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Quantum frogs and jumping to conclusions

  1. Aug 28, 2003 #1
    [SOLVED] Quantum frogs and jumping to conclusions

    What does quantum mean exactly? What is a 'quantum leap'? And this urban myth going around about quantum matter changing upon observation (as though it knows it is being observed and therefore plays little tricks on the observer), what is this really about?
  2. jcsd
  3. Aug 28, 2003 #2


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    There is a book "Alice in Quantumland" (a lot of fun to read) by Robert Gilmore, which presents a good description of the subject for the general reader. It also includes the observation that nobody really understands quantum mechanics.
  4. Aug 29, 2003 #3


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    A discrete amount of something.

    A radical change in thinking. Usually scientific/technological.

    The uncertainty principle, which is a conclusion of QM. Basically, it states that (a) position and momentum, or time and energy, are not entirely separate concepts. and (b) there is no absolute value for each of them and (c) The degree of uncertainty is governed by Uncertainty in P * Uncerrtainty in Q > half h-bar. Note: This uncertainty is not just an error in measurement, but a statement of the actual nature of stuff.

    Based on this, we get the idea that every thing is expressed as a wave equation of probability, which collapses down to a specific particle on observation. Until it is observed, the nature of the entity is indeterminate. Hence the schrodinger's cat paradox, when we expand this concept to larger size objects, and conclude the cat is neither exactly dead nor alive, until we observe it and collapse it's wave function.

    Several solutions exist. One is that the particles don't really do that, but Hidden Variables give it the impression of doing so. Numerous experiments (like the Aspect experiment) have disproven the so-called EPR paradox. Another is that our consciousness somehow is significant, and collapses the waveform. The Universe is Participatory. This produces some philosophical nastiness as to whether the cat is conscious, and whether we are neccessary etc etc. Another is that Many Worlds exist so all the possibilities are simultaneously real. This however has little evidence. There may almost certainly be others.

    A demonstration of the problem is easy to arrange - get 3 polarised filters. They, as you may recall, allow light through in only one alignment. If you put two of the filters aligned at right angles to each other, as predicted, no light goes through. The only light that gets through the first is cut out by the second filter. But if you put another filter in between the two aligned at 45 degrees to each, some light does in fact penetrate! This is completely contrary to classical theory. The quantum explanation is that some how, the act of passing through the filters introduced uncertainty to the light, giving it a probability of penetrating the other filter.

    EDIT: Fixed HEP formula. Thanks Jcsd
    Last edited: Aug 29, 2003
  5. Aug 29, 2003 #4


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    A quantum more specifically is the smallest amount of energy a system can gain or lose, though it has come to mean the smallest possible amount of any quantity.

    A quantum leap or jump is the transmission from one stationary state of an atom or molecule accompanied by the admission or absorbtion of energy.

    Properties are undefined until you measure them and the act of measuring properties can also change them.

    Also for FZ+, in the most precise form of the Hesienburg Uncertainity principle the unceratinity in a pair of complemtary variables is equal to half h-bar.
    Last edited: Aug 29, 2003
  6. Aug 29, 2003 #5
    Heisenberg realized there was no
    way to observe an electron in
    orbit without interfering with it.
    There was no way to measure it's
    position or speed, not even with
    the most sensitive measuring tool
    available: light. Imagine looking
    through the best possible micro-
    scope at a atom and trying to see
    what an electron is doing by means
    of the light bouncing off the
    electron up into the lens:

    "The electron apparently doesn't
    like the new turn of events. It is
    having a rough time of it. We are
    not leaving it in peace any more.
    We are not just looking at it, we
    are hurling huge boulders of ener-
    gy at it and it is being badly
    knocked about. What sort of scien-
    tific experiment is this? It is
    certainly far from delicate. Sup-
    pose we do manage to see the electron and note its position? It is an empty victory. The very fact that we see it means we have
    scored a direct hit with a photon.
    The electron is a very light par-
    ticle unable to withstand a
    particle of light. It is badly
    jolted by the impact. In observing
    the electron's position we give it
    a jolt which alters its velocity. We defeat our own object."

    The Strange Story Of The Quantum
    by Banesh Hoffman
    1947 Dover Publications NY

    So it is not correct to character-
    ize the situation as one of tricks
    being played on the observer.
    Heisenberg was pointing out that,
    with the means available any
    attempt to observe the phenomenon
    will change the phenomenon and
    what we see will be a misrepre-
    sentation of its usual position
    and/or speed, whatever that may
    Last edited: Aug 29, 2003
  7. Aug 29, 2003 #6

    Chi Meson

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    I won't even mention Heisenburg or Schroedinger. Oops.

    It is true that you can NOT make any observation of any thing without interacting with it. In the most obvious form, you can not tell how heavy something is without lifting it.

    But even looking at something is interacting with it. You can not see something (like a passing car) unless light has struck it and subsequently bounced off toward your eye. When light hit that car, a change to that car occurred. It got a tiny bit hotter. The car even recoiled an extremely tiny bit due to the momentum of the photon. With large objects this tiny change is not noticeable.

    But with single particles, the change of momentum and energy is very significant when they are struck by photons or other particles.

    The point is, no matter what, if you wnat to observe something, you must hit it with a particle, and that collision will change it.
  8. Aug 30, 2003 #7

    Sounds good. Thanks...
  9. Aug 30, 2003 #8
    i wouldnt recommend alice in quantumland if allegories and the like arent ur thing or if u just dont understand quantum mechanics. if u do understand it good but some of the things in the book are relatively vague compared to the actually principles and laws. the footnotes do help so read them!
  10. Aug 30, 2003 #9

    Thankyou very much for that informative and thoughtful response FZ!

    I can not profess to understand everything you have said but that's ok and all part of my own learning process.

    Regards the filter and light problem, does the introduction of the third filter act as some sort of reflector off the the other two filters thereby allowing light through? Light bending?
  11. Aug 30, 2003 #10
    no it is half silvered on one side so as to let light only come through one side and its angled
  12. Aug 30, 2003 #11
    also the uncertainty principle makes it clear that it penetrates and doesnt because the photons must take both paths but since we are lookin at it we can only use the probability of light passing through. it goes back to the whole electron thing and the superpostion of states
  13. Aug 30, 2003 #12
    What Zoobyshoe and Chi Meson said seem to make the most sense. Analogy that comes to mind. Someone decides to find out how much sealife there is in very deep oceans not usually reached by light. The introduction of light kills of the life because this is life not adapted to light (as we know it) and therefore the quantity equals zero although it is known that sea-life exists via discovery using other methods..

    hey...I'm not a scientist so I'm trying to grasp the problem with layperson understanding.
  14. Aug 30, 2003 #13
    thats understandable everyone learns differently and meson did describe it very well better than i did i think oh well another question answered at least by someone
  15. Aug 30, 2003 #14
    if u have any more question ud like answered id be more than happy to try and explain it
  16. Aug 30, 2003 #15
    carla uve got mail
  17. Aug 30, 2003 #16

    Thanks pop...it's hard to respond to everyone's posts but I do read them all and I read yours too.
  18. Aug 30, 2003 #17
    sry im just really bored and i need sumtin to do
  19. Aug 30, 2003 #18
    Reflection was the wrong wrong wrong term to use. I meant, refraction, I think. That is light BENDING via all possible avenues open to it to allow it to travel through!
  20. Aug 30, 2003 #19
    yes thats quite a crucial difference in the wording
  21. Aug 31, 2003 #20


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    Not so easy. If it did so in the classical sense, it would mean that all of the photons would go through since on the other side of the first filter, they would be all the same and bent the same way. But experiments show that only about 25% of the photons that passed the first filter were outputted.
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