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posted by Fnord666 on Wednesday May 03 2017, @07:39AM   Printer-friendly
from the so-it's-what-you-know,-not-who dept.

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


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  • (Score: 2) by sjames on Wednesday May 03 2017, @05:11PM (8 children)

    by sjames (2882) on Wednesday May 03 2017, @05:11PM (#503771) Journal
    Once and for all, it's not just a matter of what we know. Sure, that makes it all seem much simpler, but the fact is, we not only have arcane experiments but every day applications that depend on the "quantum weirdness" being a reality.

    If it was just what we know, tunnel diodes wouldn't tunnel and die shrinks wouldn't be so challenging. On the experimental side, if it was just what we know, sodium atoms wouldn't tunnel through a diffraction grating too fine to pass a sodium atom.

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  • (Score: 0) by Anonymous Coward on Wednesday May 03 2017, @05:45PM (2 children)

    by Anonymous Coward on Wednesday May 03 2017, @05:45PM (#503790)

    If it was just what we know, tunnel diodes wouldn't tunnel and die shrinks wouldn't be so challenging. On the experimental side, if it was just what we know, sodium atoms wouldn't tunnel through a diffraction grating too fine to pass a sodium atom.

    I'm no expert either way but I would be very surprised if no one could come up with a non-quantum explanation for the observations behind these claims if significant motivation was involved.

    • (Score: 2) by sjames on Wednesday May 03 2017, @06:44PM

      by sjames (2882) on Wednesday May 03 2017, @06:44PM (#503836) Journal

      There's been a long time to do so and plenty of motivation. But each and every experiment shows that Heisenberg's uncertainty isn't just a limitation of measurement, or measurability. In a sense, it's a limitation on the resolution of reality itself.

      If there's a deterministic layer below that, we haven't the slightest clue on how to probe it.

    • (Score: 1) by khallow on Wednesday May 03 2017, @07:54PM

      by khallow (3766) Subscriber Badge on Wednesday May 03 2017, @07:54PM (#503897) Journal

      but I would be very surprised if no one could come up with a non-quantum explanation for the observations behind these claims

      Sure, God willed it so, for example.

      But if you're looking for an explanation that will pass empirical muster, you will inherently have a non-classical explanation. Quantum is merely the label for that.

  • (Score: 2) by kaszz on Wednesday May 03 2017, @06:41PM (2 children)

    by kaszz (4211) on Wednesday May 03 2017, @06:41PM (#503828) Journal

    Perhaps that is because there are no sodium atoms just wave packets that mimic the behavior of one? And those wave will have no problem passing through a material made up of other waves.

    • (Score: 2) by sjames on Wednesday May 03 2017, @06:46PM (1 child)

      by sjames (2882) on Wednesday May 03 2017, @06:46PM (#503839) Journal

      And that puts us right back to quantum mechanics being about reality rather than our ability to measure reality.

      • (Score: 2) by kaszz on Wednesday May 03 2017, @08:13PM

        by kaszz (4211) on Wednesday May 03 2017, @08:13PM (#503917) Journal

        In that case the theory should be able to predict physical phenomena such that one can build things without some measurements that because of practical reasons can't be had.

  • (Score: 1, Touché) by Anonymous Coward on Wednesday May 03 2017, @08:46PM (1 child)

    by Anonymous Coward on Wednesday May 03 2017, @08:46PM (#503932)

    It seems to be that what they're arguing is that it is almost as if there were some information we don't know about, like some kind of variables that are hidden to us. And if we were somehow cognizant of what these variables were, we could put them into our equations and have a fully deterministic model. I can't believe nobody has ever thought of this, this Hidden Variable explanation of quantum mechanics! It is so devilishly obvious, somebody must have thought of it before, and I'd swear I've heard of it before, but it just doesn't seem to ring a Bell.

    • (Score: 1) by Demena on Sunday May 07 2017, @09:16AM

      by Demena (5637) on Sunday May 07 2017, @09:16AM (#505770)

      A straw man perhaps? I read it as suggesting other mechanisms not hidden variables. And since we do not yet have an explanation for something as simple as inertia it is clear we do not know all the mechanisms.

      If you are going to make a forlorn attempt to irrationally scathing please do not abuse the names of good people.