B Why isn’t quantum entanglement just correlation?

[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]

 

[Nugatory]

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.

Yes the wave function changes over time, always. It is by definition a solution of Schrodinger’s equation, and all solutions of that equation must change over time.

It is true that “photons don’t feel time”, but only in the trivial sense that they aren’t thinking feeling conscious beings so they don’t feel anything, just like a rock doesn’t feel time or anything else either. The state of a photon changes over time just like everything else.

 

[Dale]

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.

I don’t know how much math you know but the left hand side of $$i \hbar \frac{\partial}{\partial t} \Psi =\left( -\frac{\hbar^2}{2m} \frac{\partial^2}{\partial x^2}+V \right) \Psi $$ literally means “$i\hbar$ times the rate of change of the wavefunction”

 

[bhobba]

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.

An isolated wavefunction of a single particle spreads with time:
https://en.wikipedia.org/wiki/Wave_packet

For entangled particles, the whole idea of wave-particle duality makes no sense. This is one of the issues with beginning books/popularisations in QM. The beginning books talk about wave-particle duality, but when you move onto more advanced material, they do not point out wave-particle duality is only correct in exceptional circumstances, i.e. a single isolated particle. The following may help in that it explains the double-slit experiment without using wave-particle duality:
https://arxiv.org/pdf/quant-ph/0703126.pdf

Basically, it is a good idea to forget about the whole wave-particle duality idea.

If you have done a bit of calculus at the high school level, THE book to get on QM is by Susskind. It is designed specifically for people like you that want to go beyond popularisations and study the real deal, yet do not feel ready to tackle a more advanced textbook:
https://www.amazon.com.au/dp/0465062903/

When finished, you will have an understanding way beyond popularisations or even HS textbooks. You need a bit of calculus. If you do not know any calculus, rest assured it is not hard - the following will suffice:
https://www.mathsisfun.com/calculus/introduction.html

Thanks
Bill

 

[lukephysics]

An isolated wavefunction of a single particle spreads with time:

how does the wave function change in momentum space, which is what matters when measuring spin. does it change over time when entangled in isolation? i know its at least conserved (conservation of momentum)?

 

[bhobba]

It starts with an initial quite accurate (although not 100% accurate) position like a spike (technically a Dirac delta function) then spreads in position space. A momentum measurement will give a value as per the Heisenberg Uncertainty principle, which, since the position is known fairly accurately, will give a range of values. The spreading intuitively can be viewed as the larger momentum 'components' racing ahead of, the slower ones. That is just intuitively:
https://web.phys.ksu.edu/vqm/vqmnextgen/qmbasics/wavepackets.pdf

When entangled, you can't view it in a two-dimensional graph like the article I linked to. Wave-particle duality has run out of puff in that situation. You need a more powerful formalism - the formalism of Hilbert-Spaces (or for those really advanced Rigged Hilbert Spaces).

Thanks
Bill

 

[PeroK]

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.

Bell's theorem has nothing directly to do with QM. It's an elementary (albeit very clever) result about correlations in classical probability theory.

You haven't even taken the time to understand Bell's theorem before you've dismissed it. It really is nothing more than elementary probability theory.

I suspect you don't understand what correlations mean in the context of spin or polarization measurements.

Some people come here to learn about maths, science and QM and some people come here to argue and reinforce their ignorance. It's your choice what you want to get out of this site.

 

[RUTA]

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?

As others have told you, the same-setting measurements can be explained in this way, so they do not provide any real mystery per se. The mystery resides in the combination of same-setting measurements and different-setting measurements. Here is Mermin's famous explanation of the mystery: https://www.jstor.org/stable/2026482?seq=1#metadata_info_tab_contents
or
http://hep.ucsb.edu/courses/ph125_02/mermin.pdf

 

[lukephysics]

Ok I have studied Ruta’s links and the answer I was looking for was hinted by Dale earlier, that the particle doesn’t know at the time it’s entangled what settings both measuring devices are going to use. This is the real difference that prevents hidden variables.

mermins paper describes it without having to trust anything to do with polarity et all its simple logic! thanks all it really solved my problems.

 

[lukephysics]

in the bell tests there are detectors at either end, and we are getting a clustered dot pattern. doesn't a dot pattern mean we have measured the position of the partcle? and if so, due to complimentarity wouldn't we not be able to measure or know the spin? how does the experiment still work if it appears we know the spin and we know the position as well?

 

[hutchphd]

Please look up commuting operators.
It is very good to be curious. But depth of thought requires depth of knowledge and you need to answer some shallower questions first.

 

[lukephysics]

Please look up commuting operators.

it took me 5 years of reading crappy explanations of bells test, and 5 minutes of reading the right explanation. I assume there are simple good explanations of all of these concepts without spending semesters learning how to calculate the maths. Maths is not explanative its functional.

 

[StevieTNZ]

it took me 5 years of reading crappy explanations of bells test, and 5 minutes of reading the right explanation. I assume there are simple good explanations of all of these concepts without spending semesters learning how to calculate the maths. Maths is not explanative its functional.

Have you looked at the books I suggested here - https://www.physicsforums.com/threa...glement-just-correlation.1007029/post-6540289 ?

 

[lukephysics]

Have you looked at the books I suggested here - https://www.physicsforums.com/threa...glement-just-correlation.1007029/post-6540289 ?

sure thanks i haven't read them yet though. you can probably close this thread i have enough to go on.

really suspicious of multiverse explanations. want to get to the point where i can come up with my own interpretations.

 

[hutchphd]

it took me 5 years of reading crappy explanations of bells test, and 5 minutes of reading the right explanation.

As you search for another explanation from some authority I will leave you with R P Feynman's definition of science as "the belief in the ignorance of experts". Take the time to learn stuff.

 

[lukephysics]

As you search for another explanation from some authority I will leave you with R P Feynman's definition of science as "the belief in the ignorance of experts". Take the time to learn stuff.

dood that's exactly why i search for answers from first principles, not trust the guys on here who say "just trust us dummy". they get angry when i ignore the authority. and even give me a warning lol.

or guys who say "trust this function right here". needs a foundation.

 

[Dale]

doesnt a dot pattern mean we have measured the position of the partcle? and if so, due to complimentarity wouldn't we not be able to measure or know the spin? how does the experiment still work if it appears we know the spin and we know the position as well?

Yes. The uncertainty principle is about the uncertainty in the states, not the uncertainty in the measurements. So even if you measure both sides of a complementary pair precisely, any state you can prepare will have at most one be precise.

 

[Dale]

dood that's exactly why i search for answers from first principles, not trust the guys on here who say "just trust us dummy". they get angry when i ignore the authority. and even give me a warning lol.

or guys who say "trust this function right here". needs a foundation.

That is a pretty big mischaracterization of what actually happened here. You were given a first principles answer by post 3. You nevertheless continued to repeatedly make a false assertion. Even then, you were given several well-reasoned explanations why your continued assertion was wrong.

Some participants then did begin to become impatient and snippy. But to assert that you are looking for first principles explanations and that you were instead told “just trust us dummy” is simply not what happened.

With that I think we will close this thread before it degenerates further. You are certainly welcome to post new threads as new questions come up.