader feel bad and so the reader might not "like" that explanation.
But my point above is more specific. Since the two interpretations have the same practical implications, a practical prediction can't really "need" one interpretation - the other interpretation gives you the result. This is the point about all the hidden objects/states explanations have classical analogues.
And if we're getting metaphysical, Copenhagen doesn't say live cat or dead cat but says superimposed state.
If you are only worried about analyzing a specific quantum system, then yes, for most part the interpretation does probably not matter. But I think in general the differences are very important, especially as we do not understand quantum mechanics and different interpretations will direct future research in different directions. If you believe in Copenhagen, you will try to figure out how to reconcile unitary evolution with wave function collapse. If you believe in Bohmian mechanics, you will think about the quantum equilibrium hypothesis. If you believe in many worlds, you might be thinking about energy conservation.
If you believe in Copenhagen, you will try to figure out how to reconcile unitary evolution with wave function collapse.
This is saying the interpretations "exist". The interpretation like intermediate values in some calculation process that are never returned. If the interpretations are true, they can be seen, not just "locally".
It seems like a lot of this basically involves people who've "suspended disbelief" provisionally, accepted a violation of their intuition provisionally but still are hankering for their intuition to spring back into validity. It's like people who accept that general relativity specifies curved space, understand the implications but are still expecting to somehow find a higher dimensional space that all this is suspended in 'cause that's what a fundamental reality feels like to them.
I am not really sure what you are trying to say. Are you talking about the difference between an interpretation as a way of thinking about something and an interpretation as describing what something really is?
It's like people who accept that general relativity specifies curved space, understand the implications but are still expecting to somehow find a higher dimensional space that all this is suspended in 'cause that's what a fundamental reality feels like to them.
General relativity works just fine without an embedding space but there still could be one. Not that you should think about it in this way without good reasons but only because it is more intuitive. But in the case of quantum mechanics we do - at least that is what I think - understand things so poorly that it is hard to judge what one should reasonably consider while making contact with questions about the fundamental nature of things.
"Interpretation", in the way that Copenhagen and Many-worlds are called interpretation, is just a way to add human meaning to an existing formalism. Like adding the label "energy" whatever quantity in Newtonian mechanics.
If many worlds is true, then there are actually countless parallel worlds, if pilot wave is true, then there is only one world. Those are metaphysical claims about the most fundamental aspects of the cosmos, I don't think it is fair to just file this under human meaning.
I don't think it is helpful to talk about multiple universes, that makes a strong implication towards a many world interpretation. It is better to say that the difference is that in the classical case the decision who gets which marble happens when one of the marbles is taken out of the pouch while in the case of entanglement we do not really know when the decision happens but it does provably work differently than in the classical case. It might be that the decision is never truly made, that both outcomes happen in two parallel worlds, it might be that the decision is only made when one party inspects their marble, it might be that it happens at the same time as in the classical example, ...we don't know.