Shrug. “If a tree falls in a forest…”
What is the role of error in computation? If one misinterprets a symbol…what, exactly, by your (or anyone’s) theory of communication, has happened?
(A side issue to the side issue!)
I’m not sure I see why, can you elaborate? I would say it’s just the opposite: all observations are explained by an observer finding themselves in different branches of the wave function. So it’s the branching that decides the observations being made, not the observer.
I mean, it’s true of course that a given object only assumes a given definite state relative to a particular state of the observer, but that complex of correlations is perfectly objective, and just given by the wave function. It’s really not very much different from the observer making different observations at different times—there, too, I have to specify the observer in order to get a definite answer regarding which observation is made, but that doesn’t give the observer an active role.
Well, that’s the point: there was only an error in a computation if it failed to achieve its desired function; but what supplies that function, if not the programmer, or the one interpreting the computer’s outputs? The computer doesn’t care if it sets a bit or fails to; in either case, it’s just done what the physical rules of evolution governing it required it to do.
My point is more that the “structure” of the branches in the universal wavefunction seem to be highly non-trivial and at the moment no one yet has derived the branching structure without making assumptions as to the kind of observations that they correspond to.
I can’t claim to know as much you about the issue, but from what few things I have read it seems that the most plausible explanation is that this branching structure is related to the ‘mental’ structure of the observer themselves - the observer can only have certain mental states and therefore the branches must correspond to those states. Of course there’s still plenty of devils in the detail.
This isn’t to suggest that mental states have a privileged position and/or guide the evolution of the universal wavefunction.
Yes, you’re right about analogies and I’m aware of their pitfalls. But, the point you are making about the delayed quantum eraser could be a lot clearer and unless I can see what your objection is am unable to really make a rational response. Remember, I’m not a PhD so you have to indulge me a bit. If you could allow me to seem my misunderstanding it would be a step forward. Basically, you need to spell it out. 
Well, according to Tom Campbell, when the detectors register which-way information and the back screen is looked at you see a particle configuration.
If you first destroy any information picked up by the detectors showing particles and then look at the screen (which might involve developing it), you will see an interference pattern. Is this correct or not? If correct, then the fact that the information regarding which-way no longer exists leads to an interference pattern, however, that information can never see the light of day until looked at either directly or indirectly. So what Tom is saying is that although information is separate from consciousness you need both to make reality. What is so wrong with this view. It seems the most simple and clean explanation and makes no assumptions beyond what the experiment actually shows.
Hardly. 
But, again, in regard to what you have said, Tom Campbell is saying you QM researchers (at least most) are not looking at the big picture by attempting to construct a theory of QM from ‘physicality’, i.e. from where you are starting from. That you need to look at this from a much more overarching all inclusive level; one of some kind of external consciousness perspective.
Something like hidden variables is an attempt to reconcile this ‘classical’ perspective by attributing quantum behaviour to more or less know parameters that can be traced back to traditional notions of causality. But you can’t really get anywhere this way because the origin of all of it is coming from somewhere else, not here. All we can do is ask nature questions by constructing shrewd experiments thereby forcing it to give us data. Okay, we can organize this data into nice mathematical patterns but they are a consequence of the information we are given, not the other way round.
The BB saw the inception of universe but the BB gave rise to our reality, therefore, whatever it was that caused the BB must have been outside our reality since it did not exist before the BB. So Tom is saying whatever causes the quantum effects does not originate in our reality but outside of it via some kind of consciousness system and we, also being part of this system, can pick and choose what we want to gleen from this grand scheme.
I realize this could appear contradictory to what I said, so to clarify I mean I don’t suggest mental states are privileged as compared to any other way of breaking down the description of the true physical state given by the universal wavefunction, but they may be privileged in the sense they are involved in recovering our quasi-classical observations.
I’m not sure what you mean by the ‘structure’ of the branches. Do you mean in what basis one decomposes the wave function? If so, you’re right that there’s something that’s usually swept under the rug in many worlds, namely, the question of why I would use that particular decomposition to define the branches, rather than any other.
So when typically, you’d start out with a system in the state |s>, that evolves into a state a|s[sub]1[/sub]> + b|s[sub]2[/sub]>, and an observer O making a measurement on the system, you’ll have an evolution such as
|O>(a|s[sub]1[/sub]> + b|s[sub]2[/sub]>) = a|O,s[sub]1[/sub]> + b|O,s[sub]2[/sub]>
---> a|O[sub]1[/sub],s[sub]1[/sub]> + b|O[sub]2[/sub],s[sub]2[/sub]>.
Many accounts then gloss this as the world ‘splitting in two’, with one branch containing the system in state |s[sub]1[/sub]>, the other in state |s[sub]2[/sub]>; then, the observer, after coupling with the system, likewise splits, and sees |s[sub]1[/sub]> in one branch, and |s[sub]2[/sub]> in the other.
But of course, there’s no reason to prefer the basis in which we’ve written down things so far. In particular, there’s a basis in which the system is just in an eigenstate of a particular observable, and thus, there’s just one world. So, which is ‘the right’ basis to define the branching structure?
Here, the usual claim is that this problem is solved by decoherence. You don’t just have the system and the observer, you’ve also got an environment. This leads to a process called einselection (environment-induced superselection): only certain kinds of basis states—so-called pointer states—are stable against interaction with the environment, and thus, form a preferred basis which can be used to describe macroscopically enduring structures.
However, this is a dubious claim: einselection relies on a decomposition of the ‘world’ into system, observer, and environment; but that itself is only possible in a certain basis (having a tensor product structure for the three components). So one can argue that the whole thing becomes circular.
So I think that indeed the question of what, exactly, are the branches of the MWI is a difficult one (making the interpretation pretty unattractive to me); but I’m not sure what the observer could do to remove that difficulty.
Let’s try to break this down. There are four possible detectors that the entangled partner of the signal (the idler) can be detected. Call them A, B, C, and D. Things are set up such that if the idler is detected at A, then the photon giving rise to the entangled pair must have gone through slit 1; if it is detected at B, then it must have gone through slit 2. If a detection is made at either C or D, no information about the path taken is recoverable.
Now just start the experiment, and randomly divert the idler either to be detected at the which-path conserving detectors A and B, or at the which-path destroying detectors C and D. Then, arriving at the screen, we have four sets of signal photons:
[ol]
[li]signal photons where the corresponding idler is detected at A,[/li][li]signal photons where the corresponding idler is detected at B,[/li][li]signal photons where the corresponding idler is detected at C,[/li][li]signal photons where the corresponding idler is detected at D.[/li][/ol]
Call these sets S[sub]A[/sub]-S[sub]D[/sub]. Now, in the experiment done like that, we will see no interference pattern on the screen. The reason is as follows: clearly, S[sub]A[/sub] and S[sub]B[/sub], and consequently, their sum, will not show any interference, as which path information is available. However, the set S[sub]C[/sub] should show interference, and likewise, the set S[sub]D[/sub]. But the thing is, if you do the math, you’ll find that their sum, S[sub]C[/sub] + S[sub]D[/sub], will not show interference—in fact, the pattern that the set S[sub]C[/sub] + S[sub]D[/sub] gives rise to is identical to the pattern S[sub]A[/sub] + S[sub]B[/sub] gives rise to.
Consequently, you’ll only be able to see interference once you can identify which photons that have impinged on the screen belong to S[sub]C[/sub], say; before that, everything looks just as if ordinary particles had hit the screen.
This holds true even if the idler is detected long after the signal has hit the screen, that is, if the signal is detected long before it has been decided which of the four sets it belongs to. This is surprising, to be sure: how did the signal ‘know’ whether the idler would be detected at, say, D, so as to not fall into the ‘dark’ bands for that particular interference pattern?
But in fact, you have a symmetry here: while you could say that whether the idler is detected at A-D determines the probability distribution of the signal hitting a certain spot on the screen, you can just as well say that where the signal hits the screen determines the probability of the idler being detected at A-D. The important thing is that the experiment does not allow you to extract information about the future: where the signal hits the screen does not allow you to tell whether the idler will be detected at A, B, C, or D.
And neither of this involves the human mind in any way. I wonder, did you have a look at the answers beneath the Quora question I linked to? Essentially the same points I’ve raised here are being made there, in part by well-known scientists (I’ve already pointed to Scott Aaronson, but there’s also Charley Bennett, who’s a pioneer of quantum information theory) with the exception of a single ‘artist and engineer’, who basically cherry-picks some quotes to fit his preconceptions.
This is the point at which the discussion always goes off the rails. People who don’t know math sincerely believe that if the experts were just to explain things a little more clearly the lightbulb would go on over their heads. And therefore if the lightbulb stays dark it’s the fault of the experts. Because obviously all things are explainable through words and common sense.
That’s simply not true. Most advanced math and physics do not have any analogies to everyday experience and common sense. Science is not explainable in ordinary words. Science is only explainable through math. Not high school math, either. Difficult to comprehend math whose meaning may take decades of immersion to fully understand.
Little wonder that people will gravitate to charlatans who promise that they can explain everything in simple words. The charlatans are always wrong. Always. 100% of the time. You can’t explain this stuff in simple words. Every scientist in the world knows this, so when people come in claiming that they have a word answer they are slapped down before anyone listens. Of course they think this is terribly unfair, which builds up further resentment of the experts, and drives the non-math people back to the charlatans.
I can’t think of a solution. People here at the Dope have been dealing with this problem for almost two decades and as we see here it never changes. The non experts have to admit they aren’t getting it - and that it’s OK if they don’t get it. It’s a specialized skill that takes years of training. It’s like not being able to speak Mayan. Nobody feels they are stupid if they can’t understand a conversation in Mayan, nor should they. Conversely, nobody walks up to a group of expert Mayan speakers and tells them that a guy on YouTube says that their Mayan is all wrong.
Bottom line. The responsibility is all yours, not theirs.
abashed, Expano Mapcase isn’t kidding. Here’s a few classic well-cited papers on quantum erasers that are open source, i.e. not behind a paywall. What you’ll observe is that they all have math. Lots and lots of complex math. To really understand these phenomena requires mathematics. And whenever I see math like this I say to myself “Thank God I’m in computer science.”
Kim, Y. H., Yu, R., Kulik, S. P., Shih, Y., & Scully, M. O. (2000). Delayed “choice” quantum eraser. Physical Review Letters, 84(1), 1. https://arxiv.org/pdf/quant-ph/9903047.pdf
Walborn, S. P., Cunha, M. T., Pádua, S., & Monken, C. H. (2002). Double-slit quantum eraser. Physical Review A, 65(3), 033818. https://arxiv.org/pdf/quant-ph/0106078.pdfÂ
Scarcelli, G., Zhou, Y., & Shih, Y. (2007). Random delayed-choice quantum eraser via two-photon imaging. European Physical Journal D–Atoms, Molecules, Clusters & Optical Physics, 45(1). https://arxiv.org/pdf/quant-ph/0512207.pdf
It’s from his website, I see it in multiple places, the original one I quoted was from one of his slides available on his website, here is a different quote from him for the same topic:
We need to take a couple of steps back. At least I do. As I understand this thread, the OP has some theories inspired from findings in quantum mechanics research; for instance, from the well-known double split expirement.
Half Man Half Wit and others says that he cannot draw these conclusions, because 1) QM is really no mystery at all, and 2) which the OP himself would know if he or she knew the math. Therefore his/her theories are invalid.
Thay mat be good or bad, right or wrong. I have no dog in this fight. I’m reading the thread and writing this post to learn (hell, I even ordered the book!, which hasn’t arrived yet).
But over many years we’ve all seen all those TV documentaries or YouTube clips and what not, and they are all very consistent; which is pretty much: ”This is how QM works. Weird, huh. We don’t understand it.” And the guys who says they do not understand it are QM scientists (assuming there’s no conspiracy). I find this a stark contrast to the arguments the OP gets in this thread, where dopers pretty much says it’s no more mysterious than 1+2=3, if you know the maths.
Take for instance the flick linked to earlier: Double Slit Experiment explained! by Jim Al-Khalili - YouTube, where Jim Al-Khahliliexplains the double slit experiment. This presentation is just an example. As we all know there are hundreds of its kind where scientists explain or help explain this from a classical physics standpoint unexpected phenomena just about the same way, year after year.
This I do not get. It would mean that Al-Khahlili et al’s presentation of QM is ”fake science”? Of course there are simplifications etcetera, often needed to explain something complicated, but is for instance Al Khahlili simply wrong in his presentation of the double slit experiment or even deliberately telling us lies, like just about any other scientist in presentations like this one?
(The clip is only nine minutes long and seems to support the OP’s conclusions. For me it would be valuable if Half Man Half Wit have time to watch it and respond, though I understand if he or she does not want to review YouTube clips on demand. :))
My interpretation of this dichotomy is that it comes from conflating different definitions of what “understanding” means. Scientific understanding means that you have a viable consistent theory that makes useful predictions. This is not the same as intuitive “understanding” or the ability to relate phenomena to behaviors on familiar macroscopic scales. We cannot “understand” how a particle can be in several places at once, but we can mathematically describe its wavefunction. We cannot intuitively “understand” the Higgs field, and indeed as already said, attempts to explain it intuitively are worse than useless because they lead to incorrect predictions, yet the math represents a sufficient level of understanding that it directly and successfully led to its experimental detection. In a real sense, a mathematical theory is the ultimate understanding, because it transcends the limitations of human intuition, and allows us to make inferences and predictions that are sometimes stranger than anything we might have been able to imagine.
Independently of that, another scientific interpretation of lack of understanding is lack of sufficiently complete understanding – the meaning that we have useful theories with predictive value, but recognize that the theories are incomplete, and that more complete and robust theories have yet to be developed.
That’s a good post, wolfpup, thank you. But I want to take a couple of steps back here to the crux of the matter (of my post).
If the scientists, admittedly in a popular scientific context, are saying (as in the clip linked to above, which is only an example) that there is a Nobel Prize to win here for the one who can explain this with common sense and logic, on the one hand, and dopers on the other, says that they can explain this with common sense and logic – then, you know… something doesn’t seems to be right.
So I would like a clarification, assuming that there is something I’ve misunderstood.
I’m not saying that quantum mechanics doesn’t still contain puzzles. The measurement problem, for instance, still demands further research—at least according to most; there are also those that think it’s simply a pseudoproblem. But so far, there’s nothing about QM that substantiates the sort of leaps of the imagination abashed proposes.
There are, however, certainly many phenomena that are contrary to intuition. But then, what does that really tell us: that quantum mechanics is fundamentally mysterious—or that our intuition is simply a bad tool when it comes to fundamental physics? I think a good case may be made towards the latter. After all, our intuition has evolved to supply us with quick and dirty resources in everyday life, which is largely unaffected by quantum mechanics; so it’s not really that surprising that our intuition might have problems with this.
Many presentations of quantum mechanics play up this conflict between intuition and reality. This stuff is weird, it’s not what you’d expect, but it’s how things are, and fundamentally, we never had anything but prejudice stipulating that it couldn’t be that way. So the presentation by Jim Al-Khalili isn’t wrong anywhere, although I would have phrased some things differently; but it does somewhat emphasize QM’s weirdness. And as I said earlier, the most popular science presentations aren’t necessarily those that hew the closest to current scientific understanding, but rather, those that can hold an audience.
This isn’t in any way different from other theories in physics that have counterintuitive effects. Take special relativity, and the twin paradox, for example. That’s also profoundly counterintuitive; but at the same part, it’s well-understood, settled science. The same goes for the double-slit experiment, or the delayed-choice quantum eraser.
I’m not sure exactly what you’re asking, but to address these specific points:
I’ve never known anyone to have received a Nobel Prize in science for explaining anything “with common sense and logic”. The awards are for conducting actual science leading to breakthrough discoveries. Whether or not some principle can be cast in intuitive terms is completely independent of scientific understanding.
Perhaps really excellent science communicators deserve recognition – people like the late Carl Sagan. Science communicators help the lay person achieve some basic grasp of important scientific ideas, and may spark an interest that inspires young people to pursue science as a career. But whatever its virtues, science communication is not science – it’s a kind of journalism, or maybe sometimes, a kind of evangelism, but it’s not a path to discovery. The doper here (if you’re referring to the OP) who claims to explain QM “with common sense and logic” has no understanding at all of QM, as has already been amply demonstrated, and is just engaged in baseless speculation around a lot of wrong ideas.
Thanks for taking time, Half Man Half Wit, I understand your standpoint much better now.
wolfpup, I think I agree with you. My questions were not in defense of the OP, or in regards of him/her, so to speak, but out my own curiousity sparked by this thread. Half Man Half Wit settled my inner dilemma so I’m good (for now). 
Thank you for taking the time to spell things out a bit, although I still don’t fully understand it.
However, the central point is still that all this clever mathematics is a form of information, no matter how complex. And such information can (theoretically at least) be considered to exist as a possibility, probably one of many, that consciousness has to interact with in order to become ‘real.’ So we still have information, on one hand, and consciousness that needs to interact with it to form reality, on the other. So this is why some people like the idea of us living in a ‘virtual reality’, i.e. one that comes about as a choice or choices we make between any number of possible alternatives. In a simulation such as Mindcraft, for example, we have a choice of which characters to use and how to use them. It all depends on what we choose to do. This is reflected in real life by what choice of experiment to perform and what we do with the information therein gained. This is all mediated by consciousness and assuming we have freewill will be indeterminate, and it’s striking (at least to me) the way this is reflected in the indeterminacy of QM. But in this ‘game’ we have a multiple-choice participants, so many different people take part in the construction of the overall game, so to speak. In other words, we may be living in a simulation that is determined by the choices we make and it is not that ‘reality’ is caused by matter; rather it is matter that is caused by ideas and these ideas originate somewhere other than in the simulation itself, rather analogous to an external ‘server’ providing the software for the game-player (us) to interact with. But without a game-player the simulation cannot work and so with reality. The information is always there, waiting for us to interact with it, but unless we consciously do can never be anything other than a ‘potential,’