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Th moon is still there right?

  1. Oct 18, 2011 #1
    The moon is still there right?

    I understand how the act of measuring can cause subatoms to collapse into fixed positions but they do not have mass. And the moon is made of atoms not a flurry of electrons its made of atoms. My question is are atoms (which have mass) there even when they havent been "observed". I mean atoms arnt in a wave like superposition like electrons an atom cant be in two places at the same time right? An atom is there even if youre not observing it right?
     
    Last edited: Oct 18, 2011
  2. jcsd
  3. Oct 18, 2011 #2

    xts

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    Electrons have their mass too.
    Atoms exhibit QM behaviour (may interfere, are subjects to uncertainity principle, etc).
    Even large molecules exhibit measureable quantum behaviour.

    The Moon is there because it is measured very frequently (even when Einstein does not look at it) and those measurements affect its further movement quite a little.

    (Anyway - to be quite sure about Moon, you must ask Aussies. I can't tell you if it is still there right now...)
     
    Last edited: Oct 18, 2011
  4. Oct 18, 2011 #3

    HallsofIvy

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    No, you don't understand. The "act of measuring" causes anything to "collapse into fixed position", not just things that "have no mass". What, exactly do you mean by "subatoms"? Electrons? Neutrons? Protons? They certainly do have mass!

    Of course, for a "large" (larger than atomic size) object the possible variation in position is very small- it has such a very short wavelength, it can always be treated as an "object" rather than a wave.
     
  5. Oct 18, 2011 #4
    Scuese me isnt "measuring" just the physical interaction with other subatoms that irreversibly forces electrons and things into fixed positions. Dont wave functions collapse naturrally in the universe even when no physicist "observes" them, I mean were just recreating a process that happens in nature anyways. So atomic structures which are not isolated from the rest of the universe are constantly bombarded by subatoms forcing it into a static position. Correct?

    "Atoms exhibit QM behaviour (may interfere, are subjects to uncertainity principle, etc).
    Even large molecules exhibit measureable quantum behaviour." Examples please?

    So atoms are waves I thought that only applied to electrons? Everything in the universe is a accumulation of wavefunctions that concentrate in certian "possible" positions where the bigger they become the smallr their superposition. Im sorry no way this has to be an oversimplification.
     
  6. Oct 18, 2011 #5

    DrChinese

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    All quantum objects exhibit a mixture of wave and particle behavior, and this is often described by the Heisenberg Uncertainty Principle (HUP). There is no specific size at which this behavior is no longer evident, it simply tends to vanish quickly as the objects get larger (atoms are bigger than electrons, molecules are bigger than atoms, etc. up the chain). Below is a reference to quantum properties of a reasonably large molecule.

    http://hexagon.physics.wisc.edu/teaching/2010s%20ph531%20quantum%20mechanics/interesting%20papers/zeilinger%20large%20molecule%20interference%20ajp%202003.pdf [Broken]
     
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  7. Oct 18, 2011 #6
    What about this part?

    Scuese me isnt "measuring" just the physical interaction with other subatoms that irreversibly forces electrons and things into fixed positions. Dont wave functions collapse naturrally in the universe even when no physicist "observes" them, I mean were just recreating a process that happens in nature anyways. So atomic structures which are not isolated from the rest of the universe are constantly bombarded by subatoms forcing it into a static position. Correct?
     
  8. Oct 18, 2011 #7

    xts

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    Thenewdeal: could you, finally, answer yourself (but also to me) what do you mean by 'collapse'?
    E.g. Anton Zeilinger's experiments with interference of fulerenes.
    http://www.univie.ac.at/qfp/research/matterwave/c60/index.html - pop-science version (but I like it!)
    http://arxiv.org/PS_cache/quant-ph/pdf/0110/0110012v1.pdf - serious publication

    You definitely should avoid the verb 'to be'. It carries too much metaphysical baggage. Hamlet was one of those who assigned to much meaning to that verb...
    Say it rather: In some situations atoms may be successfully described in terms of wave mechanics and in those cases the "miniature snooker ball" model fails.
     
    Last edited: Oct 18, 2011
  9. Oct 18, 2011 #8

    DrChinese

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    As far as we know, physicists are not required for the moon to have a position. Nonetheless, unobserved particles can be anywhere - although they are more likely to be in some places than others.
     
  10. Oct 18, 2011 #9

    atyy

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    The collapse of the wave function is an unsolved problem in quantum mechanics. It works, but doesn't make sense.

    The best coherent alternatives are the many-worlds interpretation (which is aesthetically terrible to many, including Yakir Aharonov and Steven Weinberg), and Bohmian mechanics (which I am told doesn't work for relativistic situations - I expect Demystifier to say something here:)
     
  11. Oct 18, 2011 #10
    First of all: Electrons have mass. Atoms, molecules, both massless and massive particles, all exhibit quantum behaviour.

    Secondly: The act of measuring a physical observable(like position) forces the wavefunction to an(or multiple) allowed state of that observable. This does mean that electrons are forced into a small volume when it's position is observed. However, when you observe it's energy it becomes less confined. There are a lot of physical observables like energy, position, spin etc. Measuring any one of these will force the particle to take an allowed state of that observable. Measuring one observable hence will change the state of the electron with respect to another. Ie: if you measure position, its energy is no longer what it was when you measured it.

    Thirdly: The electron/atom is really there regardless of whether you observe it or not. Your detector for position cannot be infinitely small. Even after you measure position, the electron still exists in many places at once. Except now, all the places where it exists are confined to a small region.
     
  12. Oct 18, 2011 #11
    ...and this gives rise to a multitude of incredible suggestions about reality. That's all there is to the question "is the Moon there when nobody is observing?"
     
  13. Oct 18, 2011 #12
    I basically agree, but would change "doesn't make ssense" to something like "is not completely understood"...but of course a lot of things in physics and QM are like that.

    I'd generally agree with that...when particles interact which they typically do, wavefunctions( if that is the description one is using" collapse....so where there is a lot of interaction, stuff
    tends to be localized. For example, an electron bopund to an atom is expetced to be found within the electron cloud and exists in quantized (discrete energy states)....but when it is flying around freely in outer space it spreads everywhere.....

    "Measuring any one of these will force the particle to take an allowed state of that observable." is one nice way to summarize that.

    confined to some localized area, yes, but not "a static position". An instantaneous position to an accuracy dictated by the Heisenberg uncertainty principle,yes, but another measurement in general will reflect a distribution of values. Even after all measurement corrections are made, HUP prevents you from achieving some perfect result...even 'instantaneously'.
     
  14. Oct 18, 2011 #13

    xts

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    I would rather say: it is a lexical problem rather than physical. There is nothing more to understand in 'collapse' except of lack of commonly accepted definition of that word. People use the term 'collapse' without defining it clearly. There are severeal different meanings commonly used. There is quite a little mystery in it if we agree to use only one of those meanings. In some contextes the meaning is obvious, but more often it is not clear and people tend to take a metaphysical baggage frokm one meaning and assign it to other uses of that word.

    That is why I hate to speak about 'collapse'... Vast majority of minunderstandings and paradoxes may be avoided if we don't use such words and speak about well described experiments (or thought-experiments) in terms of their initial conditions and outcomes.
     
  15. Oct 18, 2011 #14
    Folks,

    May I input my understanding of 'the act of measuring'? When a particle has to be examined, some measuring device which emits a light beam is required. Since the particles we are measuring are so so small they are sensitive to any interaction from a light beam...ie the particle gets pushed around due to the interaction of the light beam, the particle is sensitive to the imposing elecromagnetic field.

    Hence we can never know about the particles position because the very act of measuring disturbs it. I dont see anything mysterious about this.....I dont see why we need to use the word 'collapse'....
     
  16. Oct 18, 2011 #15
    What about interaction-free measurements?

    It is also often stated that measurement creates the outcome, not reveals an existing definite property.
     
  17. Oct 18, 2011 #16
    Interaction free measurements...? hmmmmm...I have never heard of that before....:-)

    All experiments I have heard of are not interaction free ie, double slit, stern...
     
  18. Oct 18, 2011 #17

    xts

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    Yeah? What about them? Could you explain what do you mean by 'collapse' in context of interaction free measurement?

    Seems like you never flew El Al - they invented interaction free bomb-tester to make your flight safe :
    http://en.wikipedia.org/wiki/Elitzur–Vaidman_bomb_tester
     
    Last edited: Oct 18, 2011
  19. Oct 18, 2011 #18

    DrChinese

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    This is somewhat misleading, because we know (from Bell) that it is not the measurement which causes collapse. Rather it is the gaining of knowledge. In other words, I can confine a particle to a certain position (volume) without directly disturbing or affecting it in any way - if I am able to similarly observe (confine) its entangled partner. So it is not the measurement apparatus, it is something more fundamental than that.

    Keep in mind that partial collapse is possible as well! Which shouldn't be possible if the apparatus is disturbing our target.
     
  20. Oct 18, 2011 #19
    Is this equivalent to the delayed choice experiment in the context of 'interaction free'?
     
  21. Oct 18, 2011 #20
    but how else would you similarly observe its entangled partner without the aid of an apparatus?
     
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