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Einstein vs quantum mechanics … and why he’d be a convert today

Albert Einstein may be most famous for his mass-energy equivalence formula E = mc2, but his work also laid down the foundation for modern quantum mechanics. His analysis of the “spookiness” of quantum…

It’s mind-blowing stuff, but Einstein wasn’t completely convinced by quantum mechanics. Travis Morgan/Flickr, CC BY-NC-ND

Albert Einstein may be most famous for his mass-energy equivalence formula E = mc2, but his work also laid down the foundation for modern quantum mechanics.

His analysis of the “spookiness” of quantum mechanics opened up a whole range of applications including quantum teleportation and quantum cryptography, but he wasn’t completely convinced by the theory of quantum mechanics – and that story is as fascinating as the theory he attempted to nail down.

Quantum mechanics is downright bizarre. It implies that a particle, such as an electron, can pass through two holes at the same time.

More famously, German physicist Erwin Schrödinger’s equations proved that a cat could end up in a peculiar sort of quantum state, being neither dead nor alive.

None of this impressed Einstein. He believed quantum mechanics was correct, but desperately wanted to find a way to “complete” quantum mechanics so it made sense.

At the time, most quantum physicists adopted the “shut up and calculate” philosophy: get on with the job, and don’t worry about philosophical issues – just get the predictions.

Gaining momentum (and position)

bitznbitez ( was lucias_clay )/Flickr, CC BY-NC-ND

Einstein’s opponents used Heisenberg’s Uncertainty Principle against him, which (among other things) states it is not possible to measure both the position and the momentum of a particle simultaneously to arbitrary accuracy.

If someone measures the position of a particle, the particle is disturbed, so its momentum changes. If it’s impossible to measure those two things at once, how can they be defined together?

Einstein’s opponents thought he simply didn’t understand quantum mechanics – but he knew the problem was deeper.

Then Eureka! In 1935, Einstein thought of a way to explain the problems with quantum mechanics. He would give a strong argument to show how position could indeed be measured without disturbing the particle!

Einstein (with American physicists Boris Podolsky and Nathan Rosen) discovered quantum entanglement.

Quantum entanglement of two particles means – bear with me here – that the quantum wave function describing them cannot be mathematically factorised into two separate parts, one for each particle.

This has an important consequence. Once two particles undergo entanglement, they become specially connected in a “spooky” kind of way that was eventually made clear by Einstein’s arguments and the experiments that followed.

A video explaining quantum entanglement, if that’s easier.

Einstein, Podolsky and Rosen – known collectively as EPR – realised that quantum mechanics predicted entangled states, where the positions and the momenta for two particles are perfectly correlated, no matter how far apart the two particles are.

That’s what was important to Einstein, who believed there could be no immediate disturbance to the second particle, as a result of anything that was done to the first particle. He called this “no-spooky-action-at-a-distance”.

So, suppose a girl called Alice measures the position of the first particle and a boy called Bob simultaneously measures the position of the second particle. Then because of the perfect correlation, once Alice makes her measurement, she knows immediately the result of Bob’s measurement.

For Einstein’s magical entangled states, her prediction is absolutely spot on – no error at all.

Joanídea Sodret/Flickr, CC BY-NC

Then, Einstein argued that can only happen because Bob’s particle did actually have that precise position that Alice predicted. Nothing at Bob’s location can change because of Alice’s measurement, which cannot disturb the second particle.

As Bob and Alice’s measurements are separated by space, Einstein concluded there had to be a hidden variable to describe the precisely specified value of the position of the second particle measured by Bob.

Now, similarly, Alice can predict with absolute precision the momentum of Bob’s particle without disturbing it. Then, assuming no spooky action, Einstein claimed the momentum of Bob’s particle could also be precisely specified, regardless of Alice’s measurement.

This leaves us with Bob’s particle having simultaneously precise values for position and momentum – which contradicts the Heisenberg Uncertainty Principle.

Resolving spooky action

Einstein’s argument illustrated the contradiction between quantum mechanics as we know it and the assumption of “no-spooky-action-at-a-distance”. Einstein’s belief was to resolve the problem in the simplest way: to introduce hidden variables consistent with no spooky action that would complete quantum mechanics.

Of course, by far the simplest resolution would be that Einstein’s entanglement simply doesn’t exist in nature. There were proposals that maybe entanglement decays with the spatial separation of the particles, then there would be no conflict between quantum mechanics and spooky action.

There was the need to experimentally confirm Einstein’s entanglement.

Chien-Shiung Wu.

Chien-Shiung Wu – often referred to as Madame Wu or the First Lady of Physics – from the University of Columbia was first to give evidence of Einstein’s entanglement in the laboratory. She showed an Einstein-type correlation between the polarisation of two well-separated photons, which are tiny localised particles of light.

John Bell, a physicist working at CERN, took Einstein very seriously and wanted to develop a hidden variable theory along the lines Einstein suggested.

He examined the states Madame Wu had created, but on looking closely at their predictions for some small adjustment of measurements, he came across a startling result.

According to quantum mechanics, finding such a hidden variable theory would be impossible. The results of measurements in the laboratory would be different for Einstein’s hidden variables and quantum mechanics.

This meant that quantum mechanics was simply wrong, or else that any hidden variable theory enabling a completion of quantum mechanics would have to allow a “spooky-action-at-a-distance”.

Back to the lab

In a nutshell, experimentalists John Clauser, Alain Aspect, Anton Zeilinger, Paul Kwiat and colleagues have performed the Bell proposal for a test of Einstein’s hidden variable theories. All results so far support quantum mechanics. It seems that when two particles undergo entanglement, whatever happens to one of the particles can instantly affect the other, even if the particles are separated!

Have Einstein’s dreams of a better theory have been dashed by experiments?

Not quite. The experiments to date focus on photons, not massive particles such as electrons or atoms. Nor do they deal with very large systems.

So I don’t think Einstein would give up just yet. He’d think that maybe laws are different for real particles.

Australian scientists are examining ways to test Einstein’s and Bell’s ideas, using atoms and even miniature objects that have been cooled so much they have lost all their thermal jittering. Who knows what they’ll find?

And as for my contribution? On working with squeezed states of light in the 1980s, I thought of a way to test for the original Einstein’s entanglement, after noting scientists were able to amplify and detect the tiny quantum fluctuations of optical amplitudes.

In quantum mechanics, these are just like “position" and “momentum” and the experiment opened up a whole new way of testing Einstein’s entanglement.

Experiments since have confirmed this mesoscopic type of Einstein’s entanglement in a range of environments, which brings us closer to understanding Schrödinger’s cat.


This article is based on presentations given at Australian Academy of Science: new fellows and medallists symposium, June 12, 2014 and Science at the Shine Dome, May 27-29, 2014.

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39 Comments sorted by

  1. Mark Harrigan

    PhD Physicist

    Thanks - great article :)

    Would love your comments on a problem I have with the analogy of Schrodinger's Cat.

    Put simply it is that it seems to rest on the assumption that the "Cat" is not an observer?

    I guess I am more a fan of the so called "rational interpretation" - that the "superposition" of states of decay or non-decay collapses when there is an interaction with something rather than an observation by an conscious entity - however I am ware this view has some inherent problems? :)

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    1. Ben Marshall
      Ben Marshall is a Friend of The Conversation.

      Writer

      In reply to Mark Harrigan

      I think this is a good point. If a wave function isn't considered to have collapsed until 'observed' by a human animal then any sentient being can be that 'observer' - a cat or a cow. Take that thought further and make it that 'observer' can be any living organism - even, say, a bacterium. What about a virus? Or, frankly, a neighbouring bunch of molecules?

      While my understanding of quantum mechanics is, clearly, deficient, I often feel that we're trying to make nature fit into words that are not capable or appropriate to transmit meaning that truly reflects it.

      I was intrigued by a recent article in New Scientist on quantum Bayesianism in which immutable 'uncertainty' seems to be, basically, replaced by more reasonable context-specific 'probability'.

      arvix.org/abs/1311.5253

      I can read any number of articles, but I still wonder how much of quantum physics is actually mangled semantics.

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    2. Mark Harrigan

      PhD Physicist

      In reply to Ben Marshall

      To be fair on Schrodinger - I think he was looking at the Copenhagen interpretation and the implications of the EPR work which showed that, if a particle can exists simultaneously in a superposition of states (and the evidence of a number of experiments - including ground breaking work on Quantum Computing - now shows is exactly what DOES happen) - the this creates a potential paradox.

      It perhaps illustrates well the famous Feynman quote "If you think you understand quantum mechanics, you don't"

      I love it - because it is a limitation of our own experience (being neither microscopic nor macroscopic in scale or perception) that tries to "make sense" of it. I just doubt you need an "observer" you just need a "measurement" which can just mean interaction with another particle or system

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    3. Ben Marshall
      Ben Marshall is a Friend of The Conversation.

      Writer

      In reply to Mark Harrigan

      I'm torn between goggling at, and marveling at, an aspect of nature I'll never physically or mentally be able to apprehend or comprehend, and scowling as I, yet again, fail to 'get' the most basic aspects of particle physics.

      If words won't do it, I need pictures, damn it. Give me pictures, science!

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    4. account deleted

      logged in via email @gmail.com

      In reply to Mark Harrigan

      Interesting question Mark.

      Wouldn't it be the case that the cat is contained within a reference frame that is isolated from the one occupied by the observer? Effectively the interior of the box is a different universe until the two reference frames are unified by the opening of the lid. Within its own reference frame, the cat is an observer, but its observation can't influence the wider world until its state within that frame is established by observation. It's akin to the "many worlds" theory…

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    5. Mark Harrigan

      PhD Physicist

      In reply to account deleted

      Hmm - that's a many worlds interpretation. Which I don't favour any more than the Copenhagen interpretation.

      I think these interpretations came about because the implication of what Schrodinger has to say - that QM showed that a particle literally could be in two states at once - was unacceptable to some. So you "were not permitted to ask".

      I don't think so. The interior of the box in a different universe? I think that's absurd. A different universe is just that - not a different frame of…

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    6. account deleted

      logged in via email @gmail.com

      In reply to Mark Harrigan

      "A different universe is just that - not a different frame of reference."

      It's a frame that is entirely isolated until the lid is opened. to that extent it's a different universe, effectively. It's also causally directional, since the cat's observation can influence the outcome of the experimenter's observation, but the experimenter's can't influence the cat.

      "if we put the experimenter in a box? Are they also in a superposition?"

      Sure, why not? That was the point of my post above. The collapse…

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    7. account deleted

      logged in via email @gmail.com

      In reply to account deleted

      correction, sorry

      "The cat's state is indeterminate within the reference frame of the observer, but not from within that of the cat, where it is merely [determined causally]. "

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    8. account deleted

      logged in via email @gmail.com

      In reply to account deleted

      Sheesh, "when the world-lines [con]verge". Trying to listen to Zappa while typing can only ever end in tears...

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    9. Mark Harrigan

      PhD Physicist

      In reply to account deleted

      Maybe - but I doubt it. I can't refute the many worlds interpretation - which I think you are advocating - where the wave function never "collapses" but instead all possible pasts and futures are real - but I don't like it because currently it defies feasible testing. Mind you it's the source of so many great Sci-Fi stories :).

      But I wonder if a superposition of quantum states (like spin for example) can be better explained by extent to which the particle is present (or rotated into) extra dimensions…

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    10. account deleted

      logged in via email @gmail.com

      In reply to Mark Harrigan

      Hmmm, the properties of the circle observable from within Flatland would be interesting, that's for sure. The plane of the circle would be seen as a line as soon as the axis of rotation moved away from perpendicularity. (A "string"?) What would be really interesting would be the nature of the energetics of that line with respect to the Flatland observer. There would be a shear component that varied both with radial position and with axial angle, as well as a constant force directed toward the point at which the shear vectors changed direction. It gets even more interesting if you consider the Flatlander trying to push on the spinning circle...

      I think your conception is pretty good, for all that my own maths aren't up to the task either.

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    11. Mark Harrigan

      PhD Physicist

      In reply to account deleted

      Hey Craig - glad you like the conception and I appear to have explained it okay.

      Yes if the circle remained a flat plane it would indeed, as you say, be seen as line (although to some extent that's what a circle would "look" like anyway - albeit one with properties different from a "straight" line). If the pinch was gradual it would transform slowly - which isn't very quantum of course.

      Am alternative conception is that the rotating "circle" always was a sphere perfectly bisected by the flatland…

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    12. account deleted

      logged in via email @gmail.com

      In reply to Mark Harrigan

      It's a nice idea, Mark, but I'm not sure that it's any more empirically testable in its implications than the many-worlds hypothesis. Mind you, it does certainly create an "entanglement" from the Flatlanders POV.

      Perhaps Capra's work on constructor theory will provide the necessary maths to get a real handle on this stuff? From what little I know, it seems a really interesting approach. I hope he can get it to produce some results that entice some others to stick their own necks out a bit and do some serious work on it.

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    13. Tim Allman

      Medical Software Developer

      In reply to Mark Harrigan

      "More famously, German physicist Erwin Schrödinger’s equations proved that a cat could end up in a peculiar sort of quantum state, being neither dead nor alive."

      Hardly. It was a reductio ad absurdum thought experiment aimed at the proponents of the Copenhagen interpretation. I doubt that anyone believes that a large object such as a cat would - or even could - be in a superposition of |dead> and |alive>.

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    14. Allan Gardiner

      Dr

      In reply to Mark Harrigan

      It's interesting that you both use the word 'collapse' and ask as to when it actually d_oth'erwise [ceteris paribus etc., not that they ever are] obtains. It's far better to visualise 'it' as being but a *colla_pse'udo* given that an hypothesis is but one thing...and one thing only.

      As Ben has already said: " If words won't do it, give me pictures...", and Craig knows it to be but a 'thought experiment'. This is why the old saying "In thought, word AND deed" will always take a lot of beating, leaving many who are out-of-whack just beating around the bush.

      I trust Schrodinger will forgive me for letting the cat out of the bag..er..box. ;^)

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    15. Vladislav Zorov

      logged in via Facebook

      In reply to Mark Harrigan

      No, consciousness is not required - just measurement. This has been proven by quantum eraser experiment, the interference pattern goes away when full information about the photon's path can be recovered and it doesn't matter if a human is watching or not (they actually tested this, left the device to switch one of the detectors on and off while nobody was watching and then looked at the video recordings).

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    16. John Nicol

      logged in via email @bigpond.com

      In reply to Ben Marshall

      Ben Marshall

      There are many who would agree with your last statement, Ben. In the period after the Hanbury-Brown and Twiss experiment, many quantum theorists went into a frenzy trying to explain the effects only in terms of photon theory, which turned out to be both difficult and useful. Difficult because the ideas from Quantum Electro Dynamics at the time could not comfortably handle the results in any coherent fashion, and useful because it stimulated a lot more thought which brought out some…

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  2. Michael Shand

    Software Tester

    Really really interesting, thanks for sharing

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  3. Peter Ormonde
    Peter Ormonde is a Friend of The Conversation.

    Farmer

    Lovely article - many thanks ... let's hope it inspires a few young minds to break the bars on their imprisoned thinking.

    Poor old Albert - caught on the hinge of history like that - straddling the barbed wire scientific fence - most uncomfortable.

    You realise that this sort of stuff has absolutely no commercial application at all (according to the utterly wrong common-sense) and will probably be defunded now that we know what you're up to... the cat's out of the bag. Or the box. Over there or there or maybe both.

    Looks like God plays dice after all ... come on number 7!!!

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    1. Eden Ukrivljen

      Principal Retro-encabulator at Sinusoidal Deplenurators

      In reply to Peter Ormonde

      What is a farmer from whaddi... whadevah ... doing reading this sort of an article? Aren't you aware that our new ideotocracy do not approve of an educated serfdom. A farmer with notions of quantum entagled meat and three veg - what next!? Harumphh!

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    2. Peter Ormonde
      Peter Ormonde is a Friend of The Conversation.

      Farmer

      In reply to Eden Ukrivljen

      Having spent the last few days pruning my grapes I am much more aware than I would like of this notion of entanglement.

      In fact I was so taken by this gentle article I was drawn back to some of old Albert's philosophical and scientific musings, like these:

      "[I do not] carry such information in my mind since it is readily available in books. ...The value of a college education is not the learning of many facts but the training of the mind to think."

      " I was sitting in a chair in the patent…

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    3. Allan Gardiner

      Dr

      In reply to Peter Ormonde

      Methinks it's a plum job you have there Peter, pruning your grapes, and let this comment of mine prove conclusively how very little I knew about viticulture -- dry wit_iculture being no stranger to me of course -- because I'd always thought [p_un'til now] that prunes could only be had from plums.

      Don't quote me, but I have heard it said that some fine Fellow who walked the Earth just over two millenia ago, was [f]able to turn some water into wine, but your being able to prune grapes could well result in your soon being b_rort some plum_p prophets..err..profits.

      It's my belief that if Albert was weighed down with having to fully comprehend someone's falling freely sans feeling [is that reeling?] their weight, then he but perhaps merely stumbled upon it by acci*dent* [being the *deep impression* thusly made upon him ;^)] before being held in suspen_se'rio-comic.

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  4. John Crest

    logged in via email @live.com.au

    "More famously, German physicist Erwin Schrödinger’s equations proved that a cat could end up in a peculiar sort of quantum state, being neither dead nor alive."

    Rubbish. The so called "thought experiment" proved no such thing. It was merely an analogy to help lay people understand the uncertainty of quantum states.

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    1. Vladislav Zorov

      logged in via Facebook

      In reply to John Crest

      Nope, it was an analogy to illustrate the absurdity of quantum mechanics :) The dude was AGAINST that idea, he wasn't trying to bring it to laypeople. Eventually QM turned out to be correct, and I bet Heisenberg got really pissed off when people started talking about his cat as a fine example of QM :)

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  5. Craig Myatt

    Industrial Designer / R&D

    Good article...Einstein should be happy with his relative success...

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  6. Tom Lowe

    Research Officer

    While it is often stated that Einstein was wrong on Quantum theory, the fact is that he remains largely correct. The majority of Quantum physicists use the Many Worlds Interpretation by Everett, therefore there is no spooky action at a distance, there is no magical wave-function-collapse event that occurs at some vague observation event that no-one can agree on.
    There is simply one universal wave function. The waves of quantum field theory exist at macroscopic scales as well, it is just they are…

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    1. account deleted

      logged in via email @gmail.com

      In reply to Tom Lowe

      I find the many worlds model appealling, I must say. While there is no universal wave-function collapse, as I understand it there is a localised collapse that manifests as deterministically causal convergence of the world-lines of the observer.

      In simple terms, the course of events leading up to an observation has to be that way or the observation could not have been made. In a way, the observation creates the history!

      It's a kind of super-anthropic principle.

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    2. Mark Harrigan

      PhD Physicist

      In reply to account deleted

      "It's a kind of super-anthropic principle" - which is why I do not like it :)

      Of course my objection is in one sense purely aesthetic.

      I think what the majority prefer is, however, irrelevant. I know Murray Gell-Mann hated it - and I think at a relevant conference, despite the majority voting for MWI they all agreed this is not how it is decided. It has to be based on testability - which is the nub of the problem. How to devise feasible tests to determine which interpretation is real? I think David Deutsch - one of the leading MWI proponents and a real pioneer of Quantum Computing - has proposed some tests but they are not able to be carried out.

      Tom - I'm not sure that MWI rules out "spooky action at a distance" though. Even in MWI you get that - at least in the "local" universe.

      Regardless it's a fascinating area

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  7. Henning Dekant

    logged in via Twitter

    It's often overlooked that Schrödinger came up with his cat in a box example to mock the orthodox interpretation of QM. In 1952 he penned an article on the meaning of wave-mechanics that is very explicit in his criticism (http://wp.me/p2lHU6-Df).

    Entanglement facilitates correlation but not causation, special relativity is still as valid as when the EPR paradox was formulated, yet while the experimental confirmation of entanglement has been making great strides there is still no deeper understanding. Local hidden variables can be ruled out, but we are completely devoid of a satisfying theoretical explanation of what facilitates entanglement (that is unless one subscribes to the De Broglie–Bohm pilot wave description).

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  8. Roslyn Ross

    logged in via Twitter

    No-one gets it all right, not even Einstein and science will never get it mostly right until it can move beyond the prison of its materialistic/mechanistic paradigm. Still, despite opposition from Einstein Quantum Mechanics developed and thrived and science will be forced to broaden its belief system.

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  9. Allan Coop

    logged in via email @gmail.com

    Margret,
    It is rather duplicitous (careless?) to suggest as you do that Bell's Theorem demonstrates: "According to quantum mechanics, finding such a hidden variable theory would be impossible." when in fact Bell showed: "No physical theory of "local" hidden variables can ever reproduce all of the predictions of quantum mechanics." This statement does not proscribe "non-local" hidden variables which may actually exist as has been shown by de Broglie-Bohm.

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  10. Andrew Bromage

    Research Engineer

    This is just speculation in my part, but the "interpretation" that I think Einstein would be most interested in, were he around today, would be the retrocausality interpretation.

    I'm going to oversimplify it here, but the general idea is that the laws of physics are manifestly time-symmetric (well, spacetime-symmetric), so there is no reason why "causality" couldn't happen backwards in time. In this picture, there is no "spooky action at a distance", because entangled particles actually do interact locally, it just happened in the past.

    There is no contradiction with causality happening backwards in time as long as information doesn't flow backwards in time, so I suspect that quantum information theory would be one of his pet topics.

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  11. Doug Alder

    logged in via Twitter

    So unless I missed the answer in the video or article here's a question: It's great to talk about entanglement and from a purely non-scientist perspective I get the general gist, but how can they tell two photons, or two atoms, or any two particles are actually entangled?

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    1. Vladislav Zorov

      logged in via Facebook

      In reply to Doug Alder

      There are statistical tests that can prove whether the system of particles can be separated or not, i.e. if you'd be able to get the same results for the overall system if the qubits were independent from each other. Here's a web page that can test arbitrary density matrices - http://physics.technion.ac.il/~oded/documentation.html

      For maximally entangled states (like the Bell states), you have a 50-50 chance to measure one qubit as a 0 or 1, but depending on the result of this measurement the other qubit *always* measures some particular value (depending on which Bell state exactly you're using). If they were not entangled, you'd have a 50-50 chance for both and no correlation between their measured values.

      P.S. Sorry for mixing particles and qubits above... I'm referring to the same thing.

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  12. John Nicol

    logged in via email @bigpond.com

    Margaret Reid

    This is an excellent article the type of which there are all too few on the conversation.

    As a long retired (17 years)but interested physicist, I am wondering if I could ask a few basic questions and make some comments as well. I apologise in advance for being so verbose and thank you in advance if you should be able to read it and respond..

    There is no doubt that great progress continues to be made in the applications of Quantum theory but I don't think it is unrealistic to nominate…

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  13. Kurt Gminder

    logged in via Facebook

    • From the perspective of philosophy, the question is what entanglement probably meant to be answered for 2500 years. Heraclitus: Units were around the Gentiles so that they are active club end and nothing is isolated in itself, something can not emerge out of nowhere. Only when the A consists of Allen and all of the One can (the causeless nothingness) draw on their own bootstraps something. By extending over each other, so they can stick together. Units extend over the entire Now as wholes and the proof is the entanglement. There extending structures which are present at the same time here as there. So the child particle model must, however, be completely forgotten the exchange particles and the Various kinds of forces. With an Expansive atomic model to get all the features of physics.

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