Why isn’t quantum entanglement just correlation?

[lukephysics]

TL;DR Summary

Why isn’t it just a random shuffle plus locked in correlation?

They say spin up and spin down is correlated at at any distance and that it can’t be explained by basic logic.

say I rip a photo in two, shuffle them and put them in two boxes and send them light years away. No matter which box I open, I can’t know which half I have but when I open it the photo in the other box is always the opposite. this seems completely normal and not spooky. And in my understanding, this is the same as what is happening in the spin measurement of a photon.

So why is there anything quantum being invoked when it’s being done with photon spins and not with photographs?

now I am told it’s necessary by bells experiment, but bell talks about polarity and measuring at angles. Which complicates the problem because I am not an expert on polarisation and I can’t argue about it and creates thoughts that perhaps polarisation is spooky (and bells experiment flawed!). So I really don’t follow from first principle what bell is offering.

In regards to my analogy vs bell, I can’t measure my photos at angles so the analogy seems to be missing something. So is there a better version of this analogy which can describe what the whole polarisation/ angles are doing in this context?

 

[PeroK]

Summary:: Why isn’t it just a random shuffle plus locked in correlation?

They say spin up and spin down is correlated at at any distance and that it can’t be explained by basic logic.

say I rip a photo in two, shuffle them and put them in two boxes and send them light years away. No matter which box I open, I can’t know which half I have but when I open it the photo in the other box is always the opposite. this seems completely normal and not spooky. And in my understanding, this is the same as what is happening in the spin measurement of a photon.

So why is there anything quantum being invoked when it’s being done with photon spins and not with photographs?

This question gets asked a lot. First, before you study entanglement, you need to study spin and the nature of measurements in QM. According to QM the specific values for spin of a particle are not decided when the particle is created or otherwise prepared. And, indeed, spin measurements about different axes are what is known as incompatible measurements: the uncertainty principle applies and you cannot know the spin about every axis in any case.

Unlike your example of the photograph, when entangled quantum particles are separated they have no well-defined values of spin that are inevitably what will be measured.

This is what creates the apparent "spookiness". It's the correlation of values that are not predefined before measurement.

As to Bell's theorem, QM predicts correlations that cannot be achieved by predefined values (also known as hidden variables). This is what Bell's inequality encapsulates. The test of Bell's theorem is then a significant test of QM and, if QM is confirmed, it also eliminates all theories of local hidden variables - as they cannot match the predictions of QM.

The actual experiments of Bell's theorem use photon polarization, as that is experimentally easier to produce. But, the QM calculations and the implications of Bell's theorem remain the same for particle spin or photon polarization.

 

[Dale]

Summary:: Why isn’t it just a random shuffle plus locked in correlation?

So why is there anything quantum being invoked when it’s being done with photon spins and not with photographs?

Here is a relatively accessible paper that explains exactly in what way your specific analogy fails. Basically, if there is some “hidden variable” then the probability laws can be written in a specific form and that form constrains the possible probabilities. Actual QM experiments violate that constraint, so there can not be any such “hidden variable”.

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

 

[lukephysics]

Here is a relatively accessible paper that explains exactly in what way your specific analogy fails.

Thanks I will read it. Hopefully it answers it.

 

[lukephysics]

Unfortunately it did not help me. You can't logically separate these two explanations of the state before measurement:

- two un-measured un-determined entanglemed spins (quantum explanation)
- two un-measured yet set-at-entanglement spins, (my explanation)

They are both logically equivalent and experimentally equivalent upon measurement. so you can't break that simple logic no matter what experiment you try to create.

Can someone please help a bit further? I swear I am proving action at a distance at least un-provable here?

 

[PeroK]

Unfortunately it did not help me. You can't logically separate these two states:
- two un-measured un-determined spins (quantum explanation)
- two un-measured yet set-at-entanglement spins, (my explanation)

They are both logically equivalent and experimentally equivalent. so you can't break that simple logic no matter what experiment you try to create.

Well, no they are not. That's what Bell's Theorem is all about. And, the experiments have been carried out.

 

[lukephysics]

Well, no they are not. That's what Bell's Theorem is all about. And, the experiments have been carried out.

What is the logical difference? Do you know the difference or just assume since others have stated that?

 

[PeroK]

What is the logical difference? Do you know the difference or just assume since others have stated that?

I've calculated it for myself.

The difference is that QM uses complex probability amplitudes and the key to it all is the factor $\cos^2 \frac \theta 2$, which is ultimately the correlation that classical hidden variables cannot achieve.

You can find the entire calculation in Modern QM by JJ Sakurai, for example.

 

[lukephysics]

The difference is that QM uses complex probability amplitudes and the key to it all is that the factor $\cos^2 \frac \theta 2$, which is ultimately the correlation that classical hidden variables cannot achieve.

yes I understand that, but during entanglement the two particles can pre-calculate that and say "lets remain a wave function until measurement, but when you get measured let's follow what the super-entangled world expects of us, so your position will be here and when i get measured ill do the complementary of that and go here".

so from the super-entangled observed world, both pre-made arrangement and an un-determined at entanglement arrangement return the same outcome.

 

[PeroK]

yes I understand that, but during entanglement the two particles can pre-calculate that and say "lets remain a wave function until measurement, but when you get measured let's follow factor $\cos^2 \frac \theta 2$, so you go here and when i get measured ill go there".

But not for all angles. The trick is to use different measurement angles, which pull classical probabilities one way and the other and they can't do both. You need to see the calculations.

 

[lukephysics]

But not for all angles. The trick is to use differnet measurement angles, which pull classical probabilities one way and the other and they can't do both. You need to see the calculations.

it doesn't matter. you can build in a quantum model into a classical algorithm, call it a quantum simulator, to produce the same outcomes, and do this up-front.

 

[bhobba]

It is a correlation, just like putting a red slip in one envelope and a green slip in another. Regardless of how far away the envelopes are, open one. You know the slip in the other. The same thing is happening. But here is the rub. Red and green slips are two distinct entities. Not so entangled particles. When entangled, they form a single system and, as separate particles, do not exist. It is sometimes said to be 'holistic'. When you work through the math of QM, this leads to statistical properties different to the red and green slips. However, want it to be like the red and green slips where each has a separate existence? Then you have to introduce non-local communication between the particles that violates relativity, at least in spirit. It may not strictly do so because the communication can't send information, but it certainly makes it a bit of a puzzle. Best to stick with the holism idea, which has no issues with relativity at all.

Thanks
Bill

 

[PeroK]

it doesn't matter. you can build in a quantum model into a classical algorithm, say a quantum simulator to do the same thing.

What matters is whether you have come here to learn about physics or to tell us that modern physics has got it al wrong. If it's the latter, then we are all wasting our time.

 

[lukephysics]

What matters is whether you have come here to learn about physics or to tell us that modern physics has got it al wrong. If it's the latter, then we are all wasting our time.

go back to the topic at hand. you said "But not for all angles. The trick is to use differnet measurement angles. which pull classical probabilities one way and the other and they can't do both".

quantum mechanics can be programmed into a classical computer that simulates quantum effects via the same quantum formulas. The computer that calculates the Quantum world can do it at the point of entanglement or at the point of measurement, both of which are logically equivalient from the perspective of the observer/super-entangled world.

 

[PeroK]

The computer that calculates the Quantum world can do it at the point of entanglement or at the point of measurement, both of which are logically equivalient from the perspective of the observer/super-entangled world.

Well, as I said, you are just wasting everyone's time.

 

[lukephysics]

Well, as I said, you are just wasting everyone's time.

you seem to have disengaged. hopefully someone can pick up the thread.

From what I undertstand the wave function of entanglement is not affected by time between entanglement and measurement. So it doesn't matter when the spin is determined internally (hidden until measurement), at entanglement or at the measurement.

 

[bhobba]

yes I understand that, but during entanglement the two particles can pre-calculate that and say "lets remain a wave function until measurement, but when you get measured let's follow what the super-entangled world expects of us, so your position will be here and when i get measured ill do the complementary of that and go here".

As I said, once entangled, they are not separate particles. If you want the gory detail see:
https://plato.stanford.edu/entries/qm-action-distance/

Here you see if they are two separate particles while entangled, then factorizability holds. It can be shown, as pointed out in the article, factorizability is the conjunction of two properties. Parameter Independence (PI) and Output Independence (OI). Output Indepenance is compatible with relativity; Parameter Independence is not. Bell showed factorizability does not hold, so one or both of OI and PI does not hold. OI is compatible with relativity, but PI is not, so out goes PI. To preserve relativity, you must consider them not as separate particles but as a holistic combination of each.

I will also mention it also depends on what you mean by locality, but that leads into often long and pointless debates, even heated arguments, that IMHO is counterproductive to understanding. Many threads have gone down that path on this forum. You can read them at your convenience. But be warned - I doubt they will really resolve anything.

Thanks
Bill

 

[bhobba]

So it doesn't matter when the spin is determined internally (hidden until measurement), at entanglement or at the measurement.

He is alluding to, while entangled, each particle not only loses its separate identity; it is even deeper than that. They do not even have an identity until measured. Certain interpretations try to get around this (e.g. Bohmian Mechanics), but none have proved entirely satisfactory to all physicists.

Thanks
Bill

 

[lukephysics]

As I said, once entangled, they are not separate particles. If you want the gory detail see:
https://plato.stanford.edu/entries/qm-action-distance/

thanks ill read that it looks good.

 

[Dale]

You can't logically separate these two explanations of the state before measurement:

- two un-measured un-determined entanglemed spins (quantum explanation)
- two un-measured yet set-at-entanglement spins, (my explanation)

The two are logically distinct. The second implies that the probability law can be written in a specific form. We can’t observe the unmeasured variable, but the fact that it exists places constraints on the probability law. Those constraints are experimentally violated.

Your assertion that they are logically identical is simply wrong.

 

[lukephysics]

The two are logically distinct. The second implies that the probability law can be written in a specific form. We can’t observe the unmeasured variable, but the fact that it exists places constraints on the probability law. Those constraints are experimentally violated.

thank you for your answer. so why can't you define the probability law as a local quantum law that matches bell's rules? since nothing happens to the particle while its entangled (definition of entanglement), no interference occurs, and from the perspective of an entangle particle, no time has passed between entanglement and measurement. so it wouldn't make any difference if its measured right after entanglement, or moved across the galaxy for eons and then measured. its only an emergent macro property to say the particle has moved away from it's pair.

 

[phinds]

@lukephysics you really would do well to quit while you are behind. The first rule of holes is that when you find yourself in one STOP DIGGING. Read some actual physics, as others keep suggesting, and you will see for yourself that your argument is baseless, as has been described here of PF, and elsewhere, in detail many times.

At the very least do a forum search to see all the other threads where this has been put to rest and read the links provided above. Stop digging.

 

[lukephysics]

what is the minimum requirement for someone to start a High School marked thread? You can see some people are offering good faith answers, while others are bored at a noob. I am not a complete noob I have been reading and had an interest for about it for a decade at least.

There is something very powerful having a discussion that you can't get from reading textbooks.

 

[Dale]

so why can't you define the probability law as a local quantum law that matches bell's rules?

I don’t understand your question here. The quantum probability rules are governed by Schrodinger’s equation, so you cannot just change that probability law. It is what it is.

Bell’s theorem is not a theorem about QM, it is a theorem about any possible probability law that is consistent with local hidden variables. So it encompasses any hidden variable law you could imagine.

QM (fixed law) violates Bell’s inequality (any possible local hidden variable law). Therefore QM cannot have local hidden variables.

 

[phinds]

There is something very powerful having a discussion that you can't get from reading textbooks.

But you CAN get this from textbooks. Entanglement is a well known phenomenon.

 

[phinds]

others are bored at a noob

It's not at all a matter of your being a nooby or not, it's that you kept making the same wrong argument after being repeatedly told that it is wrong. Entanglement is WEIRD and goes against our "common sense" and "intuition" and so forth, but it is what it is. Read about it rather than argue against it.

 

[lukephysics]

I don’t understand your question here. The quantum probability rules are governed by Schrodinger’s equation, so you cannot just change that probability law. It is what it is.

I am suggesting to make the fixed law the Shrodinger Equation. Store the result of the collapsed wave function at time of entanglement in hidden variables, rather than doing it at time of measurement.

Since entangled particles are isolated from time and space (interference collapses them), calculating Shrodinger Equation at entanglement (when they are local) will yield the same result as calculating Shrodinger Equation at measurement (when they are distant).

This is why I stated the two postulates are experimentally equivalent.

 

[Dale]

Store the result of the collapsed wave function at time of entanglement in hidden variables, rather than doing it at time of measurement.

That is exactly what Bell’s theorem discusses. That sort of stored result is precisely the type of local hidden variable envisioned by Bell. He proved exactly that any such law has an experimental signature. This signature is violated by QM and by experiment.

The problem that you seem to be having is that you appear unwilling to believe that local hidden variables have an experimental signature. I am not sure why this is so surprising. Local hidden variables would be a physical path for information. I would expect that information to leave an experimental trace. Bell proved that it does and showed what that signature is.

I am not sure why you would even start with that assumption. That such an information channel would have no possible experimental trace. Why not? I am further surprised that you continue to adhere to that assumption when it is proven false. You have no evidence or logic for your assumption, the assumption itself is a bit of a stretch, and it is mathematically proven false.

This is why I stated the two postulates are experimentally equivalent.

But they are mathematically provably experimentally distinct. Please cease repeating this false claim. This is physics, not politics, so something does not become true simply by virtue of repeating it.

 

[Jarvis323]

As a QM layperson, I found this video to be insightful. The story of John Bell's discovery starts at about 45 minutes. The details start around 50.

 

[lukephysics]

The problem that you seem to be having is that you appear unwilling to believe that local hidden variables have an experimental signature. I am not sure why this is so surprising. Local hidden variables would be a physical path for information.

Forgive me I like to work things from first principles. I am not sure what I am proposing can be said to be hidden variables. So it might be exempt from bell. i need some help with that.

Can you clarify if I am correct that the wave function of an entangled particle pair does not change as you move them apart in spacetime? From what I understand you have to do it without interference to avoid decoherence. So the state of the wave function when they are next to each other is the same as when they are far apart, no matter how much time and space has occured. and it follows logically that a hidden QM machine can simulate the collapse of that exact entangled wave function at time of entanglement, bring that result with each particle so that they collapse according to that agreement when the time comes. so in this set up an experimenter would never be able to know whether the result was determined by a QM simulation at entanglement or at measurement via spooky action at a distance since the wave function, and thus the result, is identical in both.

 

[Dale]

I am not sure what I am proposing can be said to be hidden variables. So it might be exempt from bell.

Everything that you have written thus far appears to be a completely standard hidden variable.

Can you clarify if I am correct that the wave function of an entangled particle pair does not change as you move them apart in spacetime?

You are incorrect. The wave function absolutely does change. That is what Schrodinger’s equation describes: the way that any wavefunction changes over time.

a hidden QM machine can simulate the collapse of that exact entangled wave function at time of entanglement, bring that result with each particle so that they collapse according to that result when the time comes.

This is exactly the sort of hidden information channel that Bell envisioned.

Note that there is some additional constraint here that you may not be considering. The measurement to be performed can be selected after the entangled state is prepared. So whatever information is stored by the simulation cannot depend on which of all possible measurements will be done. It must be a generic piece of information that will correctly produce the results of any possible measurement.

so in this set up an experimenter would never be able to know whether the result was determined by a QM simulation at entanglement or at measurement via spooky action at a distance.

Again, this is false. There is an unavoidable experimental signature of any local hidden variable.

 

[lukephysics]

information.

perfect. thanks for explaining. i will think about these things some more.

 

[Nugatory]

i will think about these things some more.

We have many older threads here, and this Scientific American article and the web site maintained by our own @DrChinese are also good resources.

 

[lukephysics]

i am not sure a wave function changes if a particle is isolated from its environment, maybe after 10 to the billion years? photons don't feel time. anyway don't worry i am going to go away and finish reading some of those articles.

 

[StevieTNZ]

Two good books on quantum entanglement:
1. https://www.bookdepository.com/Quantum-Chance-Nicolas-Gisin/9783319054728
[https://www.springer.com/gp/about-springer/media/press-releases/corporate/quantum-chance/32820]

2. https://www.bookdepository.com/Quantum-Entanglement-Jed-Brody/9780262538442
[https://mitpress.mit.edu/books/quantum-entanglement]