What is the truth behind quantum entanglement?

[Rodrigo Cesar]

http://www.engadget.com/2015/03/30/spooky-experiment-proves-quantum-entanglement-is-real/

what is the best way to understand entanglement ? I just want to know your opinions on the subject, because I'm a bit confused

There are people who say that "must be some another dimension in which information travel from one to other particle" and others who say this proves that "Distance is an illusion" lol ? There's even a video about it

 

[bhobba]

There are people who say that "must be some another dimension in which information travel from one to other particle" and others who say this proves that "Distance is an illusion" lol ? There's even a video about it

Its nothing like that.

Entanglement is simply what separates standard probability theory from QM:
http://arxiv.org/pdf/0911.0695v1.pdf

All this stuff like EPR etc is, is a different type of correlation than you find in standard probability theory.

Thanks
Bill

 

[Rodrigo Cesar]

thanks bhobba, you always helping!

 

[craigi]

http://www.engadget.com/2015/03/30/spooky-experiment-proves-quantum-entanglement-is-real/

what is the best way to understand entanglement ? I just want to know your opinions on the subject, because I'm a bit confused

There are people who say that "must be some another dimension in which information travel from one to other particle" and others who say this proves that "Distance is an illusion" lol ? There's even a video about it

The truth is, we don't fully understand entanglement yet. The general gist of what you're talking about isn't all that crazy.

You might be interested in this:

 

[bhobba]

The truth is, we don't fully understand entanglement yet.

We do as, for example, the link I gave explains.

Its just that it implies a different type of correlation as per Bells Theorem that is counter intuitive to everyday experience.

Thanks
Bill

 

[Rodrigo Cesar]

The general gist of what you're talking about isn't all that crazy.

calling distance an illusion because of the experiment that maybe we don't know yet, yes It's 100% CRAZY.

 

[craigi]

calling distance an illusion because of the experiment that maybe we don't know yet, yes It's 100% CRAZY.

This stuff is really more Beyond the Standard Model, or Quantum Foundations and to some extent General Relativity than Quantum Physics, but one thing we know for certain is that space and time are not as they seem intuitively. You can call it crazy, but it is supported by experiment and by our best theories.

 

[Rodrigo Cesar]

I'm calling crazy the video saying "distance is an illusion" (whatever that means), which has nothing to do with entanglement. I'm not calling entanglement 'crazy', which is just a tool of the universe that we don't know how it happens, yet. that's all

 

[craigi]

I'm calling crazy the video saying "distance is an illusion" (whatever that means), which has nothing to do with entanglement. I'm not calling entanglement 'crazy', which is just a tool of the universe that we don't know how it happens, yet. that's all

Is it this?

If so, it's totally fine. It's just many of the most notorious physicists discussing the Bohr-Einstein debates and the Bell Tests. It's all well accepted mainstream quantum mechanics.

 

[Strilanc]

I'd be interested in knowing if you found the post Entangled States are like Unitary Matrices useful or not. It covers how operations on maximally-entangled two-party states act isomorphic to a unitary matrix w.r.t. how they get operated on by either side independently.

 

[Rodrigo Cesar]

I think

Is it this?

If so, it's totally fine. It's just many of the most notorious physicists discussing the Bohr-Einstein debates and the Bell Tests. It's all well accepted mainstream quantum mechanics.

Totally fine? So answer me. what does means "Distance is an illusion"? It makes no sense. If it's something that no one understands how it works. why someone make claims without evidence? btw the guy who created this video, ins't a physicist. he is a new age follower.
As bhobba said, we do understand a lot of the experiment , It just that is counter intuitive to everyday experience.

 

[bhobba]

If so, it's totally fine.

Its a bit better than many popularisations but totally fine - no I wouldn't say that. For example they talk in slogans like the world isn't there unless you look - which isn't what QM says - its actually silent on it if you don't observe.

Always be wary of popular accounts - take what they say with a grain of salt - come here for what's really happening - but your thinking cap will need to be on because easy this stuff aren't - but it has to be said understandable with effort.

Thanks
Bill

 

[zonde]

I think

Totally fine? So answer me. what does means "Distance is an illusion"? It makes no sense. If it's something that no one understands how it works. why someone make claims without evidence? btw the guy who created this video, ins't a physicist. he is a new age follower.
As bhobba said, we do understand a lot of the experiment , It just that is counter intuitive to everyday experience.

Conclusions of that video are not correct. First of all in science nothing is proved - only verified or falsified. And second even now experiments have not yet falsified local realism.

 

[Ahmad Khanehzar]

In order to understand the meaning of entanglement, let me give you an example: "We have 6 marbles then we put them in two boxes. So give one box to Bob and the other to Alice, Alice goes to New York and Bob goes to Paris. They don't know how many marbles there are in their boxes. they open their own boxes when they arrive to their destinations. Then Bob opens his box and looks at 4 marbles so he understands instantly there are 2 marbles in Alice's box". However it can not be a process as a traveling information, so we can't define the" velocity" quantity for this knowing. But it's just a classical example and difference phenomenon exactly happens in quantum mechanics.

 

[craigi]

In order to understand the meaning of entanglement, let me give you an example: "We have 6 marbles then we put them in two boxes. So give one box to Bob and the other to Alice, Alice goes to New York and Bob goes to Paris. They don't know how many marbles there are in their boxes. they open their own boxes when they arrive to their destinations. Then Bob opens his box and looks at 4 marbles so he understands instantly there are 2 marbles in Alice's box". However it can not be a process as a traveling information, so we can't define the" velocity" quantity for this knowing. But it's just a classical example and difference phenomenon exactly happens in quantum mechanics.

That's fine from a QM perspective. The picture in QFT is more complex, but it becomes really interesting when we start to look for a quantum theory which is compatible with General Relativity, where it should be no surprise that space and time take on a counterintuitive form.

 

[Ahmad Khanehzar]

That's fine from a QM perspective. The picture in QFT is more complex, but it becomes really interesting when we start to look for a quantum theory which is compatible with General Relativity, where it should be no surprise that space and time take on a counterintuitive form.

yeah, It's more complex in QFT but more interesting.

 

[georgir]

In order to understand the meaning of entanglement, let me give you an example: "We have 6 marbles then we put them in two boxes. So give one box to Bob and the other to Alice, Alice goes to New York and Bob goes to Paris. They don't know how many marbles there are in their boxes. they open their own boxes when they arrive to their destinations. Then Bob opens his box and looks at 4 marbles so he understands instantly there are 2 marbles in Alice's box". However it can not be a process as a traveling information, so we can't define the" velocity" quantity for this knowing. But it's just a classical example and difference phenomenon exactly happens in quantum mechanics.

This is a bad example. It only shows a situation that is easily explainable with hidden variables and does not violate Bell's inequality.

Here is the example that I would give:
Imagine you have three doors, and behind each you have a hidden binary value. If someone told you that no matter which two you open, they will show opposite values to each other, you'll say this does not make sense.
Well, QM tells you something like that. It does not go as far as to say they will be 100% opposite, but it can say that the probability for different values is 75% for example, and this is still just as insane. Any classical or "hidden variable" explanations that do not involve a value behind some door changing because of you opening another door can only get to 66%.
It's a bit hard to think in probabilities for such simplified example, so you have to extend it a bit. Instead of just one set of three doors, you can have 100 or 1000 sets each of 3 doors.
QM additionally muddles the water by saying you can never open the third door in a set after having opened the first two.

 

[Jilang]

Wouldn't it be that having opened the first two doors you will find nothing behind the third?

 

[georgir]

Wouldn't it be that having opened the first two doors you will find nothing behind the third?

Not really. The three doors represent three possible angles of measuring polarization, for example. Opening two represents measuring the polarization on two different angles on each of two entangled particles. You can not have a set of three entangled particles, so you just can not "open three doors".
[EDIT: I think you can have a set of three or even more entangled particles, but only with other kinds of entanglement than assumed here - i.e. with a known sum of spins, instead of the equal spins we need here.]

On one particle you can only measure one angle and then its state has been changed. Measuring more than once on the same particle does not count as opening a door because its result is after the changes caused by the first measurement, and is thus irrelevant.

 

[Rajkovic]

I think the OP question was if entanglement proves that "distance is an illusion", .. this question makes no sense, even "distance is an illusion" makes no sense, could someone explain to me?

 

[georgir]

well, entanglement is a sort of non-local interaction. its a far cry from saying distance is an illusion, but i hope you can see how it at least points in that general direction :p

 

[Jilang]

From a third party point of view there is clear space between the entangled particles. The book-keeping will demand that they are space-separated on measurement (think consevation of momentum). Without measurement though, I wonder how it would appear from the particle's point of view.?

 

[bhobba]

From a third party point of view there is clear space between the entangled particles.

Why do you think it must be viewed as particles with a definite position separated by 'clear space'?

Thanks
Bill

 

[Jilang]

Because when they are measured they are found to be space separated and the later they are measured the farther part they are found to be. Is that the wrong way to think about them?

 

[bhobba]

Because when they are measured they are found to be space separated and the later they are measured the farther part they are found to be. Is that the wrong way to think about them?

Yes.

You are falling into the trap of ascribing properties when not observed.

Of course the first position measurement will break entanglement.

Thanks
Bill

 

[craigi]

From a third party point of view there is clear space between the entangled particles. The book-keeping will demand that they are space-separated on measurement (think consevation of momentum). Without measurement though, I wonder how it would appear from the particle's point of view.?

In QM particles aren't complex enough to make observations alone (no decoherence), but in SR we can consider an observer in the rest frame of a particle. Such an observer sees separation too, in fact, in all rest frames, observers see separation. Does this answer your question?

 

[zonde]

I think the OP question was if entanglement proves that "distance is an illusion", .. this question makes no sense, even "distance is an illusion" makes no sense, could someone explain to me?

Yes, I can. "distance is an illusion" is crappy philosophy that has nothing to do with physics.

 

[craigi]

Yes, I can. "distance is an illusion" is crappy philosophy that has nothing to do with physics.

The only contexts in which "distance is an illusion" makes sense in physics (to my knowledge at least) are, Special Relativity, General Relativity and the EPR=ER paper. Only the latter pertains to quantum entanglement.

I'd urge everyone to stop quoting it without context, but if you are actually arguing that the EPR=ER paper is wrong, then either you don't understand it or your contribution would be gratefully accepted by journals.

 

[Jilang]

In QM particles aren't complex enough to make observations alone (no decoherence), but in SR we can consider an observer in the rest frame of a particle. Such an observer sees separation too, in fact, in all rest frames, observers see separation. Does this answer your question?

Thank it helps.

 

[zonde]

The only contexts in which "distance is an illusion" makes sense in physics (to my knowledge at least) are, Special Relativity, General Relativity and the EPR=ER paper. Only the latter pertains to quantum entanglement.

I'd urge everyone to stop quoting it without context, but if you are actually arguing that the EPR=ER paper is wrong, then either you don't understand it or your contribution would be gratefully accepted by journals.

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?

 

[craigi]

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.

 

[my2cts]

The truth is, we don't fully understand entanglement yet.

More accurately, we don't fully understand quantum mechanics yet.

 

[Ahmad Khanehzar]

I would suggest you to read-
http://bookfi.org/book/547546
http://bookfi.org/g/George%20Greenstein , http://bookfi.org/g/%20Arthur%20G.%20Zajonc

 

[my2cts]

Why, do these authors understand quantum mechanics?

 

[Ahmad Khanehzar]

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.

 

[bhobba]

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.

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

 

[zonde]

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.

 

[my2cts]

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.

 

[my2cts]

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.

 

[bhobba]

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

 

[bhobba]

No one understands normalisation.

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

 

[Swamp Thing]

Is there anything really new about the work referred to in the report that the op has linked to:

http://www.engadget.com/2015/03/30/spooky-experiment-proves-quantum-entanglement-is-real/

I thought entanglement has been experimentally demonstrated at least since the 70s?

:

 

[bhobba]

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

 

[my2cts]

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. "

 

[my2cts]

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.

 

[bhobba]

The fact that the volume integral of the square of the wave function has to be unity does not follow from any equation.

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

 

[bhobba]

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

 

[my2cts]

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.

 

[bhobba]

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

 

[TrickyDicky]

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)?