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Science

A Boost For Quantum Reality 241

Posted by Soulskill
from the superposition-of-super-positions dept.
Eponymous Hero sends this excerpt from Nature: "The philosophical status of the wavefunction — the entity that determines the probability of different outcomes of measurements on quantum-mechanical particles — would seem to be an unlikely subject for emotional debate. Yet online discussion of a paper claiming to show mathematically that the wavefunction is real has ranged from ardently star-struck to downright vitriolic since the article was first released as a preprint in November 2011. ... [The authors] say that the mathematics leaves no doubt that the wavefunction is not just a statistical tool, but rather, a real, objective state of a quantum system."
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A Boost For Quantum Reality

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  • by notgm (1069012) on Wednesday May 09, 2012 @01:18AM (#39938039)

    it is, and it isn't.

  • Elephants! (Score:5, Funny)

    by Black Parrot (19622) on Wednesday May 09, 2012 @01:27AM (#39938079)

    the mathematics leaves no doubt that the wavefunction is not just a statistical tool, but rather, a real, objective state of a quantum system.

    If that's the case, I would suppose that wavefunctions have wavefunctions.

  • Emotional debate (Score:4, Insightful)

    by mwissel (869864) on Wednesday May 09, 2012 @01:29AM (#39938087) Homepage

    > The philosophical status of the wavefunction [..] would seem to be an unlikely subject for emotional debate

    Well not to me. I guess any subject a given amount of people put lots of effort in can arise emotional debates. *Especially* if the subject in question is discussed philosophically.

    • by glorybe (946151)
      I'm not certain people are quite done fussing over the reality of the flat Earth notion. That means that it may be a while before they are up to any arguments over quantum mechanics and the actuality of various realities. As for myself I'm still stuk on the fabric of space notions. What is the grid size of the fabric? How small a fish can swim trough the net? how many nets are there and do they hold a certain distance between layers. Are these fabrics in numerous colors? Are the fabrics laid out all ov
      • by jd (1658)

        The Flat Earth debate only started in the 1800s, until then people had believed in this globe thing.

  • Most people think of matter as a solid when in fact there is no fundamental solid but matter is in it's base form a vibration which is roughly the same as a wavefunction. In some ways a wavefunction is no different a vibrating string so it's not as crazy as it sounds.
    • I can cut a string. I can not cut a fundamental wave/particle.

      matter is in it's base form a vibration

      What is the thing that is vibrating? The equation E=MC^2 seems to indicate that matter and energy are just two expressions of the same thing. I acknowledge that my confusion "might be" just my inability to imagine nothing vibrating. That doesn't sound right. I mean "nothing" vibrating. See there, I have done it again. No matter how I say it it only makes sense if something is vibrating. If it is a th

      • I am not convinced that the particles regarded as fundamental actually are. I'm not even completely convinced that "particles" at that level even exist in the normal sense, since we know interference patterns exist when the gap is in time rather than in space. That makes no logical sense when using a corpuscular model.

        It is my suspicion (IANAQMPBTIBO) that in precisely the same way that matter is merely energy that has "condensed" and entangled, particles are merely waves that have "condensed" and entangled. This is based on the fact that fundamental particles of the same type are totally interchangeable and no two particles of the same type are in the same state. To me, that does not appear distinguishable from saying that a single wave appears to be every particle of that type, since that would give you what is observed without having to have any new or excessively complex physics to explain it.

        If that is correct, then neither space nor time are particularly important in QM. Which has been theorized by better minds than mine. You would be able to map everything into waveforms and not need spacetime for them to exist in. Rather, spacetime would be one way an observer could interpret those waveforms - it would be subjective, not objective. The waves themselves would be the only "reality". Again, there's a branch of QM based on just such a notion.

        To answer your question as to what is "vibrating", in this line of thought there wouldn't be anything TO vibrate, per-se, no time for it to be vibrate in and no space in which the vibrations could take place. You'd simply have a multidimensional waveform where if you made some axis space and another one time, you could treat it as though something was vibrating. In practice, though, it would be a static n-dimensional waveform whose existence was logical rather than physical.

        I like this particular branch of QM, as it means physics is a branch of mathematics, a specific group with specific properties and specific operations, and that the universe is a specific set of functions that wholly reside in that group. It makes maths the "ultimate" reality, which means these sorts of philosophical musings about the world can be answered through mathematical analysis (although maths permits that answer to be rigorously undefined).

        • I am not convinced that the particles regarded as fundamental actually are.

          This is in fact an older problem than people expect. Ancient Greeks talked about problems with the continuity of space, which lead them to note spacial paradoxes like "Achilles and the tortoise" [wikipedia.org]. One of the proposed solutions to deal with issues of the infinitesimal was to assume that objects are all made of very small particles called "atoms", which were not able to be split any further. Scientists later adopted the word when they discovered that there were indivisible pieces of chemical elements which

  • by Anonymous Coward on Wednesday May 09, 2012 @03:35AM (#39938609)

    The paper is related to Einsten-Podolsky-Rozen (EPR) paradox and the related "hidden variables" hypothesis which AFAIU states that there are some hidden variables apart from wave function that we can not observe directly. However, under some assumptions it can be proven that their existence affects some statistical properties of a particular type of measurements and therefore can be experimentally tested. One of such theorem was Bell inequalities published in 1964. In the Nature paper in question authors prove similar "no-go" theorem but under different assumptions. To quote:

    The result is in the same spirit as Bell’s theorem[13], which
    states that no local theory can reproduce the predictions
    of quantum theory. Both theorems need to assume that
    a system has a objective physical state such that prob-
    abilities for measurement outcomes depend only on .
    But our theorem only assumes this for systems prepared
    in isolation from the rest of the universe in a quantum
    pure state. This is unlike Bell’s theorem, which needs
    to assume the same thing for entangled systems. Fur-
    thermore, our result does not assume locality in general.
    Instead we assume only that systems can be prepared
    so that their physical states are independent. Neither
    theorem assumes underlying determinism.

    There is, however, another theorem by Kochen and Specker that is not cited in this paper but also does not assume locality. From wikipedia

    The essential difference from Bell's approach is that
    the possibility of underpinning quantum mechanics
    by a hidden variable theory is dealt with independently
    of any reference to locality or nonlocality, but instead
    a stronger restriction than locality is made, namely
    that hidden variables are exclusively associated with
    the quantum system being measured; none are associated
    with the measurement apparatus. This is called the
    assumption of non-contextuality.

    It would be interesting to know what would be the relation of results from the paper to that theorem...

  • by Intrepid imaginaut (1970940) on Wednesday May 09, 2012 @03:54AM (#39938725)

    Its that there's no such thing as an unlikely subject for emotional debate.

  • Thought (Score:4, Interesting)

    by should_be_linear (779431) on Wednesday May 09, 2012 @05:13AM (#39938999)
    Here is thought I had the other day: assume mathematical "function" that defines our universe and underlying physics (function that "theory of everything" is trying to find), works in _reverse_ direction of time. So that every particle (or whatever) at t is calculated from local state at (t+1). We usually thinks of laws of physics going in "natural" direction of time. Now, after the inevitable final end of intelligent civilizations in this universe, surely there will be some artifacts made by durable nanomaterials, that persists long after stars and even black holes evaporate into 'nothing". Universe calculated from backwards will therefore have such "intelligently designed" artifact at the _beginning_, as sort of input parameter, so it have to find a mathematically plausible way going forward (which is backwards in time for us) how these artifacts were created. Intelligent life and physical laws supporting intelligent life might be _result_ of something strange at the function input. That means if you have function where random "state" is input and set of equations ("laws of physics") is output, as soon as you put something looking improbable at input, say set of large prime numbers, function might find it is easier to create universe with intelligent civilization, which created this prime numbers, then to create universe where laws of physics created such improbable outcome by chance.
    • by gshegosh (1587463)
      You should read about CPT symmetry and breaking it. If you reverse time flow, you have to make some other changes to apply the same laws of physics.
      • Re: (Score:3, Interesting)

        by Anonymous Coward

        Indeed, the problem with reversing time is that you suddenly have to change physics to handle the fact that the time has already been used. I remember Einstein expressing the view that we get 3 dimensions of space which can be reused and one dimension of time which can't be reused. And physics generally works and the equations are written for that reality. If suddenly, you can reused time and particles can go back and potentially interfere with themselves, then a ton of work would have to be put into making

  • Summary (Score:5, Informative)

    by FrootLoops (1817694) on Wednesday May 09, 2012 @06:14AM (#39939239)

    The article confused me greatly so I read some of the arxiv preprint linked above. Here's the idea and context as I understand it. I've included some basic quantum background since most people here don't have it.

    * Intro to wavefunctions via an example. Electrons have a property called "spin" which has two states, "up" or "down". These can be measured in, for instance, the Stern-Gerlach experiment [wikipedia.org] where those electrons with spin up are deflected up by a magnetic field and those with spin down go down. The wavefunction corresponds to a list of the probability of each outcome occurring. The probabilities evolve through time via the Schrodinger equation which allows predictions to be made. One might prepare an electron where its spin wavefunction corresponds to the list [1/3, 2/3], so 1/3 of the particles go up and 2/3rds go down. [I've oversimplified; wavefunctions are actually elements of an abstract Hilbert space and complex-number amplitudes are used instead of real-number probabilities. I love Hilbert space but it's too much to explain here.]

    * Spin is not a classical property. One can measure spin "left" and "right" in addition to "up" and "down" by rotating the Stern-Gerlach (SG) device mentioned above and measuring left/right deflection. Suppose you run a stream of electrons through an up/down SG device which gives 80% of them "up". You then run those "up" electrons through a left/right SG device--it will always come out with 50% "left" and 50% "right". Even more strangely, if you then run the "left" electrons through another up/down SG device, the probabilities will now be 50%/50%, even though you selected only spin up electrons at the first stage so you'd expect 100%/0%. The act of going through the left/right device altered the spin up/down state somehow.

    * Hidden variables. Perhaps the electrons above have definite "spin vertical" and "spin horizontal" properties before the experiment starts. The act of going through a device must change the other property, though everything might be deterministic if there is some further hidden property controlling which electrons have their spin up/down states altered in which ways by passing through the "left" SG device. The alternative is that there are no definite properties which determine the wavefunction; the wavefunction is all there is, reality is somehow fundamentally probabilistic, and the wavefunction is "real" instead of a statistical construct.

    * Bell's theorem. Suppose spin up/down and spin left/right are definite properties and some hidden variables explain the above results. Using entanglement (which I'll leave undefined) and the assumption that information cannot travel faster than light, one can measure both the spin left/right and spin up/down values of a particle before the hidden variables have a chance to act (note: they might act in a very bizarre, perhaps even non-deterministic, manner, but we get to measure things before they have that chance). This gives a testable prediction which differs from quantum mechanics. If the experiment is performed, the "definite property" theory does not predict reality while the use of wavefunctions does predict reality. This is strong evidence for the reality of wavefunctions, though it's not completely conclusive.

    * The paper. It derives Bell's fundamental contradiction from fewer assumptions. In its own words,

    The result is in the same spirit as Bell's theorem, which states that no local theory [i.e. one without faster-than-light communication] can reproduce the predictions of quantum theory. Both theorems need to assume that a system has a objective physical state L such that probabilities for measurement outcomes depend only on L. But our theorem only assumes this for systems prepared in isolation from the rest of the universe in a quantum pure state [e.g. a particle measured as spin "up" right after the SG experiment above]. This is unlike

    • Re:Summary (Score:4, Insightful)

      by radtea (464814) on Wednesday May 09, 2012 @10:45AM (#39941335)

      My own take as a physicist who knows a bit about this stuff can be found here: http://www.tjradcliffe.com/?p=621 [tjradcliffe.com]

      The important fact is at the end: "That is, 'Preparing a photon in the same quantum state will sometimes result in photons in different physical states' does not imply 'Preparing a photon in different quantum states will sometimes result in photons that are in the same physical state'. The former proposition is the statistical interpretation. The latter is the assumption that the authorâ(TM)s argument depends on."

      Since the author's assumption has nothing to do with the statistical interpretation, their argument says nothing about the statistical interpretation.

  • Of course it's real. There isn't an imaginary term in the wavefunction equation.

  • I intend to patent the direct manipulation of the quantum wave function, which will, among other outcomes, be the basis for my infinite improbability drive.

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