What is the truth behind quantum entanglement?

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Quantum entanglement remains a complex and not fully understood phenomenon, often leading to misconceptions about its implications, such as the idea that "distance is an illusion." Some discussions suggest that entanglement represents a different type of correlation that defies classical probability theories, as highlighted by Bell's Theorem. While entangled particles exhibit non-local interactions, this does not imply that physical distance is meaningless; rather, it challenges our intuitive understanding of space and time. Misinterpretations often arise from popular science accounts that oversimplify or misrepresent quantum mechanics. Overall, entanglement is a fundamental aspect of quantum physics that continues to intrigue and puzzle scientists.
  • #31
zonde said:
Well, this is thread about entanglement in quantum physics subforum. This gives context enough, no?
And what is EPR=ER paper? Is it EPR paradox paper?

It's much more recent than the EPR paradox. Try the lecture which I posted at the start of the thread.
 
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  • #32
craigi said:
The truth is, we don't fully understand entanglement yet.
More accurately, we don't fully understand quantum mechanics yet.
 
  • #33
I would suggest you to read-
http://bookfi.org/book/547546
http://bookfi.org/g/George%20Greenstein , http://bookfi.org/g/%20Arthur%20G.%20Zajonc
 
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  • #34
Why, do these authors understand quantum mechanics?
 
  • #35
my2cts said:
Why, do these authors understand quantum mechanics?
No one could understand quantum mechanics.
It's an engaging and thorough treatment of the extraordinary phenomena of quantum mechanics and of the enormous challenge they present to our conception of the physical world. Traditionally, the thrill of grappling with such issues is reserved for practicing scientists, while physical science, mathematics, and engineering students are often isolated from these inspiring questions. This book was written to remove this isolation.
 
  • #36
my2cts said:
More accurately, we don't fully understand quantum mechanics yet.

I think we do:
http://arxiv.org/pdf/quant-ph/0101012.pdf

The issue is if its the kind of understanding you are happy with.

That varies from person to person and why we end up with threads like this.

Ahmad Khanehzar said:
No one could understand quantum mechanics

Again that depends on what you mean by 'understand' which is the kind of thing philosophers argue about and get nowhere. Its easiest to simply take the dictionary definition: 'interpret or view (something) in a particular way'.

We have met the enemy and he is us - Pogo.

Instead of getting caught up in this we don't understand QM stuff - understand the problem lies in you - not QM. There are plenty of ways of viewing QM that 'makes sense'. Choose one, examine how it compares to others which is an interesting thing to do, maybe change the one you choose from that, but don't get stuck in this we don't understand QM thing - because we clearly do.

The bottom line issue with QM, the rock bottom problem, if it is in fact a problem, is we have all these different interpretations. It doesn't matter what issue particularly worries you there is an interpretation that fixes it. What we don't have is a way to decide between them experimentally.

Thanks
Bill
 
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  • #37
craigi said:
It's much more recent than the EPR paradox. Try the lecture which I posted at the start of the thread.
I tried. In this lecture Susskind refers to some paper so I assume it makes no sense to view this lecture if you don't know the paper.
But I watched anyways and got to the point where two people from two non-interacting worlds are jumping into the black hole and meeting in the middle. And I do not understand what I was supposed to take from this lecture.
 
  • #38
This book probably can help a lot of people, but I have been practicing science long enough to know that I fundamentally do not understand QM.
 
  • #39
bhobba said:
I think we do:
http://arxiv.org/pdf/quant-ph/0101012.pdf

The issue is if its the kind of understanding you are happy with.

That varies from person to person and why we end up with threads like this.
Again that depends on what you mean by 'understand' which is the kind of thing philosophers argue about and get nowhere. Its easiest to simply take the dictionary definition: 'interpret or view (something) in a particular way'.

We have met the enemy and he is us - Pogo.

Instead of getting caught up in this we don't understand QM stuff - understand the problem lies in you - not QM. There are plenty of ways of viewing QM that 'makes sense'. Choose one, examine how it compares to others which is an interesting thing to do, maybe change the one you choose from that, but don't get stuck in this we don't understand QM thing - because we clearly do.

The bottom line issue with QM, the rock bottom problem, if it is in fact a problem, is we have all these different interpretations. It doesn't matter what issue particularly worries you there is an interpretation that fixes it. What we don't have is a way to decide between them experimentally.

Thanks
Bill
I could not disagree more ;-).
Pauli did not understand the Pauli principle and was unhappy about that. I side with him.
No one understands quantum interference. No one understands normalisation.
I happily use QM without understanding it, but I don't kid myself into believing that there is nothing more to understand.
 
  • #40
my2cts said:
No one understands quantum interference.
Speak for yourself:
https://www.physicsforums.com/threads/entanglement-question.811193/page-2#post-5104952

Precisely what don't you understand in the above?

Remember we are speaking about UNDERSTANDING - not what you find weird or counter intuitive. Nor are we speaking about quotes by famous physicists along those lines which by and large are taken out of context.

Thanks
Bill
 
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  • #43
Swamp Thing said:
I thought entanglement has been experimentally demonstrated at least since the 70s?

Entanglement is part of the very foundations of QM, following pretty much from the principle of superposition.

It has been known about and verified from the early days of QM.

What happened in the 70's is experimental verification of the consequences of Bells theorem which is a particular type of entanglement.

Despite the carry on in this thread that it's mysterious, needs explanation yada, yada, yada - what's going on is well understood.

I think I mentioned it earlier in the thread but all that's really happening is entanglement allows a different type of correlation than classical probability theory does.

Thanks
Bill
 
  • #44
bhobba said:
Speak for yourself:
https://www.physicsforums.com/threads/entanglement-question.811193/page-2#post-5104952

Precisely what don't you understand in the above?

Remember we are speaking about UNDERSTANDING - not what you find weird or counter intuitive. Nor are we speaking about quotes by famous physicists along those lines which by and large are taken out of context.

Thanks
Bill
Some notes on good debating practice:
Do not put the words ("weird", "counterintuitive") in my mouth.
Your statement that I take statements of famous physicists out of context is without ground, see below.
Refrain from excessive use of capitals.
About QM, I understand it in the sense that I can use it and I understand its logic.
If, as it appears, you claim full understanding of QM, that would require you to understand the Pauli principle better than Pauli himself.
I do not believe that you do.
Please check this statement in Pauli's Nobel prize lecture of 1945:
"Already in my original paper I stressed the circumstance that I was unable to give a logical reason for the exclusion principle or to deduce it from more general assumptions. I had always the feeling and I still have it today, that this is a deficiency. "
 
  • #45
bhobba said:
Do you mean renormalisation?

That no one understands it is a myth - it was sorted out by Wilson who got a Nobel prize for it.

It's actually very straightforward:
https://www.physicsforums.com/insights/renormalisation-made-easy/

Thanks
Bill
No I did not mean renormalisation. I mean normalisation.
The fact that the volume integral of the square of the wave function has to be unity does not follow from any equation.
Anyway, your link explains _how_ to renormalize. Similarly, Pauli knew _how_ to antisymmetrize wave functions.
Her considered that an incomplete understanding and I subscribe to that.
 
  • #47
my2cts said:
If, as it appears, you claim full understanding of QM, that would require you to understand the Pauli principle better than Pauli himself.

Precisely what about the spin statistics theorem don't you believe is understood?

I don't mean a feeling Pauli had - but an actual reason.

BTW things have moved on a lot since Pauli wrote that. We now have watertight proofs:
http://www.worldscientific.com/doi/abs/10.1142/9789812817037_0018

Thanks
Bill
 
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  • #48
bhobba said:
Its follows from the Born rule which is a foundational axiom of QM.

Although one can base it on Gleason - see post 137:
https://www.physicsforums.com/threads/the-born-rule-in-many-worlds.763139/page-7

Thanks
Bill
Thus an essential property of the solutions of the Schrödinger equation, which itself follows from a postulate,
does not follow from the equation, but has to be postulated separately.
That is formalisation not understanding.
 
  • #49
my2cts said:
Thus an essential property of the solutions of the Schrödinger equation, which itself follows from a postulate,
does not follow from the equation,

The essential property follows from the definition of a wave-function being the expansion of the state in terms of position eigenstates - it has nothing to do with Scrodingers equation.

|u> = ∑ |xi><xi|u> - by definition <xi|u> is the wave-function. From Borns rule |<u|xi>|^2 is the probability of the particle being in position xi if it was observed:
http://www.math.ru.nl/~landsman/Born.pdf

Hence, because probabilities sum to 1, ∑|<u|xi>|^2 = ∑ <u|xi><xi|u> = <u|u> = 1. It also follows from the fact Born's rule applies to vectors of unit length for pure states.

Note - for simplicity and not to get bogged down with technicalities like the Dirac Delta function I have approximated a position measurement by a large number of discreet outcomes such as if it was displayed on a digital readout.

Regarding Schroedinger's equation, it actually follows from Born as well. It turns out that the probabilities, and the rather obvious fact they shouldn't depend on your frame of reference, determine Schroedinger's equation - you can find the detail in Chapter 3 - Ballentine - Quantum Mechanics - A Modern Development.

Thanks
Bill
 
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  • #50
bhobba said:
The bottom line issue with QM, the rock bottom problem, if it is in fact a problem, is we have all these different interpretations. It doesn't matter what issue particularly worries you there is an interpretation that fixes it.
What interpretations fix the preferred basis issue (definite outcomes I think you call it)?
 
  • #51
TrickyDicky said:
What interpretations fix the preferred basis issue (definite outcomes I think you call it)?

Why do you think its interpretation dependant?

But aside from that it might be a good idea not to derail this thread with a side issue.

Thanks
Bill
 
  • #52
bhobba said:
Why do you think its interpretation dependant?

But aside from that it might be a good idea not to derail this thread with a side issue.

Thanks
Bill
You said:"It doesn't matter what issue particularly worries you there is an interpretation that fixes it." I just gave an example of an issue, and you just have to say what interpretation fixes it. Otherwise it means your assertion was false.
 
  • #53
zonde said:
I tried. In this lecture Susskind refers to some paper so I assume it makes no sense to view this lecture if you don't know the paper.
But I watched anyways and got to the point where two people from two non-interacting worlds are jumping into the black hole and meeting in the middle. And I do not understand what I was supposed to take from this lecture.

It's worth following it to the end, even if you don't fully understand it. The conjecture at the end, is that entangled particles are connected by Planck scale wormholes.

In Quantum Gravity space and time are much more complicated than in Quantum Mechanics.

I'm not directing this at you. Your comment was just a recent comment along the relevant vein of discussion, but more generally, the point I'm making is that we shouldn't dismiss something which we do not understand as crazy, cranky, crack-pottery etc. We've all been there. When we know a little bit about a subject, we presume to know more than we do. It might seem like it makes us appear more knowlegable or more scientific, but that's not how science works and it's not a good example to set others.
 
  • #54
TrickyDicky said:
\Otherwise it means your assertion was false.

Decocherence is interpretation independent and it explains the preferred basis part of the measurement problem. It doesn't explain all the other parts - but it does that one.

Thanks
Bill
 
  • #55
bhobba said:
The essential property follows from the definition of a wave-function being the expansion of the state in terms of position eigenstates - it has nothing to do with Scrodingers equation.
Staye of what? Eigenstates of what? The reason is that you discuss the wave function without referring to the equation that it is the solution of (!),
is that you tacitly presuppose the QM formalism. My point is that we do understand the formalism, but not QM.
 
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  • #56
my2cts said:
Staye of what? Eigenstates of what? The reason is that you discuss the wave function without referring to the equation that it is the solution of (!), is that you tacitly presuppose the QM formalism. My point is that we do understand the formalism, but not QM.

It's as I said - position eigenstates. Why didn't you note the key word I said before eigenstate?

The problem is you have got the cart before the horse. What a wave-function is has nothing to do with Schroedinger's equation - its got to do with the foundational axioms of QM which are (as per Ballentine):

1. To every observation there exists a Hermitian operator O such that the possible outcomes are the eigenvalues of the observation.

2. There exists a positive operator P, of unit trace, called the state of the system, such that the expected outcome of an observation O, E(O), is E(O) = Trace (PO). This is called the Born Rule.

Axiom 2 is to some extent determined by axiom 1 via Gleason's theorem (you have to add non contextuality to it).

States of the form P= |u><u| are called pure and because Trace (|u><u|) = 1, <u|u> = 1, hence they are of unit length, which is the normalisation condition you were talking about. It follows directly from the Born Rule.

Consider the eigenvectors |xi> of the position operator X. These, by definition, are position eigenstates. Then, by the properties of Hermitian operators (ie its eigenvectors form an orthonormal basis) we have |u> = Σ |xi>< xi|u> with what I said in the previous post following.

That the wave-function is normalised to 1 and |< xi|u>|^2 is the probability of getting position xi follows directly from the axioms of QM. The Schroedinger equation is not involved.

Where the Schroedinger equation comes from is that the probabilities defined by the Born Rule should not depend on the frame.

I think you are a bit confused about understanding in science. In science you understand something when its explained by known premises - which is exactly what we have in QM. The two axioms I gave are the known premises from which we explain QM phenomena.

The issue here is they are not premises in the mould of the usual ones in physics. That's why people say no one understands QM. But what they really mean is no one understands it in terms of everyday intuitive pictures. That is different from understanding - and why I say - We have met the enemy and he is us.

Thanks
Bill
 
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  • #57
bhobba said:
Decocherence is interpretation independent and it explains the preferred basis part of the measurement problem. It doesn't explain all the other parts - but it does that one.

Thanks
Bill
In the interest of not confusing people you should add that there is no agreement in the quantum foundations community about decoherence explaining preferred basis and that basically only proponents of the Everett approach like Zurek claim that decoherence solves the issue.
 
  • #58
TrickyDicky said:
In the interest of not confusing people you should add that there is no agreement in the quantum foundations community about decoherence explaining preferred basis and that basically only proponents of the Everett approach like Zurek claim that decoherence solves the issue.

Where did you get that from? It wasn't from my text on the matter:
https://www.amazon.com/dp/3540357734/?tag=pfamazon01-20

See page 83 to 85.

Thanks
Bill
 
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  • #59
craigi said:
It's worth following it to the end, even if you don't fully understand it. The conjecture at the end, is that entangled particles are connected by Planck scale wormholes.
Reasoning along the lines of black holes, wormholes, parallel worlds and such is armchair science in n-th iteration. You have to have quite specific beliefs to consider it interesting.
You have to believe that mathematics is not just very useful baseline for describing real world but that mathematics is some sort of super reality that determines how reality is "functioning". And this is without considering poor assumptions and erroneous applications of math.
Well, I don't have these beliefs so I don't consider that kind of reasoning worth following.

craigi said:
I'm not directing this at you. Your comment was just a recent comment along the relevant vein of discussion, but more generally, the point I'm making is that we shouldn't dismiss something which we do not understand as crazy, cranky, crack-pottery etc. We've all been there. When we know a little bit about a subject, we presume to know more than we do. It might seem like it makes us appear more knowlegable or more scientific, but that's not how science works and it's not a good example to set others.
The problem is not in understanding of the idea but it's relevance to reality.
 
  • #60
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