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Arthur T Knackerbracket has found the following story:
One of the most striking features of quantum theory is that its predictions are, under virtually all circumstances, probabilistic. If you set up an experiment in a laboratory, and then you use quantum theory to predict the outcomes of various measurements you might perform, the best the theory can offer is probabilities—say, a 50 percent chance that you'll get one outcome, and a 50 percent chance that you'll get a different one. The role the quantum state plays in the theory is to determine, or at least encode, these probabilities. If you know the quantum state, then you can compute the probability of getting any possible outcome to any possible experiment.
But does the quantum state ultimately represent some objective aspect of reality, or is it a way of characterizing something about us, namely, something about what some person knows about reality? This question stretches back to the earliest history of quantum theory, but has recently become an active topic again, inspiring a slew of new theoretical results and even some experimental tests.
If it is just your knowledge that changes, things don't seem so strange.
To see why the quantum state might represent what someone knows, consider another case where we use probabilities. Before your friend rolls a die, you guess what side will face up. If your friend rolls a standard six-sided die, you'd usually say there is about a 17 percent (or one in six) chance that you'll be right, whatever you guess. Here the probability represents something about you: your state of knowledge about the die. Let's say your back is turned while she rolls it, so that she sees the result—a six, say—but not you. As far as you are concerned, the outcome remains uncertain, even though she knows it. Probabilities that represent a person's uncertainty, even though there is some fact of the matter, are called epistemic, from one of the Greek words for knowledge.
This means that you and your friend could assign very different probabilities, without either of you being wrong. You say the probability of the die showing a six is 17 percent, whereas your friend, who has seen the outcome already, says that it is 100 percent. That is because each of you knows different things, and the probabilities are representations of your respective states of knowledge. The only incorrect assignments, in fact, would be ones that said there was no chance at all that the die showed a six.
-- submitted from IRC
(Score: 2) by AthanasiusKircher on Wednesday May 03 2017, @02:03PM (4 children)
But doesn't quantum-computing rely on the fact that quantum systems have several states at the same time?
Yes, and this is one of two places in the extended research article (linked about halfway through the summary) that they bring up these very practical issues (to me). The first is interference and the double-slit experiment, where that article quotes Richard Feynman saying "In reality it contains the only mystery" about quantum mechanics (p. 78). And the second is David Deutsch quoted a little later (p. 81) on quantum computing: "if the universe we see around us is all there is, where are quantum computations performed? I have yet to receive a plausible reply."
The reply to the latter seems to be that the algorithms for quantum computing so far only deal in NP problems, but not NP complete problems, and that there might (I guess) be an "efficient classical algorithm for these problems" which presumably we haven't figured out yet. In other words, the argument seems to be (if I understand it correctly): quantum computing may only appear to be doing really cool stuff because we've only solved hard problems with it, but not really hard problems, and maybe it's possible solve just the hard ones classically anyway (which maybe would explain how quantum computing appears so efficient?). It frankly sounds like a bunch of handwaving to me. The section concludes by just saying "explaining quantum computation ought to be viewed as a challenge."
In other words, we have no idea.
As for the interference/double-slit issue, it seems rather than the "wave function" traveling through both slits, they want to claim that "a bit of information" or an "influence" is traveling through one slit while the photon goes through the other, hence leading to interference: "There is an influence that travels through both arms [i.e.,slits], but that influence is not a wavefunction."
I don't quite know what the heck difference it makes to claim that "information" or some vague "influence" is traveling through the other slit, compared to the wavefunction itself, other than we can handwave the interference problem away too. I assume some explanation of what they're proposing in terms of this "information" might be buried in the other 89 pages of the article, but my quantum theory knowledge is rusty (and some of this is I'm sure above my head) -- and I stopped reading after the whole "We should just view explaining quantum computing as a challenge" nonsense. To me, interference phenomena and quantum computing are the PRIMARY issues that need to be explained for someone arguing for a pure epistemic theory, and both seem to just be met with handwaving, unless there's something buried in all the equations and theorizing elsewhere in the article that lends insight into these.
Note that the interference section ends with: "In combination with the fact that interference phenomena can be modeled ψ-epistemically, the argument from interference is far from compelling." So I assume there's something buried later in that article... if anyone has the knowledge and initiative to go digging for their explanation of interference other than handwaving "information" traveling through slits, I'd be interested in knowing what the gist of their explanation is.
(Score: 3, Informative) by AthanasiusKircher on Wednesday May 03 2017, @02:23PM
Also, I should note that the summary is very misleading about TFA (i.e., the Nautilus article linked at the beginning of the summary). The excerpt in the summary seems to suggest TFA is advocating an epistemic view, but actually by the end TFA basically is arguing that recent evidence might start to RULE OUT an epistemic view. From the conclusion:
Spekkens and his collaborators managed to take an interpretation of the quantum state and turn it into a precise hypothesis—a hypothesis that was then refuted with mathematical and experimental results. That does not mean epistemic approaches are dead, but it does force their advocates to come up with a new hypothesis. And that is unambiguous progress—both scientific and philosophical.
The bit in the summary is presenting a simplified version of the perspective the article later points out is more likely to be wrong....
(Score: 0) by Anonymous Coward on Wednesday May 03 2017, @04:16PM (1 child)
Can't we say that we simply don't understand (can't see) the root mechanisms of QM and that probability-based models are the best models we have right now?
Until the math behind gravity was understood, nested epicycles were the best model of the time to "explain" orbits, or at least model them. We probably are just missing some key clues and/or haven't spotted a key relationship yet, and thus are mucking around with probabilities and ghost multiverses. Probabilities could be our generation's epicycles. ("Dark-matter" could be in a similar boat.)
(Score: 2) by maxwell demon on Wednesday May 03 2017, @10:35PM
Probabilistic behaviour isn't the real problem. The real problem is that one can mathematically prove that anything following the rules of quantum mechanics has to violate at least one of the conditions that one would expect to hold. The debate essentially is which of those is the right one to drop.
Basically those conditions are:
Quantum mechanics essentially says: Realism, Locality, Single world: Choose two. And the experiments confirm it.
Moreover, and related to this, there is the measurement problem: Quantum mechanics has two qualitatively different sets of rules: One set that describes what happens when you don't observe a system, and one that describes what happens on observation. The first one is deterministic, the second one probabilistic. The problem here is that ultimately observations are ultimately also just normal physical interactions, and therefore one would expect the rules for unobserved evolution to also apply to observations. But that would contradict the rules for observation; in particular you'll never get a definite measurement result out of unobserved evolution.
The Tao of math: The numbers you can count are not the real numbers.
(Score: 1) by Demena on Sunday May 07 2017, @05:41AM
Are you talking about Pilot Wave Theory?