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Quantum boundaries

  1. Aug 6, 2009 #1
    It has been said that I have not made it clear what I mean by boundaries. So I will try to explain it here. It has been said that at the quantum level boundaries are not clear. First imagine an object. By definition a clear boundary means that every point in the visual field* is part of the object or not part of the object. Therefore, an indefinite boundary means that there are points in the visual field that are part of the object and not part of the object. I repeatedly asked if the indefiniteness of the boundaries was actually there or not. I was told that they are. Therefore, there are points in the visual field that are both part of the object under investigation and not part of the object.
    * By visual field I mean the area that is being looked at.
    PS I fail to see how what I just said is such an outrageous speculation.
     
  2. jcsd
  3. Aug 6, 2009 #2
    It has also been alleged that I am anti-QM. That is far from the truth. I was only trying to understand what it means when QM says that boundaries are indefinite. I have only said that QM goes against our common sense ( I think that even ZapperZ would agree with that.) My last statement is not a condemnation of QM. It is simply saying that common sense is inadequate to the task of comprehending QM.* I am still baffled by the anger my above comments have caused.
    * I even mentioned that I am familiar with paraconsistent logic. So I am obviously open to the idea that classical logic might not be the only logic in town.
     
  4. Aug 6, 2009 #3

    DrChinese

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    You are actually in the middle of some interesting areas in QM. You are probably familiar with the term "wave-particle" duality. This maps to the HUP in the formalism. The HUP essentially says that there is an inverse relationship between the definiteness of the position of a particle and the momentum of a particle. You can wrap whatever words you want around this, but the HUP is still the guiding principle. Because of this, it is not really possible to *strictly* speak of a fundamental particle's size. The reason it is called a point particle is because in the limit it acts as one. Further, it has no internal structure (as opposed to protons, for example, which consist of quarks and clearly display a discernable structure). So clearly, the boundaries are cloudy here.

    Further: there is a virtual field which surrounds matter (atomic particles: electron, proton, and neutron also - but that nets to essentialy zero). So it is difficult to discuss the boundary here without reference to the charge field, which can technically be said to extend everywhere.

    And further: most particles are members of a variety of larger systems. This means they cannot fully be described independently, although they can be setup so their other associations can be mostly ignored.

    Now, as to whether these facts violate classical logic or not, you can take your pick. Either they do, because we classicly define objects such that they have clear boundaries. Or they don't, because such definition defies the facts.

    As to the "anger" you are feeling instead of "love"... :smile: Actually you will find this board a very welcoming place and good people (like ZapperZ) will spend time trying to help you. We have a lot of folks come here that try to say things like: "Have you considered X?" That doesn't sit too well; try re-phrasing things so it is YOU (rather than us) that is considering other things - you will find a warmer reception that does not bump into the guidelines. (I can personally tell you, as an amateur in the field, that just about any crazy idea that you can think of has in fact been considered by serious top pros. And a paper published to boot!) Take advantage of the great folks here and you will both enjoy and learn. And there is plenty of vigorous debate and discussion too, a lot of different viewpoints!! Plenty of room for yours too, if expressed properly.
     
  5. Aug 7, 2009 #4

    Not only bounderies, but the existence of space as we perceive it is disputed, and our perception of it is incomplete at best, totally wrong at worst.



    Classical logic belongs in the classical realm. What is an electron? Don't try to picture it, you can't. No one can. We might use orbitals to show the probability density of it being somewhere with dots on your screen, but this is imprecise and only for visualisation purposes. No one can imagine something that has zero size and zero structure and is spread all over the universe(provided we know the wavelength). Even the animation of a 1-dimensional string, that has phyisical size, is pretty misleading.



    Yes, and your "inadequate" eye-sight does not tell you that the spread of the wavefunction can mean that the probability for some of the electrons in atoms(higher energy electrons), is not zero for said electron to be on Mars or on Alpha Centauri or at the other end of the universe at some point in time. Now you can extend your objection that the boundery is not fixed, by asking why is a part of the boundery(electron) on Alpha Centaury?


    There is most certainly something that resembles a boundery, but it's neither rigid nor permanent. Again, how you see things in your daily life is misleading to how nature behaves at more fundamental levels. And it's not a fault of quantum mechanics that we perceive matter as solidly physical when its 'solidity' is simply an electromagnetic phenomenon. Our classical perception is pretty weird, as physical matter would be more precisely and unambiguously defined as an electromagnetic phenomenon, than as solid stuff as our perception dictates.
     
    Last edited: Aug 7, 2009
  6. Aug 7, 2009 #5
    Wittgenstein,
    I don't think what you say is so outrageous either, though, for myself, I'm on the fence on the issue of clear boundaries. However, I'm a little unclear about the way you're setting the issue up.

    I'm not sure I would talk about parts of the visual field, even understood as the area being looked at, being part of the object or not part of the object. If, for example, there's such a thing as space (as some such as Newton believed), and objects are located at points or areas of space (with these areas and points being treated as having genuine existence), those areas are nevertheless not *parts* of the located object. Similarly, if we're serious about the electromagnetic field, then objects and parts of the field can be co-located but that doesn't mean that the field is thereby part of the object.

    Rather than it being an issue about whether one thing is a part of another, the question of boundaries can also be understood as a question about the *location* of an object and/or that object's parts. There is a line of thought that quantum mechanics tells us that, in many situations, objects don't have precise locations. In particular, in many quantum states, it is thought that there is no precise fact of the matter as to the distance an atom's orbiting electron is from the nucleus. On this view, the idea that an electron is either precisely located at this region or precisely located at another region is not correct. The idea that objects have precise locations is undermined.

    I would give the following caution - though I don't think everyone sees it this way so don't take my word for it - I don't think QM proves electrons lack determinate position. It's true that Quantum mechanics postulates and uses states where objects are described as being in superpositions - something wholly non-classical and difficult to describe. Mathematically, they are a kind of weighted sum of an object's having various properties. Physically, they appear to be indispensible to predicting the results of experimental situations, and are key to quantum mechanics. But the physical interpretation of superpositional states is a contentious one. Different interpretations of quantum mechanics interpret the notion of a superposition differently; only on some views are objects understood to literally lack a precise, determinate position.

    Yossell
     
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