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Waves are real waves or what?

  1. Feb 25, 2008 #1
    Hi,

    Some writers on quantum physics, especially on copenhagen interpretation, talk about waves as "real" before measurement/hitting the screen/disturbing. Electron, in nature, is nothing but a real physical wave. They say particle like appearence as normally collapsing of this wave function.

    Others are talking about this waves before measurement have no reality as we used to. It's a mathematical wave in hilbert space. And write "this waves before the measurement are not waves as we know. These are probability waves.

    I'm really confused. Are waves waves? or is it something else? If answer is second, then ı have to ask what's interfering with itself? Because interference occurs physically real...
    And it's said to have no definite position for electron before measurement, because it doesn't have any. Before measurement, there's a wave which locations spread space. I understand that but doesn't it have to be a real wave for this description?



    Thank's for answers.
     
    Last edited: Feb 25, 2008
  2. jcsd
  3. Feb 25, 2008 #2

    Demystifier

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    There are several different interpretations concerning the question what the waves/particles really are. Let me briefly describe the 3 main types of interpretations:

    1. No reality interpretations
    There is no reality, there are only observations. The wave describes only the information (probability) on future observations.

    2. Single reality interpretations
    Wave is the only real entity. In the case of a position measurement the wave collapses to a localized wave, but it is still a wave.

    3. Double reality interpretations
    This is basically the Bohmian interpretation. The wave (that never really collapses) and the pointlike particle exist separately. The directly measured stuff is the particle, while the wave plays a role of an external force on the particle.
     
  4. Feb 25, 2008 #3

    vanesch

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    Uh ? I feel left out :cry: o:)
     
  5. Feb 25, 2008 #4

    Demystifier

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    Well, the many world interpretation is also a kind of a double reality interpretation. It involves the bird reality (the uncollapsing wave) and the frog reality (the observed).

    Alternatively, this interpretation could be classified as
    4. Infinite reality interpretations

    Which do you prefer?
     
    Last edited: Feb 25, 2008
  6. Feb 25, 2008 #5
    I think the orthodox copenhagen interpretation says only about observations. So according to this, wave structure of electron is mentioned as a probability distributions to find particles on screen, right?

    In a recent article about myths in quantum mechanics Nikolic says "electrons
    and photons always behave as waves, while a particlelike behavior corresponds only to a
    special case." (http://arxiv.org/PS_cache/quant-ph/pdf/0609/0609163v2.pdf)

    But john Gribbin write that there's nothing as real about waves before measurement. Waves mentioned is not a classical manner, It's an abstraction. Schrodinger was confused with this too.

    I want to ask from the point of view of copenhagen interpretation, Which is right? waves are strange-but-real waves or an abstaction?
     
  7. Feb 25, 2008 #6

    Mentz114

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    Quantum theory itself says that something is observable (real?) only if a corresponding hermitian operator that acts on the state vector can be found. There are operators for position and momentum but not for the wave function. One could therefore say that the wave function is an abstraction.
     
  8. Feb 25, 2008 #7
    scientists explain wave collapse using real waves picture. Interference experiment is being explained with waves' interference, coherence and decoherence. but when it's time to name it, they say it is not a classical wave! I can't understand.

    What's the difference between classical waves and quantum waves?
     
  9. Feb 25, 2008 #8
    [POST=1615794]Here's a post[/POST] from the philosophy forum where I mention why the general usage of the term “wave” in QM seems iffy to me. The “wave function” wave, the particle entity in QM, is mathematically not at all the same thing as the waves that are studied in wave mechanics (though wave mechanics do apply to light waves and radio waves, et cetera, of course.)
     
  10. Feb 25, 2008 #9
    Is there an e-text that explains differences between classical wave and quantum wave, conceptually and physically?
     
  11. Feb 25, 2008 #10
    If I'm correct in what I'm saying above, I think you may be unlikely to find a text like that.

    The basic thing about a classical wave is that on some level it's periodic - it's a structure or phenomenon that repeats in some manner that can be expressed mathematically.

    Whereas, on the other hand, if you look at the forms of atomic orbitals derived from the Schrödinger equation / wave function for example, they don't appear periodic. And having worked with them mathematically in the past I don't believe they contain the trigonometric functions (sin, cos, tan) anywhere, which are usually how waves are expressed in wave mechanics.
     

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  12. Feb 26, 2008 #11

    Demystifier

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    This paper actually says that it is so from a specific widely accepted point of view. Other possibilities are discussed in the paper as well.
     
  13. Feb 26, 2008 #12

    Fra

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    is information real?

    I consider my personal interpretation to be close to this one, however I wouldn't phrase it as "no reality".

    Is information real? Of course that begs a definition for "real" and for "information". But loosely speaking I think of information (~predictive power about something) as subjectively real, where the "subject" refers to the observer.

    This would suggest that the wavefunction, or what the correspondence to the "information state" will be in a final theory, must have a real basis, although a subjective one.

    Objective information, such as classical information can be seen as emergent when a group of observers interact. The interaction selects/creates this objective reference. Objective emergent information are required for sensible communication.

    I imagine the "information" of an observer, to be subjectively real in the sense that it can be encoded in the combinatorics of it's internal microstate. So an observer is specificed by it's microstructure, which does not only speciy a information capacity of encoding, but also an internal relation and transformation rules - ie it defines how the microstructure and it's microstates responds probabilistically to perturbation.

    The next quesstion is what makes certain microstructure appear in the first place in a certain environment? How can a microstructure "grow" and gain information capacity? And what makes a microstructure destabilise, and ultimately loose information capacity? And more importantly, to what extent is that predictable by a real "inside observer". To what extent can the predictability be acquired by evolving the observer by selecting for the most fit one? (See quantum darwinism) And what sets a bound to the predictability of a given observer? Is the observers information capacity, somehow related to it's mass? There are many extremely interesting questions here.

    I personally see these interpretational issue as a result of our still incomplete understanding of the world, and perhaps more importantly, an incomplete understanding of our own understanding so to speak - this latter case is what I think is the task of theoretical physics. Theoretical contemplation never replaces real experiments, but OTOH real experiment also never replaces contemplation and reflection. That's just my personal view.

    /Fredrik
     
  14. Feb 26, 2008 #13

    Demystifier

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    The Copenhagen interpretation is a no-reality interpretation of my classification in #2.
     
  15. Feb 26, 2008 #14

    Demystifier

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    They are mathematically the same. Their physical interpretation is different. Classical waves are directly observable. Quantum waves are not, unless you introduce an additional non-classical mathematical property of waves corresponding to their collapse.
     
  16. Feb 26, 2008 #15

    Fra

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    So what's gained by "understanding our understanding"? It does not provide answers to our questions, but it may help use ask us better questions in the future, because spending resources to trying to answer unthoughtful questions doesn't seem like a good idea, and this in the long run gives us a better information gain / investment ratio.

    So I personally feel that the efforts in doing "interpretations" like the above, well spent time, becauase I picture that it doesn't end with an interpretation, the "interpretation" to me is more like a guidance for future expansion and improvement. So I would expect "benefit" from a sensible interpretation.

    /Fredrik
     
  17. Feb 26, 2008 #16
    Let's clarify

    You say the differences between a classical(mechanical and electro-magnetic) wave and quantum wave is

    1. You can not see quantum waves.
    2. It has a property of collapse to a particle

    Right? all the problems is that?
     
  18. Feb 26, 2008 #17

    Demystifier

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    More or less that's it. However, two additional notes are in order:
    a) Between 1. and 2. is OR (not AND), depending on the interpretation.
    b) The collapse does not necessarily need to be to a particle, but in general to an eigenstate of an arbitrary measured operator.
     
  19. Feb 26, 2008 #18

    vanesch

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    I would like to make an observation: "quantum waves" "wave" in configuration space, while "classical waves" wave in 3-D (or 4-D) Euclidean (Lorentzian) space.
     
  20. Feb 26, 2008 #19

    vanesch

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    I like that :smile: ... although there are now many different frogs...
     
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