SoylentNews is people
SoylentNews
from the quantum-reality-is-just-classical-reality-in-really-tiny-bits? dept.
For nearly a century, "reality" has been a murky concept. The laws of quantum physics seem to suggest that particles spend much of their time in a ghostly state, lacking even basic properties such as a definite location and instead existing everywhere and nowhere at once. Only when a particle is measured does it suddenly materialize, appearing to pick its position as if by a roll of the dice. This idea that nature is inherently probabilistic -- that particles have no hard properties, only likelihoods, until they are observed -- is directly implied by the standard equations of quantum mechanics. But now a set of surprising experiments with fluids has revived old skepticism about that world-view. The bizarre results are fueling interest in an almost forgotten version of quantum mechanics, one that never gave up the idea of a single, concrete reality.
In a groundbreaking experiment, the Paris researchers used the droplet setup to demonstrate single- and double-slit interference. They discovered that when a droplet bounces toward a pair of openings in a damlike barrier, it passes through only one slit or the other, while the pilot wave passes through both. Repeated trials show that the overlapping wavefronts of the pilot wave steer the droplets to certain places and never to locations in between — an apparent replication of the interference pattern in the quantum double-slit experiment that Feynman described as "impossible ... to explain in any classical way." 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.
Droplets can also seem to "tunnel" through barriers, orbit each other in stable "bound states," and exhibit properties analogous to quantum spin and electromagnetic attraction. When confined to circular areas called corrals, they form concentric rings analogous to the standing waves generated by electrons in quantum corrals. They even annihilate with subsurface bubbles, an effect reminiscent of the mutual destruction of matter and antimatter particles.
How about it Soylentils. Is there anyone here who groks Quantum Mechanics who would care to explain this in layman's terms? What shortcomings and/or benefits do you see with this theory?
(Score: 5, Informative) by kebes on Tuesday July 01 2014, @03:34PM
There is indeed disagreement [preposterousuniverse.com] about fundamental interpretational issues even among quantum experts. But, importantly, no one is disagreeing about the mathematics or the predictions, there are only disagreements about what words to use to describe them, and how to interpret the results. (Of course there are also people looking for experimental deviations from QM, but that's another issue.) Yes, there are some physicists still using older phraseology; though even they typically wouldn't say things like "ghostly state" or "suddenly materialize". They would instead restrict themselves to saying things like "If you do experiment X you will get result Y. Whether or not the intermediate function psi 'really exists' is not a meaningful question."
At a minimum, I am making the point that there are clear-cut interpretations of modern QM that don't invoke silly notions like those espoused in the article. Also that modern QM mathematics (which includes decoherence) is deterministic.
Wave-particle duality is another historical misconception, rooted in classical thinking. As I noted above, it's more meaningful to say that real quantum entities are wave-packets. The 'ideal wave' (infinite/perfect sine-wave) and the 'ideal particle' (delta function at one specific location) are limiting cases that can be useful conceptually, but never actually exist in the real universe. It's always a wave-packet. Never an ideal wave. Never a pointlike particle.
One objection I have is that the article uses confusing language needlessly. Is reader comprehension enhanced by using words like "ghostly"? Even if you want to provide a probabilistic interpretation of QM, you can do so without invoking confusing metaphors.
However I also think the article is factually wrong. Saying that particles "[lack] even basic properties such as a definite location" is incorrect. As I said, the properties of the particle can all be meaningfully enumerated. Besides, why are "definite locations" a "basic property" of a sensible reality? Does it really make sense to have zero-volume entities localized to arbitrary precision? Just because classical physical theories assumed such a thing? Regardless of what your intuition tells you, when one measures Nature, one finds that it obeys QM. The properties enumerated by QM are the actual "basic properties" that reality consists of. (With the usual caveats that QM may one day be superseded by a better theory.)
P.S.: I know I'm being a bit confrontational about this. But bad pop-sci descriptions of QM, which ignore the last few decades of progress, are one of my pet-peeves!
(Score: 2) by Theophrastus on Tuesday July 01 2014, @03:44PM
With respects, (for someone demonstrating a profound passion about science reporting), i'd say quantum mechanics, even without a strong dose of then Copenhagen interpretation, does require the location of a particle to be indefinite. That is a the clear understanding that one gains by accepting the Heisenberg uncertainty principle. Or are you hereby declaring your rejection of the uncertainty principle?
(Score: 2) by kebes on Tuesday July 01 2014, @03:49PM
Sorry for being unclear.
(Score: 2) by Theophrastus on Tuesday July 01 2014, @03:59PM
So you're suggesting that location should not be considered a 'basic property of reality'..? Do you have a substitute set of these properties? Because if you don't then i'd say your interpretation is rather classically Copenhagen. "not that there's anything wrong with that" -- but you did seem to be rejecting the deep probabilistic nature presented there.
(Score: 4, Informative) by kebes on Tuesday July 01 2014, @04:37PM
I'm saying that empirically, when we study reality, we observe that it matches QM and not classical physics. How one interprets that is a matter of philosophical preference, but usually in science we accept that this is just how reality behaves. I.e.: the properties of the wavefunction are the "basic properties" we should care about.
(Score: 2) by Theophrastus on Tuesday July 01 2014, @04:47PM
again, i understand you're passionate, but it's the foibles of language which seems more what you're passionate about, so i'll join you insomuch to assert that a lot of "1930's thinking" contains thought that transcends my own (and would that i could get "stuck" there rather than suffer a quantum of uncertainty)
(Score: 2) by romanr on Tuesday July 01 2014, @03:49PM
He doesn't say that particles do have definite location, he says that particles have basic properties and that location isn't necessarily basic property, which is matter of opinion i would say.
(Score: 2) by romanr on Tuesday July 01 2014, @03:45PM
Thanks for clarifying your objections! I'm not defending that article or the whole summary, I just mentioned that one thing. Anyways, your notions are most insightful, I would upvote you if that was possible :).
(Score: 5, Interesting) by Anonymous Coward on Tuesday July 01 2014, @06:20PM
I think kebes' critique of the article above and here is a little heavy-handed. I'm not familiar with the Simons Foundation publication, but the article was reprinted in Wired, which although not a science magazine, is a top-notch magazine that often deals with scientific topics with high journalistic standards. The article actually covers the ideas fairly well, including descriptions of theories and commentary from important physicists. The intro gives an accurate description of the Copenhagen interpretation, which is the most widely accepted position among physicists, and yes as given in textbooks. It is weird and confusing, difficult to accept, and yet it appears to be the correct interpretation. As Seth Lloyd, physicist at MIT, is quoted in the article, "Quantum mechanics is just counterintuitive and we just have to suck it up." I think 'ghostly state' is probably a very apt description for public discussion. It also describes alternatives to that theory, primarily led by David Bohm and supported by this paper. Another alternative would be that alluded to by kebes.
Whether a measurement can be described within the context of wavefunction evolution appears to be indirectly in question here, but generally not a settled question, and seems to me to contradict the Copenhagen interpretation.
Wave functions describe everything there is to describe about a particle, but that doesn't imply that it describes everything about the particle. E.g. one wave function cannot detail/define both the position and momentum. Again, here, 'ghostly state' is appropriate.
Kebes' general description of decoherence as an explanation is probably a highly viable one, but afaik not yet widely accepted among physicists. But to say that the currently widely accepted Copenhagen interpretation is 'silly' or "1930's thinking" is factually incorrect.
My understanding has been that wave-particle duality had settled into "particles that obey QM, which is sometimes wave-like," but that effectively the particle side had won out. To very high precision, ie limited to extremely small scales, the electron for example is a point particle. But it is also observed to obey quantum wave-like principles. But see my link below to 'delayed choice' experiments, which apparently bring this question back into play. Ask any random physicist, though, and they won't be able to give you a solid answer on this.
That is because most students of physics and most physicists who study quantum mechanics never go deeper into the interpretation of quantum mechanics than the cursory explanation at the level of Griffith's Intro to QM, which is likely what kebes is using for his/her course as it's the most popular text at that level. While virtually all other areas of physics are motivated by direct insight, QM motivations are generally hand-wavy, at the textbook level, and almost all quantum texts, from Griffiths' intro on up thru advanced graduate texts, basically ignore the philosophical quandaries surrounding quantum theory and focus on how to do quantum mechanics. It's sort of like teaching/learning to ride a bicycle or swing on a swingset--the physics is fairly complex, but you can actually do it fairly easily. The practice of QM itself has been so effectual in its precise theoretical predictions and its broad application as to be evidence of its correctness.
So it is with some reluctance that I join the fray here, being a physicist that works in quantum mechanics regularly, and yet I, like so many, don't feel I have a completely solid understanding of the theoretical implications. But this also makes me doubt anyone that asserts that they understand it themselves, having only prepared lectures on introductory QM. Certainly as far as I understand it, the QM community has basically settled on one interpretation of QM, the Copenhagen interpretation, while admitting the possibility of others and noting that for the vast majority of scientific investigations, the QM interpretation itself is moot. As Griffiths says in the afterword to his text, "In light of this, it is no wonder that generations of physicists retreated to the agnostic position, and advised their students not to waste their time worrying about the conceptual foundations of the theory."
Having said this, I hope to say what my position is without implying that I speak for physicists in general or that my confidence in my position is extremely high. The original poster, martyb, borrows language from the original article, and is basically correct in his description of the story. It's almost the exact story you get in an undergraduate chemistry course--the Bohr model is introduced, depicting electrons circling an atom, and then is corrected with imprecise language, saying that what really happens is there is an "electron cloud" and the electron is actually smeared out, not just over all angles, but over a wide range in radius (technically infinite) as well, and does not possess a precise location. A similar line of description is used to explain the double-slit experiment, that particles, a single particle even, must travel through both slits in order to produce the interference pattern that we observe. You can not say that the particle went through either slit A or slit B, and therefore the particle did not have a precise location until it was observed at the sensor.
The fluid test apparently is now challenging this very argument, which is critical to quantum theory because of its simplicity and yet the apparent impossibility to explain it classically. So it is with great surprise that I read the summary and skimmed the article, although I'd heard of alternatives to quantum mechanics, probably the most well-known being David Bohm's. The implications of the present work are possibly extreme.
It's not clear whether a change in the fundamental interpretation of quantum theory would affect the general practice of quantum mechanics--my guess is that it wouldn't, but perhaps it could. The implications may only be noticed at subatomic and extremely small scales, leaving quantum theory capable of explaining most subatomic and all atomic, molecular, and solid state physics, but incapable of describing extreme fundamental considerations, just as Newtonian mechanics adequately covers a great variety of phenomena but is incomplete when relativistic or quantum scales become active. From the article, and as Bohm says, "we could easily be kept on the wrong track for a long time by restricting ourselves to the usual interpretation of quantum theory." On the other hand, some of the top physicists, such as Leggett, believe this is a waste of time, so I don't know.
But also importantly, this is a very interesting set of experiments that really does challenge the fundamental interpretation of QM. One test I would propose as an extension of the present work is to reproduce a delayed choice experiment. See http://en.wikipedia.org/wiki/Wheeler's_delayed_choice_experiment [wikipedia.org] . Further questions would be whether they can produce superpositions in their bound states, and entanglement/EPR paradox/Bell inequality stuff. My guess here is that they will not be able to reproduce these, but who knows. According to TFA, unfortunately it won't be soon, "an experimental test of droplet entanglement remains a distant goal."
The philosophical implications pertaining to human concerns are very important, for it resurrects Einstein's doubts of quantum indeterminacy, which we had all thought was settled and had come to terms with, though with difficulty. If indeterminacy holds, then there remains some (vanishing, thanks to recent neuroscience experiments) possibility of free will. But if the present fluid experiments and Einstein's previously held view of determinism is correct, that the evolution of nature, including all matter and non-matter, is determined, ie pre-determined and post-determined, then free will is indeed an illusion as Einstein believed.
(Score: 3, Informative) by kebes on Tuesday July 01 2014, @10:33PM
In any case, I'll happily defer to the real experts on QM theory. Such as:
Sean Carroll [wikipedia.org] (CalTech) has some nice general-audience blog entries:
Why the Many-Worlds Formulation of Quantum Mechanics Is Probably Correct [preposterousuniverse.com]
Does This Ontological Commitment Make Me Look Fat? [preposterousuniverse.com]
Quantum Mechanics Made Easy [preposterousuniverse.com]
(This blog post [lesswrong.com] covers some similar ground.)
Allan Adams [mit.edu] has an intro to quantum [mit.edu] course that is freely available on MIT Open Courseware, and provides a good intro (without resorting to the any Copenhagen verbiage).
Max Tegmark [mit.edu] (MIT) wrote a Nature commentary, Many lives in many worlds [mit.edu], that provides a general-audience into to a deterministic interpretation of QM. He also has some more technical papers on these topics (the intros and conclusions may be useful even to non-specialists):
Many Worlds in Context [arxiv.org]
Parallel Universes [arxiv.org]
Yet more technical, but still interesting, are the results of decoherence (in large part from Wojciech H. Zurek [wikipedia.org] ):
Decoherence, the measurement problem, and interpretations of quantum mechanics [arxiv.org]
The quantum-to-classical transition and decoherence [arxiv.org]
Decoherence and the transition from quantum to classical [arxiv.org]
(Score: 2) by melikamp on Wednesday July 02 2014, @06:03AM
Fascinating thread. I have a few general comments.
The interpretation does matter, even to physicists. It seems at least possible that the universe (or some aspect of it) is in fact a mathematical object. If it is, then it may be possible to ascertain this fact statistically. (For example, if it is deterministic, and there is a way to check the outcomes experimentally, and to re-stage these experiments many times over.) That would be an interesting discovery, and one worth making. And talking about deterministic interpretations and deterministic mathematical formalism is a basic heuristic for getting there.
As a mathematician, I don't see how a deterministic mathematical universe is more or less surprising than a non-deterministic mathematical universe. It would be far more surprising to me if the universe was discovered to be a mathematical object at all (although also deeply satisfying). Exactly what kind of mathematical object? Well, right now it looks like it is geometrical on large scales and algebraic/probabilistic on small scales (algebraic geometry is hot, incidentally), but it in the end it may turn out to be something really silly, like a finite sequence of distance matrices. All of these possibilities have exactly the same ontological status for mathematicians, and in particular, the probability theory with its perfect theoretical distributions is no less true or concrete than the integer arithmetic.
Alas, if this is the grail, then the odds are against us. Assuming that the world is mathematical, it may simply be too complicated to pin down. What if it is a bit longer than Newton's laws or the field equations? For example, it may be fully described as an algorithm (a Turing machine program) operating on a very large array of simple "particles", kind of like in the world of Newton. And this algorithm just happens to be a fancy PRNG-like gizmo with emergent macroscopic properties we are observing (so yeah, it sucks as an RNG, but because of that we have galaxies, planets and life instead of a homogenous soup). And the shortest description of this algorithm in, say, Lisp, is 10 MiB in length: a modest program by modern standards, but far more complicated than any PRNG devised so far. So a physicist may be faced with reverse-engineering a PRNG of incredible complexity, and that without even a perfect way to read the output.
The odds are against us in the sense that there are far more long programs than there are short ones, and a 10 MiB, or even a 10 TiB program is absolutely tiny when faced with the infinity of all possible programs. I don't mean to call actual odds here, only to express the cautiousness of my optimism.
(Score: 2) by kebes on Wednesday July 02 2014, @12:56PM
The Mathematical Universe [arxiv.org]
Is "the theory of everything'' merely the ultimate ensemble theory? [arxiv.org]
Shut up and calculate [arxiv.org]
On Math, Matter and Mind [arxiv.org]
The ideas he's putting forth are that:
1. The universe is, inescapably, a mathematical structure. One way of thinking about this is to say that if the universe is consistent, then it must be described by some set of non-contradictory rules, in which case one can find a mathematical formalism isomorphic to those rules. Tegmark phrases it slightly differently: explaining instead that if one accepts that the universe has independent reality (not just a product of our minds), then it must have some observer-independent description; which again implies one can find a mathematical formalism isomorphic to that description. (Read the paper for the rigorous version of this argument.)
2. If our universe is actually a mathematical structure, why does it have physical reality? Perhaps other mathematical structures also do? Perhaps all consistent mathematical structures have physical reality?
It's a long way from being a sound scientific theory, but interestingly Tegmark sketches out how this idea could be tested experimentally. It also has some interesting implications, e.g. that the description of the universe becomes so generalized, compact, and elegant, that its information content is nearly zero [arxiv.org].
Again, this is all just speculation at this point, but these are fun concepts to think about.
(Score: 0) by Anonymous Coward on Wednesday July 02 2014, @04:03PM
Thank you so much for your comments, those are one of the most interesting/insightful comments I've seen on any "nerdy" website about a particular topic so far. If you had some website/blog with interesting ideas/facts about QM or other topics, I'd be very happy to have it in my RSS feed :)
--
Posting anonymously, because I'm clearly OT - feel free to downvote :).