# Entanglement

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Entanglement:

1) How do physicists physically separate two particles?

2) When we read that when one particle is observed it will instantly mirror its partner on opposite ends of the Earth or the universe, how would they transsport that particle to that far distant location?

I would appreciate a reply in more layman's terms.

Thanks.

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Entanglement:

1) How do physicists physically separate two particles?

2) When we read that when one particle is observed it will instantly mirror its partner on opposite ends of the Earth or the universe, how would they transsport that particle to that far distant location?

1) 'Pulling them' using different techniques. E.g. if the particles are charged you can use magnetic fields. Using magnetic fields is as you move particles at the LHC.

2) Nobody is transporting a particle at the end of the universe, whatever what you mean by "end". Transporting material to the opposite part of the Earth is not so difficult, but I suspect that you refer to some statement in some book where the author is really trying to explain that entanglement is independent of the distance.

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2) The experiment war really done. By Alain Aspect, at Orsay University, less than three decades ago.

The observation is that not only do the states of both particles match: they do it so quickly that even light would not have enough time to propagate from one detector to the other to tell the other detector what it shall do.

This was a serious interrogation (Einstein, Podolsky and Rosen called it a "paradox" then), because Relativity supposes that no information can propagate faster than light does. If we could exchange information without delay, compare clocks and rulers instantly, then Relativity couldn't claim "A sees B's clock go slower, and B sees A's clock go slower as well, and this is normal". This situation would become imposible, and Relativity would collapse.

The (most common) interpretation of Aspect's observations (repeated many times meanwhile) is that the entangled pair of particles "decides" in which state it is. The rest of the world, and notably the observers, can't decide the state of a detected particle. So entangled particles are not a means to transmit information, hence their too fast behaviour doesn't ruin Relativity.

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1) 'Pulling them' using different techniques. E.g. if the particles are charged you can use magnetic fields. Using magnetic fields is as you move particles at the LHC.

2) Nobody is transporting a particle at the end of the universe, whatever what you mean by "end". Transporting material to the opposite part of the Earth is not so difficult, but I suspect that you refer to some statement in some book where the author is really trying to explain that entanglement is independent of the distance.

How would they transport material to the opposite part of the Earth?

Yes. I've read that entanglement is independent of distance. Is this a fact or a theory?

2) The experiment war really done. By Alain Aspect, at Orsay University, less than three decades ago.

The observation is that not only do the states of both particles match: they do it so quickly that even light would not have enough time to propagate from one detector to the other to tell the other detector what it shall do.

This was a serious interrogation (Einstein, Podolsky and Rosen called it a "paradox" then), because Relativity supposes that no information can propagate faster than light does. If we could exchange information without delay, compare clocks and rulers instantly, then Relativity couldn't claim "A sees B's clock go slower, and B sees A's clock go slower as well, and this is normal". This situation would become imposible, and Relativity would collapse.

The (most common) interpretation of Aspect's observations (repeated many times meanwhile) is that the entangled pair of particles "decides" in which state it is. The rest of the world, and notably the observers, can't decide the state of a detected particle. So entangled particles are not a means to transmit information, hence their too fast behaviour doesn't ruin Relativity.

So, entanglement and its purpose remain a mystery—yes?

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What do you call a mystery?

People debated whether who had interpreted it properly, someone made an experiment that was conclusive, case closed.

We're talking about basic phenomena here, which probably won't be "explained" through something else like the datestamp by the Post office or the age of the tree. At that level, we can check if our description fits what happens, or which description is best.

And: a "purpose"? Does our world have a purpose? This isn't the scope of successful science.

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How would they transport material to the opposite part of the Earth?

Using a plane?

Yes. I've read that entanglement is independent of distance. Is this a fact or a theory?

Both.

So, entanglement and its purpose remain a mystery—yes?

I do not think it was a mystery. I think that it is just the quantum version of classical correlations.

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...I think [entanglement] it is just the quantum version of classical correlations.

There is more than correlation in entanglement, and QM gets disturbing here. Entangled particles prove that mixed states are not just a definite state that we don't know precisely.

Photons can have linear (say, vertical or horizontal) or circular (right or left) polarization or any elliptic combination. All combinations can be written as the sum of two linear or two circular polarizations, for instance pure circular is vertical+horizontal with a 90° phase shift. Linear is a good base, circular an other, equally good.

You can observe the perfect correlation between two entangled photons using linearly polarized detectors.

BUT with the same source of photons,

you can observe the perfect correlation between two entangled photons using circularly polarized detectors.

Here QM is radically different (and disturbing), because the photon pair cannot decide when it's emitted to be vertical, horizontal or anything else: this would explain the correlation with linear detectors, but not with circular detectors. (Nor could they decide to be right or left).

That is, vertical photons would be seen by circular detectors, but equally as right or as left photons, and with no correlation between the detectors. Experiments contradict that.

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Entangled particles prove that mixed states are not just a definite state that we don't know precisely.

This is also true for a kind of classical systems named LPSs.

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This is also true for a kind of classical systems named LPSs.

Would you have a link or a more complete name? I couldn't find LPS because three letters aren't specific enough for a search.

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1351365141[/url]' post='710710']

1) 'Pulling them' using different techniques. E.g. if the particles are charged you can use magnetic fields. Using magnetic fields is as you move particles at the LHC.

2) Nobody is transporting a particle at the end of the universe, whatever what you mean by "end". Transporting material to the opposite part of the Earth is not so difficult, but I suspect that you refer to some statement in some book where the author is really trying to explain that entanglement is independent of the distance.

How this 'pulling-apart' into two, of an uncharged particle such as photon takes place, either naturally or in the lab in the context of this real phenomenon called ENTANGLEMENT? Your thoughts?

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• 1 month later...

I don't know how it is done for photons but I can describe an example that gives entanglement (this way is very impractical to do, but...).

If you take an atom of helium and shoot away the nucleus by a high speed neutron, the electrons that formed a pair are then free to go apart, and their spins are entangled.

You must NOT use a magnetic field to separate them (otherwise you will get each in a definite state of spin, without entanglement).

Instead you must just let them repel each other, then verify that you have exactly one electron on "each side" in some way.

But many other processes give entanglements, even if they are not very practical either.

For example if you take an atom in a definite excited state, and then wait the fraction of second so that there is 50% chance for it to have come down to the ground state by emitting a photon, then the state of the atom is entangled with the state of the electromagnetic field around (that may contain or not contain the photon).

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I would argue that entanglement is correlation.

Consider the following experiment.

Take a coin and carefully slice it along the circumference so that one slice has the 'head' on it and the other slice has the 'tail' on it. Place each slice in a separate envelope. Hand one envelope to person A and the second envelope to person B. Person B hops on a plane, or a spaceship, and travels to the other side of the world, or the universe for that matter. An agreed on period of time later, person A opens his envelope and, depending on which slice of the coin he finds inside, immediately knows which slice person B has in his envelope, no matter what the distance separating them. Now consider the experiment repeated with electron spin or photon polarization instead of coin slices.

Notice that this is equivalent to the wavefunction collapsing when the envelope is opened, but there is no actual instantaneous information exchange, ie. it is a correlation. The determination of the first coin slice does NOT cause the other to assume the opposite slice.

Edited by MigL
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I would argue that entanglement is correlation.

Consider the following experiment.

Take a coin and carefully slice it along the circumference so that one slice has the 'head' on it and the other slice has the 'tail' on it. Place each slice in a separate envelope. Hand one envelope to person A and the second envelope to person B. Person B hops on a plane, or a spaceship, and travels to the other side of the world, or the universe for that matter. An agreed on period of time later, person A opens his envelope and, depending on which slice of the coin he finds inside, immediately knows which slice person B has in his envelope, no matter what the distance separating them. Now consider the experiment repeated with electron spin or photon polarization instead of coin slices.

Notice that this is equivalent to the wavefunction collapsing when the envelope is opened, but there is no actual instantaneous information exchange, ie. it is a correlation. The determination of the first coin slice does NOT cause the other to assume the opposite slice.

This seems like a very reasonable explanation (hidden variable theory)...unfortunately the Aspect experiments proved it does not work that way

Using your example: it would be like them opening the envelope in such a way, say opening the left side facing the closure, they would find they could induce the head twice as often as the tail...and still the other envelope automatically has the opposite...quantum weirdness

Edited by J.C.MacSwell
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Don't have the exact details of the Aspect experiments, but I do know a little about entanglement as first brought up by Einstein, Podowlsky and Rosen at one of Bohr's conferences. It involves, as Einstein put it, "the spooky action at a adistance" of entangled particles. All I'm suggesting is that there is no actual action at a distance since the so called entanglement is actually a correlation and has nothing to do with the how the wavefunction is collapsed or the outcome of the collapse.

When you collapse the wavefunction of one electron or open the one envelope you automatically know the spin of the other electron or the contents of the other envelope. There is no transfer of information across vast distances at superluminal speeds.

Edited by MigL
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Don't have the exact details of the Aspect experiments, but I do know a little about entanglement as first brought up by Einstein, Podowlsky and Rosen at one of Bohr's conferences. It involves, as Einstein put it, "the spooky action at a adistance" of entangled particles. All I'm suggesting is that there is no actual action at a distance since the so called entanglement is actually a correlation and has nothing to do with the how the wavefunction is collapsed or the outcome of the collapse.

When you collapse the wavefunction of one electron or open the one envelope you automatically know the spin of the other electron or the contents of the other envelope. There is no transfer of information across vast distances at superluminal speeds.

You are making the same argument Einstein made (so you are in good company!), but real experiments trump thought experiments.

Note that "vast" distances have not been proven and it is said that information is not transferred superluminally, but you may want to check out the Aspect details. They certainly indicate a non local effect and no prior fixed correlation or hidden variable.

Edited by J.C.MacSwell
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No, actually I'm making Bohr's argument. There iis no EPR paradox because there is no causality, one event does not cause the other no matter what the separation.There is a pre-existing relation between the spin or polarity observations, ie a correlation.

I'm either not explaining myself well enough or you're not understanding my ideas ( same thing ? ).

Edited by MigL
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This seems like a very reasonable explanation (hidden variable theory)...unfortunately the Aspect experiments proved it does not work that way

Quantum correlations are no part of a hidden variable theory, but part of standard quantum mechanics. As stated in #6 entanglement is just a quantum correlation. Moreover, the Aspect experiment has not ruled out hidden variable theories.

You are making the same argument Einstein made (so you are in good company!), but real experiments trump thought experiments.

Note that "vast" distances have not been proven and it is said that information is not transferred superluminally, but you may want to check out the Aspect details. They certainly indicate a non local effect and no prior fixed correlation or hidden variable.

MigL is correct. What Aspect found experimentally is that we are saying to you [http://en.wikipedia.org/wiki/Alain_Aspect]:

A correlation between their wave functions remained, as they were once part of the same wave-function that was not disturbed before one of the child particles was measured.

Edited by juanrga
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No, actually I'm making Bohr's argument. There iis no EPR paradox because there is no causality, one event does not cause the other no matter what the separation.There is a pre-existing relation between the spin or polarity observations, ie a correlation.

I'm either not explaining myself well enough or you're not understanding my ideas ( same thing ? ).

Observing one collapses the wave function of the other, not merely exposing a pre-existing condition, like in your envelope analogy.

Did I take your analogy out of context?

"Stated more simply, the experiment provides strong evidence that a quantum event at one location can affect an event at another location without any obvious mechanism for communication between the two locations. This has been called "spooky action at a distance" by Einstein (who doubted the physical reality of this effect). However, these experiments do not allow faster-than-light communication, as the events themselves appear to be inherently random."

Edited by J.C.MacSwell
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Observing one collapses the wave function of the other, not merely exposing a pre-existing condition, like in your envelope analogy.

Did I take your analogy out of context?

"Stated more simply, the experiment provides strong evidence that a quantum event at one location can affect an event at another location without any obvious mechanism for communication between the two locations. This has been called "spooky action at a distance" by Einstein (who doubted the physical reality of this effect). However, these experiments do not allow faster-than-light communication, as the events themselves appear to be inherently random."

It is untrue that one event at one location affects in non local fashion another event. There is no "spooky action at a distance" in quantum mechanics. A rather good study of all this is given in the textbook Consistent Quantum Theory by Robert B. Griffiths:

Quantum mechanics, like classical mechanics, is a local theory in the sense that the world can be understood without supposing that there are mysterious influences which propagate over long distances more rapidly than the speed of light. See the discussion in Chs. 23–25 of the EPR paradox, Bell’s inequalities, and Hardy’s paradox. The idea that the quantum world is permeated by superluminal influences has come about because of an inadequate understanding of quantum measurements — in particular, the assumption that wave function collapse is a physical process — or through assuming the existence of hidden variables instead of (or in addition to) the quantum Hilbert space, or by employing counterfactual arguments which do not satisfy the single-framework rule. By contrast, a consistent application of quantum principles provides a positive demonstration of the absence of nonlocal influences, as in the example discussed in Sec. 23.4.

Unsurprisingly, the chapter 23 is titled "Singlet state correlations". As explained before in this thread, the quantum state is not given by a mere tensor product, there are correlations, between the measured variables.

Edited by juanrga
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It is untrue that one event at one location affects in non local fashion another event. There is no "spooky action at a distance" in quantum mechanics. A rather good study of all this is given in the textbook Consistent Quantum Theory by Robert B. Griffiths:

I don't have that text (edit: found this on line: http://quantum.phys.cmu.edu/CQT/) , but here is a non technical lecture by the David Griffiths I found while looking for info on Robert Griffiths and thought it was interesting with regard to the collapse of the wave function. His entanglement analogy is frisbees thrown with both left and right hand (warning: it is 53 minutes long!)

He seems to say that non local effects are in fact indicated (consistently) by experiment, though without information or causality, and "barely" (he holds his fingers barely apart) compatible with SR (personally I don't see the compatibility)

Edited by J.C.MacSwell
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He seems to say that non local effects are in fact indicated (consistently) by experiment, though without information or causality, and "barely" (he holds his fingers barely apart) compatible with SR (personally I don't see the compatibility)

Did you read the full quote from his textbook given above? What part of

Quantum mechanics, like classical mechanics, is a local theory [...] By contrast, a consistent application of quantum principles provides a positive demonstration of the absence of nonlocal influences

do you believe support the claim that non local effects are indicated by experiment?

Regarding the 'bare' compatibility of quantum theory with SR, the mix of both is quantum field theory. This is a local theory, as shown in any textbook on QFT.

Edited by juanrga
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Did you read the full quote from his textbook given above? What part of

do you believe support the claim that non local effects are indicated by experiment?

Regarding the 'bare' compatibility of quantum theory with SR, the mix of both is quantum field theory. This is a local theory, as shown in any textbook on QFT.

No part of it. David Griffith in the video referred to, not the author of your text.

As for "absence of nonlocal influences", what exactly does that mean? Does it mean 1. the wave function cannot be collapsed non locally FTL(so no non locality at all?), or does it mean that, 2. when or if it does, it cannot effect information or causality FTL?

I believe that the 2nd is the most commonly accepted. Is it not? Is your author suggesting the first? I am not familiar with his approach but will have to look into it.

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No part of it. David Griffith in the video referred to, not the author of your text.

As for "absence of nonlocal influences", what exactly does that mean? Does it mean 1. the wave function cannot be collapsed non locally FTL(so no non locality at all?), or does it mean that, 2. when or if it does, it cannot effect information or causality FTL?

I believe that the 2nd is the most commonly accepted. Is it not? Is your author suggesting the first? I am not familiar with his approach but will have to look into it.

The wave function is not a real physical wave and hence there is no collapse and no nonlocal influences, it is just an abstract mathematical function to determine the possible outcomes of the quantum system.

The quantum correlations are indeed instantaneous but it neither has a hidden cause means no hidden variable theories or nonlocal communication can account for such correlations. One cannot hold a multitudinous view of the world where things can be separable into parts or subsystems, the entangled pair of photons doesn't exist in my opinion, if the two photons do not have well defined spin values and if their spin states are in a superposition of states then there is no sense in assuming that the photons have the attribute of spin and if the property of spin cannot be attributed to a photon then it is incorrect to even call it a particle or a photon. Bernard is right photons, quarks, electrons etc are not self existent. The only way that these quantum correlations make sense is to assume we are living in some kind of immersive virtual reality.

The wavefunction is an abstraction and that's the only reality, there is no absolute reality out there.

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No part of it. David Griffith in the video referred to, not the author of your text.

Ok

As for "absence of nonlocal influences", what exactly does that mean? Does it mean 1. the wave function cannot be collapsed non locally FTL(so no non locality at all?), or does it mean that, 2. when or if it does, it cannot effect information or causality FTL?

I believe that the 2nd is the most commonly accepted. Is it not? Is your author suggesting the first? I am not familiar with his approach but will have to look into it.

It means that both the experiment and the theory describing it do not use/require any nonlocal influence. There is no collapse of the wavefunction associated to the Schrödinger equation, but if even if you consider a description with a collapse beyond the Schrödinger equation, the collapse continues without transmitting any information or causality neither FTL nor in any other way.

One cannot hold a multitudinous view of the world where things can be separable into parts or subsystems, the entangled pair of photons doesn't exist in my opinion, if the two photons do not have well defined spin values and if their spin states are in a superposition of states then there is no sense in assuming that the photons have the attribute of spin and if the property of spin cannot be attributed to a photon then it is incorrect to even call it a particle or a photon. Bernard is right photons, quarks, electrons etc are not self existent. The only way that these quantum correlations make sense is to assume we are living in some kind of immersive virtual reality.

This is nonsense. There was a long thread in speculation forum where it was shown that your 'opinions' are unfounded and based in a misunderstanding of (i) the scientific method and (ii) quantum theory. That thread eventually died because you were unable to answer the critics. You would not hijack this thread with the same nonsense.

Edited by juanrga

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