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.
(This is assuming the "multiple universes" interpretation- In the other interpretations there is "spooky action at a distance", but this action happens in EXACTLY THE RIGHT WAY to prevent you from transmitting 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.
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.
The contention seems to be that the Copenhagen interpretation elevates wave function collapse from mathematical artifact to real phenomenon.
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 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 "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.
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.
>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!
[1] https://en.wikipedia.org/wiki/The_Demon-Haunted_World#Dragon...
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.
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.
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
Not wrong >_>
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.
> Not wrong >_>
Woooo! I won an argument on the internet!!
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.
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.
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.
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.
You take a marble out of the bag without looking and give the bag to your friend. Your friend also take a marble out of the bag without looking. Both you and your friend now look at your marbles and they will both be the same colour every time.
You can't send information this way because you don't know the colour until your friend has already taken a marble too. You cannot do anything with the fact that they are both the same colour unless you can control or know the colour in advance, which you can't. When you look at your marble and see that its red, your friend looks at their marble and sees that its red, all you know is you both have red marbles.
The only way to send information would be if you took multiple marbles, looked at them until you found one that's blue (say, the third) and then told your friend to look at the third marble. But since you can't tell your friend to do that without using traditional information sending, you may as well just tell them that the third marble is blue and forego the marbles altogether, you're not sending faster than light information anymore anyway.
Your above experiment could be done with just a simple bag and two marbles without the need to contort yourself around looking or not looking.
Here's an attempt to fix your example:
You and your friend each take a marble out of the bag, go very far away from each other and make an agreement to look at the marble at a given time in the future and not before or after. If you look at the marble before or after the agreed upon time, the result will be random. If you both look at it at the exact same time, the marbles will be equal.
Maybe you have many bags of marbles so you can do this experiment many times over. You decide to fudge some results by looking at some marbles before or after the agreed up time. You and your friend will see the same marble color for all marbles seen at the agreed upon time and potentially different marble colors for ones that were opened at different times.
How do you tell if your friend fudged the result? How does your friend tell if you fudged the result? The marbles have a 50-50 distribution of being red and blue, and the extra probability of it flipping to one color or no if it's fudged is lost in that noise.
If you then reconvene and compare notes on what you observed, you can see a very clear correlation of which experiments were fudged and which weren't but now you've done the work of getting in close proximity and destroyed any chance of faster than light communication.
I'm not a physicist and I don't have a deep knowledge of this stuff. This is a toy example and may or may not be a valid reduction of quantum entanglement. The above explanation is my current understanding, which could be wrong.
Here: "they will both be the same colour every time"
> Your above experiment could be done with just a simple bag and two marbles without the need to contort yourself around looking or not looking.
They won't be the same colour every time, despite being able to be either colour prior to looking.
But ok, I see what you're saying in the attempted fix. Thanks.
Somehow they still manage to align themselves so if one person sees red the other sees blue.
Although it's even more accurate to say that if one person sees [colour] the other person sees [opposite colour].
The colours are random, but the relationship between them is fixed.
Very crudely (and rather misleadingly but never mind) this is why you can't communicate at FTL.
You need the other marble to know whether you had [colour] or [opposite colour]. And that info can't travel faster than the speed of light.
It's even more accurate to say there are no marbles anywhere - only interaction events between marble objects and people-looking-at-marble objects, and the API does not allow you to look inside either to see state.
(The state has to exist somewhere otherwise none of this would work. But Bell proves it's not inside the marbles. So it's "non-local" which is code for "we have no idea where it is".)
It is probably encoded in the cosmic horizon a la Green's Theorem.
For it to be encoded at the cosmic horizon, it has to communicate with the cosmic horizon. It's hard to see it doing so, within the time frame of the experiments, without superluminal communication.
In fact, Bell’s inequality was stated as a collaboration game that can only succeed if you use entangled particles. No classical object will get you the same results.
You still can’t communicate faster than light but the reason is more subtle. The article does a good job but for a deeper explanation I’d refer to Sean Carrol: https://youtu.be/yZ1KSJbJAng
An analogy I like for entanglement is to picture two atoms that will both decay at the same time. You could place them on other sides of the planet and until one is observed to decay nobody learns anything because the timing is unpredictable. After the observation people agree with that timing independent of distance but can’t communicate anything because the timing was random. Still, having two people both knowing some fact at the same time which can’t be observed by outsiders is a useful in it’s own way.
What I like about this is it’s clear what’s going on is different from what’s being described, it’s describing a property of something, and it separates information from communication. On the other hand it’s got plenty of it’s own problems.
The whole point of Bell’s inequality is that quantum entanglement is fundamentally different than classical correlation between two objects which have some opposite properties the observer simply does not know about before observing one of them.
It’s not helpful to use an analogy which teaches the reader the exact opposite of the point you are trying to make.
Your example with decaying atoms suffers from the same misunderstanding. Quantum entanglement is not about lack of information about some specific states, if that was the case, why would anyone talk about loss of locality?
Understanding entanglement and Bell’s inequality requires a completely different ontology than your everyday experience with classical objects. I highly recommend the video I linked above for an approachable explanation. It is not as simple as these analogies but at least it gets to the actual point of this result which tells us something profound about how nature works.
The many worlds interpretation is analogous to global hidden variables, and while out of favor, perfectly consistent with modern physics. That said, the core issue is IMO only a one dimensional property was correlated which hides a lot of the oddities involved.
https://en.wikipedia.org/wiki/Bell_test though none of them are quite definitive on their own https://en.wikipedia.org/wiki/Loopholes_in_Bell_tests. I wouldn’t read much into the loopholes, but they do demonstrate just how difficult this stuff is.
Yes, a classical bag containing classical balls doesn't reproduce quantum behavior, because of Bell's theorem. But GP's description isn't classical; it explicitly invokes multiple universes. Once you've done that, quantum behavior is reproducible, because (just as Bell's theorem says) it's no longer possible to ascribe a single hidden state to the ball/bag system, because you can't eliminate the extra universes.
What exactly does it mean? This sentence always feel like as if these particals are sentient beings and can react to when someone sees them.
If observing means performing an experiment and finding out is this information, that an experiment to observe the actual state has been performed, sent to the other half? To know the state of other half another experiment had to be carried out on it.
How is it any different from picking randomly from a set of pairs? We won't know what other half is until one of them is observed.
Number of atoms in 8 billion human brains:
about 10^35
number of atoms in the universe
about 10^82
according to search results.
Maybe there are no aliens but homo sapiens are just the first stage of the universe becoming self aware.
For one, if we don’t have free will, no one could bother one way or the other whether they argue about it.
It would also just be a feature of the universe that clouds of atoms take one position or the other :)
Neither of you can choose what the state of either particle is. You have no control, so there's no way to transmit information.
What you can do is agree in advance that you will both take certain actions based on the measured state of the particles. There's no way to be sure the person at the other end actually does so though.
Indeed, you are only sure about the state of the other particle in the instant just after they measured it. Whoever measures first instantly destroys the entanglement link, so if they chose to manipulate the particle after measurement, you will have no knowledge of these manipulations.
More generally, note that in quantum mechanics "reading" the state of a particle (i.e. performing a measurement) is drastically different than "writing" information by manipulating a particle. Most entanglement-related weirdness hinges on this fundamental asymmetry between "read" and "write" operations for quantum information.
Bell's Theorem is just a model showing how two sets of measurements of certain properties of entangled particles would differ in a “Quantum Physics” regime where there is spooky action at a distance changing the measured properties versus a “common sense” regime where both particles leave the entanglement site with those properties already set.
[0] is a fairly easy to understand graph of the correlation of the measurements at the two sites in both regimes, even if the math that generates those charts is beyond me.
Given that, just knowing what measurements you got only gives you an idea of what the other guy across the universe would see were he to measure your particle’s entangled partner - he has no more control over what those measurements actually are than you do, you just know that there’s a certain correlation between what you saw and what he saw. That’s why you can’t use Bell’s Theorem to communicate, neither one of you is actually controlling the measured property, there’s just a certain correlation between the measurements both of you got.
It seems to this layman that in order to communicate FTL using QM, you’d need a way to determine that the property being measured has already been “collapsed” (if that’s the right word) by spooky action at a distance, e.g if the other guy had already measured it. Bell’s Theorem gives us no way to determine if the other guy has actually made the measurements, it assumes that both you and he have both made those measurements.
[0]: https://en.wikipedia.org/wiki/Bell's_theorem#/media/File:Bel...
What happens in entanglement is that the two entangled objects receive say an entangled photon, it is at this point where the two objects are entangled.
Entanglement is a dance of the statistical limits and position of a particle/object given a specific space/energy configuration (initial condition). From this we know the probability of where it can be, what states it can assume, and the limits of both—given the energy it takes to traverse space and assume those states at once.
They are entangled because once information of the states of one of the entangled objects is measured (mainly by analyzing the exiting photon), we can apodictically discern the state of the other.
Say you and Bob share a bunch of entangled particles. Bob wants to send you a message using those particles, so he takes one particle at a time and encodes his information. How would you know he did so? At the very least, Bob would still have to send you a classical signal to say he did something.
There are more subtle arguments why this doesn't work even at the particle level, but that at least should give you an idea why superluminal communication won't work.
When the two detectors are set to the same, but arbitrary, angle, the detectors give the same answer. This is normal correlation. Quantum correlation says that as one dial moves away from the other reference point, the correlation falls off as a sine wave, not a linear decrease as would be expected by classic probability.
To see how bonkers this is, do the following experiment:
Set detector X to be at angle 0 and detector Y to give a 1% error rate. Call that 1% angle 'a'. So a sample experiment run might be:
X(0): 0001000101001110...111010
Y(a): 0011000101001110...111010
Now let let's change both X and Y by the same angle so the relative error rate between them is still 1%, this might give something like:
X(a): 0011000001001110...111010
Y(2a): 0011000101001110...111010
Now, we know that from X(0) to Y(a) there's one change, from X(a) to Y(2a) there's one change. Classic probability says that there can be at most two flipped bits from X(0) to Y(2a). Quantum mechanics predicts three.
To convince yourself, try making a list of bits such that there's one difference between X(0) and Y(a), one difference between X(a) and Y(2a) but three differences from X(0) to Y(2a). It's impossible and this is the heart of Bell's theorem.
Bell's theorem is a classical probability statement, generalized from my above statement that if |X(0)-Y(a)|=1, |X(a)-Y(2a)|=1 then |X(0)-Y(2a)|<=2. Quantum entanglement violates Bell's inequality.
The 0 reference point has to be arbitrary (in the above it should really be X(ref_angle + a), Y(ref_angle + 2a), etc.) and you have to assume no faster than light communication (that is, independence) to get the contradiction. There are some further subtleties with the above argument but hopefully that's intuitive enough to follow why quantum entanglement is so counter intuitive.
EDIT: corrected X(a) bit string
I'm also a bit confused by why it can't be used to send information but here's a try:
In the above scenario, if the particle (pair) has completely random spin, one that can only be observed by detection and not by some sort of construction, then each observer sees a completely random bit, regardless of whether it gets "flipped" by the "non-local" observation/communication of the other particle. They'll only be able to discover the correlation after the fact, if they compare notes and thus have to meet up, destroying any non-local benefit.
Put another way, if you have a bit with probability p of being 1 ((1-p) of being 0) that you're communicating over the wire but the wire is so noisy as to flip it with probability 1/2, then you won't be able to recover what the transmitted information was.
You'll be able to discover the correlation between the bits if you compare notes after the fact but since the "wire" acts as a completely noisy channel, you can't recover the transmitted bit.
X(0) = 0000, Y(a) = 0001, X(a) = 0011, Y(2a)= 0111.
The subtler issue is that it's a counterfactual question. What would have happened if I had measured or been able to measure all three angles 0, a, 2a? In this case the bit string is the same except for the 1% difference. In other words, X(t) = Y(t), for all t.
The argument is essentially trying to construct a "hidden variable" model and showing that it can't work.
edit: I didn't mean it as an explanation of entanglement. Just thought it was a convenient joke.