7 ms·

Imagine you have a pouch with a red and a blue marble in it, then take out a marble without looking at it and hand the pouch to a friend. Later, if you look at

• danbruc 5y ago
  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.
  • narrator 5y ago
    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.
  • tbabb 5y ago
    > 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.
    • danbruc 5y ago
      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.
      • tbabb 5y ago
        it actually introduces many worlds 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.
  • fnord77 5y ago
    in this example, what evidence is there that it works differently than in the classical case?
    • ammon 5y ago
      That's exactly what the article we're all commenting on is about!
  • joe_the_user 5y ago
    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.
    • danbruc 5y ago
      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.
      • joe_the_user 5y ago
        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.
        • 5y ago
• fnord77 5y ago
  this seems a little fast and loose. you already had the information the instant you picked the marble, but you didn't observe it until later.
  • cycomanic 5y ago
    But that's essentially the point. The information is not information until it is observed.
• abetusk 5y ago
  This is incorrect. What you've described is classical correlation, not quantum correlation.
  • dkersten 5y ago
    Would this be correct: 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.
    • abetusk 5y ago
      This is, as stated, again classical correlation. Where have you used the fact that the observation of one marble affects the observation of the other? 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.
• gpsx 5y ago
  I think your analogy works for both many worlds and Copenhagen. You can view each universe in your analogy as states in the wave function. The two interpretations diverge only when the "observation" occurs. In the Copanhagen interpretation the other universe disappears. In the many worlds interpretation they both remain.
• 0majors 5y ago
  That’s not quite correct. There are no good analogies between classical objects like marbles or socks and entanglement. 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 https://youtu.be/yZ1KSJbJAng
  • Retric 5y ago
    All analogies are flawed because the underlying reality is different. They can still be useful if they can communicate some more abstract idea. 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.
    • 0majors 5y ago
      The problem with that analogy is it gives an illusion of understanding while being completely misleading about what Bell’s inequality actually tells us about nature. 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.
• TheOtherHobbes 5y ago
  The only difference is that in QM the marbles don't really exist until you look at them. 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".)
  • morpheos137 5y ago
    >we have no idea where it is. It is probably encoded in the cosmic horizon a la Green's Theorem.
    • AnimalMuppet 5y ago
      You're giving "probably" quite a workout there. 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.
      • morpheos137 5y ago
        I said like greens theorem. so the boundary encodes the interior. No communication necessary.
• tinus_hn 5y ago
  Also you could look up some other, related quality instead and the result on the other side would still be the reverse. This cannot be explained using some hidden variable in the particles (like the color of your marbles), so it requires an action to happen in the distant particle dependent on which quality you chose to look up in the local particle.
• morpheos137 5y ago
  Personally I prefer the superdeterminism arguement: i.e. the state of every "future" entanglement was already set "before" the big bang. The anthropocentric corollary is that "free will" is an illusion.
  • mensetmanusman 5y ago
    The universe seems to have surely went to great lengths to trick its own atoms into believing they have free will. :)
    • morpheos137 5y ago
      I don't know what you mean. The number of atoms involved in the free will delusion is infintesimal compared to the number of atoms in the universe. 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.
      • intrbelle 5y ago
        You shouldn't even bother responding to the person that's claiming to be a research scientist in his/her profile and while he/she wrote such garbage at an attempt to justify such nonsense. HN is overcroweded with people that are against the notion of free will being an illusion. Even dang has a bias. All comments like yours just get unfairly downvoted until they're not visible.
        • morpheos137 5y ago
          Yeah there is definite a SF hippie/beat woo vibe here at times. Maybe "consciousness" is fundamental and free will is real but so far we haven't see any evidence. I think superdeterminism is more parsiminous than multiple universes.
        • mensetmanusman 5y ago
          There are so many fun consequences of no free will, 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 :)
• zby 5y ago
  This marble setup is a 'hidden variable' theory - the article is how quantum is different from that.
• joe_the_user 5y ago
  Your discussion is inherently within the classical realm and so it doesn't explain the uniqueness of quantum phenomena. You easily have "classical many worlds" where unknown information makes a model "split" and gaining the information decides which split you get. That's still not quantum in particular.
• smusamashah 5y ago
  > until you look at 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.