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Fluid Tests Hint at Concrete Quantum Reality (simonsfoundation.org)
171 points by karolisd on June 30, 2014 | hide | past | favorite | 117 comments

I've liked:

Clearing up Mysteries - The Original Goal http://bayes.wustl.edu/etj/articles/cmystery.pdf

" ...we must keep in mind that Einstein's thinking is always on the ontological level; the purpose of the EPR argument was to show that the QM state vector cannot be a representation of the "real physical situation" of a system. Bohr had never claimed that it was, although his strange way of expressing himself often led others to think that he was claiming this.

From his reply to EPR, we find that Bohr's position was like this:

"You may decide, of your own free will, which experiment to do. If you do experiment E1 you will get result R1. If you do E2 you will get R2. Since it is fundamentally impossible to do both on the same system, and the present theory correctly predicts the results of either, how can you say that the theory is incomplete? What more can one ask of a theory?"

While it is easy to understand and agree with this on the epistemological level, the answer that I and many others would give is that we expect a physical theory to do more than merely predict experimental results in the manner of an empirical equation; we want to come down to Einstein's ontological level and understand what is happening when an atom emits light, when a spin enters a Stern-Gerlach magnet, etc. The Copenhagen theory, having no answer to any question of the form: "What is really happening when - - - ?", forbids us to ask such questions and tries to persuade us that it is philosophically naive to want to know what is happening. But I do want to know, and I do not think this is naive; and so for me QM is not a physical theory at all, only an empty mathematical shell in which a future theory may, perhaps, be built."

My favorite quote on the topic is from chapter 10 of Jaynes' Probability Theory: (http://omega.albany.edu:8008/ETJ-PS/cc10i.ps)

"Biologists have a mechanistic picture of the world because, being trained to believe in causes, they continue to search for them and find them. Quantum physicists have only probability laws because for two generations we have been indoctrinated not to believe in causes - and so we have stopped looking for them. Indeed, any attempt to search for the causes of microphenomena is met with scorn and a charge of professional incompetence and "obsolete mechanistic materialism." Therefore, to explain the indeterminacy in current quantum theory we need not suppose there is any indeterminacy in Nature; the mental attitude of quantum physicists is already sufficient to guarantee it."

You are misrepresenting physicists. There have been many many different attempts to talk about quantum theory on an ontological level. Case in point, the widely debated interpretations: copenhagen, many worlds, bohemian etc.

However, one reason that physicists don't spend too much time thinking about what quantum theory really means (besides laziness and bad science) is because we know that quantum theory is not the ultimate theory of reality. Why try to force ontology on a theory that can not even answer all empirical questions without ad hoc additions (standard model of particle physics) or force mating with another theory (general relativity)? We are all waiting for a better theory of physics. Of course rewriting the quantum theory in different forms, like this attempt, might help in getting there.

I disagree completely. Putting quantum mechanics on firm ontological footing will probably aid developing intuition for whatever the real truth of the matter is.

Note Einstein developed his theories of relativity based on very scant evidence just by contemplating the aesthetics of what physics should be. I think there's a big lesson to be learned here - that some heuristics are generalizable.

I agree with you. Read my reply to the other comment. I should have been clearer in my first comment.

I agree and disagree with what you're saying. I agree it is laziness and bad science that physicists stopped caring about the philosophical underpinnings of quantum mechanics but I disagree that it was because quantum mechanics not being the ultimate theory.

The reason most physicists don't think about the philosophical underpinnings has more to do with the way quantum mechanics is taught. Many of the founders were interested in the philosophical questions but after WWII you can see a sharp decrease in that aspect and more of the hardcore calculation and the philosophy kind of disappeared. Here [1] is a great talk that discusses this very point. Unfortunate for quantum mechanics because understanding the philosophical underpinnings is critical to understanding what quantum mechanics says and doesn't say. But, thanks to things like quantum computing and better experiments, interpretations is becoming more mainstream since there are experiments being planned (at least when I was heavily involved in the area in 2010) that would be able to test some of the claims made by different interpretations.

>We are all waiting for a better theory of physics. Of course rewriting the quantum theory in different forms, like this attempt, might help in getting there.

To me this is a contradiction. If we are all waiting for a better theory, then why are we re-writing the theory? A critical part of all the interpretations I've encountered [2] is making different ontological claims about quantum mechanics. Take Bohmian, which posits a guiding wave. Sure we can make some of the mathematics nicer, but we can't go very far without explaining what this 'guiding wave' is, which itself is rather ad hoc. Plus, how do we rectify it with Bell's theorem on hidden variables? To me, we can't really begin re-writing quantum mechanics without inevitably running into ontological statements.

But I still agree with your premise. I believe that by pushing the limits of quantum mechanics both scientifically and philosophically we will get a better grasp of what things to look for / where to look, for a deeper theory. Case in point, wavefunction collapse. Some might consider it a huge problem of quantum mechanics, but if you take a different ontological view, wavefunction collapse isn't that interesting or important. So maybe wavefunction collapse isn't as important and trying to develop a theory that doesn't have it is the wrong idea.

[1] http://pirsa.org/08090000/ [2] http://pirsa.org/C10002

I agree that progress in experimental abilities and QC is really forcing physicists to think carefully about what QM means. In fact the month of May saw three papers by major physicists about these deeper issues in QM [1,2,3] In my own humble capacity as a graduate student in physics, my research is about clearly understanding the divide between classical and quantum physics.

On the rewriting of theory, I should have been clearer. I meant that when you rewrite the theory you should think about ontology. Differences in ontology between different rewrites, as well as interpretations of those rewrites, will point to how quantum theory can be generalized.

I agree completely that quantum mechanics is taught horribly. Strangest of all are introductory quantum/modern physics courses that are meant to give students an intuition of quantum theory use the Copanhagen interpretation -the one that says the least about ontology and emphasizes calculations and empiricism.

I still stand behind my central claim. Let me be clearer. The mathematical model of classical physics says something about physical reality. The mathematical model of quantum mechanics says something very different about reality. Even if there are disagreements, we all agree that the ontology of classical physics is very much incorrect. Extrapolating from this I claim that the next theory of physics will have an ontology that will look nothing like that of quantum theory. As I have said previously, the ontology of quantum theory can be a means to an end (the next theory), but not the end itself.

[1] http://arxiv.org/abs/1405.1548 [2] http://arxiv.org/abs/1405.3483 [3] http://arxiv.org/abs/1405.7577

My understanding is that the pilot wave theory is a non-local hidden variable theory, which doesn't contradict Bell's theorem at all. Of course, this presents ftl information transfer, presumed to be impossible, which is part of the reason such theories have been unpopular for so long.

Non-locality does not imply the possibility of FTL information transfer - a theory can be non-local but make exploiting the non-locality impossible.

This is a vitally important point. Many people still naively believe entanglement allows communication. Simultaneous randomness doesn't permit a signal.

>Plus, how do we rectify it with Bell's theorem on hidden variables?

I'm not a physicist, but I've always wondered about Caroline Thompson's work, like:

"Chaotic Ball" model, local realism and the Bell test loopholes


>You are misrepresenting physicists.

Small correction. Except for the first two lines of the parent post, the rest is a quote from Jaynes' paper linked above. It may be hard to tell because of the limited formatting options on HN.

If that was the case, they will be waiting for eternity. There will never be a complete theory of everything, just ask Godel. :)

I'm not sure how science can expect to answer questions of the form "What is really happening when... ?", since that is entirely unknowable.

Science looks at results and provides useful models. Whether those models are representations of the 'real', or whether the entire universe is actually a bunch of magic pixies who just happen to arrange the universe to look as if it follows a particular theory, or whether the whole thing is a simulation embedded in the real universe which is entirely different are questions not actually accessible to science.

Could it be that you're not prepared for "What is really happening"? It seems like you'r expecting that QM's unintuitive properties will go away with a better theory.

They may, but it doesn't have to be that way. It may just be that reality at that scale is not made of trajectories, balls, and simple causality. As evolved monkeys with 5 senses we are used to intuitive concepts like continuity, predictions, trajectories, objects.

Now imagine a world 1,000,000 times smaller. Will it behave, look and feel the same way? Maybe. Let's shrink it 10,000 times again. Now we are at the size of an atom. Does the landscape look familiar? It may be so unfamiliar that trying to shoehorn our monkey intuitive world-view is helpless.

No objects, no concept of position, no matter as "substance occupying a space", no limitations regarding being in different places at the same time. It may well be that things are so different that it's best described as a mathematical abstract world ... because no intuitive structures and behaviours exist there. (Heisenberg has some interesting correspondence with Bohr on the matter)

It would be like trying to search for Mario inside your nintendo. Mario and his physics only live as abstract equations. Taking apart the computer will not show us what Mario's flesh is made of. The analogy can't be taken too far of course, because there is no known "chip" besides the mathematical structure of the universe.

There is plenty of research on quantum reality (from one of my colleagues, for eg. http://arxiv.org/abs/0709.1149), but it may turn out that there isn't an easy "particle does X when it enters Stern-Gerlach". The concept of particle is probably completely inadequate.

This goes back a long ways before QM. Even Newton, observing gravity, concluded that he had no idea why it worked, he could only describe how to numerically model the results of it working.

We've been modeling ever since, hoping that enough pieces of the puzzle come together for a cogent picture to emerge.

FAQ about Bohmian Mechanics (it's very accessible): http://www.mathematik.uni-muenchen.de/~bohmmech/BohmHome/fil...

Its certainly accessible, but I'm not sure how much I trust the authors to understand what they're talking about after point 3.

> But very often the conclusion that is drawn from this observation is that our _knowledge_ about the path followed by the electron causes the interference pattern to disappear.

Although we couch the idea in terms of "knowledge", the usual meaning of that word is not what anyone thinks is actually the trigger. What exactly is meant by "knowledge" is not well-agreed upon, but no one thinks it is something particular to humans (or sentient beings, I suppose) that causes the wavefunction to collapse. It has more to do with whether the information exists - if the particle interacts with something in such a way that its position suddenly becomes deducible, then it collapses. Like I said, the specifics of this are a known hole in the Copenhagen theory - but this "knowledge" tangent is a clear strawman.

That FAQ is generated by the students of a very trustworthy group. Students typically setup strawman when explaining things. Look more to the source. Also, they are trying to argue against a theory which no one wants to fully specify. So they have to come up with something definite. And as soon as they do, the other side says "Strawman!" And then proceeds, as you have just done, to refuse to give a definite theory.

As a student of that group as well, I can't say that I am all that pleased by that kind of argument.

The bottom line is that collapse is untenable. There is no good place to put it and yet it is needed by the standard theory.

There is a range of possibilities of what might cause a wavefunction collapse, and the constructed strawman is nowhere near that. It would be like if we claimed the speed of light was 3.2e8 +/- 5e7 m/s and someone decided our whole theory was bunk because the speed of light is obviously faster than 100 m/s. Well yeah, it is, but there's no contradiction here.

"if the particle interacts with something in such a way that its position suddenly becomes deducible, then it collapses" would be a theory in the range of possibilities. It would be much more convincing to argue against something like that.

Yes! But the best theory is not Copenhagen or Pilot Wave.

Eliezer Yudkowski has a brilliant treatise on the Many Worlds interpretation here: http://lesswrong.com/lw/r5/the_quantum_physics_sequence/ that really should be required reading for anyone that wants to talk intelligently on the subject.

Edit: seriously, don't even bother reading the article. It (like most science journalism) is garbage. Take the time to work through Eliezer's sequence.

So please explain in a few sentences what is that that Yudkowski writes, did he make any new contribution to the standard model or made something else or do you agree that the standard model is the most researched and most usable model up to now, and he just made a lot of posts where he just writes a lot of text?

I see a lot of links in the article you gave, but I don't understand what we're supposed to discover in Yudkowski's writings after trying to follow most of them. There's a lot of free text, not much physics. The standard model is a lot of smart formulas supported by the decades of expensive elaborate measurements (and vice versa), however his texts look more like writings of some philosophy student who knows a little of the math than like a physicist's material. I'd also really welcome opinions of professional physicists.

Edit: Wikipedia entry about him seems to fit my impression: http://en.wikipedia.org/wiki/Eliezer_Yudkowsky "Yudkowsky (...) is an American blogger, writer, and advocate for Friendly artificial intelligence (...) Largely self-educated."

The Standard Model does not concern itself with the various interpretations of quantum mechanics. It can, as of now, neither verify or falsify any of the interpretations which, consequently, are not scientific theories or even hypotheses, but firmly on the side of philosophy of physics. The interpretations are attempts to answer why Nature works as it does, within the framework of the Standard Model which only appears to answer the question how.

Yudkowsky certainly hasn't invented the many worlds interpretation, which was originally formulated by the physicist Hugh Everett in 1957. Even though originally scorned, in the more recent times it has gained popularity among physicists. The series of blog posts by Yudkowsky are (in my opinion, at least) a persuasive argument in its favor against the competing interpretations, and are very much recommended reading for anyone who would like to better understand the issue.

> the interpretations which, consequently, are not scientific theories or even hypotheses, but firmly on the side of philosophy of physics.

Thanks, that's exactly what I wanted to know.

I'm completely satisfied with the "shut up and calculate" approach. For me, until somebody shows that he/she can calculate (that is, predict) more than what physicists achieve, they are the ones the closest to "the truth" and not the "interpreter."

Bring it back to the scientific method. The way to differentiate between competing hypotheses is by devising experiments that falsify some of them, and then running the experiments and either falsifying or failing to falsify them.

The issue here is subtle, and it's that the most popular interpretation, Copenhagen, isn't a complete theory because it doesn't tell you algorithmically when collapse occurs. For any possible algorithmic way to handle collapse, there's a corresponding experiment that could (at least in theory) differentiate between Copenhagen and Many Worlds. But the Copenhagen is inordinately slippery in that collapse is defined to occur ex post facto in whatever way is needed to make the experimental results match the theoretical results.

It's perhaps not so surprising that this shortcoming was overlooked in the beginning because Copenhagen was hypothesized before we really had a clear handle on the study of algorithms. But the fact that Copenhagen is still as popular as it is means that Yudkowski needs to spend a lot of time on philosophy of science, because that's what's holding back most people from seeing the problems with Copenhagen, and why at first glance it looks like philosophy.

spacehome suggested reading Elizer's posts on quantum physics precisely to not bother to deal with outdated statements like "the most popular interpretation, Copenhagen". Afaik, no serious physicist uses this any more, not even to explain quantum theory. To use wavefunction collapse to "explain" QM is like invoking "God" to explain the universe - i.e. it is not much of an explanation, and it raises more questions than it answers.

Text books ought to be rewritten to teach decoherence instead of outdated stuff like wave-particle duality, wavefunction collapse and such. That is the history of the development of QM and not QM as it is known today, in my limited knowledge.

If you get decoherence, much of the "mysteriousness" and "spookiness" that's talked about in such magazines just disappears and you find them all, every one of them, shallow.

I think I can agree with your first paragraph, even if I'd phrase it somewhat differently, but I don't see the connection between it and the next two.

I certainly meant those three paragraphs to be connected; perhaps I'm just poor at explaining. As I mentioned, the shortcomings of the Copenhagen Interpretation are subtle, and I originally linked to Yudkowski's treatment because I believe he does much better job of explaining it than I ever could.

Allow me to not agree with your classification of the top article as "garbage." I've got from it more than from the text you linked to and I'm even less satisfied with your explanations whereas Natalie Wolchover did a rather good article. Please tell me if you have any background in physics, or what is your background?

The major thing missing from the article for me is that she didn't mention that the co-author of the most recent paper on the topic is much more known as the security researcher than as somebody who does anything related to quantum physics:




He's author of (to me) very useful book "Security Engineering":


Last week, I wrote a comment replying to someone asking, if we can't use experiments to determine which interpretation is better, what's the point? Rather than repeat myself, I'll just link to it: https://news.ycombinator.com/item?id=7940647

You say there: "How they lean will influence what sort of questions they investigate, how they investigate it, and what sort of outcomes they will look for."

If I understand you, we can expect that somebody is maybe going to be the first to discover something new thanks to the way he's used to think about the subject. Still, before that happens, what do we have?

Edit: Only real hard science. The discoverer we expect of course must decide from which side to attack the matter to reach the new discoveries.

I don't know, but I'm not sure what you're asking. Kuhn's point (and mine, by transitivity) is that scientists are human with unavoidable biases, and these biases will influence what work they do.

The reason I don't know how to answer your question is that I interpret it to mean: what tangible result do we have before we have our first tangible result?

Of course, if that's not what you mean, then please clarify. You may want to read his book, though, as this concept is central to some of it. If you're saying, "what is the effect of such paradigms before they change," then it's best to read his book. One point he makes is that everyone operates under a paradigm, whether conscious of it or not. That is, we must think about our scientific work in some way, and whatever way we think about it will influence what scientific work we do. It is generally the case, though, that many people have some form of agreement on that "some way." When we do, that sets informal bounds on what is "acceptable science."

He does use quantum mechanics as an example, but that's loaded because we're still hashing it out. Another example he uses is phlogiston chemistry (http://en.wikipedia.org/wiki/Phlogiston_theory), which has thoroughly been supplanted. Those who first encountered oxygen were unable to recognize what it was because the paradigm in which they operated didn't contain the concept.

Thanks Scott. I understand that phlogiston thrived until real honest scientific experiments disproved it. Thanks to which "interpretation" before that point? Can't we call it "shut up, experiment and calculate"? Or, simpler, "do the science."

There was none, that I'm aware of. It was a long, drawn out affair over more than a century, as far as I know. If you're looking for a direct analog to the situation with interpreting quantum mechanics, then I'm afraid you won't find it.

The analogy I draw from it is not direct, but merely: scientists unavoidably think of their work in certain ways that are themselves not tested. This thinking influences their work.

(You've made some changes since I responded, so let me respond to your last question: No, they cannot. Just as choosing what articles goes into the newspaper makes the news inherently subjective, scientists themselves choose what problems to work on, and experiments to carry out. You want them to just "do the science," but what science? The mental framework that helps them decide what science to do is what Kuhn calls a paradigm, and my claim is that the interpretations of quantum mechanics are such a paradigm. That, then, means that the interpretations are important, even if we can't yet directly test them.)

Just by looking at the Wikipedia entry, it appears that Kuhn's book is more philosophical than the carefully researched history of science. How do you see it?

Mostly philosophical. It's given me ways to think about how science is done in practice.

If you can get the same answers, with a simpler or more elegant theory, you might consider that a "better" theory. IIRC, initially the heliocentric theory of the solar system gave worse approximations of planetary positions than the highly refined epicycles of the geocentric models.

IMHO the many worlds interpretation is one of the least appealing interpretation - millions of millions of millions of universes for nothing. What do you think about the following similar argument? Why are the laws of physics and the physical constants the way they are? There is a universe for every possible set of laws and constants and our universe just happens to be compatible with life. You can just render almost every interesting question moot with similar arguments - I would really prefer something more interesting at the heart of the universe.

On that level I don't have any problem with "many worlds" as it can be seen just a one more extrapolation of "we're not in the designed special position" principle: first we're not the center around which the planets circle, then our Sun also isn't the center of our set of stars, then our set of stars is one of many other galaxies... if our physical universe is just a lucky variant of other possible (and mostly uninteresting) universes, it at least doesn't appear surprising.

But there is an important difference - you can observe the other stuff in our universe and therefore know that we are not (exceptionally) special in our universe. Other universes are usually per definition completely outside of our reach, as hard to grab as a god or Russell's teapot.

> completely outside of our reach, as hard to grab as a god or Russell's teapot.

I agree! As in my other comments here, as long as there isn't any scientific result, I also won't regard the products of the proponents of such "interpretations" better than you.

Well, they're not completely outside our reach. Interference between our worlds and nearby ones is very testable, and the reason the theory exists.

That makes it out of reach. You cannot falsify the many-worlds-interpretation simply with interference. All other interpretations make the very same predictions.

What's more likely? That quantum interference appears because many worlds perpetually exist, or because they seem to exist but somehow don't, or because they sometimes briefly come into existence, hang around for a bit and then randomly collapse breaking all sorts of laws (Liouville thereom, CPT invariance etc.)?

Many Worlds is much tidier than all the alternatives.hh

Actually, I'd say the "illusion" option is the simplest and tidiest. It is more likely that we interpret a complicated reality from incomplete information than that reality truly is complicated.

Yudkowsky's contribution is pedagogical rather than anything fundamentally new. He explains quite well in short the process of "decoherence" and links it well to the many worlds interpretation. He makes a case for why the "many worlds interpretation" is not really an "interpretation", but is just stating what the mathematics says on its face anyway .. and hence is the one we should adopt. In particular, he does a good job of connecting this to "timeless physics" - i.e. the Weyl equation for the "wavefunction of the universe" which does not have time in it.

edit: Sorry! Tongue of the slip. I was referring to the "Wheeler-DeWitt equation" [1] and not the "Weyl equation" [2].

[1] http://en.wikipedia.org/wiki/Wheeler%E2%80%93DeWitt_equation

[2] http://en.wikipedia.org/wiki/Weyl_equation

Have you read Feynman's QED? It's a bit like that, in that it's meant for a layman, but with more algebra. Instead of abstract arrows, EY goes straight to complex numbers. It goes through a basic approach to quantum mechanics (taking diagrams from here for instance: http://www.qi.damtp.cam.ac.uk/node/60) in a modern way and tries to build an intuitive understanding of the subject, especially demolishing a lot of confusions one may have gained through popular media. He then departs from the basic theory to elaborate on why the collapse interpretation is ridiculous and why Many-Worlds makes more sense. You can stop when you get to the Timeless stuff.

You're not really going to find anything ground-breaking, but it may help your intuitions about QM even if you're a physicist. As a chapter from the Aaronson book (http://www.scottaaronson.com/democritus/lec9.html) I linked to in a cousin comment says in the first two paragraphs:

There are two ways to teach quantum mechanics. The first way -- which for most physicists today is still the only way -- follows the historical order in which the ideas were discovered. So, you start with classical mechanics and electrodynamics, solving lots of grueling differential equations at every step. Then you learn about the "blackbody paradox" and various strange experimental results, and the great crisis these things posed for physics. Next you learn a complicated patchwork of ideas that physicists invented between 1900 and 1926 to try to make the crisis go away. Then, if you're lucky, after years of study you finally get around to the central conceptual point: that nature is described not by probabilities (which are always nonnegative), but by numbers called amplitudes that can be positive, negative, or even complex.

Today, in the quantum information age, the fact that all the physicists had to learn quantum this way seems increasingly humorous. For example, I've had experts in quantum field theory -- people who've spent years calculating path integrals of mind-boggling complexity -- ask me to explain the Bell inequality to them. That's like Andrew Wiles asking me to explain the Pythagorean Theorem.

(And on EY himself, just read the million or so words of the Sequences and you'll see he is actually really smart across multiple domains. ;))

I never claimed that EY isn't smart, only that what he writes is not actually physics.

As much as I remember Feynman's lectures, he introduced the amplitudes very early?

Fair enough. I agree a large part of it isn't strictly physics, though I think in the beginning it's like the first couple weeks of a high school Mechanics class. Low on math, but you get to do or hear about experiments and how they ought to shape your view of what's going on. (Thinking of masses and acceleration and forces, thinking of complex amplitudes and decoherence.)

Yudkowsky is more of an expert in reasoning than in Physics. For example, imagine you are a world class expert in Quantum Mechanics and you are asked to give a talk at a science club. You cover a lot of stuff and think "that was great". But somebody else tells you "that was terrible, you mumbled, you wrote illegible stuff on the board etc." and you realise it wasn't about QM, it was about public speaking.

Similarly physicists have done amazing work over the decades, and now it's time to draw conclusions from all that evidence. But it turns out that physicists are not domain experts in drawing conclusions from evidence. That kind of skill is a separate domain in its own right.

The Standard Model is a model, not an interpretation. Whether Yudkowsky is right or wrong, anyone's interpretation is going to be text, not physics.

May I add David Deutsch's (not yet peer-reviewed, but promising) paper on the Contructor Theory of Information?

The Physics arXiv Blog writes about it here: https://medium.com/the-physics-arxiv-blog/deeper-than-quantu...

For the curious, the link to the paper can be found at the end of the article.

I remember being downvoted by circle of HN folks who weren't comfortable with Information transfer using Quantum Teleportation. Depite efforts to refute it, here's a team that suceeded doing exactly this => http://phys.org/news/2014-05-team-accurately-teleported-quan...

While we're giving suggestions, I'd again point to Scott Aaronson's http://www.scottaaronson.com/democritus/default.html (also available in book form) and his blog in general (especially when quantum computers are in the news). His approach is unique to me in that it's a strongly computer science point of view.

Edit: Another link I remembered that's strongly in the vein of QM for programmers is (suitably titled) here: http://oyhus.no/QuantumMechanicsForProgrammers.html

I'm skeptical, if only because this should be a big deal and hasn't gotten much press.

> We're able to set the spin (rotational direction) of these particles in a predetermined state, verify this spin and subsequently read out the data.

This is terribly unclear. I don't have access to the article - maybe its better explained there?

The problem with using quantum entanglement for information transfer is that you can't cause a particular spin at either location - they're just both reading random data that correlates. Nothing about this allows any actual transfer of information. Why are they not explaining how they got around this? That's the interesting bit, as far as I can tell.

Paper in short: http://download.repubblica.it/pdf/2014/scienze/science-xpres...

Paper in full: http://arxiv.org/abs/1404.4369

To the physicists among the us, please share your opinion on the paper with the rest of us.

It's not easy to digest the paper. The finding is not only going to change finance, but also the whole data economy will speed up. Now 'Quants' can be located anywhere and everyone has can enjoy same advantage of datacenter-closeness, evening out the prestigious role of the select few. Well, not really. It's going to take quite some time until the mid-class can access this technology unless, someone finds out a way for mass-production. That would be stellar.

That's it. I hate nobody with a rational mind of whatever kind, but naysayers really itch me.

Luboš Motl reviewed it. http://motls.blogspot.com/2014/05/constructor-theory-deutsch...

He didn't like it, as if there were any doubt.

Not a physicist, but I don't think this is much to get excited about... There's no faster-than-light communication happening -- you'll note in the second paragraph that Alice must send the outcome of her measurement to Bob. It's a step towards impressive quantum computers, which will be cool to have so we can do things like Shor's algorithm or Grover's algorithm, but there's still a long way to go.

> Alice sends the outcome via a classical communication channel to Bob, who can then recover the original state by applying the corresponding local transformation.

This seems to be the key point - there is absolutely no FTL information transfer going on, which is pretty much what I expected. As I said before, it would've been much bigger news if there was. The press article was clearly written by someone who didn't understand what was going on (or was intentionally deceitful, but that seems unlikely).

I'd like to propose a thought experiment that in my opinion speaks for a many world interpretation.

Imagine Schrödingers Cat. Only we have a physicist in the box instead! Also inside the box is a photon emitter programmed to send a photon after 30 seconds. Our dear physicist has been instructed to, if he is still alive after 20 seconds (adjusted to have 50% prob) to move the emitter just a tiny little bit to the left. Now according to the quantum laws, the photon from the original position should create an interference pattern with the photon from the slightly moved position... provided we succeeded in creating a superposition of dead/alive physicist. Of course, every single experiment would just give one detection, so we would have to do it many times, to really verify that it worked.

The experiment could of course be scaled up, so that we instead proved that we had superpositions of planets being blown up or something. At some point, when the system in superposition is large enough - or significant enough -, I don't see how you could refrain from calling that many worlds.

Since you only post a link with no comment text I don't really know what you mean to say with it.

Anyway, I think my proposal had a key advantage. The interference pattern could in theory be measured. We could get tables and graphs and sigma values.

It was just a link to something that seemed very similar to what you said.

The clearest version of Many Worlds that I have seen is from the "Bohmians": http://arxiv.org/pdf/0903.2211.pdf

Basically, you get a mass density on physical space by integrating over the wave function. Then you have correlations of changes in the mass density. Reality becomes a bit like fuzzy tv reception with overlapping channels.

There is no splitting of the universe or any other nonsense.

I still don't like the theory, but at least it is a well-defined theory.

I want to upvote out of agreement, but I really, really hate when people recommend a very long work they expect me to read on faith without a summary of what it offers so I can know Of it's worthwhile.

Here's what I would have added: Yudkowsky explains quantum mechanics in terms of the decoherence interpretation and how it makes logical sense as following from a simple rule that nonetheless contradicted widely held philosophical assumptions that kept its simple truth from being appreciated.

And why should we read Yudkowski instead, rather than other physicists on this matter, as far as I know he's not in a position of authority in this area.

I would never say instead of. Read whomever you like; read them all. Though, I would suggest that anyone who still takes Copenhagen seriously really needs to digest that sequence.

Eliezer is a smart guy with great insights into how to think about questions. His "position of authority" is that he writes in a way that's clear and conveys meaningful novel insights. If you think that only Ph.D.s have insights worth reading you're missing out. There's nothing "new" or "publishable" in the sequence, but Eliezer collates a lot of good ideas and presents them well.

For what it's worth, I personally feel like I've learned a lot from reading him. I was just trying to share it in the hopes that someone else on HN may find him as insightful as I do, and the quantum physics sequence is one of his better works. It was just a friendly pointer. If you're not interested, disregard.

>>And why should we read Yudkowski instead

>I would never say instead of.

When you recommend something, you're implicitly advocating that it be read in preference to other works on the matter.

It makes no sense to say "I recommend this, but it's no better than anything else."

"I recommend this. Reading it is better than not reading it, and you may not have been aware of its existence."

The recommender is in no position to know whether your next-best alternative was reading another work on the subject or reloading hacker news or taking a nap.

The recommendation and its follow-up question were about alternatives in the context of learning about quantum mechanics.

Another anti-Copenhagen argument by Carver Mead: http://www.cns.caltech.edu/people/faculty/mead/Nature_Of_Lig...

Liquid dynamics sounds like an interesting hypothesis, but I agree with the critics that entanglement presents a tough analogy. If Pilot Wave theory suggests that the particle-wave duality is actually two distinct actors - a wave and a particle riding the wave - then 1) each particle has an independently generated wave? and 2) it's difficult to see how that wave could be exactly preserved for both particles over the long distances of entanglement experiments. That's like envisioning a ripple on the far side of a large lake exactly mimicking a ripple here. Granted, the entanglement result of simultaneous collapse of probabilities is also tough to rationally understand. But Pilot Waves maintaining their effect, regardless of various asynchronous interactions encountered between the distances of entanglement experiments, seems naive. Then again, I'm not a particle physicist.

In interpretation of dual slit experiment people do the main mistake of mixing up the 2 waves - one is wave like quantization of position and another is de Broglie wave-equivalent of the particle. The mix up is caused by photon where both have the same wavelength. They are different in the case of all other particles. Hitachi experiment shows its the best - it clearly shows interference of the positional waves of different particles, not the de Brogile wave (ie. not the particles themselves). It thus also lends very credible support to statistical ensemble interpretation (which, as far as i know, Einstein favored)

People frequently bring up single-particle self-interference as explanation for Hitachi too. Well, in case of photon the self-interference pattern looks the same as positional interference because of the same wavelength. In case of electron and especially neutron the self-interference pattern would be much more dense than observed one because of the much higher de Brogile frequency than the frequency of the positional quantization.

The wave-like quantization of position is the Pivot Wave. The really important thing here is to understand that Pivot Wave isn't a real wave/object. It is just a description of possible positions of the particle at specific times. Like a trajectory of a bullet isn't a real thing, it is just description of possible positions of the bullet at specific times. It is just in QM the trajectories are wave-like quantized and probabilistically spread - that the non-smooth at small scales structure of our Universe shows its "ugly" face :)

Again, taking the quantized wave-like trajectory description of a particle for a real thing, the particle itself, is the main misinterpretation that has been going for years, especially in Copenghagen interpretation.

>>The really important thing here is to understand that Pilot Wave isn't a real wave/object. It is just a description of possible positions of the particle at specific times.

Is it possible that pilot waves are ripples in space-time? We should not make strong assertions especially when a theory is young. You make it sound like nothing more than the a regular QM wave function.

Ripples in space-time are gravitational waves. It could be a real wave but the danger is that one introduces a preferred references frame. Not that it would be the end of physics but it would surely be a big surprise.

Do you have any more information or references about this, in particular your claim that there are two different wavelengths in play?

it is actually doesn't matter. The de Brogile wavelength can be excluded from consideration completely - imagine an electron as just a very small metal speck whose position function is wave-like quantized - you'd still get the same interference pattern in the double-slit experiment. [The position function is wave-like quantized because it is QM, and thus all the interactions of the particle with environment - ie. electron with the EM field sending it to the screen in this experiment - happens in "quanta"s. ]

Don't get me wrong - i'm not denying de Brogile, i'm just saying that wave-like nature of a particle isn't necessary (and thus isn't proven by ) the double-slit experiment. The single-particle double-slits are never really "single" particle - they always show statistical aggregate of many single particles and thus they are explainable by positional quantization alone (of small specks as described above).

What is a "Pivot Wave"? Do you mean "Pilot Wave"?

thanks, it was a mistype, i guess i see "pivot" much frequently than "pilot"

As an answer to the question on entanglement, the FAQ posted above had this to say:

In contrast, Bell’s Theorem can be formulated without even speaking about hidden variable theories: the theorem states that some predictions of QM, well confirmed by several experiments, can not be explained by any local theory. And BM is nonlocal, as well as QM is. In fact BM inspired Bell to investigate non-locality, finally leading himto discover his famous inequalities. Bell was one of the most prominent proponents of BM and wrote many articles explaining it in great detail.

Also here's a wikipedia article that talks about Bohmian Mechanics and entanglement. http://en.wikipedia.org/wiki/De_Broglie%E2%80%93Bohm_theory#...

I was under the impression that Bohmian mechanics were known to be equivalent if and only if hidden variables were strictly non-local (in the case of the macrophysical observations, the "universe" can for a reasonable higher-order approximation be the limits of the chamber being observed).

IIRC, There is also an interesting 'alternative' relativity which has non-local effects and a universal frame of reference formulated by a physicist named Frank Tangherlini (I'll be interviewing him this month). It also has weird properties like anisotropy of the vacuum speed of light!!

Might be interesting if Bohmian and Tangherlini mechanics provided a better mathematical rapprochement of quantum mechanics with relativity than Copenhagen/Lorentz/Einstein

I was under the impression that Bohmian mechanics were known to be equivalent if and only if hidden variables were strictly non-local

Correct. The same is true of Bell's theorem; it shows that no local hidden variable theory can reproduce the predictions of standard quantum mechanics. It's true that the "local" part often gets left out in pop science treatments of Bell's theorem; but Bell himself was quite clear about it, and about the fact that Bohm's pilot wave theory is nonlocal. The article completely fails to mention this, which IMO is a huge omission.

An interesting response to the fluid experiments discussed in the article:


Comment on Y. Couder and E. Fort: "Single-Particle Diffraction and Interference at a Macroscopic Scale", Phys. Rev. Lett. (2006)

I have been contemplating Julian Barbour's work which has a frame of reference in it. This would make it natural as to what the "now" of Bohmian mechanics would be. That "now" is the mystery at the moment.

This sums up my thoughts exactly. The pilot-wave theory "...seems to me so natural and simple, to resolve the wave-particle dilemma in such a clear and ordinary way, that it is a great mystery to me that it was so generally ignored."

I don't run in experimental-physicist circles, granted, but I've definitely encountered countless cases of a clear, obvious, correct solution being brought up and summarily ignored for what proves to be a poor solution. The probabilistic theories have always made for good Science Fiction, but that should hardly matter.

This seems to be common problem. Watzlawick describes an experiment where two people (A and B) are given the task to classify tissue samples as "healthy" or "sick".

There is an initial training session where a light tells them whether their answer was correct, with one little caveat: only A gets real feedback. B's light just duplicates A's, so the answers that B receives are essentially random (I think they get different picture as well).

Since the task is not very difficult (on purpose), the As learn the task in 80% of the cases. The Bs have a much more difficult task, they are required to try and find order in a random world. They form very complex theories to account for this.

However, that's not the experiment quite yet. The real experiment is that As and Bs are then put together to discuss their results. What happens then is stunning: instead of rejecting the B's theories as unnecessarily complex, the As are usually so impressed with the subtle complexity and detailed brilliance of the B's theories, that they change their mind and accept the B theories!

When asked who will improve in the next round, all the Bs and most of the As pick the Bs. And they are right, because the As will have accepted at least some of the Bs ideas and thus perform more poorly.

Reference: http://omg.pytalhost.net/dls/ebk_wwidw.pdf (German)

By far the most fantastic and unbelievable part of this article is the calm thoughtfulness and collaborative criticism in its comments. Thank you, OP, for sharing such a rare gem.

I'm impressed by the probability distribution graph in the video. I would never expect the random motion to form a wave!

that's the point, it's chaotic, not random.

Interesting. Maybe this will discourage chuckleheads and ne'er-do-wells from citing "quantum physics" as the justifixation for their particular variety of woo?

Probably not, but one can hope!

If you can't bullshit your way through criticism with vague-sounding phrases like “As the particles move along, they feel the wave field generated by them in the past and all other particles in the past”, you're not trying hard enough. :v

This magical bracelet is powered by the wave field shaping the contours of the superfluid of space time! Carrying you through life on positive pilot waves!

I can imagine supporters of homeopathy would have a great time drawing unfounded conclusions from the bit in that article that talks about fluids exhibiting a "path memory".

Would be even worse if it was proven that molecule actually have this kind of memory.

This brings to mind a talk by someone I saw once who talked of telling the Dalai Lama about Bohmian mechanics. The response was one of gratitude for he had always felt like standard QM was a bit nonsensical.

I hope so, mostly because it makes it harder to claim that the universe is non-deterministic; it was QM that really pushed that from vague spiritualism into accepted science.

If you try to create a classical situation that obeys similar mathematics to a quantum situation, you're bound to get "quantum-like behaviour" isn't it? Wouldn't this be some kind of an analog computer for simulating the two-slit experiment in the classical realm? If so, what would this offer to the interpretation of quantum mechanics that cannot be gleaned from the mathematics itself?

The significance is that this is a model of a special, alternate formulation of quantum mechanics called Pilot-wave Theory.

pwt is an interpretation. All interpretations of QM share the same mathematics .. or effectively the same maths. Different "interpretations" cannot produce different predictions in experiments. So using an experiment designed to emulate pwt (which afaik violates special relativity) can only say one thing possibly - that if we find it convenient to think in this way in some limited cases of quantum mechanics, we may do so. In other words, it may at best serve as a heuristic to teach children, that they can later grow out of.

> All interpretations of QM share the same mathematics .. or effectively the same maths.

No they do not.

> Different "interpretations" cannot produce different predictions in experiments.

Yes they do.


I was unable to figure out one key thing about this article. Are they claiming that if the droplet is observed while going through one of the slits that the interference pattern will vanish? If not can we really claim a strong analogy with QM?


"And just as measuring the trajectories of particles seems to “collapse” their simultaneous realities, disturbing the pilot wave in the bouncing-droplet experiment destroys the interference pattern."

Excellent. Hope this gets support and more attention. I'm getting tired of explaining how hand-waving about probabilistic magic of subatomic behavior is successful guessing of reality without explaining it.

Any ideas on how quantum computing would be explained with this model?

If quantum behavior is really classical like I think they're claiming, wouldn't that mean quantum computers wouldn't provide any benefit?

First keep in mind that quantum computers have not been proven to be more powerful than classical computers. Keep also in mind that "quantum computing", like "classical computing", is a mathematical model that exists outside of reality.

That said, I think the answer to your question is that the "test particle" in the pilot-wave model is always reaching its destination at the speed of light. If however you model the pilot wave with Newtonian physics and place a literal test particle in it, well, even if the particle is moving very fast, its meandering route will all but guarantee a much slower (likely asymptotically slower) traversal than the pilot-wave test particle.

But IANAP and welcome corrections.

Bohmian mechanics is not classical mechanics. The wave is guiding the particles. If quantum computing is based on the standard quantum formalism then it is also present in Bohmian mechanics. That's a proven fact.

So quantum computing is rather orthogonal to this.

I'm not sure how quantum computing would be explained, but if it is classical then yes, quantum computers really wouldn't provide any benefit. This brings to mind headlines recently on how so-called quantum computers could not outperform optimized classical computers on algorithms specifically designed to cater to a quantum computer's strengths. Of course, then you would have to argue whether there actually are any quantum computers in existence today, or the quantum computers that are claimed to exist are actually classical computers exploiting some quantum effects.

The D-Wave devices that you are referencing are actually quantum annealing machines, not universal quantum computers. A quantum annealing machine is to a universal quantum computer what a mechanical computer is to a digital processor.

Are there any experiments where the pilot wave theory and the Copenhagen interpretation predict different results?

It depends a bit how you define things. The Copenhagen interpretation has a collapsing wave function doing so in a way where we can't observe it collapse and the pilot wave theory has pilot waves all over the place that have no effect other than guiding the particle in question then effectively disappearing when not needed. If either the collapses or pilot waves were real things I would kind of expect them to be observable in some way, but in both interpretations they are not observable, which leads me to suspect they are not real things and will go the way of the "luminiferous aether".

Funny that you say that since the particles are pretty much all we do observe. You never "see" the wave function, but you do see where stuff is.

To be charitable to Copenhagen, we would say that we do see the results of collapse. The main problem with the collapse is that is simply not well-specified by the theory.

In pilot wave theory, one can deduce the standard quantum formalism. Operators, collapse, and all the rest just pop right out.

So no, there is no experiment that can tell the difference. It is possible that pilot wave theory has predictions that the standard formalism is silent on. However, something that is inspired by pilot wave theory can be easily co-opted by standard approaches, e.g., Bell's theorem.

As an example, it is very easy to place pilot wave theory on manifolds as these are just differential equations. For the standard theory, it is not at all clear what the main operators ought to be such as momentum. Position, yes, but not other things.

Of course, once one has pilot wave theory on it, then whatever needs to be deduced can be and then given to the standard theory.

If the pilot wave really drives the behavior of the particle to a very large extent, finding such experiments would be exceedingly difficult.

I would not be surprised if it turns out that the Copenhagen interpretation is a modern phlogiston theory.

We changed the url from http://www.wired.com/2014/06/the-new-quantum-reality to be the original source.

Thanks, because that is a really well written article so I added that magazine to my RSS.

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