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Random events?

  1. Sep 15, 2003 #1
    I'm a physics novice who has a question for all you gurus out there. I've heard that following the laws of quantum mechanics, an event could occur twice while producing two different outcomes. How is this possible? Or mayby I've completely misunderstood. Can someone explain for me?
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  3. Sep 15, 2003 #2


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    If you do a quantum experiment, that means you operate somehow on some quantum entity to produce a result in the form of some measured quantity. The nature of the quantum world is such that that measured quantity can come in more than one value. Which one you find is a question of probability, and if you did the experiment again, exactly the same, tomorrow, maybe one of the other values would show up on your meter.

    Quantum Mechanics gives you the probability of each possible outcome, and if you did the same experiment thousands of times, the number of times each value came up, divided be the number of experiments, would approsimate to the quantum probability.
  4. Sep 16, 2003 #3
    It sounds like quantum probability is really just an inability to measure events accurately at the quantum level. I should have known there's no such thing as randomness in a system.
  5. Sep 16, 2003 #4


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    No it's much deeper than this, quantum mechanics is trult stochastic, in other words it is truly random.
  6. Sep 16, 2003 #5
    Can you explain why please?, I'm very interested
  7. Sep 16, 2003 #6


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    Basically, the idea of HUP being a limit on measurement is really inaccurate. What HUP says is that there is no real absolute, distinguishable values of velocity and position, or time and energy on such small scales. It isn't just measurement - it is the reality of the situation. (someone should really go through the textbooks, and change Heisenberg's analogy of the photons bouncing off the electrons...)

    But this is still somewhat open to interpretation, though the evidence does most point to the existence of such quantum randomness. (IMHO, of course)
  8. Sep 17, 2003 #7
    What is HUP?
    I think I know what you're talking about. You're saying that the variables involved in quantum mechanics need to be measured to an infinitesimal degree before you can predict events - and since we can't do that the events are random.

    Ok, here's my problem. Measurement is not a property of things, it's a tool we use. Therefore this stumbling block does not necessarily indicate that events on the quantum level are random. We may never be able to accurately depict them, but to say they are random seems to be a fallacy.
  9. Sep 17, 2003 #8


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    It is not a fallacy -- it is proven. Read up on the Bell inequalities and the Aspect experiment. It turns out that is it absolutely not possible for a deterministic quantum theory to agree with experiment. (Many people are stunned when they first learn of this -- you included, I'm sure -- but it is fact.)

    Nor is it a stumbling block -- it's part of the fundamental character of microscopic things. There can be no advance in technology or technique that can allow us to find a precise position and a precise momentum for a particle simultaneously. The particles simply do not have precise positions and momenta at the same time. This conclusion was reached some 80 years ago now.

    If you're looking for some way to understand the Heisenberg uncertainty principle (HUP) qualitatively, here's how Griffiths describes it:
    In quantum mechanics, particles are considered (roughly speaking) by oscillatory probability waves. All particles obey the HUP.

    Richard Feynman speaks at great length about these matters in his Lectures on Physics. If you're still unwilling to accept experimental fact, you might want to do some reading.

    - Warren
  10. Sep 17, 2003 #9
    I accept experimental fact, I accept the analogy you used.

    My argument is logic based. I said: Measurement is not a property of things, it's a tool we use.

    You said: The particles simply do not have precise positions and momenta at the same time. I understand this, but the problem is an artificial one. It has been created by attempting to quantify.

    If you attempt to quantify - the events are not just random, they are inexplicable.

    Without any sort of measurement, they are still theoretically deterministic even though they can never be determined.

    What I'm espousing here isn't scientific, but it's still interesting.
  11. Sep 18, 2003 #10


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    Chroot, I disagree Bell's theorum most certainly rules out a local hidden variables theory, but it still allows a non-local hidden variables theory such as De-Broglie-Bohm theory (though as this predicts that atoms have electric dipole moments this is probably incorrect).

    That said to me the ASpect experiments are compelling evidence that the Copenhagen Interpretation is the best current interpretation of quantum mechanics.
  12. Sep 18, 2003 #11


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    I have no idea what this means.
    Scientists are usually local positivists. If you can't measure it, even in principle, then it doesn't exist.

    - Warren
  13. Sep 18, 2003 #12
    I Said: I understand this, but the problem is an artificial one. It has been created by attempting to quantify.

    I mean that the problem is created by attempting to impose an artificial process on a physical system.

    Chroot said: Scientists are usually local positivists. If you can't measure it, even in principle, then it doesn't exist.

    There's nothing new age or metaphysical about what I said. I called QM randomness a fallacy becuase my argument was based on logical reasoning, something used all the time in science.
  14. Sep 18, 2003 #13


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    No... you don't understand. The Heisenberg uncertainty principle has nothing to do with measurement. It makes a statement as to what the particles actually ARE.
  15. Sep 18, 2003 #14


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    I think what steersman is trying to say is that the randomness only appers when we try to pigeonhole the world into things like position & momentum.

    IOW things really are deterministic, but cannot be described deterministically with position & momentum-like descriptions.
  16. Sep 18, 2003 #15
    FZ, I would like to know something of that statement. I have to disagree with you though on HUP and measurement. Consider this:

    The uncertainty relations may be expressed in words as follows.
    The simultaneous measurement of two conjugate variables (such as the momentum and position or the energy and time for a moving particle) entails a limitation on the precision (standard deviation) of each measurement. Namely: the more precise the measurement of position, the more imprecise the measurement of momentum, and vice versa. In the most extreme case, absolute precision of one variable would entail absolute imprecision regarding the other.

    from this site: http://www.aip.org/history/heisenberg/p08a.htm

    Also, consider this:

    Heisenberg realized that the uncertainty relations had profound implications. First, if we accept Heisenberg's argument that every concept has a meaning only in terms of the experiments used to measure it, we must agree that things that cannot be measured really have no meaning in physics. Thus, for instance, the path of a particle has no meaning beyond the precision with which it is observed. But a basic assumption of physics since Newton has been that a "real world" exists independently of us, regardless of whether or not we observe it. (This assumption did not go unchallenged, however, by some philsophers.) Heisenberg now argued that such concepts as orbits of electrons do not exist in nature unless and until we observe them.

    from: http://www.aip.org/history/heisenberg/p08c.htm

    I think people need to seperate Heisenberg's uncertainity principal and Heisenberg's assumptions. The topic of discussion has resulted from a combination of the two.
  17. Sep 19, 2003 #16


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    Greetings !
    I think you need to separate what is being explained
    to you from your own explanations...:wink:

    What you are basicly saying, and that's understandable, is
    that there is no real uncertainty. It is so because the
    uncertainty is only a matter or measuremtn and when
    it is not conducted the uncertainty in nature does not exist.

    Well, basicly that idea is violated by even the simplest
    and most basic of QM's experiments the Double Slit experiment:
    Consider a single particle sent on its way towards
    two slits with a small distance between them(small enough for
    the wavefunction of the particle to "pass" through both).
    Then position a screen behind the slits and measure the impacts
    of many particles sent through the slits one by one.
    You'll see a diffraction pattern - the impacts will show
    that each particle(or wave-particle = wavicle) passed through
    BOTH slits. On the other hand if you send a partcile
    through a single slit with the other one closed OR place
    a detector behind one of the slits and detect the partcile
    OR NOT detect it behind THAT slit the wavefunction will
    collapse and the particles will hit the screen around some
    particuilar point - as a "normal" tiny ball like particle would.

    In conclusion, you will SEE that nature itself, measured or not,
    has wavicles and follows the HUP (for the quantized theories
    and their particles).

    Live long and prosper.
  18. Sep 19, 2003 #17


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    Listen, guy:

    You obviously don't know quantum mechanics. You don't realize WHY the theory directly predicts that particles do not have precise positions and momenta simultaneously. You're reading a bunch of pseudo-technical prose descriptions of things, and thinking you understand it.

    Learn the math of quantum mechanics, and you'll understand immediately why things have to be as they are. Until then, you simply don't know what the hell you're talking about. You'd do well to stop arguing with those who do.

    - Warren
  19. Sep 19, 2003 #18


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    Let's be a little gentler on steerman, shall we?

    You have alas fallen into the no.1 public misconception about quantum mechanics - what uncertainty is actually about. It's probably not your fault, but the fault of heisenberg's initial explanation of the effect which is hugely misleading, but still often reproduced.

    Heisenberg's initial inspiration for the existence of the uncertainty effect was based on knowledge. True. But the mathematical derivation of this from fundamental facts is based on matrices, and from quantum laws of how particles behaved, not on the accuracy of the measurement. This is an essential point. Just as Einstein began with light clocks, Heisenberg's use of "measurement precision" is only an analogy for the real world significance of the HUP. The HUP, in it's useful form, in it's experimentally verified form, in the form we talk about when we say HUP, in the form we use it in most modern theories is based on it as a state of the actual existence of the particles, not of knowledge.

    If I has my way, the analogy of the photons and the electrons shouldn't be used at all, but we should just go straight into no absolute position/momentum as a derived and confirmed physical fact.
  20. Sep 20, 2003 #19

    I'm certain there is uncertainity. I'm not denying HUP or any experiment that has validated it. My question is: How can an event occur twice and produce two different outcomes while maintaining the exact same variables?


    As I said in my first post to this board, I am a physics novice. I don't claim to understand the math involved in Heisenburg's theory as I believe it inconsequential to what I posit. I'm probably wrong. Forgive me if it seems I'm arguing with you.


    In a previous post I wrongly interpreted what you termed as 'HUP' to be randomess based on an inordinate precision of measurement. I hope you see that what I assert now is completely different to that.

    Even though HUP is solid and proven, I believe it is incorrect to assign the proprety 'random' to these particles. Hurkyl summed up my position , except when he said things were deterministic - this implies they can be found.

    How can quantum particles be intrinsically random? I'll have a look at hidden variable theory and Bell's inequalities to see if they have relevance to what I'm saying.

    For now though: God does not play dice!
  21. Sep 20, 2003 #20


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    Greetings !
    :smile: Well, that's what uncertainty means - with the same
    parameters you only have probabilities for the various
    results - which means that according to uncertainty you
    can get different results.

    chroot, I think it's always a type of shock for people when
    they start learning QM and understand its couter-intuative
    and illogical (according to common logic) results. Don't be
    so hard on him. It's one thing to argue and another thing to
    express strong doubt and require some convincing, after all
    people are here to learn (and teach). :wink:

    Live long and prosper.
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