Krauss on entanglement

Doesn’t the New Yorker need someone to write about evolution as well as cosmology? Once again, I’m both impressed by and jealous of Lawrence Krauss’s performance at that venue, and this week he writes about the new physics experiment that pretty much verified the idea of quantum entanglement and nonlocality. The piece, “Tangled up in entanglement.”

I like it when scientists rather than science journalists write about such things, for you know that at least they understand what is going on. (Don’t get me wrong: not all science journalists are ignorant of their subject: Faye Flam and Carl Zimmer are notable exceptions.). And Krauss does about as good a job as possible of explaining entanglement to the layperson. When I say “as good as possible,” I mean that entanglement is such a bizarre, counterintuitive concept that understanding it intellectually is not the same thing as fully grasping its bizarreness, which simply can’t be done in words.

I’ll leave it to you to read Krauss’s very clear description of the experiment, and why it doesn’t mean that we can send information at superluminal speeds. I’ll just put in one bit in which Krauss disses the misuse of quantum mechanics:

Entanglement is so spooky that it’s tempting, when thinking about it, to draw nonsensical conclusions. Deepak Chopra, for example, keeps implying that quantum mechanics means that objective reality doesn’t exist apart from conscious experience. The truth, however, is that consciousness is irrelevant to the act of measurement, which can be done by machines, or even by single photons. If consciousness matters, then the inner thoughts of the experimenter who operates the machines would also have to be reported when we write up the results of our experiments. We’d need to know whether they were daydreaming about sex, for example. We don’t. The machines can record data and print it out whether or not a person is in the room, and those printouts, which behave classically, don’t change when the humans come back.
Similarly, last week, the Pulitzer prize-winning writer Marilynne Robinson published an essay in which she challenges the nature and relevance of modern science. The essay argued that entanglement “raises fundamental questions about time and space, and therefore about causality.” She went on to say that this called into question the ability of science to explain reality as a whole. It’s easy to understand how Robinson arrived at this incorrect idea: when a measurement of one electron here can instantaneously affect the measurement of another electron on the opposite side of the universe, faster than the speed of light, it does seem as though causality has been thrown out the window.
Now I will predict—and I’m doing this without looking—that Deepity Chopra will already have attacked Krauss on his site. But forget the niggling and carping of the risible Chopra. What’s amazing is that nonlocality exists at all: that getting information about one electron a gazillion miles away form its entangled twin will affect the state of that electron instantly. This now appears to be the case, but I still can’t get my head around it.

117 Comments

  1. Militant Scientist
    Posted November 1, 2015 at 1:14 pm | Permalink

    The experiment shows that one must give up on local realism. You have a choice: give up on locality or give up on realism (a classical idea of reality). Most people I know have chosen to give up on realism. The physics is still local.

    Bohm took the opposite choice. He gave up on locality. There are some physicists who still adhere to this view.

    • Torbjörn Larsson
      Posted November 1, 2015 at 1:31 pm | Permalink

      As I understood it the difference is that the new Bell test experiments forces one to give up both. The correlations are mediated faster than the universal speed limit. (And, it seems, so is tunneling.)

      “Reality” is easy to give up on since it is a philosophic idea. Whether or not observables have an independent existence apart from when they are measured is rather irrelevant, a quirk of nature if you will.

      Penrose’s twistors framed spacetime as an inherently non-local system as far as I understand (not much), so that seems easy to give up on too.

      As long as I can have my causality to make sense of nature, I’m good.

      • Militant Scientist
        Posted November 1, 2015 at 2:11 pm | Permalink

        Before this experiment, there was always a possibility that there was an extra local realistic signal between the two sides. A non-local realistic model can still describe the results.

        By locality, I mean that interactions are local. As long as interactions are local, there will always be something like the speed of light limiting the speed of information transfer.

        I actually think this is an example where a many-worlds interpretation of quantum mechanics makes things less strange. Of course, you can’t then calculate the probabilities without adding some Born rule anyway, but in terms of locality of signals and non-violation of the speed of light, I think it is clearer.

      • Posted November 2, 2015 at 11:15 am | Permalink

        There’s no debate over realism here at all, as ordinarily understood.

        As was pointed out 30 years ago (at the minimum) the debate is over *classicality*, or one might say “sharp properties”. Quantons do not have them (in all circumstances), and somehow Bohr and Einstein used “realism” to mean this. This hasn’t anything more to do with epistemic or ontological realism in philosophy than realism in politics or art.

        • nightgaunt49
          Posted November 4, 2015 at 2:07 pm | Permalink

          It must be recalled that the two electrons must have interacted at one time directly before in order for “spooky action at a distance” to work in relation to quantum inseparability via entanglement.

          Still I understand that quantum affects aren’t perceived on our level or greater, only at the sub-atomic level.

  2. Barry Lyons
    Posted November 1, 2015 at 1:34 pm | Permalink

    Yes, a once-per-month (or whatever) column by you on evolution for The New Yorker would be great!

    • Marilee Lovit
      Posted November 2, 2015 at 6:44 am | Permalink

      Agreed! Or more often.

  3. sshort
    Posted November 1, 2015 at 1:48 pm | Permalink

    I’d say as long as two parts gin reliably mixes with one part vermouth, the constituents can spin however they choose.

  4. GBJames
    Posted November 1, 2015 at 1:50 pm | Permalink

    Sub

  5. Posted November 1, 2015 at 1:52 pm | Permalink

    I like that Krause linked to a review of Rhonda Byrne’s “book”, The Power. I read (and blogged about) it when it came out. It really is a remarkably stupid piece of writing. In fact, I would argue that it is the stupidest piece of writing ever in the history of writing.

    She says that everything — every particle and every event — is governed by the law of attraction. There is no repulsion in her universe (making her understanding of cosmology pre-Empedoclean, if not pre-Sumerian). The law of attraction governs all chemical reactions.

    Gravity is love, according to Byrne. Literally. Gravity is identical to love, which is identical to magnetism. The heart has a higher magnetic field than the brain, so we attract the things we love through magnetism/gravity. Everything has a positive or a negative charge (events are simply either positive or negative), and with magnetism, if we think positive thoughts, we attract positive events.

    So the positive poles of a magnet attract each other, according to Byrne… and she also understands quantum physics without problem.

    And the book is dedicated to *her lawyers*. Yes, her lawyers. Who “introduced positivity” into her life.

    • Heather Hastie
      Posted November 1, 2015 at 2:20 pm | Permalink

      She sounds like she should be uncontrollably attracted to Deepak. With a bit of luck they’ll be so firmly stuck together by magnetism that neither will be able to do anything.

      • Diana MacPherson
        Posted November 1, 2015 at 2:34 pm | Permalink

        Or quantum locked. Deepak would like that.

      • Posted November 1, 2015 at 2:34 pm | Permalink

        As far as I know, they tend to avoid each other. I do know that Chopra looked down his nose at The Secret, as being populist and cheap (and competition). (He claimed to have ignored it completely at one point.)

        There is plenty of unquestioning cross-promotion among all these scammers, but there is some serious competition too. (There was a massive falling out among some of the Secret-makers too. One of them dropped out and tried to copyright the law of attraction, to stop Byrne using the term!)

        • Heather Hastie
          Posted November 1, 2015 at 2:57 pm | Permalink

          It’s weird because Chopra, for example, clearly cares that serious scientists think his theories are a joke, but at the same time he competes with others in his “field” for new and wondrous ways to interpret science. To use Diana’s very cool term, he’s “quantum-locked” deterministically.

        • Scott Draper
          Posted November 1, 2015 at 10:07 pm | Permalink

          “Chopra looked down his nose at The Secret, as being populist and cheap (and competition). ”

          Narcissism of small differences

    • Sastra
      Posted November 1, 2015 at 3:46 pm | Permalink

      New Thought beliefs like those in The Secret or A Course in Miracles pull a lot of their credibility from both traditional religion and good old secular common sense.

      Most concepts of God, Eastern and Western, entail world views which make thoughts, feelings, and intentions central aspects of reality. It’s not much of a leap to make our minds aspects of God, as opposed to lesser versions formed in His image. Physical reality is always hiding or obscuring the central cosmic importance of Mind per se.

      The common sense reading of “if we think positive thoughts, we attract positive events” is that well, yes: a positive attitude can be motivating to the self or attractive to others in the usual psychological way, invoking cause and effect which is neither spooky nor mysterious. You got the job not because you visualized yourself getting it, but because your confidence and smile made a good impression on the interviewer.

      Woo is when they entangle the one meaning with the other. If one is rejected, the hope is that the skeptic must also be rejecting the other, thus allowing the goofy version its own quantum connection with what people already find plausible.

      There are a lot of believers who find increasing comfort in modern physics. I really appreciate the New Yorker allowing a known atheist an opportunity to describe the new discoveries in quantum science. It cuts off their ability to insist that atheists either ignore, haven’t heard about, or admit they’re overset by them.

      • Posted November 2, 2015 at 4:28 am | Permalink

        Yes, they *always* retreat to that fall back position when challenged, and pretend that’s all anyone has ever claimed…. before hopping quickly back to “it’s a law, just as real as gravity and it works for me” etc.

        I’m glad to see physicists fighting back against this misappropriation of their culture!

    • Ralph
      Posted November 1, 2015 at 4:08 pm | Permalink

      The idea that Local Realism is wrong is profound and utterly bizarre. So much so, that if all we to go on was Occam’s Razor, I’d feel that it’s more likely that everything that Chopra, Byrne and the Bible have to say is more likely true than that Local Realism is wrong.

      We do, however have evidence. And it turns out that the true nature of the universe is far more fascinating and weird than anything that the woo-peddlers could invent even in their wildest evidence-purged dreams.

      • Posted November 1, 2015 at 5:10 pm | Permalink

        Totally unrelated point, but I really think that evidence-purged dreams should be the title of Nancy Grace’s biography.

        • Ken Kukec
          Posted November 1, 2015 at 7:09 pm | Permalink

          I’d like to be able to inquire about her one day, as she regularly does about so many others, “Where is that missing [white woman]?”

      • Posted November 2, 2015 at 4:18 am | Permalink

        The sentence from Krause that Chopra & Co would have skipped over in a trance is

        “Classical objects are large enough, or interact strongly enough with their environments, for quantum-mechanical effects to wash out.”

        As he famously told Richard Dawkikns, “Quantum physics has been hijacked by physicists.”

        • Ralph
          Posted November 2, 2015 at 4:46 am | Permalink

          This sounds like it could be developed into a corollary to Stephen Colbert’s maxim, “Reality has a well-known liberal bias.”.

          The Chopra corollary:
          “You can’t entrust science to the people who understand it!”

  6. Joe Dickinson
    Posted November 1, 2015 at 1:52 pm | Permalink

    A comment probably from ignorance from which I would love to be rescued by a careful explanation. This bit seems to be the key assumption: “Quantum mechanics says that the actual spin direction of either electron is not determined in advance of the measurement”. How do we know that the spin direction does not exist before the measurement as opposed to we can’t know the direction? It always seems to me that quantum uncertainty could be a bit like the solutions to a quadratic equation: two solutions are possible but we can’t know which is true until we have some additional information. What, other than a preferred interpretation of quantum equations, rules out that sort of interpretation of Non-locality? Please help!!

    • Militant Scientist
      Posted November 1, 2015 at 2:18 pm | Permalink

      I think about this the other way. The equations of quantum mechanics explain many easy to understand things: why an electric burner turns red, why a sodium light is yellow, etc. All of these things rely on matter acting like waves. Everyone is happy with the results.

      Then Einstein, Podolsky, and Rosen show that the same equations that describe these everyday things leads to entanglement and this strange correlation. John Bell then shows that one can perform a measurement that rules out local realistic theories. This recent result closes most of the loopholes.

      You can have a non-local realistic theory that gives you the same results. See Bohm for instance.

    • Ralph
      Posted November 1, 2015 at 3:16 pm | Permalink

      Joe, some definitions may help.

      REALISM. This is what you’re talking about – it means (loosely) that a definite “true” state of the universe underlies the uncertainty prior to measurement. It’s as though a subatomic die has been cast, with one number showing, but it is hidden from view under a cup. Measurement just lifts the cup.

      LOCALITY. Excludes instantaneous (faster-than-light) action at a distance.

      Bell’s inequality, if all loopholes are closed, shows that Local Realism is wrong. You can keep either Realism or Locality, but not both. It does not speak to which is wrong.

      If Realism is wrong, that means that the quantum superposition prior to measurement IS reality. In other words, prior to measurement, it’s not just that we don’t KNOW the result. All possible states really exist simultaneously. Schrodinger’s cat really is both dead and alive. The measurement process is not then just “lifting the cup” to reveal a definite state of the quantum die that was always there – it is in some bizarre and counterintuitive sense “picking” between multiple realities that all existed simultaneously prior to the measurement.

      If Locality is wrong, it means that one particle can influence another particle instantaneously on the other side of the universe – faster than the speed of light, and counter to Special Relativity.

      So, you have two bizarre and counterintuitive possibilities – and you have to pick one!

      The concepts are explained pretty well here:

      https://en.wikipedia.org/wiki/Principle_of_locality#Local_realism

      However, neither the above Wiki nor Krauss’s article attempt to explain just how Bell’s Theorem works, and why it rules out Local Realism. That’s more subtle, and I’m really not sure I follow it – if somebody more qualified could have a go, that would be great.

    • eric
      Posted November 1, 2015 at 4:42 pm | Permalink

      How do we know that the spin direction does not exist before the measurement as opposed to we can’t know the direction?

      AIUI, because without the measurement quantum particles act differently than with the measurement. Electron spins may be a bad example because it’s hard to imagine a case where it interacts in away that isn’t a ‘measurement’; photons paths are a much better example. Look up the two-slit experiment: without measuring the path, you get a result different from it taking either path. So with an experiment where you sent a photon through a screen with two slits in it, the experiment can actually yield three possible results: left-passage (when you measure), right-passage (also when you measure), or it appears to go through both at the same time (when you don’t measure).

      This is inconsistent with the idea that when we “don’t look,” the photon still takes a path. And since the same math governs electons, its inconsistent with an entangled electron having a defined direction of spin when we “don’t look.”

      • Walt Jones
        Posted November 1, 2015 at 7:13 pm | Permalink

        Thanks, eric, that helps me a lot! (Even if my head is about to explode from the implications.)

        • eric
          Posted November 2, 2015 at 7:01 am | Permalink

          Oh its much worse than taking two paths at once. 🙂 According to Feynman, the photon takes every possible path through the universe from the emitter to the screen; it looks like it travels through the two slits to us because the contributions (to the final state) from all the other paths cancel out.

    • Posted November 1, 2015 at 5:49 pm | Permalink

      In fact we don’t have to assume anything about the underlying mechanics of the spin direction. That’s the beauty and power of Bell’s theorem. It takes some empirically observed correlations, and shows that these correlations are impossible if local realism is true. If matching socks or people’s preferences for pizza toppings followed these statistics, we could prove that they also don’t obey local realism. In fact there are good articles on the internet and in print explaining Bell’s Theorem in terms of pizza topping preferences, and others explaining it in terms of matching socks.

      The important thing to remember is that none of this depends on the details of QM and none of it can be resolved by saying that maybe the mysteries are due to our ignorance about those details.

    • Posted November 1, 2015 at 6:48 pm | Permalink

      I posed your question to Krauss, and he gave this response, saying I was free to pass it on:

      We actually use the correlations that are measured in the Bell-Inequality experiment to demonstrate that the electrons cannot have any fixed spin direction but are in a linear superposition of different spin states.. it is part of the calculation, and the reason the measured correlation comes up as a larger number than is possible if the electrons were in some specific state that was unknown before the measurement.

      • Joe Dickinson
        Posted November 1, 2015 at 10:14 pm | Permalink

        Sorry, it all still comes across to me as assertion rather than explanation. I ask again: why not just consider the “wave function” as a probabilistic description of what we don’t/can’t know, not a physical reality? I guess I’m just too dense to understand. I will say this: any explanation that hinges on “observation” to resolve entangled states is deeply unsatisfying. The universe could, and did, evolve up to and including human life before the last couple of centuries without anyone making observations and would, I believe, continue to function just fine without the invention of quantum theory.

        • Ralph
          Posted November 1, 2015 at 10:52 pm | Permalink

          The “observations” can be automatic detections by an experimental setup without direct human intervention, and everything still works fine. I don’t think “observation” implies any kind of dependence on consciousness, if that’s your concern.

          But it’s not just assertion, it’s data. Bell’s Theorem rules out the possibility that the entangled system is just something uncertain and “waiting to be discovered”. I don’t think it’s enough to say that the wavefunction represents our uncertainty, it still can’t account for the experimental results. The only way to account for the data is that the process of detection affects the entire entangled system instantaneously – including the state of the second particle at arbitrary distance.

          • Joe Dickinson
            Posted November 1, 2015 at 11:10 pm | Permalink

            Sorry, Ralph, that last bit, to me reads a good deal more like an assertion than an explanation. And requiring an “experimental setup” is at least as problematic as requiring consciousness.

            • Ralph
              Posted November 2, 2015 at 12:01 am | Permalink

              None of this is just assertion or a vague handwaving metaphysical discussion. It’s experimental data that really do rule out all but totally weird possibilities for the way that the universe works. If others’ attempts to summarize the results look like unfounded assertions to you, I’d say go look first hand at how the experiments work. It’s pretty accessible – try the link I put up below.

              For sure, nobody knows the ultimate metaphysical interpretation — but the data do now definitively rule out all non-weird accounts.

            • Richard Bond
              Posted November 2, 2015 at 3:44 am | Permalink

              Joe, we only need an experimental setup to detect the phenomenon. One of the electrons could interact (with certain provisions) with any stray charged particle or photon and the other electron would still have its spin determined by the interaction. Once the determination has occurred, the pair of electrons are no longer entangled. The phenomenon is called “decoherence”, and is a pretty good explanation of why classical mechanics holds at the macroscopic level.

              It is also an excellent explanation of why Deepak Chopra’s weird ideas about the whole universe being interconnected by quantum entanglement are utter nonsense.

        • darrelle
          Posted November 2, 2015 at 9:55 am | Permalink

          Joe,

          What Krauss was explaining in his response to you is that the the electrons actually being in a state of superposition rather than their state merely being unknown to us is a specific, key aspect of the model, and that experimental results verify that the model is accurate. Furthermore, if the model specifically included that the actual state of the electrons was merely unkown then the predictions of the model would be different, and the expected observed results of the experiment would have been different.

          There is no particular reason that that feature of the model couldn’t have been an assertion*, so to speak, but however it came to be a key feature of the model it has been verified by experiment. That means that even if it ever were merely an assertion it is no longer so. It is now empirically verified.

          *Actually it is something that falls out from the math. It is inextricably part of the theory. If everything else that quantum theory describes is accurate, and so far it is very accurate indeed, then it is highly probable that this is accurate as well.

    • vince
      Posted November 1, 2015 at 10:11 pm | Permalink

      That’s what the Bell inequality thing is all about, distinguishing between having the properties all along and not. I’ll give a link to a video at the end to see how Bell’s inequality is derived (it is very simple), but try to describe it here too.

      Imagine you have a collection of objects (could be anything) that each have three binary properties. Let’s say it is children on a playground. Property A is ‘has a hat’ which is either yes or no. B is ‘has gloves’. C is ‘has a scarf’. We are to imagine those properties as fixed and existing before we measure them.

      We are going to sample from the total collection but only measure two of the properties for each child we sample. Either A and B , or B and C, or A and C.

      Bell’s inequality just says there is a certain relationship that _must_ hold. Namely that percentage(A & not B) + percentage(B & not C) >= percentage(A & not C).

      If you watch the video I post you’ll see that this a completely trivial and obvious statement. But guess what, it doesn’t hold in quantum mechanics! So our assumption of fixed properties must be wrong.

      A, B , and C are measurements of spin of a particle around different axes, which in quantum mechanics is a binary property. The experiments must also make sure the measurements occur far apart spatially so we can rule out there being communication of some sort passing between the measured particles.

      That’s the in a nutshell version and here’s the link I promised. https://youtu.be/qd-tKr0LJTM?t=4m38s

  7. Canoe
    Posted November 1, 2015 at 2:05 pm | Permalink

    For decades I’ve read commentary to the effect that entanglement does not mean that the speed of light is not breached, but I have yet to see an attempt at explaining this apparent descrepancy. Can someone please point me to an explanation? A good book, with or without equations, will do.

    • Militant Scientist
      Posted November 1, 2015 at 2:30 pm | Permalink

      The speed of light is a speed limit on information. When I measure my half of an entangled system, I get an answer that allows me to predict what you will measure. You don’t know that I have made the measurement, so you don’t know that I can make the prediction.

      There might be an explanation in this book. I don’t have the book with me, but it is a good introduction to quantum strangeness with a small amount of math
      http://www.amazon.com/The-Quantum-Challenge-Foundations-Mechanics/dp/076372470X

  8. EvolvedDutchie
    Posted November 1, 2015 at 2:18 pm | Permalink

    That moment when you realize Americans are better at explaining stuff that goes on at a university in your country than your local newspapers.

  9. John D Stackpole
    Posted November 1, 2015 at 2:20 pm | Permalink

    Ir recently read (don’t recall where, sorry) a comment on this entanglement business to the effect that when the electron pair is separated they “actually” remain together, “attached”, or right next to each other, through a weenie-teeny undetectable (of course) quantum wormhole stretched out “between” them. So there is no “space” between them (from the wormhole’s point of view) and hence nothing spooky. I have no idea if Bell’s Theorem rules that out too.

    Fanciful to be sure, but sort of satisfying. It’s nice to picture the universe all cris-crossed with spider webs of wormholes entangling everything. (Think Chopra will offer me a job to work on this notion?)

    • Militant Scientist
      Posted November 1, 2015 at 2:36 pm | Permalink

      There is a connection between wormholes and entangled black holes

      https://en.wikipedia.org/wiki/ER%3DEPR

      as Wikipedia says “The authors pushed this conjecture even further by claiming any entangled pair of particles — even particles not ordinarily considered to be black holes, and pairs of particles with different masses or spin, or with charges which aren’t opposite — are connected by Planck scale wormholes.”

      I think that is really unnecessary. Entanglement arises from two things: superposition and multiple particles. You don’t need any strange space-time geometry.

  10. JonLynnHarvey
    Posted November 1, 2015 at 2:58 pm | Permalink

    The New York Times write up of this same experiment stated that quantum physics was at the heart of “laser beams and modern computers”.

    Now I knew right off what they meant about laser beams, but was stymied by the note on computers. I then realized (it took a minute) that quantum physics is at the heart of the theory underlying semiconductors and transistors, and that these in turn are at the heart of the modern computer. But it seemed like an additional degree of separation than in the case of laser beams.

    • Posted November 1, 2015 at 5:52 pm | Permalink

      Yes, that’s a weird thing to say. Nothing about today’s computers is essentially quantum-mechanical. You could do it all with gears and switches if you had to. Tomorrow’s quantum computers may be another matter.

      • Ralph
        Posted November 1, 2015 at 6:35 pm | Permalink

        Semiconductors?

        It’s not really correct to say that you could build a modern computer with gears and switches, unless you ignore ALL practical considerations! All known materials in your mechanical CPU would melt when you try to open a spreadsheet.

        • Posted November 1, 2015 at 8:10 pm | Permalink

          People DID build simple computers without semiconductors. But agreed, semiconductors make modern computers possible.

          • Ralph
            Posted November 1, 2015 at 8:40 pm | Permalink

            If you haven’t come across it, you’d probably enjoy Neal Stephenson’s “Cryptonomicon”. Several intersecting stories, loosely based around Turing et al and early computing & codebreaking, all ludicrous to varying degrees, and just incredibly funny.

            • Posted November 1, 2015 at 8:45 pm | Permalink

              Thanks for the tip, sounds interesting.

  11. JonLynnHarvey
    Posted November 1, 2015 at 3:00 pm | Permalink

    Re:
    Now I will predict—and I’m doing this without looking—that Deepity Chopra will already have attacked Krauss on his site.

    Perhaps Jerry’s brain and Deepak’s brain are quantum entangled. As one resolves its spin, the other spins in the opposite direction.

    But only Chopra’s brain is tangled up in knots.

    • Ken Phelps
      Posted November 1, 2015 at 4:07 pm | Permalink

      “..Chopra’s brain is tangled up in knots.”

      ..of strings. Let’s keep the physics theme going.

  12. John Frum
    Posted November 1, 2015 at 3:30 pm | Permalink

    If I understand this correctly, entanglement depends upon superposition. If superposition did not exist then the electrons are in fixed opposite spins so of course if you measure one as being up then the other would be down no matter how far apart they are.
    So what is the evidence for superposition?

    • Ralph
      Posted November 1, 2015 at 3:54 pm | Permalink

      John,

      “Realism” means more-or less what you say – that superposition is not “reality”, but just represents lack of knowledge, and there is a “true” underlying state that we just don’t know prior to measurement.

      “Locality” excludes faster-than-light action.

      The evidence from this experiment consists of observations that violate Bell’s inequality, with (it is claimed) no loopholes remaining. That means, according to Bell’s Theorem, that either Realism or Locality is wrong.

      The tricky part is understanding how Bell’s Theorem works and just how a violation of Bell’s inequality rules out Local Realism. This is more technical, Krauss doesn’t attempt to explain it, and I don’t understand it well enough to try.

    • eric
      Posted November 1, 2015 at 5:01 pm | Permalink

      There’s a bunch (as I understand superposition). The two-slit experiment would be evidence of superposition. Another example would be alpha decay of radioactive elements when they’re just sitting there (i.e., without it getting smacked hard by some other particle). This is classically impossible since the energy barrier for a part of the nuclei to separate itself from the rest is just too high; it only occurs because of quantum tunneling, which is an example of superposition. The alpha particle portion of the nucleus must be able to exist in multiple places at once – some inside the nucleus and some outside the nucleus – in order for the decay to occur.

  13. Jiten
    Posted November 1, 2015 at 4:11 pm | Permalink

    Jerry, you say that explaining entanglement simply can’t be done using words. But this can’t be true. The only things that can’t be done are those which are not permitted by the laws of physics. And there is no law of physics that says that mathematical symbols can’t be translated into words. Now it’s true that currently we don’t know how to do this. But in 400 years? In 1000 years? We just lack the knowledge at present.

    • Diana MacPherson
      Posted November 1, 2015 at 4:44 pm | Permalink

      I am compelled by determinism to drag out this Battlestar Galactica quote by Cavil the Cylon where he bemoans the limitations of his meat brain and body. I think language is one of those limitations when it comes to physics.

      I don’t want to be human! I want to see gamma rays! I want to hear X-rays! And I want to – I want to smell dark matter! Do you see the absurdity of what I am? I can’t even express these things properly because I have to – I have to conceptualize complex ideas in this stupid limiting spoken language! But I know I want to reach out with something other than these prehensile paws! And feel the wind of a supernova flowing over me! I’m a machine! And I can know much more! I can experience so much more. But I’m trapped in this absurd body!

    • Ralph
      Posted November 1, 2015 at 5:19 pm | Permalink

      It’s not even clear what “explaining” would mean, beyond an instrumental definition. How would you determine if someone truly understands the concepts, other then giving them a math test?

      In the case of QM, it’s particularly baffling. The consensus even among physicists who CAN do the math of QM seems to be that nobody knows for sure what it “really” means. The Copenhagen Interpretation has been jokingly described as “Shut Up And Calculate” – in other words, it isn’t really a philosophically satisfying interpretation at all; but we can do the math, make predictions, and then do experiments to show that the universe really does work this way.

      As to your more general point, that in 1000 years we will be able to express all mathematical ideas in words – this seems like (?) a category error, or something like that. Mathematics IS just a form of ordinary language, modified to suit its purpose – to have much greater precision and rigor. If you try to write math in English, you can sort-of do so – but even a simple statement ends up with multiple footnotes (and footnotes to the footnotes) to account for every possible ambiguity that’s present in everyday English. It’s incredibly inefficient, and certainly doesn’t become easier to understand. If you want to (fully) understand math, you need to learn how to speak the language of math, it does not help to try to use a language that’s unsuited to the purpose.

      If you don’t speak “math”, talented didacts like Feynman or Krauss can give the layman a hint of what’s going on conceptually, but in something as bizarre as QM it’s never going to be easy.

    • Ralph
      Posted November 1, 2015 at 5:44 pm | Permalink

      Another way to look at this:

      Human natural language is an way to communicate ideas that are intuitive to humans. There’s no reason to expect that natural language can express ideas that are outside of our experience at all – on a size scale that’s outside of everyday experience (atomic or cosmic) or on a timescale that’s outside of our experience (particle interactions or the geologic/evolutionary timescale).

      By analogy, some ants communicate largely with pheromones, which are suited to the niche of their existence. Is there any reason to suppose that in 1000 years (or ever) we will be able to explain even easy ideas like (say) the reason for the change of seasons to an ant?

      To me, it’s already quite astonishing that humans can understand as much as we do, including at scales of size and time that are entirely outside of our everyday experience. The fact that we have even a hint of a clue about our cosmic origins is utterly amazing. Ultimately, we will probably reach our limits, and the way forward will be through artificial intelligence, or augmentation. The AIs that we create will probably soon understand far more about the universe than even the most talented human; and they, in turn, will learn how to create ever smarter AIs that can understand yet more. And the AIs won’t use English to reason or communicate, that’s for sure – if they retain it at all, it will just be to talk (down) to us, if we’re still around.

  14. Ken Phelps
    Posted November 1, 2015 at 4:28 pm | Permalink

    I just take comfort in having read Flatland years ago. It helped me understand the concept of reality being utterly non-intuitive to beings whose perceptions are the product of only a part of the set of things that exist. The inability of the two dimensional inhabitants of Flatland to understand or visualize or fully interact with a third dimension has always helped me understand that incomprehensible is not synonymous with supernatural. Or maybe not even all that weird, given the very narrow slice of things that we can perceive.

    Oh dear, I think I’m having a “the wall is fucking brown” moment.

  15. Diana MacPherson
    Posted November 1, 2015 at 4:45 pm | Permalink

    Can we use entanglement for encryption and as a replacement for passwords?

    • Richard Bond
      Posted November 2, 2015 at 4:14 am | Permalink

      Probably not, because there is no actual information in the entangled state (at least, none accessible to us). Anyway, identifying the state of entanglement destroys it. (See my reference to decoherence in response to comment #6 above.) In practical terms, setting up an entangled state is extraordinarily difficult, which is why it has taken half a century to (probably) verify the conclusions from Bell’s theorem.

      On the other hand, quantum encryption might be possible: see Chapter 8 of The Code Book by Simon Singh.

  16. Posted November 1, 2015 at 4:58 pm | Permalink

    In much the same way that Newton and Liebniz had to develop calculus in order to do their work, I think a new word, something well beyond bizarre or weird, should be developed to properly describe entanglement.
    I submit, “perfunkulated.”

  17. Posted November 1, 2015 at 5:54 pm | Permalink

    A nice way to think about it, in my opinion, is to think about entangled objects as multiple manifestations of a same variable. Observing one will tell you the state of the others. There’s no causality issues.

    • Posted November 1, 2015 at 5:57 pm | Permalink

      Simpler: entangled objects are correlated due to a common cause. Observing one will tell you the state of the others.

      • gravelinspector-Aidan
        Posted November 2, 2015 at 5:31 am | Permalink

        Simpler: entangled objects are correlated due to a common cause. Observing one will tell you the state of the others.

        Woah! hold on there. I always thought that entangled particles came in exactly pairs.

    • Ralph
      Posted November 1, 2015 at 6:18 pm | Permalink

      But as I understand it, this is precisely the “intuitive” explanation that is ruled out by Bell’s Theorem and these experiments.

      https://en.wikipedia.org/wiki/Local_hidden_variable_theory

      That’s why it’s all so weird.

      • Posted November 1, 2015 at 7:29 pm | Permalink

        Ralph, I’m not talking about hidden variables, but simple probabilistic correlations due to a common cause (entangled particles have a common origin). If I always wear pairs of socks of the same color, I just need to observe one sock to know the color of the other. It’s nothing like “observing blue sock on the right foot” causes “blue sock is observed on the left foot”.

        • Ralph
          Posted November 1, 2015 at 8:19 pm | Permalink

          Do you agree that this is what these experiments distinguish between:

          (a) both particles are in the same unrevealed but determined state, carried from the source to the detector; the detection of the state of one particle tells you about the other one, but they had always been that way;

          (b) superposition, no single state prior to detection; the measurement of one particle determines the state of the other particle instantaneously.

          The violation of Bell’s inequality in these experiments show that (a) is wrong, (b) is correct.

          Your description – “simple probabilistic correlations due to a common cause” sounds like (a) to me – it sounds like you are ruling out the “spooky action at a distance” of (b).

          • Posted November 2, 2015 at 1:47 am | Permalink

            I agree with you that b) is right and a) is wrong.

            In my example, my socks are in an undetermined color until I observe one of them. Observing just one is enough to know the color of the other.

            Another (probably far-fetched) analogy: the state of two entangled particles is “stored” in the same memory block of the universe hard disk. The content of this memory block is undetermined/random (follows a probability distribution) until I observe one of the particles. At this point, something is written on the memory block and both particles assume a (correlated) state.

            • Ralph
              Posted November 2, 2015 at 4:10 am | Permalink

              And how are the two entangled particles and the “hard disk of the universe” instantaneously synchronized? If the answer is — we don’t know, that’s just the nature of reality because QM works that way — then i think that’s the very definition of “spooky action at a distance”.

              • Posted November 2, 2015 at 6:25 am | Permalink

                Yes, we don’t know. But I wouldn’t call it “spooky action at a distance”. It’s the way Nature works. Properties of particles are random variables, following a probability distribution and undetermined until “observed”. And there are properties of different variables, separated in space, which are correlated (due to having a common cause). Once we observe one, we know the other. (Btw, I’m not physicist, just curious.)

              • Posted November 2, 2015 at 6:40 am | Permalink

                If there is an algorithm running the universe, these correlations don’t look that mysterious. Particles properties follow probability distributions, assume a value when observed (or when they interact with something else), and different particles are correlated, even if far away in space.

            • Posted November 2, 2015 at 6:41 am | Permalink

              *may be correlated

      • Posted November 1, 2015 at 7:44 pm | Permalink

        I was reading this wikipedia entry https://en.wikipedia.org/wiki/Bell%27s_theorem, and in my view, the following excerpt is a clear example of the misunderstanding of this topic. Why assume there must be causality between the two particles? We can apply this same argument to the socks example above.

        “Imagine a pair of particles that can be measured at distant locations. Suppose that the measurement devices have settings, which are angles—e.g., the devices measure something called spin in some direction. The experimenter chooses the directions, one for each particle, separately. Suppose the measurement outcome is binary (e.g., spin up, spin down). Suppose the two particles are perfectly anti-correlated—in the sense that whenever both measured in the same direction, one gets identically opposite outcomes, when both measured in opposite directions they always give the same outcome. The only way to imagine how this works is that both particles leave their common source with, somehow, the outcomes they will deliver when measured in any possible direction. (How else could particle 1 know how to deliver the same answer as particle 2 when measured in the same direction? They don’t know in advance how they are going to be measured…). The measurement on particle 2 (after switching its sign) can be thought of as telling us what the same measurement on particle 1 would have given.”

        • Ralph
          Posted November 1, 2015 at 8:31 pm | Permalink

          I find the example in that Wiki to be confusing. I think the quote you give may be laying out the assumption that is then DISPROVED by the violation of Bell’s Inequality.

          • Posted November 2, 2015 at 1:49 am | Permalink

            My point was that that assumption was already extremely suspect..

      • Posted November 1, 2015 at 8:16 pm | Permalink

        Ralph, that’s right. The whole point of Bell’s theorem is that it is impossible for these correlations to arise from some common cause independent of the measurements that will later be made on the photons. And the proof requires no math beyond gradeschool algebra, and no assumptions about how QM works, and no assumptions about its completeness. It is one of the most brilliant and far-reaching deductions in the entire history of science and philosophy.

    • eric
      Posted November 2, 2015 at 7:08 am | Permalink

      Yes, AIUI entangled particles are one big wavefunction spread across space. Asking how one particle can ‘know’ that the other has been measured is (to use an inapt Newtonian metaphor) kind of like asking how the left side of the electron knew the right side did something.

  18. Ralph
    Posted November 1, 2015 at 9:18 pm | Permalink

    I just came across this. I think it’s the best explanation that I’ve seen. I’d be interested in comments from the experts here – it makes sense to me, so I hope it’s actually correct! The author Gary Felder seems to be a prof at Smith College.

    As has been mentioned, the math is trivial arithmetic so I think almost anyone should be able to “get” this. Once you’ve got the terminology, the tricky part is following the logic rigorously, thinking carefully through the underlying assumptions. He does a good job of taking you through that.

    http://www.felderbooks.com/papers/bell.html

    • Posted November 1, 2015 at 9:34 pm | Permalink

      Yes, that’s a good explanation that stresses the simplicity and generality of the result. The result does NOT depend on the details or assumptions of QM. That’s the most important take-home message. The universe itself is bizarrely different than we would expect based on our macroscopic intuitions.

      • Ralph
        Posted November 1, 2015 at 9:58 pm | Permalink

        That account shows how sacrificing Locality explains the data.

        But apparently the other option is to keep Locality and sacrifice Realism, and on reflection I don’t think I grasp how that works. I took it to mean something along the lines of just giving up on hidden variables, but that doesn’t make sense.

        So what does it mean to sacrifice Realism? What are the interpretations that “save” Locality?

        • Militant Scientist
          Posted November 3, 2015 at 7:09 am | Permalink

          The cards that the electrons hold in this example are the “realism”. Since the electrons have a classical instruction set, non-locality is required to update the instruction set.

          Quantum correlations are generated between two-electrons at the same location. Quantum correlations are the lack of “realism”. In other words, the electrons don’t carry instruction cards. The electrons are then separated and measured each individually locally. Comparing the measurement correlations later reveals the electrons had quantum correlations (did not have cards).

    • Posted November 1, 2015 at 9:46 pm | Permalink

      Here’s an even simpler explanation, by analogy with the reality of a person’s pizza topping preferences:
      http://physics.illinois.edu/outreach/saturdayphysics/2005/weissman.pdf

    • phoffman56
      Posted November 2, 2015 at 9:33 am | Permalink

      Not surprisingly, this reference to the ‘professional scoffer physicist’ Motl has elicited much comment. The following, direct from the start of Motl’s web page referred to:

      ” …they try to present quantum mechanics as a suspect sitting on the bench – while some alternative theory proposed by … or Everett or … plays the role of the judge. The only problem is that there exists no viable alternative theory….”

      already shows why Motl is pretty much just shooting his mouth off. The many-worlds or Everett so-called interpretation simply IS straightforward quantum mechanics, not an “alternative theory”, predicting some experimental differences, as Motl so incorrectly puts it and implies.

      And so is the Bohm theory (in both cases with ‘extra’ ontology, not physics, to replace the fig-leaf from Copenhagen). Bohm-deBroglie hidden variables are, as David Deutsch puts it sarcastically but after a careful analysis, ‘the Bohm theory is a many-worlds theory in a state of chronic denial’.

      But, to go back to the scoffer, I wouldn’t, once seeing the above, waste time reading Motl in his characteristic put-down of Krauss (nor of Einstein, whose work in questioning Quantum Mechanics was more crucial than anything else in leading Bell to his important work and in pointing out what came to be understood as entanglement).

      I gave references earlier here related to David Wallace mainly so won’t repeat, except to mention, with respect to another comment earlier above mentioning Everett, that the Born rule is now considered pretty solidly to be a logical consequence to ‘many-worlds’, not something, as had been true 30 to 55 years ago, something which seemed to be added in an ad-hoc manner.

      • phoffman56
        Posted November 2, 2015 at 9:37 am | Permalink

        Sorry,this reply should be to comment 19 just below.

  19. Posted November 2, 2015 at 1:09 am | Permalink

    On these matters (i.e. quantum mechanics), Krauss is as deluded as Chopra (now, maybe not as deluded, but for a real physicist it is really embarrassing)
    Please read this:
    http://motls.blogspot.ro/2015/10/new-painful-hype-on-spooky-nonlocalities.html

    • Posted November 2, 2015 at 1:52 am | Permalink

      Yes, Motls seems to be right, imo.

    • Ralph
      Posted November 2, 2015 at 2:08 am | Permalink

      Well, that’s just silly hyperbole. I’m certainly not qualified to attempt to adjudicate between two prominent physicists, but so far as I can tell, Krauss’s description is pretty much in accord with the mainstream view of this experiment.

      And to give a little political background: Lubos Motl (the physicist blogger you linked to) is at the extreme end of the pro-string theory school; and Krauss has been one of the voices who followed Woit (“Not Even Wrong”) and Smolin in giving string theorists a hard time. So there’s no love lost.

      Motl seems to be saying that the results of this experiment are so obvious that the experiment was not worth doing. Clearly, many others disagree. Motl completely ignores the important technical acheivement here – closing the two loopholes SIMULTANEOUSLY.

      And he claims that “spooky action at a distance” doesn’t really happen, because the correlations just follow what has always been predicted by QM & entanglement. Well, sure – but if I understand correctly, the point is that to produce these QM correlations in a classical framework you DO require the spooky action. So the point is, if QM works (and we know it does) it is intrinsically spooky, when viewed from the perspective of our classical intuition.

      • Posted November 2, 2015 at 2:23 am | Permalink

        “Well, sure – but if I understand correctly, the point is that to produce these QM correlations in a classical framework you DO require the spooky action.”

        But why a classical framework? The world is quantum, not classical.

        • Ralph
          Posted November 2, 2015 at 2:53 am | Permalink

          Because we want to gain insight into the nature of reality, and our intuition is classical.

          Imagine the world before QM was discovered. QM comes along, Einstein notices that QM (seen in classical terms) seems to entail spooky action, and decides QM must therefore be wrong. And as we know, it turns out that Einstein was wrong. Even though, in classical terms, QM does entail spooky action, QM is a correct description of the universe.

          I think it’s perverse of Motl to turn this on its head and say that: QM is the way the universe works; therefore, by definition, the correlations predicted by QM are not spooky.

      • Posted November 2, 2015 at 3:00 am | Permalink

        The point is that for those who understand and accept QM as the right theory, the experiment is worthless, the size of it is just a matter of engineering (technical achievement, as you say).

        • Posted November 2, 2015 at 3:10 am | Permalink

          Anyway, this conclusion is utterly false, even with our classical intuition:
          “What’s amazing is that nonlocality exists at all: that getting information about one electron a gazillion miles away form its entangled twin will affect the state of that electron instantly. This now appears to be the case, but I still can’t get my head around it.”

          • Ralph
            Posted November 2, 2015 at 3:38 am | Permalink

            So far as I can tell, your argument is semantic. I think most people who understand QM find it bizarre, and describe the behavious of widely separated entangled particles as “spooky”. Since everyone agrees on the math, it would appear that the substance of your argument is just that you are now accustomed to QM, which has been well established for many decades, so you don’t personally feel that its predictions are spooky.

            The existence of this theorem

            https://en.wikipedia.org/wiki/No-communication_theorem

            would suggest that many people feel that spooky action at a distance does occur in a substantial sense, and needs to be addressed, by showing that it does not allow superluminal communication.

            If “spooky action at a distance” in the way that Krauss describes (which is the way that it’s commonly understood) simply doesn’t happen, why did anyone feel the need for a No-Communication Theorem?

            • Posted November 2, 2015 at 4:15 am | Permalink

              I think that somebody pretending there is an instanteneous affect is very wrong, it’s suggest superluminal communication, and it’s not a matter of semantics. It’s a matter of confusing correlation with causation.

        • Ralph
          Posted November 2, 2015 at 3:20 am | Permalink

          Motl discusses both loopholes separately in some detail, diminishing this experiment by noting that, in respect of either loophole alone, this experiment breaks no new ground. Why does he fail to address the most significant claim of this experiment: that both loopholes are closed SIMULTANEOUSLY? I really can’t accept that he’s discussing this in good faith if he simply ignores the central claim to make his polemics sound more convincing.

          And I think a lot of people feel that the QM description of reality – which I agree, nobody would now seriously challenge – is so weird and counterintuitive, the more evidence and certainty the better.

          And even if you think that the result is so obvious that it’s a foregone conclusion, it’s pretty clear that the technical advances here will be significant for other purposes.

          • Posted November 2, 2015 at 3:26 am | Permalink

            He discusses and correctly explains this kind of experiments a thousand times in his blog.

            • Ralph
              Posted November 2, 2015 at 3:40 am | Permalink

              I’m not questioning whether he understands the experiment. I’m questioning whether he’s describing it objectively and in good faith.

              • Posted November 2, 2015 at 4:55 am | Permalink

                Well, the answer is in his blog.

              • Posted November 2, 2015 at 6:08 am | Permalink

                Of course if one starts with the belief that QM is the perfect final theory, the experiment isn’t informative. But yes, the correlations are hard to make sense of, and there was always the chance that some new theory would find a local realistic answer via some loophole. The new experiment puts stronger constraints on all future replacements of QM, and in that sense it is interesting.

  20. Posted November 2, 2015 at 6:17 am | Permalink

    I agree that Motl is right about this one. Your woo alarm should go off any time you see an article that claims or implies a violation of relativity, and faster than light communication violates relativity.

    • Ralph
      Posted November 2, 2015 at 9:57 am | Permalink

      Did you read a different article?

      Krauss:
      “But that communication can happen, at its very fastest, at the speed of light. Entanglement may be instantaneous, but it produces no signals that can be detected instantaneously. Its detection still follows the ordinary laws of cause and effect.”

      • Posted November 2, 2015 at 12:23 pm | Permalink

        “Entanglement may be instantaneous”
        holy crap, what could this mean? 🙂
        Entanglement is the process in which two objects interact locally and as a result some of their properties will have some correlation, regardless of the distance.
        Entanglement is not the process of measurement and the woodoo (aka inexistent) process of interaction between those two objects, instantaneous or not.

        • Posted November 2, 2015 at 12:54 pm | Permalink

          Yes, I did read the article by Kraus, and consider it outstanding. Did you read the name “Motl” in my comment? Did you read his article on the Delft experiment? Nowhere in that article is there any mention, either of Kraus or his article. It follows logically that I was not criticizing that article. The only reason I brought it up was that Motl’s post was mentioned earlier in the thread.

          I don’t like to click on links in comment threads, but you can easily find the article in question by going to Motl’s blog, “The Reference Frame,” and looking for the title, “New Painful Hype on ‘Spooky Nonlocalities.'” The only part that I agreed with in my comment was the bit about “painful hype,” and in that he’s spot on. Every time an experiment like this is done, the ubiquitous woo-meisters posing as science journalists out there start publishing stuff about faster than light communication, or how Einstein’s relativity has been proved wrong, or how the laws of physics have been debunked, or how it’s all “spooky.” Motl referred to an article by Markoff in the NYT, but it’s easy to find similar stuff by Googling the experiment and checking the articles that turn up at Time, The Daily Mail, The Express, etc., etc. They get away with it because most students graduate from high school knowing even less about quantum than they do about evolution. It’s how the universe works, it’s why our electronic stuff works, and there’s nothing “spooky” about it.

  21. Mike
    Posted November 2, 2015 at 6:51 am | Permalink

    Haldane;”Not only is the Universe queerer than we think,its queerer than we can think”, says it all really.

    • Robert Bray
      Posted November 2, 2015 at 9:45 am | Permalink

      Except that he thought this and thereby made a claim he could not make, by his own admission.

      • Ralph
        Posted November 2, 2015 at 10:00 am | Permalink

        Fair enough, but we could repurpose the quote for a plaque on the wall of the Rowan County clerk’s office.

  22. Posted November 2, 2015 at 7:00 am | Permalink

    Looking for stuff on Chopra vs quantum physics, I came across this from 1986 — an affidavit written by physicist Heinz Pagels, as part of a fraud case brought against Chopra’s guru Mahesh Yogi.

    He states that anyone claiming:
    a connection between the recent ideas of theoretical physics–unified field theory, the vacuum state and collective phenomena–and states of consciousness attained by transcendental meditation are false and profoundly misleading. No qualified physicist that I know would claim to find such a connection without knowingly committing fraud. While I am not an expert on the meditation techniques advocated by the Maharishi I have experienced and studied meditation methods in the Buddhist and Hindu traditions. There is no known connection between meditation states and states of matter in physics.

    1986! I guess he didn’t state it clearly enough for Chopra to understand.

    http://web.archive.org/web/19990420021943/http:/www.trancenet.org/research/pagels.shtml

    • Sastra
      Posted November 2, 2015 at 9:21 am | Permalink

      Good catch.

      This though is where the woo proponents stuff their ears with the usual apologetics regarding how the experts in the field won’t admit, don’t understand, can’t feel, etc. the TRUE interpretation. And then they dutifully trot out a crank or two with degrees in physics to back them up. Because any minute now the paradigm is about to shift …

      It’s like people waiting for Jesus to return. A thousand failed prophesies mean nothing because THIS ONE is it.

  23. Macha
    Posted November 2, 2015 at 7:13 am | Permalink

    Entanglement is fascinating, yet when discussing the idea with non-sciencey people, the moment you get to “the electron is in both spin states at the same time and it’s only when you measure it .. etc”, their eyes either glaze over, or they assume a sceptical “Nah” expression.

    Indeed, Einstein’s idea of “hidden variables” was a sceptical approach – which was later dismissed by Bell’s inequality. Even so, the idea that somehow a “spin up” electron has always been a “spin up” electron and somehow we haven’t yet quite figured it out properly is still a tempting notion.

    Which brings me on to an experiment which as far as I can tell, hasn’t yet been done …..

    There’s a group of experiments involving photons called “quantum erasers” in which entangled photons (with polarisation either clockwise or anticlockwise, resulting in a net unpolarised system) are detected kilometres apart and are found to always have opposite polarisation, showing that the outcome of an event at place X determines the outcome of an event at place X’ (there’s an even weirder one, the delayed eraser, where the outcome of an event at time T determines the outcome of an event at time T-t, but that’s another story).

    So, the experiment I’m thinking of would be to generate two “oppositely” polarised photons and then change the polarisation of one of the photons (I think a mirror will reverse it and a quarter-wave plate will linearise it) and then see if the system’s overall non-polarised state is retained.

    Maybe it’s been done, but I can’t locate it.

  24. Posted November 3, 2015 at 12:26 pm | Permalink

    Some help needed here…

    Lawrence Krauss says “[…] when a measurement of one electron here can instantaneously affect the measurement of another electron on the opposite side of the universe, faster than the speed of light, it does seem as though causality has been thrown out the window.” And then, he mentions the cosmic Catch-22 that makes superluminal communication still out of reach.

    However, as a layman, I would rather tend to think —naively?— that causality has been nevertheless thrown out the window. I do perfectly understand that part about the fact that superluminal communication is out of reach. I would not want to wait some billions of years to hear “Yeah, spin is up now!”

    Even if detection still follows the ordinary laws of cause and effect, that electron in the opposite side of the universe does instantaneously affect this entangled electron nearby, if I’m not mistaken.

    And as such, this is extraordinary! Or quite spooky in the words of Einstein.

    Someone could explain to the layman that I am, what I don’t grasp here?

    As if the word “extraordinary” was buried because things are nevertheless “under control”: that’s the part I don’t understand. No pun intended, just an honest question.

    • Ralph
      Posted November 3, 2015 at 2:55 pm | Permalink

      I think you have pretty much got it, but to recap:

      Set up two widely separated detectors (at the ends of the universe, if you like) to receive pairs of entangled particles. Compare the results of detecting multiple pairs of entangled particles. Bell’s Theorem shows that the results cannot be explained by the entangled pairs carrying any kind of classical “instruction sets” from their origin. The only way to explain the results is that the act of detection of one part of an entangled system “fixes” the state of the entire entangled system instantaneously, however widely separated the parts may now be. However, the experimental proof consists of correlations between results at the two detectors over many trials. The Bell’s Theorem result may only be proven after the fact, by subluminally bringing together the results from the two detectors. It turns out that it is not possible, even with a prearranged scheme, to communicate any information whatsoever between two detectors instantaneously. If you don’t see how this could be so, it’s best to look at an actual experimental setup – I found this to be a clear explanation, there’s virtually no math:

      http://www.felderbooks.com/papers/bell.html

      So, it appears that the nature of Quantum Mechanics is that an entire entangled system is “changed” instantaneously in some sense when any one part of the system is measured. If you try to understand this in a Classical framework, the only way to explain it is with Einstein’s “spooky action at a distance”, i.e. one part of the entangled system seems to “tell” all parts how to behave instantaneously.

      However, it’s important to emphasize that the wavefunction that is “changed” “instantaneously” does not have any real analogy in the Classical world of our intuition, so it’s wrong to thing of it as a Classical particle at one end of the universe “changing” another Classical particle at the other end of the universe in the Classical sense of cause and effect. And, criticaly, this “change” does not entail any violation of Special Relativity. No cause/effect paradox arises, and no information may be actively transmitted superluminally.

      This feeling of spooky instantaneous action arises when you try to form an intuitive Classical account of what happens in Quantum Mechanics. But nobody has a universally accepted satisfactory interpretation of what REALLY happens (or even what “really” means). Suffice it to say that Quantum Mechanics behaves in a way that seems fundamentally spooky to our Classical intuition; and Bell’s Theorem restricts any more complete theory that may later arise to also entail these “spooky” qualities.

      In any event, if you understand it correctly, you SHOULD come away feely deeply uncomfortable. Feynman (possibly apocryphally) said something like “if you think you understand QM, you don’t”, meaning – the math works, but nobody has a satisfactory intuitive interpretation.

      • Posted November 3, 2015 at 6:05 pm | Permalink

        Thanks Ralph for such an elaborate answer to my question!

        The link you provide on the Bell’s theorem, as explained by Gary Felder, is effectively quite helpful: I stuck on the 3×3 matrix and was clear on the fact that a 5/9 result is indeed theoretically expected. Then, he “assaults” his readers with the experimental result of 1/2 instead —hence that Bell’s inequality.

        Your detailed explanation is also a nice complement to the Felder’s demonstration.

        In the end, I do feel uncomfortable.

        Then I guess I passed the test…

        Thanks again!

    • Posted November 4, 2015 at 3:20 am | Permalink

      “a measurement of one electron here can instantaneously affect the measurement of another electron on the opposite side of the universe”
      This is simply bull-shit


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