31 ms·

How Bell’s Theorem proved ‘spooky action at a distance’ is real

234 points by theafh 5y ago

miguelmurca 5y ago
> really permits instantaneous connections between far-apart location The phrasing in this article is tricky, as it wasn't FTL communication that was proven; just that there are correlations between things that would require FTL communication, were they classical processes. This is an important point: https://xkcd.com/1591/ https://xkcd.com/1591/
- eggsby 5y ago
  One reason I often hear astrology is not taken seriously by the scientific community, as in findings like 'athletes often have aries rising on their birth chart' are ignored and not evaluated further, is because there is no empirical foundation for the communication of the effects. https://en.wikipedia.org/wiki/Astrology_and_science https://en.wikipedia.org/wiki/Astrology_and_science
  - wizzwizz4 5y ago
    There actually is empirical foundation for the communication of the effects![0] But the model is strictly simpler if you remove the astrology from it; astrology has no additional explanatory power, and its novel claims (claims not predicted by any other model) are wrong. > Such children are more likely to be picked for school teams. Once they are picked, players benefit from more practice, coaching and game time — advantages denied to those not selected, who are disproportionately likely to be younger for their selection year. Once accounting for their biological age, the older players might not have been any better than later-born children when they are first picked. But after becoming part of a team, and being exposed to training and matches, they really do become better than later-born children who might be equally talented. [0]: https://fivethirtyeight.com/features/why-athletes-birthdays-affect-who-goes-pro-and-who-becomes-a-star/ https://fivethirtyeight.com/features/why-athletes-birthdays-...
    - eggsby 5y ago
      Sorry if my language was unclear, I meant to highlight this line from the wiki article: "There is no proposed mechanism of action by which the positions and motions of stars and planets could affect people and events on Earth in the way astrologers say they do that does not contradict well-understood, basic aspects of biology and physics." Relevant here because it essentially says "there is no empirical basis for spooky action at a distance" which has been grounds for dismissal of such action-at-a-distance claims like 'the relative positions of celestial bodies influence events on the earth'. This kind of empiricism has been used as grounds to not critically evaluate these claims. Everyone is certainly free to have their own reasons around why they do not want to evaluate such claims. For example some people only want to consider things that are easily falsifiable and subject to particular scientific practices. The wiki article goes on to mention how Carl Sagan refused to disavow astrology on these grounds (i.e. gravity is weak so stellar influence writ large ought to weak) while still leaving room for a disavowal if it were on firmer grounds. I do think your point about simplicity is salient here.
      - wizzwizz4 5y ago
        > 'the relative positions of celestial bodies influence events on the earth'. Who's claiming that‽ The relative positions of celestial bodies have influenced all sorts of events. For instance, the horoscopes in the newspaper, or photographs of the night sky. No, what's in doubt is astrology, which is a much more specific set of (wrong) claims.
ericb 5y ago
If we were in a simulation, would the speed of light be the processing speed of the universe as each area re-renders, and spooky action at a distance be two variables pointed to the same memory location, populated with a lazy-loaded value, with copy-on-write semantics? edit: seems like it is lazy loaded, so revised my summary.
- stouset 5y ago
  It’s more like two variables pointing to an uninitialized value that’s lazily randomly generated on dereference. And copy-on-write. Edit: OP edited their original comment to be more accurate.
- someguyorother 5y ago
  It's not really immutable as you can change the parameters of an entangled pair. You just can't communicate any information by doing so, because you need a classical signal to make sure you don't read one of the particles the wrong way.
  - SV_BubbleTime 5y ago
    I could be WAY off, but if locality isn’t entirely true, and the “read success” is 33-67%, doesn’t that still leave quite a bit of wiggle room for communicating information in some fault tolerant method?
    - wizzwizz4 5y ago
      Nobody's ever managed it, the theory all says it's impossible, and that would violate several "known laws" of physics.
    - cma 5y ago
      Correlations only but no useable communication. You can both make a decision on the same random info that isn't determined until later when you are apart, but can't know anything other than that if they followed the plan they made their choice based on the same later-determined random info, correlated with your random info. If they didn't follow the plan and measured orhogonal/same (can't remember which) spins, then your results are uncorrelated but you can't know until you meet back up (maybe barring superdeterminism that is also accessible to the individual).
willyg123 5y ago
What is the prevailing theory to explain quantum entanglement? Must there be another dimension we cannot access or measure that is not subject to the laws of relativity? (I understand the laws of relativity break down at the quantum level but please ELI5)
- benbayard 5y ago
  I’d probably start here: https://youtu.be/ZuvK-od647c https://youtu.be/ZuvK-od647c
- lisper 5y ago
  There isn't really an "explanation" for quantum entanglement. It is a fundamental property of the universe, arguably the fundamental property of the universe. But the Right Way to think about it IMHO is this: the quantum wave function is not defined over physical space, it is defined over configuration space. A wave function defined over physical space is a special case that pertains when you are dealing with a system consisting of a single particle, in which case physical space and configuration space are the same. But as soon as you add a second particle, this physical intuition breaks down. You might also want to read this: https://flownet.com/ron/QM.pdf https://flownet.com/ron/QM.pdf
- martincmartin 5y ago
  Quantum entanglement falls out of the quantum mechanics, so in some sense, the prevailing theory to explain quantum entanglement is quantum mechanics. Of course, it's unintuitive and unsettling, so you could generate other theories about other dimensions if you like. But as far as predicting the results of any experiments we can do, QM is all you need. Also, there are two very different theories of relativity, the special and the general. Special relativity is taught in 1st year undergraduate physics, you really only need high school math & physics, plus an open mind, to understand it. This has E = mc^2, twin paradox, length contraction, time dilation, speed of light as a limit. It's actually a pretty small topic, it usually doesn't have a separate course because it wouldn't fill a one semester course. QM is fully consistent with Special Relativity. The other is general relativity, which revises gravity in light of special relativity. This is a much bigger topic and typically taught in grad school, although there are some undergrad texts now that don't require math as advanced as the grad school ones. QM and GR are incompatible, and the search for a "quantum theory of gravity" is a key plank in any "theory of everything."
dQw4w9WgXcQ 5y ago
Or Superdeterminism is true: https://en.wikipedia.org/wiki/Superdeterminism https://en.wikipedia.org/wiki/Superdeterminism
- nobodyandproud 5y ago
  Those objections to super determinism seem weak, or more along the lines of “I don’t like the implications, so I won’t consider it.” Genuine question: Would quantum computers work in any deterministic framework?
  - themgt 5y ago
    I've quoted it before but I will again just because I hate superdeterminism so much: First, the logical flow: Bell’s theorem proves that no local, realistic theory can reproduce the predictions of quantum mechanics. It does so by considering a very specific situation of entangled particles being measured by spin detectors set at different angles. Critically, the angles of these spin detectors are assumed to be set independently from one another. ... Experimenters have tried to ensure independence for all practical purposes with elaborate techniques: independent quasi-random number generators running with different algorithms on different computers are one very basic example. On more advanced experiments, they use quantum sources of randomness, and try to make sure that the choice is only made once the particles are in flight. The trouble is that in principle, there will always be a point in the past at which mechanism used for the angle choice, and the mechanism used to produce the entangled particles were in causal contact with one another. (If all else fails, then the early universe will provide such a point.) The super-determination thesis says that any past causal contact can in principle provide correlation between the settings of the two detectors (or the detectors and the properties of the particles), and is the source of the violation of Bell’s inequality. Here’s a deliberately ridiculous example. Once the particles are in flight, I throw in the air a box of Newton’s notes on alchemy. I select the one that falls closest to my feet. I roll two dice, and use them to select a random word from that page. I match the word with its closest equivalent in Caesar’s commentary on the Gallic wars, or the Iliad, or the complete works of Dickens, my choice of work depending on the orientation of the Crab pulsar at the moment of measurement. I use the word position in these works to select a number in this book A Million Random Digits (take the time to read the customer reviews). And I use this number to set my detectors. I repeat this for my other measurement runs, but I substitute in Dan Brown’s Da Vinci Code for Dickens every third go. Superdetermination advocates would tell me that there is in principle a causal connection between my throwing the papers in the air, Newton, Caesar, Dickens as they sat down to write 300, 2000, and 150 years ago, the Crab pulsar and the RAND corporation’s random digit selection. And that it’s possible that these things have conspired (unknowingly) to make sure that my detector settings and a particle’s spin measurement is correlated in a particular way in my lab in a law-like way. I can only reply that yes, it’s possible. I cannot prove it wrong. But I can find it unreasonable. And I would be tempted to call these people philosophically desperate. https://www.quora.com/Why-do-some-crackpot-scientists-go-after-super-deterministic-theories-when-Bells-theorem-has-proved-that-local-realism-is-false https://www.quora.com/Why-do-some-crackpot-scientists-go-aft...
lisper 5y ago
Note that there is a very important property of entangled particles that is hardly ever mentioned in this kind of exposition, which IMHO casts a lot of light on what is really going on, and that is that entangled particles do not self-interfere the way non-entangled particles do. For more details see: https://flownet.com/ron/QM.pdf https://flownet.com/ron/QM.pdf
- jfengel 5y ago
  I don't think the paper justifies the statement as you put it, though perhaps you can point out what I'm missing. I don't think you can tell just from looking at the particle itself whether it has an entangled partner somewhere in the universe. It is, however, possible to use the entangled partners to create systems with decidedly counter-intuitive properties that change the way the un-involved partner interacts. That's also the essence of Bell's Theorem. It only works when you're controlling the experiment as a whole and thus not transmitting information faster than light... though you can set up the experiment in a way that makes the conventional transmission of information incredibly obscure. Bell's Theorem requires you to jump through a lot of hoops to exactly mimic that, which is why it took a long time to definitively rule out other interpretations of the experiments.
  - lisper 5y ago
    See section 4.2, and in particular the paragraph that starts "Here's the kicker..." It is true that you can't tell if a single particle is entangled or not. But if you have an ensemble of particles all prepared in the same state then you can tell if that state is entangled or not. Non-entangled (a.k.a. pure) states have a preferred basis that produce self-interference. Entangled (a.k.a.) mixed states do not. (The pure-mixed dichotomy is a little misleading because it depends on your point of view. A single member of an EPR pair is in a mixed state, but the pair as a whole is in a pure state.)
    - gus_massa 5y ago
      I agree with the GP. If you only have a single member of the pair, then you will see the same interference pattern in a double slit experiment than with a not-entangled particle. It doesn't matter if the other particle has collided with a brick, went thru a double slit experiment, went thru a bad double slit experiment, or is flying to Andromeda. (In spite that the calculation to get the correlation with any result in the other experiment may be much harder with the entangled pair than with two non entangled particles.)
nsxwolf 5y ago
I'll never understand entanglement. Every explanation makes me wonder why it can't be used to instantaneously send a message. I never fully understand the explanations why it can't be used to do so. I don't understand how you can be sure about the state of the other particle, what if someone already measured it and then did something to it?
- sergiotapia 5y ago
  Reminds me of the in-lore comms system in Mass Effect. I think the comms in the ship were two atoms that were entangled, allowing instant messages no matter how many lightyears away the ship was from Earth.
- deleted 5y ago
  [deleted]
- misiti3780 5y ago
  This book elucidates the concepts well: https://en.wikipedia.org/wiki/Something_Deeply_Hidden https://en.wikipedia.org/wiki/Something_Deeply_Hidden
- drcode 5y ago
  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 your marble, you instantly have information about the other marble at a speed greater than the speed of light... but you couldn't use that fact to send a message. 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)
  - 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.
wyager 5y ago
MWI allows you to have entanglement without “spooky action at a distance”. However, it requires exponential blowup in representational complexity of the universe, which also feels aesthetically displeasing.
flubert 5y ago
"This is the first of a set of papers that look at actual Einstein-Podolksy-Rosen (EPR) experiments from the point of view of a scientifically and statistically literate person who is not a specialist in quantum theory." https://arxiv.org/abs/quant-ph/9611037 https://arxiv.org/abs/quant-ph/9611037 ...I wonder if anyone has ever followed up on Caroline Thompson's work after she passed away. https://arxiv.org/abs/quant-ph/0210150 https://arxiv.org/abs/quant-ph/0210150
Zamicol 5y ago
If you "measure" the bits per character in the base 45 alphanumeric encoding used in QR code, you'd get 5.5 bits per character as 11 bits is used for two characters. How is it possible to have information less than a bit, a partial bit? What is that ".5" part? Isn't a bit indivisible? Only in the context of a character doublet is all information expressed. To know the "half bit" part, you cannot "look" at just one character, you have to look at the total. The information is shared between the two characters. Measuring the bits-per-character is only useful when considering the whole system. The "partial bits" is information smeared across the system. Changing the middle bit may change one, or both, characters. Here's a 11 bit example, where the middle bit is changed and it changes both characters: (11101001010 vs 11101101010, or '/L' vs '%8' encoded) https://convert.zamicol.com/?in=11101001010&inAlpha=01&outAlpha=0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ.%24%2B*%3A%2F%25+- https://convert.zamicol.com/?in=11101001010&inAlpha=01&outAl... https://convert.zamicol.com/?in=11101101010&inAlpha=01&outAlpha=0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ.%24%2B*%3A%2F%25+- https://convert.zamicol.com/?in=11101101010&inAlpha=01&outAl... vs changing the last bit only changes the last character: (Using the preceding example, 11101101010 vs 11101101011, or '%8' vs '%9' encoded) https://convert.zamicol.com/?in=11101101011&inAlpha=01&outAlpha=0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ.%24%2B*%3A%2F%25+- https://convert.zamicol.com/?in=11101101011&inAlpha=01&outAl... The same principle applies to information theory and cryptography. Security can be measured in "partial bits" because it's measured across something larger.
- jdb1729 5y ago
  5.5 bits is also the average information content of a single run of the GHZ experiment. In this setup three parties independently choose a binary detector setting and each observe a binary outcome. The first two parties observe an independent random bit regardless of their settings. If an odd number of the parties have their setting "on", then the third party also observes an independently random bit (6 bits total to record, 3 for the settings and 3 for the observations). But if an even number of of the three settings are "on", then the third party's observation is completely determined by the other 5 bits. When the settings are chosen randomly these two possibilities are equally likely so on average it takes 5.5 bits to record the results of the experiment.
- 5y ago
Gunax 5y ago
I've been posting this explaination for more than 10 years now: http://www.felderbooks.com/papers/bell.html http://www.felderbooks.com/papers/bell.html I think I prefer Felder's explaination more than Quanta's. It's omitting some details (eg. the angles) but is better at explaining the difficulties of Bell's Inequality--why it seems like spooky action at a distance and why it cannot be used for communication.
- subroutine 5y ago
  One thing I've not been able to clarify is whether Bell accounts for the possibility that passing through a polarization filter could effect the waveparticle in some way, like altering its polarization angle.
  - unholiness 5y ago
    Yes, the "altering" you're describing is what the theorem would call a hidden variable. Seems reasonable, but when you do the math it's exactly he kind of theory Bell's inequality rules out. There's no way to set the "polarization angle" (or any other set of variables) such that they obey the probabilistic laws we've observed (without violating some other assumption, like single measurement outcomes, statistical independence, determinism, or locality)
    - subroutine 5y ago
      As I understand the P(passing) through a second filter is related to the blue line here: https://upload.wikimedia.org/wikipedia/commons/thumb/e/e2/Bell.svg/400px-Bell.svg.png https://upload.wikimedia.org/wikipedia/commons/thumb/e/e2/Be... Couldn't the passage through the first filter effect the polarization angle in such a way that it matches the blue line instead of the red line. One could devise a physical contraption to demonstrate this is possible by sending bar magnets through slits of magnets of the same charge. Any magnet oriented such that it's too close to a slit will reorient slightly. Visually... https://ibb.co/gDnpqCb https://ibb.co/gDnpqCb That is, if both photons or magnets happen to pass through the first filter, their probability of both going through the second filter is boosted from the slight reorientation, and thus their outcome correlation will be boosted, which is what the blue line in the graph above shows.
choeger 5y ago
Does the article do justice to the hidden variables hypothesis? In case of the hidden variables, the spin is a (3-dimensional?) value that is identified by the measurement result. In case of quantum theory we have have a probability distribution. How is that probability distribution different from a hidden variables, except that it's not a straight number but a function instead? Speaking as a programmer, is the difference between hidden variables and quantum mechanics that the former postulate a real-valued property whereas the latter speak of something like a monad?
- 6gvONxR4sf7o 5y ago
  Speaking as a lay person, I think the difference might be that it's specifically about local hidden variables. If two particles are coupled, there's no per-particle hidden variable?
  - 6gvONxR4sf7o 5y ago
    edit-too-late-to-edit: I remembered something that makes me think it's actually more complicated. If I write 0 and 1 on two different pieces of paper, then flip a coin to decide which paper to give you, we have "entangled" unknowns. When I reveal my paper, we instantly know what's on yours. The joint distribution can't be described per-particle, but we don't consider it spooky. So I think there's something more to it.
    - BeKindAndLearn 5y ago
      Because your example does not contain superpositions. With entanglement the pieces of paper are both 0 and 1 at the same time until they are observed - and only then is it that the superposition of both particles become resolved.
      - 6gvONxR4sf7o 5y ago
        Tomayto - tomahto, right? Physically speaking, what’s the difference between an unresolved superposition and a different kind of unresolved random variable?
ThePhysicist 5y ago
There's no spooky action at a distance. Let's imagine we have an entangled qubit system that consists of a superposition of the states (0,1) and (1,0), i.e. either part A is in state 0 and part B in state 1 or vice versa. When we perform a measurement on the first part of the system and obtain 1, it simply means that we have "branched" into the (1,0) state of the system. This branching is usually irreversible because of the decoherence caused by the measurement (which itself is just an ordinary quantum process). There is no information exchange or any type of exchange between the two parts of the system going on, we simply branch into a part of the probability space defined for the system. The question whether the other branches still exist then leads to either the "classical" interpretation of quantum mechanics or the "many worlds" interpretation. The latter seems to be favored today as we know that there's nothing special about the measurement process that causes the collapse of a wave function (it's a quantum process in itself), but in the end there's not really a way to test this so it's really more of a philosophical question. Articles about "spooky action at a distance" should really mention this, as we have a much better understanding of the measurement process in quantum mechanics today than Einstein et. al. had when they wrote their paper.
- virgil_disgr4ce 5y ago
  If there's one single phrase I wish I could erase from history it's "Spooky action at a distance." Ugh. It bugs me a lot that Quanta made these misleading statements that just continue the confusion over what should be a more widely-understood core feature of the universe we live in. Tangentially, I wish "interaction" would come to replace "measurement," especially in the context of decoherence. The universe is branching *constantly* everywhere as various quantum systems interact.
- joe_the_user 5y ago
  What you're describing could be done with classical physics. Have one penny and two lockets. Place the penny blindly in one of the two lockets. Take one locket across the world. Opening it instantly lets you whether the other locket has contains the penny. And the point of this description is this is not what's weird about quantum entanglement. What's weird about quantum entanglement is you have two different measurement types that are non-orthogonal and they combine according to quantum logic rather than classical logic [1]. Having a particle in a state of this sort can't be explained by any analogy to discreet events occurring beforehand. [1] https://en.wikipedia.org/wiki/Quantum_logic#Quantum_logic_as_the_logic_of_observables https://en.wikipedia.org/wiki/Quantum_logic#Quantum_logic_as... Edit: Whether this is "spooky action at a distance" is in the eye of the beholder. One thing is isn't able to be is fully reducible to actions happening on something like a "classical time line" but another thing is isn't to able to do is transmit information.
naasking 5y ago
Not necessarily. Bell's theorem assumes statistical independence, but that means that either spooky action at a distance is real, OR that experimenters do not have complete freedom to configure their instruments (aka superdeterminism).
qwerty456127 5y ago
How long is it going to take until we can have a near-zero-latency Internet connection on Mars (e.g. on a Mars rover) or Moon?
- void_mint 5y ago
  At the speed of light, it would take between 4 minutes and 14 minutes to travel between Mars and Earth. A quick google says fiber optic cable can transfer data at roughly 70% of that speed. It may not be possible to get the latency you're describing. Mars is far away.
- uh_uh 5y ago
  You can not use QM for faster-than-light communication: https://en.wikipedia.org/wiki/No-communication_theorem https://en.wikipedia.org/wiki/No-communication_theorem
  - qwerty456127 5y ago
    U.S. Department of Energy unveils blueprint for the quantum internet: https://news.fnal.gov/2020/07/u-s-department-of-energy-unveils-blueprint-for-the-quantum-internet-at-launch-to-the-future-quantum-internet-event/ https://news.fnal.gov/2020/07/u-s-department-of-energy-unvei... Fermilab and partners achieve sustained, high-fidelity quantum teleportation: https://news.fnal.gov/2020/12/fermilab-and-partners-achieve-sustained-high-fidelity-quantum-teleportation/ https://news.fnal.gov/2020/12/fermilab-and-partners-achieve-...
    - BeKindAndLearn 5y ago
      Apologies that I don't have time right now to clarify further, but I highly suggest you read the Wiki for entanglement and then reread the articles with that context. These simply do not do what you think they do. Information cannot, under any currently theorized model of physics, be transmitted faster than light.
    - drdeca 5y ago
      Quantum teleportation does not allow for transmitting classical information faster than light. What it allows for is, using an entangled state, along with transmitting classical information, to send another quantum state across the distance, but it still needs the classical information to get there first. This is how quantum teleportation works. Alice and Bob have entangled qubits A and B, and also Alice has a qubit C with some state that she wants to send to Bob , and Bob has some other Qubit D. To do this, Alice does some operations between qubits A and C, which includes a measurement. Alice then sends the classical information which is the result of the measurement to Bob. Bob then does some operations with the qubits B and D, where the operations he does are chosen based on the classical information signal he received from Alice. Iirc these observations include making a measurement. As a result, the qubit D is now in the state that the qubit C was in originally. (Alice no longer has access to the state that C was in.) So, in a sense, the state teleported from C to D, using the combination of the entangled A and B, along with sending some classical information. (The classical information sent is random, and to anyone snooping on the message sent, gives them no useful information, because they don’t have Bob’s qubit B to use it with.)
jussij 5y ago
Couldn't that 'hidden variable' be just another dimension? So while those particles might have been separated in space by a large distance, on that fifth dimension they haven't moved and are still sitting side by side.
- pontus 5y ago
  The distance between two points is basically just the Pythagorean theorem: x^2 + y^2 + z^2. If you have extra dimensions, you just add more terms to this. Now, if you know that e.g. x is very large then that puts a lower bound on the distance: Distance^2 = x^2 + y^2 + z^2 + ... > x^2. In other words, if x is large there's no way that the two particles are still somehow close together.
  - platz 5y ago
    ER=EPR entanglement could be realized as a non-local distance mapping
  - mcbits 5y ago
    Put two iron pellets on a very thin, flexible fabric. Underneath the fabric, put a magnet so each pellet is stuck to one pole of the magnet through the fabric. Tug the magnet "down" in the third dimension, which will fold the fabric and move the pellets so that from their perspective (stuck to the 2D surface of the fabric) they're moving far away from each other. They also maintain a constant and very short distance in the third dimension, but they can't take advantage of it because I forgot to mention the fabric is impervious to them.
  - dreamcompiler 5y ago
    This is only true for cartesian spaces. If the low-dimensional manifold is "folded" or "crinkly" inside a higher-dimensional space then you can indeed have particles that look far apart on the manifold but which are actually close together in the higher-dimensional space. (My terminology here is largely wrong; this is not my field. Perhaps an expert can correct me.)
- gameswithgo 5y ago
kovac 5y ago
I'm not a Physicist. From what I understand, Bell's theorem only covers local hidden-variables and that this can still be explained using non-local theories like Bohm's. Can someone shed a bit of light on the non-local theories and if Bell's theorem addresses those as well?
- GoblinSlayer 5y ago
  Bell's theorem is a mathematical theorem and has narrow scope, it addresses local hidden variables theories and says they won't fly, it does nothing else, and doesn't prove spooky action at a distance either - that's author's addition.
- jostylr 5y ago
  Bell was inspired by Bohm's theory. Having seen that a rational theory of existence was compatible with QM, Bell wanted to know if the nonlocality could be removed. Bell's work showed that it could not be removed. Bell was a proponent of Bohm's work. Bohmian mechanics is a perfectly sound theory, proven to be consistent and in agreement with quantum mechanics results whenever QM has results. EPR (and more relevantly, Bohm's version of it involving spin entangled particles) showed that either one has hidden variables or nonlocality. Bell's work showed that local hidden variables is not possible. Hence, nonlocality is the conclusion and the argument to demonstrate the incompleteness of quantum mechanics falls apart. The Many Worlds interpretations kind of gets away from this by exploiting another assumption in that work: that results from experiments actually happen in the sense that the other outcomes don't. Most people at the time would have just assumed that, but MW points out that experiments need not have definitive results, instead we have multiple results of experiments correlated with multiple versions of experimenters who think they have results. Thus, the correlations we see only need to be reconciled when the two groups of experimenters get together to compare notes, a perfectly local notion. Of course, some descriptions of MW, with branching of the universe, is rather nonlocal, but a proper formulation of MW is possible without any of that. By the way, a key feature of Bohmian mechanics is the reliance on position as the "hidden variable". The spin of a particle is not an aspect of the particle itself, but rather of the wave function guiding the particle. In the spin experiments, the correlation with the environment once measurement has happened, separates the overlapping spin parts of the wave function and the particle is then being guided by that singular spin. Hence the appearance of the particle behaving as if it had that spin, but the particle is not actually spinning, whatever that would mean for a point particle.
fungiblecog 5y ago
It did not prove that. It proved that at least one of three assumptions about the universe is violated: Statistical Independence, Locality or Determinism. The usual assumption is that Locality has to go, but that is not necessarily true. It is possible that Statistical Independence is not true for the experimental systems that have been studied. See for example: https://www.frontiersin.org/articles/10.3389/fphy.2020.00139/full https://www.frontiersin.org/articles/10.3389/fphy.2020.00139...
- chopin 5y ago
  If you forgo statistical independence isn't that a kind of super-determinism?
  - goldenkey 5y ago
    Even ordinary interpretations of QM don't explain the source of randomness in QM. This randomness can be entirely deterministic which would make the entire universe deterministic. So superdeterminism can still be the case, even if it isn't the reason for entanglement correlations. The entanglement correlations would instead be due to shared RNG references across spacetime, effectively nonlocal. My point is, locality and superdeterminism aren't mutually exclusive, they are independent. All 4 combinations are logically possible with current experimental evidence. https://www.americanscientist.org/article/quantum-randomness https://www.americanscientist.org/article/quantum-randomness
    - unholiness 5y ago
      The fact that superdeterminism can violate statistical independence is what lets it violate bell's theorem, right? If so GP's statement is just backwards: Super-determinism is a kind of forgoing statistical independence
      - goldenkey 5y ago
        > The fact that superdeterminism can violate statistical independence is what lets it violate bell's theorem, right? Yes, the idea is that the past of both the detectors, experimenters, and particle producer were once interacting, relatively recently in the scheme of the human race (let alone civilization, the big bang, etc.) These correlations would be responsible for the entanglement correlations, not a shared nonlocal RNG reference, but rather an inherited local piece of an old reference. Nonetheless, superdeterminism doesn't have to be doing anything conspiratorial, but the possibility of doing so, lets it violate Bell/CHSH inequalities.
steve76 5y ago
Particles are anything with a discrete start and an end. The heat required per frequency of light emission has a discrete start and end, explained by statistical mechanics. That's a particle, even though it's very abstract and ephemeral. A material object emits light which will expand the size of the universe if unimpeded. With no discrete start and end, although very minor and subtle, it's not a particle despite the object's tangible concrete existence. Nature's internals have no mind. They are overwhelmed by the mighty surroundings and always redlining, pushing themselves to the limit. Need an electron? Nature looks all around, not just the nearest bonded atom. That's magnetism. Nature also takes the easiest path. Using it's own internals as a medium of interaction makes things easier. That's electromagnetic radiation. We're dealing with a vast ocean of the medium of spacetime, not little building blocks of matter. In terms of (space + relative clocks) EM radiation travelling through empty space has internals of it's own. Electricity will repel or attract the entire thing. No further electromagnetism with that empty space itself. But magnetism, it gets to interact again, take an easier route using it's own internals as the medium of interaction. If you want to entangle an electron, make it so interacting with the internals of one interacts with another. To exist, something has to give some resistance to surrounding interactions. If it happened immediately, it's just the surroundings. That's how we get the speed of light and the discrete nature of light frequencies. Besides electromagnetism, there's other interactions. Mass, as in nuclear decay, or triple pole "color" charge. Simulations and holograms mean that resistance to interaction will be much more simpler that the internals. Black holes do this. They're just one big 2D dot. Things that fall into them are crunched up on the surface. Bell's Theorem is like measuring from the inside of a spinning coin that's growing, and stopping it. That's where Bell's Theorem, and the speed of light has more to gain. Why fight it? It's awesome. I think of plasma applications of the sun. That star orbits a black hole which can and does zap electrons into existence from empty space by crunching up a positron of a pair. What if we get good enough at that things stop looking like electrons but look like distant stars? Would that be a drive that warps the space time around our solar system, and suddenly the universe is much more smaller.
steve76 5y ago
Particles are anything with a discrete start and an end. The heat required per frequency of light emission has a discrete start and end, explained by statistical mechanics. That's a particle, even though it's very abstract and ephemeral. A material object emits light which will expand the size of the universe if unimpeded. With no discrete start and end, although very minor and subtle, it's not a particle despite the object's tangible concrete existence. Nature's internals have no mind. They are overwhelmed by the mighty surroundings and always redlining, pushing themselves to the limit. Need an electron? Nature looks all around, not just the nearest bonded atom. That's magnetism. Nature also takes the easiest path. Using it's own internals as a medium of interaction makes things easier. That's electromagnetic radiation. We're dealing with a vast ocean of the medium of spacetime, not little building blocks of matter. In terms of (space + relative clocks) EM radiation travelling through empty space has internals of it's own. Electricity will repel or attract the entire thing. No further electromagnetism with that empty space itself. But magnetism, it gets to interact again, take an easier route using it's own internals as the medium of interaction. If you want to entangle an electron, make it so interacting with the internals of one interacts with another. To exist, something has to give some resistance to surrounding interactions. If it happened immediately, it's just the surroundings. That's how we get the speed of light and the discrete nature of light frequencies. Besides electromagnetism, there's other interactions. Mass, as in nuclear decay, or triple pole "color" charge. Simulations and holograms mean that resistance to interaction will be much more simpler that the internals. Black holes do this. They're just one big 2D dot. Things that fall into them are crunched up on the surface. Bell's Theorem is like measuring from the inside of a spinning coin that's growing, and stopping it. That's where Bell's Theorem, and the speed of light has more to gain. Think of a golf ball. Each dimple connects through the center. Perhaps instead of going through the center, it was easier to go on the surface, and let the center work itself out: Why fight it? It's awesome. I think of plasma applications of the sun. That star orbits a black hole which can and does zap electrons into existence from empty space by crunching up a positron of a pair. What if we get good enough at that things stop looking like electrons but look like distant stars? Would that be a drive that warps the space time around our solar system, and suddenly the universe is much more smaller.

234 points by theafh 5y ago

> really permits instantaneous connections between far-apart location The phrasing in this article is tricky, as it wasn't FTL communication that was proven; just that there are correlations between things that would require FTL communication, were they classical processes. This is an important point: https://xkcd.com/1591/ https://xkcd.com/1591/

eggsby 5y ago
One reason I often hear astrology is not taken seriously by the scientific community, as in findings like 'athletes often have aries rising on their birth chart' are ignored and not evaluated further, is because there is no empirical foundation for the communication of the effects. https://en.wikipedia.org/wiki/Astrology_and_science https://en.wikipedia.org/wiki/Astrology_and_science
- wizzwizz4 5y ago
  There actually is empirical foundation for the communication of the effects![0] But the model is strictly simpler if you remove the astrology from it; astrology has no additional explanatory power, and its novel claims (claims not predicted by any other model) are wrong. > Such children are more likely to be picked for school teams. Once they are picked, players benefit from more practice, coaching and game time — advantages denied to those not selected, who are disproportionately likely to be younger for their selection year. Once accounting for their biological age, the older players might not have been any better than later-born children when they are first picked. But after becoming part of a team, and being exposed to training and matches, they really do become better than later-born children who might be equally talented. [0]: https://fivethirtyeight.com/features/why-athletes-birthdays-affect-who-goes-pro-and-who-becomes-a-star/ https://fivethirtyeight.com/features/why-athletes-birthdays-...
  - eggsby 5y ago
    Sorry if my language was unclear, I meant to highlight this line from the wiki article: "There is no proposed mechanism of action by which the positions and motions of stars and planets could affect people and events on Earth in the way astrologers say they do that does not contradict well-understood, basic aspects of biology and physics." Relevant here because it essentially says "there is no empirical basis for spooky action at a distance" which has been grounds for dismissal of such action-at-a-distance claims like 'the relative positions of celestial bodies influence events on the earth'. This kind of empiricism has been used as grounds to not critically evaluate these claims. Everyone is certainly free to have their own reasons around why they do not want to evaluate such claims. For example some people only want to consider things that are easily falsifiable and subject to particular scientific practices. The wiki article goes on to mention how Carl Sagan refused to disavow astrology on these grounds (i.e. gravity is weak so stellar influence writ large ought to weak) while still leaving room for a disavowal if it were on firmer grounds. I do think your point about simplicity is salient here.
    - wizzwizz4 5y ago
      > 'the relative positions of celestial bodies influence events on the earth'. Who's claiming that‽ The relative positions of celestial bodies have influenced all sorts of events. For instance, the horoscopes in the newspaper, or photographs of the night sky. No, what's in doubt is astrology, which is a much more specific set of (wrong) claims.

ericb 5y ago

If we were in a simulation, would the speed of light be the processing speed of the universe as each area re-renders, and spooky action at a distance be two variables pointed to the same memory location, populated with a lazy-loaded value, with copy-on-write semantics? edit: seems like it is lazy loaded, so revised my summary.

stouset 5y ago
It’s more like two variables pointing to an uninitialized value that’s lazily randomly generated on dereference. And copy-on-write. Edit: OP edited their original comment to be more accurate.
someguyorother 5y ago
It's not really immutable as you can change the parameters of an entangled pair. You just can't communicate any information by doing so, because you need a classical signal to make sure you don't read one of the particles the wrong way.
- SV_BubbleTime 5y ago
  I could be WAY off, but if locality isn’t entirely true, and the “read success” is 33-67%, doesn’t that still leave quite a bit of wiggle room for communicating information in some fault tolerant method?
  - wizzwizz4 5y ago
    Nobody's ever managed it, the theory all says it's impossible, and that would violate several "known laws" of physics.
  - cma 5y ago
    Correlations only but no useable communication. You can both make a decision on the same random info that isn't determined until later when you are apart, but can't know anything other than that if they followed the plan they made their choice based on the same later-determined random info, correlated with your random info. If they didn't follow the plan and measured orhogonal/same (can't remember which) spins, then your results are uncorrelated but you can't know until you meet back up (maybe barring superdeterminism that is also accessible to the individual).

willyg123 5y ago

What is the prevailing theory to explain quantum entanglement? Must there be another dimension we cannot access or measure that is not subject to the laws of relativity? (I understand the laws of relativity break down at the quantum level but please ELI5)

benbayard 5y ago
I’d probably start here: https://youtu.be/ZuvK-od647c https://youtu.be/ZuvK-od647c
lisper 5y ago
There isn't really an "explanation" for quantum entanglement. It is a fundamental property of the universe, arguably the fundamental property of the universe. But the Right Way to think about it IMHO is this: the quantum wave function is not defined over physical space, it is defined over configuration space. A wave function defined over physical space is a special case that pertains when you are dealing with a system consisting of a single particle, in which case physical space and configuration space are the same. But as soon as you add a second particle, this physical intuition breaks down. You might also want to read this: https://flownet.com/ron/QM.pdf https://flownet.com/ron/QM.pdf
martincmartin 5y ago
Quantum entanglement falls out of the quantum mechanics, so in some sense, the prevailing theory to explain quantum entanglement is quantum mechanics. Of course, it's unintuitive and unsettling, so you could generate other theories about other dimensions if you like. But as far as predicting the results of any experiments we can do, QM is all you need. Also, there are two very different theories of relativity, the special and the general. Special relativity is taught in 1st year undergraduate physics, you really only need high school math & physics, plus an open mind, to understand it. This has E = mc^2, twin paradox, length contraction, time dilation, speed of light as a limit. It's actually a pretty small topic, it usually doesn't have a separate course because it wouldn't fill a one semester course. QM is fully consistent with Special Relativity. The other is general relativity, which revises gravity in light of special relativity. This is a much bigger topic and typically taught in grad school, although there are some undergrad texts now that don't require math as advanced as the grad school ones. QM and GR are incompatible, and the search for a "quantum theory of gravity" is a key plank in any "theory of everything."

dQw4w9WgXcQ 5y ago

Or Superdeterminism is true: https://en.wikipedia.org/wiki/Superdeterminism https://en.wikipedia.org/wiki/Superdeterminism

nobodyandproud 5y ago
Those objections to super determinism seem weak, or more along the lines of “I don’t like the implications, so I won’t consider it.” Genuine question: Would quantum computers work in any deterministic framework?
- themgt 5y ago
  I've quoted it before but I will again just because I hate superdeterminism so much: First, the logical flow: Bell’s theorem proves that no local, realistic theory can reproduce the predictions of quantum mechanics. It does so by considering a very specific situation of entangled particles being measured by spin detectors set at different angles. Critically, the angles of these spin detectors are assumed to be set independently from one another. ... Experimenters have tried to ensure independence for all practical purposes with elaborate techniques: independent quasi-random number generators running with different algorithms on different computers are one very basic example. On more advanced experiments, they use quantum sources of randomness, and try to make sure that the choice is only made once the particles are in flight. The trouble is that in principle, there will always be a point in the past at which mechanism used for the angle choice, and the mechanism used to produce the entangled particles were in causal contact with one another. (If all else fails, then the early universe will provide such a point.) The super-determination thesis says that any past causal contact can in principle provide correlation between the settings of the two detectors (or the detectors and the properties of the particles), and is the source of the violation of Bell’s inequality. Here’s a deliberately ridiculous example. Once the particles are in flight, I throw in the air a box of Newton’s notes on alchemy. I select the one that falls closest to my feet. I roll two dice, and use them to select a random word from that page. I match the word with its closest equivalent in Caesar’s commentary on the Gallic wars, or the Iliad, or the complete works of Dickens, my choice of work depending on the orientation of the Crab pulsar at the moment of measurement. I use the word position in these works to select a number in this book A Million Random Digits (take the time to read the customer reviews). And I use this number to set my detectors. I repeat this for my other measurement runs, but I substitute in Dan Brown’s Da Vinci Code for Dickens every third go. Superdetermination advocates would tell me that there is in principle a causal connection between my throwing the papers in the air, Newton, Caesar, Dickens as they sat down to write 300, 2000, and 150 years ago, the Crab pulsar and the RAND corporation’s random digit selection. And that it’s possible that these things have conspired (unknowingly) to make sure that my detector settings and a particle’s spin measurement is correlated in a particular way in my lab in a law-like way. I can only reply that yes, it’s possible. I cannot prove it wrong. But I can find it unreasonable. And I would be tempted to call these people philosophically desperate. https://www.quora.com/Why-do-some-crackpot-scientists-go-after-super-deterministic-theories-when-Bells-theorem-has-proved-that-local-realism-is-false https://www.quora.com/Why-do-some-crackpot-scientists-go-aft...

lisper 5y ago

Note that there is a very important property of entangled particles that is hardly ever mentioned in this kind of exposition, which IMHO casts a lot of light on what is really going on, and that is that entangled particles do not self-interfere the way non-entangled particles do. For more details see: https://flownet.com/ron/QM.pdf https://flownet.com/ron/QM.pdf

jfengel 5y ago
I don't think the paper justifies the statement as you put it, though perhaps you can point out what I'm missing. I don't think you can tell just from looking at the particle itself whether it has an entangled partner somewhere in the universe. It is, however, possible to use the entangled partners to create systems with decidedly counter-intuitive properties that change the way the un-involved partner interacts. That's also the essence of Bell's Theorem. It only works when you're controlling the experiment as a whole and thus not transmitting information faster than light... though you can set up the experiment in a way that makes the conventional transmission of information incredibly obscure. Bell's Theorem requires you to jump through a lot of hoops to exactly mimic that, which is why it took a long time to definitively rule out other interpretations of the experiments.
- lisper 5y ago
  See section 4.2, and in particular the paragraph that starts "Here's the kicker..." It is true that you can't tell if a single particle is entangled or not. But if you have an ensemble of particles all prepared in the same state then you can tell if that state is entangled or not. Non-entangled (a.k.a. pure) states have a preferred basis that produce self-interference. Entangled (a.k.a.) mixed states do not. (The pure-mixed dichotomy is a little misleading because it depends on your point of view. A single member of an EPR pair is in a mixed state, but the pair as a whole is in a pure state.)
  - gus_massa 5y ago
    I agree with the GP. If you only have a single member of the pair, then you will see the same interference pattern in a double slit experiment than with a not-entangled particle. It doesn't matter if the other particle has collided with a brick, went thru a double slit experiment, went thru a bad double slit experiment, or is flying to Andromeda. (In spite that the calculation to get the correlation with any result in the other experiment may be much harder with the entangled pair than with two non entangled particles.)

nsxwolf 5y ago

I'll never understand entanglement. Every explanation makes me wonder why it can't be used to instantaneously send a message. I never fully understand the explanations why it can't be used to do so. I don't understand how you can be sure about the state of the other particle, what if someone already measured it and then did something to it?

sergiotapia 5y ago
Reminds me of the in-lore comms system in Mass Effect. I think the comms in the ship were two atoms that were entangled, allowing instant messages no matter how many lightyears away the ship was from Earth.
deleted 5y ago
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misiti3780 5y ago
This book elucidates the concepts well: https://en.wikipedia.org/wiki/Something_Deeply_Hidden https://en.wikipedia.org/wiki/Something_Deeply_Hidden
drcode 5y ago
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 your marble, you instantly have information about the other marble at a speed greater than the speed of light... but you couldn't use that fact to send a message. 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)
- 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.

wyager 5y ago

MWI allows you to have entanglement without “spooky action at a distance”. However, it requires exponential blowup in representational complexity of the universe, which also feels aesthetically displeasing.

flubert 5y ago

"This is the first of a set of papers that look at actual Einstein-Podolksy-Rosen (EPR) experiments from the point of view of a scientifically and statistically literate person who is not a specialist in quantum theory." https://arxiv.org/abs/quant-ph/9611037 https://arxiv.org/abs/quant-ph/9611037 ...I wonder if anyone has ever followed up on Caroline Thompson's work after she passed away. https://arxiv.org/abs/quant-ph/0210150 https://arxiv.org/abs/quant-ph/0210150

Zamicol 5y ago

If you "measure" the bits per character in the base 45 alphanumeric encoding used in QR code, you'd get 5.5 bits per character as 11 bits is used for two characters. How is it possible to have information less than a bit, a partial bit? What is that ".5" part? Isn't a bit indivisible? Only in the context of a character doublet is all information expressed. To know the "half bit" part, you cannot "look" at just one character, you have to look at the total. The information is shared between the two characters. Measuring the bits-per-character is only useful when considering the whole system. The "partial bits" is information smeared across the system. Changing the middle bit may change one, or both, characters. Here's a 11 bit example, where the middle bit is changed and it changes both characters: (11101001010 vs 11101101010, or '/L' vs '%8' encoded) https://convert.zamicol.com/?in=11101001010&inAlpha=01&outAlpha=0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ.%24%2B*%3A%2F%25+- https://convert.zamicol.com/?in=11101001010&inAlpha=01&outAl... https://convert.zamicol.com/?in=11101101010&inAlpha=01&outAlpha=0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ.%24%2B*%3A%2F%25+- https://convert.zamicol.com/?in=11101101010&inAlpha=01&outAl... vs changing the last bit only changes the last character: (Using the preceding example, 11101101010 vs 11101101011, or '%8' vs '%9' encoded) https://convert.zamicol.com/?in=11101101011&inAlpha=01&outAlpha=0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ.%24%2B*%3A%2F%25+- https://convert.zamicol.com/?in=11101101011&inAlpha=01&outAl... The same principle applies to information theory and cryptography. Security can be measured in "partial bits" because it's measured across something larger.

jdb1729 5y ago
5.5 bits is also the average information content of a single run of the GHZ experiment. In this setup three parties independently choose a binary detector setting and each observe a binary outcome. The first two parties observe an independent random bit regardless of their settings. If an odd number of the parties have their setting "on", then the third party also observes an independently random bit (6 bits total to record, 3 for the settings and 3 for the observations). But if an even number of of the three settings are "on", then the third party's observation is completely determined by the other 5 bits. When the settings are chosen randomly these two possibilities are equally likely so on average it takes 5.5 bits to record the results of the experiment.
5y ago

Gunax 5y ago

I've been posting this explaination for more than 10 years now: http://www.felderbooks.com/papers/bell.html http://www.felderbooks.com/papers/bell.html I think I prefer Felder's explaination more than Quanta's. It's omitting some details (eg. the angles) but is better at explaining the difficulties of Bell's Inequality--why it seems like spooky action at a distance and why it cannot be used for communication.

subroutine 5y ago
One thing I've not been able to clarify is whether Bell accounts for the possibility that passing through a polarization filter could effect the waveparticle in some way, like altering its polarization angle.
- unholiness 5y ago
  Yes, the "altering" you're describing is what the theorem would call a hidden variable. Seems reasonable, but when you do the math it's exactly he kind of theory Bell's inequality rules out. There's no way to set the "polarization angle" (or any other set of variables) such that they obey the probabilistic laws we've observed (without violating some other assumption, like single measurement outcomes, statistical independence, determinism, or locality)
  - subroutine 5y ago
    As I understand the P(passing) through a second filter is related to the blue line here: https://upload.wikimedia.org/wikipedia/commons/thumb/e/e2/Bell.svg/400px-Bell.svg.png https://upload.wikimedia.org/wikipedia/commons/thumb/e/e2/Be... Couldn't the passage through the first filter effect the polarization angle in such a way that it matches the blue line instead of the red line. One could devise a physical contraption to demonstrate this is possible by sending bar magnets through slits of magnets of the same charge. Any magnet oriented such that it's too close to a slit will reorient slightly. Visually... https://ibb.co/gDnpqCb https://ibb.co/gDnpqCb That is, if both photons or magnets happen to pass through the first filter, their probability of both going through the second filter is boosted from the slight reorientation, and thus their outcome correlation will be boosted, which is what the blue line in the graph above shows.

choeger 5y ago

Does the article do justice to the hidden variables hypothesis? In case of the hidden variables, the spin is a (3-dimensional?) value that is identified by the measurement result. In case of quantum theory we have have a probability distribution. How is that probability distribution different from a hidden variables, except that it's not a straight number but a function instead? Speaking as a programmer, is the difference between hidden variables and quantum mechanics that the former postulate a real-valued property whereas the latter speak of something like a monad?

6gvONxR4sf7o 5y ago
Speaking as a lay person, I think the difference might be that it's specifically about local hidden variables. If two particles are coupled, there's no per-particle hidden variable?
- 6gvONxR4sf7o 5y ago
  edit-too-late-to-edit: I remembered something that makes me think it's actually more complicated. If I write 0 and 1 on two different pieces of paper, then flip a coin to decide which paper to give you, we have "entangled" unknowns. When I reveal my paper, we instantly know what's on yours. The joint distribution can't be described per-particle, but we don't consider it spooky. So I think there's something more to it.
  - BeKindAndLearn 5y ago
    Because your example does not contain superpositions. With entanglement the pieces of paper are both 0 and 1 at the same time until they are observed - and only then is it that the superposition of both particles become resolved.
    - 6gvONxR4sf7o 5y ago
      Tomayto - tomahto, right? Physically speaking, what’s the difference between an unresolved superposition and a different kind of unresolved random variable?

ThePhysicist 5y ago

There's no spooky action at a distance. Let's imagine we have an entangled qubit system that consists of a superposition of the states (0,1) and (1,0), i.e. either part A is in state 0 and part B in state 1 or vice versa. When we perform a measurement on the first part of the system and obtain 1, it simply means that we have "branched" into the (1,0) state of the system. This branching is usually irreversible because of the decoherence caused by the measurement (which itself is just an ordinary quantum process). There is no information exchange or any type of exchange between the two parts of the system going on, we simply branch into a part of the probability space defined for the system. The question whether the other branches still exist then leads to either the "classical" interpretation of quantum mechanics or the "many worlds" interpretation. The latter seems to be favored today as we know that there's nothing special about the measurement process that causes the collapse of a wave function (it's a quantum process in itself), but in the end there's not really a way to test this so it's really more of a philosophical question. Articles about "spooky action at a distance" should really mention this, as we have a much better understanding of the measurement process in quantum mechanics today than Einstein et. al. had when they wrote their paper.

virgil_disgr4ce 5y ago
If there's one single phrase I wish I could erase from history it's "Spooky action at a distance." Ugh. It bugs me a lot that Quanta made these misleading statements that just continue the confusion over what should be a more widely-understood core feature of the universe we live in. Tangentially, I wish "interaction" would come to replace "measurement," especially in the context of decoherence. The universe is branching *constantly* everywhere as various quantum systems interact.
joe_the_user 5y ago
What you're describing could be done with classical physics. Have one penny and two lockets. Place the penny blindly in one of the two lockets. Take one locket across the world. Opening it instantly lets you whether the other locket has contains the penny. And the point of this description is this is not what's weird about quantum entanglement. What's weird about quantum entanglement is you have two different measurement types that are non-orthogonal and they combine according to quantum logic rather than classical logic [1]. Having a particle in a state of this sort can't be explained by any analogy to discreet events occurring beforehand. [1] https://en.wikipedia.org/wiki/Quantum_logic#Quantum_logic_as_the_logic_of_observables https://en.wikipedia.org/wiki/Quantum_logic#Quantum_logic_as... Edit: Whether this is "spooky action at a distance" is in the eye of the beholder. One thing is isn't able to be is fully reducible to actions happening on something like a "classical time line" but another thing is isn't to able to do is transmit information.

naasking 5y ago

Not necessarily. Bell's theorem assumes statistical independence, but that means that either spooky action at a distance is real, OR that experimenters do not have complete freedom to configure their instruments (aka superdeterminism).

qwerty456127 5y ago

How long is it going to take until we can have a near-zero-latency Internet connection on Mars (e.g. on a Mars rover) or Moon?

void_mint 5y ago
At the speed of light, it would take between 4 minutes and 14 minutes to travel between Mars and Earth. A quick google says fiber optic cable can transfer data at roughly 70% of that speed. It may not be possible to get the latency you're describing. Mars is far away.
uh_uh 5y ago
You can not use QM for faster-than-light communication: https://en.wikipedia.org/wiki/No-communication_theorem https://en.wikipedia.org/wiki/No-communication_theorem
- qwerty456127 5y ago
  U.S. Department of Energy unveils blueprint for the quantum internet: https://news.fnal.gov/2020/07/u-s-department-of-energy-unveils-blueprint-for-the-quantum-internet-at-launch-to-the-future-quantum-internet-event/ https://news.fnal.gov/2020/07/u-s-department-of-energy-unvei... Fermilab and partners achieve sustained, high-fidelity quantum teleportation: https://news.fnal.gov/2020/12/fermilab-and-partners-achieve-sustained-high-fidelity-quantum-teleportation/ https://news.fnal.gov/2020/12/fermilab-and-partners-achieve-...
  - BeKindAndLearn 5y ago
    Apologies that I don't have time right now to clarify further, but I highly suggest you read the Wiki for entanglement and then reread the articles with that context. These simply do not do what you think they do. Information cannot, under any currently theorized model of physics, be transmitted faster than light.
  - drdeca 5y ago
    Quantum teleportation does not allow for transmitting classical information faster than light. What it allows for is, using an entangled state, along with transmitting classical information, to send another quantum state across the distance, but it still needs the classical information to get there first. This is how quantum teleportation works. Alice and Bob have entangled qubits A and B, and also Alice has a qubit C with some state that she wants to send to Bob , and Bob has some other Qubit D. To do this, Alice does some operations between qubits A and C, which includes a measurement. Alice then sends the classical information which is the result of the measurement to Bob. Bob then does some operations with the qubits B and D, where the operations he does are chosen based on the classical information signal he received from Alice. Iirc these observations include making a measurement. As a result, the qubit D is now in the state that the qubit C was in originally. (Alice no longer has access to the state that C was in.) So, in a sense, the state teleported from C to D, using the combination of the entangled A and B, along with sending some classical information. (The classical information sent is random, and to anyone snooping on the message sent, gives them no useful information, because they don’t have Bob’s qubit B to use it with.)

jussij 5y ago

Couldn't that 'hidden variable' be just another dimension? So while those particles might have been separated in space by a large distance, on that fifth dimension they haven't moved and are still sitting side by side.

pontus 5y ago
The distance between two points is basically just the Pythagorean theorem: x^2 + y^2 + z^2. If you have extra dimensions, you just add more terms to this. Now, if you know that e.g. x is very large then that puts a lower bound on the distance: Distance^2 = x^2 + y^2 + z^2 + ... > x^2. In other words, if x is large there's no way that the two particles are still somehow close together.
- platz 5y ago
  ER=EPR entanglement could be realized as a non-local distance mapping
- mcbits 5y ago
  Put two iron pellets on a very thin, flexible fabric. Underneath the fabric, put a magnet so each pellet is stuck to one pole of the magnet through the fabric. Tug the magnet "down" in the third dimension, which will fold the fabric and move the pellets so that from their perspective (stuck to the 2D surface of the fabric) they're moving far away from each other. They also maintain a constant and very short distance in the third dimension, but they can't take advantage of it because I forgot to mention the fabric is impervious to them.
- dreamcompiler 5y ago
  This is only true for cartesian spaces. If the low-dimensional manifold is "folded" or "crinkly" inside a higher-dimensional space then you can indeed have particles that look far apart on the manifold but which are actually close together in the higher-dimensional space. (My terminology here is largely wrong; this is not my field. Perhaps an expert can correct me.)
gameswithgo 5y ago

kovac 5y ago

I'm not a Physicist. From what I understand, Bell's theorem only covers local hidden-variables and that this can still be explained using non-local theories like Bohm's. Can someone shed a bit of light on the non-local theories and if Bell's theorem addresses those as well?

GoblinSlayer 5y ago
Bell's theorem is a mathematical theorem and has narrow scope, it addresses local hidden variables theories and says they won't fly, it does nothing else, and doesn't prove spooky action at a distance either - that's author's addition.
jostylr 5y ago
Bell was inspired by Bohm's theory. Having seen that a rational theory of existence was compatible with QM, Bell wanted to know if the nonlocality could be removed. Bell's work showed that it could not be removed. Bell was a proponent of Bohm's work. Bohmian mechanics is a perfectly sound theory, proven to be consistent and in agreement with quantum mechanics results whenever QM has results. EPR (and more relevantly, Bohm's version of it involving spin entangled particles) showed that either one has hidden variables or nonlocality. Bell's work showed that local hidden variables is not possible. Hence, nonlocality is the conclusion and the argument to demonstrate the incompleteness of quantum mechanics falls apart. The Many Worlds interpretations kind of gets away from this by exploiting another assumption in that work: that results from experiments actually happen in the sense that the other outcomes don't. Most people at the time would have just assumed that, but MW points out that experiments need not have definitive results, instead we have multiple results of experiments correlated with multiple versions of experimenters who think they have results. Thus, the correlations we see only need to be reconciled when the two groups of experimenters get together to compare notes, a perfectly local notion. Of course, some descriptions of MW, with branching of the universe, is rather nonlocal, but a proper formulation of MW is possible without any of that. By the way, a key feature of Bohmian mechanics is the reliance on position as the "hidden variable". The spin of a particle is not an aspect of the particle itself, but rather of the wave function guiding the particle. In the spin experiments, the correlation with the environment once measurement has happened, separates the overlapping spin parts of the wave function and the particle is then being guided by that singular spin. Hence the appearance of the particle behaving as if it had that spin, but the particle is not actually spinning, whatever that would mean for a point particle.

fungiblecog 5y ago

It did not prove that. It proved that at least one of three assumptions about the universe is violated: Statistical Independence, Locality or Determinism. The usual assumption is that Locality has to go, but that is not necessarily true. It is possible that Statistical Independence is not true for the experimental systems that have been studied. See for example: https://www.frontiersin.org/articles/10.3389/fphy.2020.00139/full https://www.frontiersin.org/articles/10.3389/fphy.2020.00139...

chopin 5y ago
If you forgo statistical independence isn't that a kind of super-determinism?
- goldenkey 5y ago
  Even ordinary interpretations of QM don't explain the source of randomness in QM. This randomness can be entirely deterministic which would make the entire universe deterministic. So superdeterminism can still be the case, even if it isn't the reason for entanglement correlations. The entanglement correlations would instead be due to shared RNG references across spacetime, effectively nonlocal. My point is, locality and superdeterminism aren't mutually exclusive, they are independent. All 4 combinations are logically possible with current experimental evidence. https://www.americanscientist.org/article/quantum-randomness https://www.americanscientist.org/article/quantum-randomness
  - unholiness 5y ago
    The fact that superdeterminism can violate statistical independence is what lets it violate bell's theorem, right? If so GP's statement is just backwards: Super-determinism is a kind of forgoing statistical independence
    - goldenkey 5y ago
      > The fact that superdeterminism can violate statistical independence is what lets it violate bell's theorem, right? Yes, the idea is that the past of both the detectors, experimenters, and particle producer were once interacting, relatively recently in the scheme of the human race (let alone civilization, the big bang, etc.) These correlations would be responsible for the entanglement correlations, not a shared nonlocal RNG reference, but rather an inherited local piece of an old reference. Nonetheless, superdeterminism doesn't have to be doing anything conspiratorial, but the possibility of doing so, lets it violate Bell/CHSH inequalities.

steve76 5y ago

Particles are anything with a discrete start and an end. The heat required per frequency of light emission has a discrete start and end, explained by statistical mechanics. That's a particle, even though it's very abstract and ephemeral. A material object emits light which will expand the size of the universe if unimpeded. With no discrete start and end, although very minor and subtle, it's not a particle despite the object's tangible concrete existence. Nature's internals have no mind. They are overwhelmed by the mighty surroundings and always redlining, pushing themselves to the limit. Need an electron? Nature looks all around, not just the nearest bonded atom. That's magnetism. Nature also takes the easiest path. Using it's own internals as a medium of interaction makes things easier. That's electromagnetic radiation. We're dealing with a vast ocean of the medium of spacetime, not little building blocks of matter. In terms of (space + relative clocks) EM radiation travelling through empty space has internals of it's own. Electricity will repel or attract the entire thing. No further electromagnetism with that empty space itself. But magnetism, it gets to interact again, take an easier route using it's own internals as the medium of interaction. If you want to entangle an electron, make it so interacting with the internals of one interacts with another. To exist, something has to give some resistance to surrounding interactions. If it happened immediately, it's just the surroundings. That's how we get the speed of light and the discrete nature of light frequencies. Besides electromagnetism, there's other interactions. Mass, as in nuclear decay, or triple pole "color" charge. Simulations and holograms mean that resistance to interaction will be much more simpler that the internals. Black holes do this. They're just one big 2D dot. Things that fall into them are crunched up on the surface. Bell's Theorem is like measuring from the inside of a spinning coin that's growing, and stopping it. That's where Bell's Theorem, and the speed of light has more to gain. Why fight it? It's awesome. I think of plasma applications of the sun. That star orbits a black hole which can and does zap electrons into existence from empty space by crunching up a positron of a pair. What if we get good enough at that things stop looking like electrons but look like distant stars? Would that be a drive that warps the space time around our solar system, and suddenly the universe is much more smaller.

steve76 5y ago

Particles are anything with a discrete start and an end. The heat required per frequency of light emission has a discrete start and end, explained by statistical mechanics. That's a particle, even though it's very abstract and ephemeral. A material object emits light which will expand the size of the universe if unimpeded. With no discrete start and end, although very minor and subtle, it's not a particle despite the object's tangible concrete existence. Nature's internals have no mind. They are overwhelmed by the mighty surroundings and always redlining, pushing themselves to the limit. Need an electron? Nature looks all around, not just the nearest bonded atom. That's magnetism. Nature also takes the easiest path. Using it's own internals as a medium of interaction makes things easier. That's electromagnetic radiation. We're dealing with a vast ocean of the medium of spacetime, not little building blocks of matter. In terms of (space + relative clocks) EM radiation travelling through empty space has internals of it's own. Electricity will repel or attract the entire thing. No further electromagnetism with that empty space itself. But magnetism, it gets to interact again, take an easier route using it's own internals as the medium of interaction. If you want to entangle an electron, make it so interacting with the internals of one interacts with another. To exist, something has to give some resistance to surrounding interactions. If it happened immediately, it's just the surroundings. That's how we get the speed of light and the discrete nature of light frequencies. Besides electromagnetism, there's other interactions. Mass, as in nuclear decay, or triple pole "color" charge. Simulations and holograms mean that resistance to interaction will be much more simpler that the internals. Black holes do this. They're just one big 2D dot. Things that fall into them are crunched up on the surface. Bell's Theorem is like measuring from the inside of a spinning coin that's growing, and stopping it. That's where Bell's Theorem, and the speed of light has more to gain. Think of a golf ball. Each dimple connects through the center. Perhaps instead of going through the center, it was easier to go on the surface, and let the center work itself out: Why fight it? It's awesome. I think of plasma applications of the sun. That star orbits a black hole which can and does zap electrons into existence from empty space by crunching up a positron of a pair. What if we get good enough at that things stop looking like electrons but look like distant stars? Would that be a drive that warps the space time around our solar system, and suddenly the universe is much more smaller.