[andomar]

Say you put a red marble and a blue marble in two envelopes. You randomly post one envelope to Australia. One year later, you open the other envelope. You now know the color of the marble in Australia.

What's the difference between this and quantum entanglement?

[cbkeller]

This is fairly far from my field, but as I understand it, that would be a hidden variable interpretation of QM [1], and specifically in that analogy a local hidden variable theory. That's what Einstein himself wanted.

There is a famous test, Bell's inequality [2], that specifically rules out local hidden variable interpretations of QM.

Nonlocal hidden variable interpretations, such as De Broglie - Bohm theory [3], are potentially still on the table, however.

It is somewhat ironic that Bell's theorem is sometimes presented in popular media as a general disproof of all hidden variable theories, in a context where locality is taken for granted -- because Bell himself seems to have been partial to nonlocal hidden variable theories. An article by the same Mermin mentioned in the OP is worth a read, on this subject [4].

[1] https://en.wikipedia.org/wiki/Hidden_variable_theory

[2] https://en.wikipedia.org/wiki/Bell%27s_theorem

[3] https://en.wikipedia.org/wiki/De_Broglie%E2%80%93Bohm_theory

[4] https://cqi.inf.usi.ch/qic/Mermin1993.pdf

[flubert]

Anyone know if anyone has followed up on Caroline Thompson's work after she passed away?

"The Chaotic Ball: An Intuitive Analogy for EPR Experiments"

https://arxiv.org/abs/quant-ph/9611037

[cbkeller]

Haven't found anything yet myself, but would love to know -- that looks quite interesting!

[Toutouxc]

The way I understand it, this is exactly the simplified (and incorrect) explanation.

Say SOMEONE ELSE puts the marbles in two envelopes and sends them to you and your friend in Australia. (it's someone else because we don't actually create the entangled particles, we just "get" them)

The marbles being red and blue (or both red or both blue, depending on what you're measuring) from the beginning would be a LOCAL hidden variable. It's local because it's been predetermined at the moment of creation and the marbles carry the property on themselves and it's hidden because you don't know how/why the person putting the marbles in those envelopes decided those colors and you can't see them until you open the envelope (measure the particle).

This way if you don't open your envelope, your friend's envelope contains a marble that's 50/50 red or blue and the color will be the predetermined one no matter what you do with your marble at home. So whatever decides the marble's color has nothing to do with your marble, it's local to the friend's one.

The actual measurements work differently. It's been experimentally proven many times that at the moment you look at your marble, the other marble's 50/50 probability of being red and blue shifts substantially to, for example 75/25. And that's without it having any way of knowing that you've seen your marble. So there are hidden variables that we don't understand, but they're not local. They somehow affect both marbles.

In real life there aren't only two colors and the probabilities aren't those nice numbers, but you get the principle.

[wnoise]

> The actual measurements work differently. It's been experimentally proven many times that at the moment you look at your marble, the other marble's 50/50 probability of being red and blue shifts substantially to, for example 75/25. And that's without it having any way of knowing that you've seen your marble. So there are hidden variables that we don't understand, but they're not local. They somehow affect both marbles.

This is completely incorrect, to the point where what you were trying to correct was actually more accurate, though incomplete.

The usual setup is that for any given axis, each person always measures 50:50. Measuring your own doesn't change the odds of the other.

Knowing the _results_ of your own does. For the same axis, the correlation is exact. For axes with an angle theta between them, we get a correlation of R ~ cos(theta/2).

The upshot is that there is no underlying (classical) probability distribution that can give rise to this that can explain things for all measurement axes. This is sometimes glossed as "correlation without correlata".

[Toutouxc]

I was trying to make the original marbles in envelopes comparison work a little better (the "holy f*ck this doesn't make sense" aspect), not to actually explain the phenomenon. I hope no one got confused. There's plenty of better sources and smarter people to get the accurate explanation from.

[Jemaclus]

> The actual measurements work differently. It's been experimentally proven many times that at the moment you look at your marble, the other marble's 50/50 probability of being red and blue shifts substantially to, for example 75/25. And that's without it having any way of knowing that you've seen your marble. So there are hidden variables that we don't understand, but they're not local. They somehow affect both marbles.

The rest of your explanation was super easy to grok (thank you!) but this part I can't wrap my head around. If the balls can be red or blue, and it's 50/50 before, how would the probability go to 75/25? I would expect it to either stay at 50/50 (no change) or to 100% (because the other ball is known).

Can you elaborate on this part? This is really fascinating.

[jayd16]

This explanation seems like it can't be correct and must be a simplification as well. If this was measurable in a way that's described here you'd be able to transmit information.

I always imagined the two "marbles" as possibly being two similar but differing clocks instead. The clocks will align more or less often depending on how similarly they're set and how fast each run. With this analogy you can come up with any distribution that fits your fancy.

Its probably a silly analogy but it lets me cling my notions of no spooky action.

[smusamashah]

So me and my friend are continuously getting those envelopes. I am opening all the envelops one after another and found that they are distributed 50/50 to red/blue.

Same if my friend is opening the envelops.

Now for all the opened envelops if I have got 10 red balls. Now if my friend open the paired envelops, he will probably get 7 blue and 3 red.

My observation of the balls had an effect on his side and shifted probabilities on his side.

If that's what you mean, what does observation or measuring even mean? How do the balls know the envelop has been opened.

[nojokes]

Can you explain how is this shift in probability measured?

[GoblinSlayer]

The experiment is repeated many times, statistics is computed over results, and gives probabilities and correlation.

[tsian2]

How much work has been put into ensuring that the observed samples aren't biased?

[iudqnolq]

What do you think? Is mainstream physics about to go "oh shit, we didn't take into account that we might be biased. Thank you random person on the internet for pointing this out".

[ikken]

It's not my field, but I remember reading that your example doesn't represent entanglement because when put into envelopes, one marble is already red and another blue.

In quantum entanglement they are both truly and really random until you measure one. And it's not random in a sense that you closed your eyes when putting them into envelope. They actually both don't have a "selected" color. They "snap into one of two colors" when you measure (look at) one. And the "unbelievable" thing is that when you measure one, the other one immediately snaps into opposite color, no matter how far it is.

[Koshkin]

I don’t think this is correct. The two particle system is prepared in a perfectly known state (e.g. both spins up). There’s nothing random about it. Randomness only occurs at the measuring device, if it not aligned with the direction of the spin of the incoming particle.

[zkmon]

Nope. They don't "have" their own state until one of them is measured. But they do have a correlated state which exists before measurement, which says they have opposite/same states. The individual states arise only after measurement. I'm not a physicist, but wrote a Quantum Simulator.

[kgwgk]

You could have something similar to quantum entanglement if these were some strange kind of marbles which cannot have a well defined color and size at the same time and are magically linked.

If you look at the marble you got and it's red (or blue) the size becomes indeterminate. Focusing now on the size you will find it's large or small, but the color becomes indeterminate. It could be red the next time you look at it.

When you take your entangled marble, look at the color and see it's red you know the other marble is in the "blue" state (and the entanglement is broken). If someone looks at the color of that marble you know they will find it's blue. But if they look at the size before looking at the color it could be large or small (and looking now at the size of your marble will tell you nothing about it) and if they look at the color later it could be red or blue.

In the classical case, if there is a large red marble in one envelope and a small blue marble in the other it doesn't matter in what order you look at the color and the size. You will always know what the other person found.

In the quantum case, if both look at color first they will find complementary colors. If they both look at size first they will find complementary sizes. But the second measurement will be uncorrelated. And if they make the measurements in a different order, everything will be uncorrelated.

[Simon321]

This would be a 'local hidden variable' theory. According to wikipedia these have largely been ruled out:

Most advocates of the hidden-variables idea believe that experiments have ruled out local hidden variables

Source: https://en.wikipedia.org/wiki/Bell%27s_theorem#Bell_inequali...

[goldenkey]

It seems that hidden variables have always been made to be simple hidden states. But we now know about RNGs and seeds and such.

A shared RNG seed is essentially entanglement.

This delves more into complex hidden variables, that normal analyses ignore: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC137470/

[kgwgk]

> A shared RNG seed is essentially entanglement.

Classical entanglement, which is not good enough to explain quantum entanglement.

[aeternum]

It can explain quantum entanglement, seems to be a version of superdeterminism.

In QM, experiments show us that entangled particle spin probabilities vary non-linearly with the angle between detectors (even if those detectors are far apart).

This means that either: 1) locality is broken.. state is somehow transmitted faster than the speed of light between particles. 2) realism is broken.. god plays dice with the universe

But there's also a 3rd, which is: the choice of detector angle is not an independent variable (a necessary assumption for Bell's inequalities to hold).. instead the state of the universe is pre-determined and the experimenter's choice of detector angle is known beforehand so there is no need for spooky action at a distance. This isn't a very popular explanation since it provides no reason as to why we don't see this weird lack of independence elsewhere.

[kgwgk]

Well, if superdeterminism is an explanation there is nothing to explain ;-)

[rssoconnor]

Welcome to Bell's Casino. You and your partner will be playing our famous two-coin game today. We have hidden two coins under these two opaque cups. You and your partner are to guess the orientation, heads or tails, of both of the coins. Guess correctly and win $1. Guess incorrectly and lose your $1 bet.

In order to help you out, after you two have made your guess we are going to give you two a chance to back out and lose nothing. After your prediction we are going to reveal one coin to you and another coin to your partner. Together you and your partner will have an opportunity to back out, but the catch is that you two are not allowed to communicate!

Instead of communicating, you can raise either a red flag or a green flag after seeing your coin. Similarly, your partner can raise either their red flag or their green flag after seeing their coin. If you both raise the same colour flag, the game keeps going and we see if you win or lose. If you both raise different colour flags, the game stops and you lose nothing.

To ensure you don't cheat, we've separated you and your partner by 200 million kilometers and you have one minute to raise one of your flags after seeing your coin, otherwise you lose the game. (Alternatively you are your partner are separated by 400 meters and you have 100 nanoseconds to raise one of your flags.)

Good luck.

---

The above casino game cannot be beaten using envelopes of marbles, but it can be beaten (i.e. positive expected value) using envelopes of entangled particles. See quantum pseudo-telepathy.

[opaque]

This argument is called "Bertlmann's Socks"

https://en.wikipedia.org/wiki/Reinhold_Bertlmann#Bertlmann%E...

The other replies explain why it's wrong, but here's a link to Bell's refutation for good measure

http://cds.cern.ch/record/142461/files/198009299.pdf

[comedowntous]

Wikipedia's Simple English page on Bell's inequality actually has a nice overview of a simplified way of thinking about quantum entanglement: https://simple.wikipedia.org/wiki/Bell%27s_theorem

[tim333]

If you do the experient with an entangled source and two Stern Gerlach detectors oriented the same way it's not so interesting a bit like the marbles in envelopes - either they both red of green. The interesting bit is if you rotate them a bit the correlation varies like cos(the angle) between them. So correlation 1 at 0 degrees, 0 at 90 degrees, -1 at 180 degrees and about 0.98 at 10 degrees. But how does nature or whatever know the angle between them when they are far apart? In most 'hidden variables' scenarios the correlation at 10 degrees is more like 0.89 or a linear change and that is basically the essence of Bell's theorem and experiments - you can't get the correlations without the particle at one end kind of knowing the set up at the other, or 'non locality' as Bell called it.

[stillsut]

Your answer seems to focus on the key un-addressed subtlety.

Why is the difference in orientation of the detector necessarily linear? What is the control aspect of this experiment where classical-system shows this linear pattern? Or can the argument be made more fundamentally?

[stillsut]

Thanks for the help. I just want to point out this [detector orientation] is likely a big area where non-physics people might get tripped up:

If you told me causally this detector which measures electrons/photons/whatever and varies by the cosine of the orientation, I don't think any (non-physics person) would bat an eye; it seems like a pretty normal thing a sensor might do.

[tim333]

You can think of an example where things are classical, the particles start with some definite orientation randomly determined at the start and if they are within say 45 degrees of the angle of the detector they go one way, over 45 the other and it's not so hard to figure in that case it will vary linearly. As to how to prove the general case I don't know. Try Bell's paper?

[tim333]

As an aside I don't think the classical 'hidden variables' situation has a prefered orientation which contradicts the premise of the featured article that you get the odd entanglement effects so as to not have a prefered orientation.

[kubanczyk]

So many confusing answers in this thread, but yours is clear and enlightening (and factually correct, afaik).

[_8091149529]

What seems to be missing from the replies posted so far is the notion of coherence, and the choice of measurement basis.

What separates a coherent "quantum" superposition, say, |0> + |1>, from a probabilistic "non-quantum" 50:50 mixture is that I can choose a measurement basis in which the coherent state always yields a definite result, say "1", whereas measuring the mixed state always yields a 50:50 mixture of "0"s and "1"s.

A continuous sweep of the angle of the measurement basis generally results in an interference pattern, the amplitude of which can be used to assess the fidelity of the quantum state.

(I get paid to work on quantum communication and related experiments.)

[haxiomic]

Sabine Hossenfelder gives the best explanation to address this that I've found

https://www.youtube.com/watch?v=j6Mw3_tOcNI&ab_channel=Sabin...

[wwarner]

The phenomenon is statistical. In your example (which is a bit too simple but still), the analogous surprise would be something like each party guesses what's in the envelope before they open it, and the accuracy of their guess is too high to explain by chance.

[unkown]

actually the marbles change colour every 1 second and you can take them really far apart, meaning one can go at a really high speed and distance trough universe, so the time would have dilated for it. when you open them both have same color

[andomar]

That sounds logical. Relativity theory allows you to age one object faster than another by changing their relative speed. Nobody would claim there was information travelling faster than light in your example.

What makes people say information travels faster than light with quantum entanglement?

[throwaway_pdp09]

If the marbles were changing colour at random but still alwasy different colours, that would suggest info is doing so.

[andomar]

If they change color in sync then that is a known property of both marbles. Say you have a marble that is blue if the number of seconds is even and red otherwise. Knowing the color of one marble is enough to know the color of the other marble without information travelling between the marbles?

[throwaway_pdp09]

Not my area. My understanding: colours sync exactly on measurement (when you look at them).

> Knowing the color of one marble is enough to know the color of the other marble

I guess so.

> without information travelling between the marbles?

The marble colours are in sync on measurement. Somehow that info has travelled instantaneously. You just can't use it to send information, at all..

above is just my understanding. I have no background in this. Just a programmer.

[pegasus]

They don't.

[Tomminn]

Roughly speaking: if you can see your marble through a purple filter, the one in Australia will turn out to be perfectly green.

[throwaway_pdp09]

Apparently this common explanation is wrong. If you actually measure one, the other actually changes. Although you can't use that to send information, so einstein's faster-than-c restriction isn't violated.

Maybe this will help https://html.duckduckgo.com/html?q=bell%27s%20inequality%20s... I imagine the youtube links might be more comprehensible.

[yetihehe]

If the other changed, you could ue those changes to send information by varying time between changes (pulse width modulation). Instead when you measure one you go from "not knowing which one is red or blue" to "knowing color of both".

[Tomminn]

No, just because something changes doesn't mean it can be used to send information. You need changes you can control.

The thing about the change of "colour" in this analogy is you don't know in which direction it changes. So let's say you observe you "marble" through a "purple filter", which gives has:

- a 50% chance of being transparent to your marble (corresponding to a red-blue superposition marble collapsing to a purple marble)

- a 50% chance of being opaque to your marble (corresponding to red-blue superposition marble collapsing to a green marble).

The issue is that when you learn your marble is purple, while you know with 100% certainty the marble in australia is green, there is no way you can send information to Australia using that. This is because the other 50% of the time, your marble will be green, and the marble in Australia is purple.

So if I'm sitting in Australia, when I measure the marbles in my envelopes with purple filters, all I see is purple marbles 50% of the time and green marbles 50% of the time no matter what measurements you are performing at your end. So you can't send me messages by performing measurements at your end because you can't change the statistics of those measurements.

But you'll know the answer to every measurement I performed, if you've measured the other marble with a purple filter too.

[yetihehe]

So, how "it changes from state we don't know to a different state after measurement" differs from "we don't know what state it is, but after measurement we know"? How do you know state changes after measurement when you don't know which state it is before measurement? Does it really change, or do our knowledge of that state changes? That's why I say that state doesn't change, we only know what state it is after measurement.

[liminvorous]

I think you can only tell if it’s changed by measuring the thing and comparing the results with the other person.

[yetihehe]

AFAIK you don't need to compare. It's like random number generator, but you have two complementary generators. When one generates 1, the other generates 0. You don't know what you will get next, but you know what was last and you know that other person got opposite number.

[Dylan16807]

Only if the measurements align. If they do you get perfectly correlated numbers. If the angles are 90 degrees apart you get completely unrelated numbers.

The problem comes in when the angle between your two measurements is anything else. The chance that the measurements match is based on the cosine of that angle. There's no way for this to happen if the measurements are independent.

If you try to write two equations, where the first equation takes the secret particle state and first angle and gives you 1 or 0, and the second equation takes the secret particle state and second angle and gives you 1 or 0, you won't be able to reproduce the odds you get in the real world. Only equations that know both angles will work.

[throwaway_pdp09]

You can't.

[yetihehe]

Exactly. Because they don't change. If you detect one color, it stays the same.

[garmaine]

Only if you assume a single universe. If, on the other hand, the universe split into both options, then nothing "changed" when you looked at one of them. You just no longer have a way of interacting with the sub-universe where the ball was the other color.