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Do weak measurement prove randomness is not inherent?

  1. Jun 15, 2011 #1
    Weak measurement show that you can get "partial/probabilistic" which-way info and get a "partial" interference pattern.

    Deduction 1:

    Does this mean that weak measurements prove that we can control the degree of randomness?

    (either of individual photons or average of a thousands of photons)

    Deduction 2:

    If we can control the degree of randomness, does it mean that randomness is not inherent BUT

    Randomness is simply a way to describe forces/phenomena/dimension that we are unaware of and thus describe/model it stochastically (for example how we do with Brownian movement)

    Deduction 1 in my opinion requires a lesser "leap of faith" as its follows out fairly logical.
  2. jcsd
  3. Jun 15, 2011 #2


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    No. The "randomness" you are referring to is a fundamental limit imposed by the HUP. That is why the quality of the interference pattern degrades as the "strength" of the weak measurement is increased.
  4. Jun 15, 2011 #3
    I doubt randomness actually exists in nature.
    It's more of a side product of the HUP, it's not ACTUAL randomness, although some crazy people do believe true randomness exists, actually quite a lot of otherwise smart people do so.
  5. Jun 15, 2011 #4


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    I don't understand what you mean ... what is "ACTUAL randomness"? Are you saying you agree with the Bohmian hypothesis that everything is deterministic, but that we can never know the initial conditions precisely enough to make predictions? I think that is the Bohmian view on the HUP ... i.e. that it restricts how well we can know the initial conditions for any quantum system, so the results of experiments appear probabilistic.
  6. Jun 15, 2011 #5
    i see your point.

    Question: when we try to get which-way info (we cause de-coherence, we create phase difference), do we increase or decrease randomness or does it remain the same?
    Last edited: Jun 15, 2011
  7. Jun 15, 2011 #6
    Yes, I am as certain as a human can be that the universe is 100% deterministic, from big bang to now.
    Every single particle is deterministic.
    I just don't see how reality could be any other way.
    If randomness were truly part of quantum theory, I doubt we could even get probabilistics out of it.

    Think about it, if something was RANDOM, how could we ever predict ANYTHING, even somethings probability?

    I'm not sure whether Bohm is correct, or some other hidden variable interpretation (gerard 't hooft is working on this) or perhaps some brand new physics will be discovered that will shed light on the issue, but what I am sure of is that realism and determinism/causality is going to be a part of whatever the truth is.
  8. Jun 15, 2011 #7


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    Nice that you are so certain when there is no evidence to support your view whatsoever. And I do mean none.

    Going back to Hume's work on causality, folks have continually assumed that which they are trying to prove. He did that explicitly. And you too, for example:

    I wonder: was the Beatles' work inevitable? Because I can't imagine a world without their music. QED! :smile:
  9. Jun 15, 2011 #8


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    Are you talking about David Hume? If you do, then let me quote his famous words very applicable here:
    "If we take in our hand any volume; of divinity or school metaphysics, for instance; let us ask, Does it contain any abstract reasoning concerning quantity or number? No. Does it contain any experimental reasoning, concerning matter of fact and existence? No. Commit it then to flames: for it can contain nothing but sophistry and illusion."
  10. Jun 15, 2011 #9


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    "THOUGH there be no such thing as Chance in the world; our ignorance of the real cause of any event has the same influence on the understanding, and begets a like species of belief or opinion."

  11. Jun 15, 2011 #10


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    I often wonder why people believe the universe is deterministic. Not only, as DrChinese says, is there no evidence for this, but our everyday experience doesn't support this view. It certainly feels like I have free will - I can choose whether to type an A or a B as the next character - here goes - B. Do you really believe that my decision to type a B was pre-ordained at the moment of the Big Bang? If there were objective evidence that the universe were deterministic, then I could understand believing this in spite of the evidence of my senses, but with the evidence of my senses telling me that the universe is not pre-determined, why would I choose to believe that it is?
  12. Jun 15, 2011 #11

    Ken G

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    What's more, I would add that both "random" and "deterministic" are always attributes of a theory or model. Science has no way to even discuss whether or not these are attributes of the real world, and I would add that a seasoned view of what science actually does warns us against extrapolating the nature of theories to the nature of reality. If the history of science has taught us nothing else, let it teach us that.
  13. Jun 15, 2011 #12
    Though IIRC, he wasn't out to prove determinism there (the futility of such an attempt, I imagine, would be something Hume of all people would have been most acutely aware of), he merely assumed it, the alternative making no sense to him. And it is a difficult thing to wrap one's head around, to the point of appearing almost self-contradictory.

    One might, for instance, quite reasonably assume that everything that happens has a way by which it does so, some mechanism through which it occurs -- but if that's the case, then randomness seems nonsense: for any indeterminate choice between alternatives A and B, either some mechanism chooses A over B (say) -- but then, obviously, that mechanism provides for determinism; or, neither A nor B gets chosen -- but then, neither happens. So (if indeterminism is real) one must renounce the assumption that everything that happens has a mechanism through which it happens, so that without a choice being made, A (or B) is chosen.

    Smarter people than me have asserted that such a thing is perfectly possible, that things can 'just happen', without sufficient cause, but to me, that's kinda like an underdetermined system of equations having an unique, and right, solution -- the information just isn't there --, and furthermore, if things just happen, then why bother with this whole physics stuff at all? If ultimately everything turns out to be the way it is because that's how it happens to be, then it seems we should cut to the chase and throw in the towel.

    But luckily, quantum mechanics is perfectly compatible with determinism -- so one might argue that parsimony tilts the scales heavily in favour of such an interpretation (since there is otherwise no indication that indeterminism is possible at all).

    And, so as not to be completely off-topic, there is of course no experiment that can decide between different interpretations of QM -- otherwise, they wouldn't be different interpretations, but rather, different theories.
  14. Jun 15, 2011 #13

    The argument for (human) free will does not require inherent/quantum randomness.

    Thought I believe in both: human free will and inherent/quantum-level randomness. But neither is a precondition for supporting other.
  15. Jun 15, 2011 #14
    Lately I've been enjoying the deterministic perspective (probably, it seems more 'relaxing' ;) ) but I think the concept of 'true randomness' sits just fine with many people. You could claim that the universe is a balance between a Pattern that is ordered/understandable/predictable and an element of Chaos that is fundamentally 'irrational'/unpredictable/arbitrary. After all, that whole concept is embedded in various mythologies which is a sign it holds some deep intuitive appeal.

    And regarding 'throwing in the towel': well, science is never going to answer the question 'why does anything whatsoever exist at all' (i.e. why do 'laws of physics' exist, why is there a 'reality' at all). So we are always going to be stuck with an element of 'arbitrariness' in the ultimate axioms of our TOE, "things just happen to be this way".
  16. Jun 15, 2011 #15
    Well weak measurement plays an important conceptual role in Aharnov's time-symmetric interpretation of quantum mechanics which is an interpretation which does away with the probabilistic nature of quantum mechanics.
  17. Jun 15, 2011 #16
    I love how people say "determinism doesn't fit our everyday life", YES IT DOES.
    More than anything.
    EVERYTHING in the classical realm has been shown to be 100% deterministic, in other words, ANYTHING you ever experience was 100% detemined.

    Now DrChinese I know you got some weird views on QM so I wont even bother.

    I never said I could prove it, because it can't be proven.
    Neither can randomness.

    However, everything we ever assumed was random, turned out to be deterministic, most likely this is so at the quantum scale too.
    Just because we, humans, can't access it doesn't mean anything.
  18. Jun 15, 2011 #17
    Ofcourse you don't got free will, what the hell, do science minded people still believe in this illusion in 2011?!
    That's beyond belief...

    It also feels like colors are objective.
    If you cut someones arm off, they can often experience "phantom sensations", experiencing that they have a hand that isn't there.
    Just because you FEEL that you make the choice free willingly doens't prove anything.
    Actually I'm pretty sure they have already proven that you have no choice through measuring brain activity.
  19. Jun 15, 2011 #18


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    Weird views, sure: I believe it.

    (By the way, that would be like the pot calling the kettle...)

  20. Jun 15, 2011 #19

    Ken G

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    No, they haven't proven any such thing. What has been shown and seems pretty reliable is that part of your brain is modeling itself, so when you make a mental decision, part of your brain has the job of "telling the story" of how you made that choice. This storytelling comes after the decision, which can easily be mistaken for evidence there wasn't a decision, but the truth is we still have no real idea of how to test for the difference between a "true free choice" and a "predetermined choice", and many people think those two things are apples and oranges that can easily coexist. But that gets into neuroscience and philosophy, not really relevant to "weak measurement" so we should probably just nip off the whole "free will" detour.
  21. Jun 15, 2011 #20

    If I watch single photons hitting a detector, or listen to a Geiger counter clicking, has it been shown that I'm experiencing something 100% determined?

    What about random mutations in DNA, or even weather patterns do to chaos principles?

    I would have thought those examples come back to the unresolved debate between different QM interpretations.
  22. Jun 15, 2011 #21

    Geiger counter clicking, has it been shown that I'm experiencing something 100% determined?
  23. Jun 15, 2011 #22

    Ken G

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    Actually, here is a different way to present your exact same argument. Throughout history, whenever we thought something was deterministic, we found a deeper level that was random. Did you really just argue that this means the people who think there will always be inherent randomness are ignoring the lessons of history?
  24. Jun 15, 2011 #23
    My point was:

    Clicks of Geiger counter are considered 'quantum random'. Clicks of a Geiger counter have not been _shown_ to be 100% deterministic (it is a matter of unresolved competing QM interpretations). Also, clicks of a Geiger counter are something I can experience.

    Therefore it is not true that 'anything I can experience was 100% determined'.

    Is there a flaw in what I'm saying?
  25. Jun 15, 2011 #24
    Do you consider thermal statistics determinate? in brownian motion, all particles take deterministic path but they are seen to be random. Was this why Einstein proposed his statistical interpretation and was it about determinism? About clicks in geiger counter being random. If one plots the distribution in time. It is no longer random. So maybe by linking it with spaceTIME. Determinism is the ultimate result as the probability distribution is only arranged in time from past present and future. Is this possible?

    Bottomline is. By taking time as illusion, determinism is retained?
  26. Jun 15, 2011 #25

    Ken G

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    Have you heard about chaos theory? Determinism is a mathematical concept that applies to mathematical models. Real systems show similar behavior to chaotic deterministic mathematical systems, but that does not mean the real systems are deterministic, again because the way they are deterministic in the mathematical models simply does not apply to real world systems. In experimental physics, unlike mathematics, determinism can only be equated with "predictable", because experimental physics has no capability to define any other concept of determinism. And what we know is, in experimental physics, Brownian motion is not predictable, so cannot be called deterministic. Mathematical models of Brownian motion can be called deterministic, but their deterministic character, because of chaos, is part of what we can never match to observations. Hence in regard to determinism, we can never know if the mathematical models correctly reflect the true nature of the reality, or not. Indeed, if we hold to the deterministic belief, we end up with the rather absurd conclusion that butterflies can cause tornadoes, even though a flapped wing never changes the statistical tendencies of any weather patterns.

    I'm not sure where you got that idea, but it's going to come as a pretty big surprise to a lot of experimental physicists who routinely model their data using Poisson (random) statistics.
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