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Delayed Choice exp and Cause-Effect

  1. Jul 20, 2005 #1
    Amateur level.

    Does the delayed-choice double slit experiment violate or put in trouble the standard cause-and-effect thinking?
  2. jcsd
  3. Jul 20, 2005 #2


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    Does it mean that there is no cause-effect relation anymore ? No.
    What does it mean ? That however this riddle is solved, it is going to be subtle :-)
  4. Jul 20, 2005 #3
    Maybe an extension of the question: "what does the delayed-choice experiment tells us about time?"
    (I know there's no answer, just opinions :-)
  5. Jul 28, 2005 #4
    No. Whichever detection method is implemented (at a
    certain time corresponding to the quantum disturbance
    being between the two-slit and the detector), it is in
    place before the disturbance reaches it.

    If single detectors, one focused on one slit and the
    other focused on the other slit, are chosen, then
    one or the other detector will register -- as if the
    disturbance had gone through only one slit.

    If a screen is chosen, then a single dot will appear
    in one of the maxima regions of an (eventually apparent
    if enough dots are accumulated) interference pattern --
    as if the disturbance had gone through both slits.

    Note the use of the term, "as if". In the case of the
    single focused detectors, it just isn't known whether
    the emitted disturbance went through both slits or
    only one. If modern physics has taught us anything,
    it's that just because you can't see something, doesn't
    mean that there's nothing there.

    In the case of the screen detector (which will
    eventually produce an interference pattern built
    up dot by dot), it also isn't known whether the emitted
    disturbance went through both slits or only one.

    Anyway, the experimental sequence of events doesn't
    contradict standard causality. Neither does the data.
    It's just so far resistant to a qualitative understanding
    via classical imagery.
  6. Jul 30, 2005 #5
    Remember that there are always two issues in epistemology - what is a useful model of something and what is the truth? A mechanical conception of causality and time can still be useful even when patently "untrue" - but yes QM nonlocality is proof that simple cause and effect thinking (which embodies the mechanical principle of locality) cannot be the whole story.

    One approach to rethinking time would be Cramer's transactional interpretation - where particles make an offer wave that travels forward in time and some absorber particles accept the offer with a wave that travels back in time.

    As a "model" the picture of a negotation going forwards and backwards in a single time dimension fits the nonlocal facts. However it is still tied to a mechanical logic and so not quite revolutionary enough for my tastes to be more than a stepping stone.

    Another related way of making sense of things would be to take Feynman's sum over histories approach very literally. All possible paths (in space and time) between two particles exist. So there is a "thick" spatiotemporal description of the world that is the wavefunction and it exists all at once. Then there is the collapse to a "thin" actual path that creates a strand of linear, cause and effect, spacetime.

    In this view, time would not be a single thin dimension but a hierarchical stack of temporal scales. The kind of time you see then depends on where you are located. If you get down to the most local scale, it will seem that you are stuck in a very small "now" - an instant - with past and future stretching out behind and before you. If you could see the world on a global scale, then the "now" would include that past and future. Like time for a photon, the world would seem frozen still with both ends of a trajectory already having happened.

    Time is about change and here we are describing a spatiotemporal hierarchy of scales in which the smallest scale is a blur of events and the largest scales appear frozen still. In our Universe, the smallest "now" would be Planck scale, the largest would be (perhaps) the lightcone of the visible universe.
  7. Jul 31, 2005 #6
    Quantum nonlocality has to do with global contexts, systems of correlated
    spatially separated measurments rather than individual (local) measurements.
    It hasn't altered our standard cause and effect thinking, or told us anything
    more about time than was already known -- and neither has the delayed
    choice double-slit experiment (re the original poster's question).

    The time of an event is the configuration (the reading) of a clock
    correlated with the event (which is itself a configuration of some
    set of objective phenomena -- eg., a ball contacting a bat, a spot
    of light appearing or disappearing, a fire igniting, a picture hanging
    on a wall, a moving car passing a stop sign, a car sitting in a
    driveway, etc.).

    Time is the indexing of changing configurations. In an expanding
    universe, each successive universal configuration is different than
    it's immediate predecessor, but also more like it than any other
    prior configurations. This is the way things seem to work from
    the largest to the smallest scales. That is, there is a definite
    direction to time (change). So, eggs don't spontaneously unfry,
    broken cups don't spontaneously reassemble, waves move
    away from (rather than toward) the disturbances that created
    them, and so on. Time doesn't reverse because it can't
    reverse in an expanding universe. Effects can't come before
    causes because that would simply be misusing the terms.
    The universe will never revisit it's past, and neither will
    anything in it -- except in our imaginations and memories.
  8. Jul 31, 2005 #7
    It is to do with temporal as well as spatial correlations. So delayed choice twin slit experiments are a stark demonstration.

    This might be how a human observer chooses to index events - motivated by a belief in cause and effect or locality. But nonlocality raises the question of when an event really occurs. The point I was making was that a global scale observer would see both ends of an event as part of the same effective moment. So both the photon emission by a star in some distant galaxy and its "much later" absorbtion by my eye.

    What the proof of nonlocality tells us is that locality-based models of causality are incomplete (though they are certainly still useful). So I was talking about the kinds of models based on hierarchy theory that might present a different view of time.

    Yeah but then you have the problem of the chicken and the egg. Cause and effect thinking runs into paradoxes - like how can a something (the universe) spring out of a nothing.

    So to get out of this, you have to look into other causal models. So for example, ones that start with a state of vague everythingness (cf: Anaximander, Peirce) and then dichotomise or symmetry-break to produce two crisp limits on being. So we would now start with a vague chicky-egginess and watch it divide asymmetrically into a chicken and egg (or if you like, the first egg inside the first chicken). So now causes are effects.

    I realise that alternatives to locality and mechanistic logic are not fashionable. But non-locality has to be accounted for within some causal model unless you want this aspect of reality to remain a mystery.
  9. Aug 1, 2005 #8
    The point is that delayed choice twin slit experiments don't
    tell us anything more about time than we already knew.

    An observer whose field of vision includes a supernova a
    billion light years from earth and also earth will observe
    the light created by the supernova taking a billion years to
    reach earth. He wouldn't see the birth of the supernova
    and the recording of a picture of it on earth as happening
    at the same time.

    Questions about when events occur arise when we use different
    clocks to index the occurance of spatially separated events.
    In a global or 'system' context, which is what 'nonlocality' refers
    to, if the spatially separated events are timed by the same
    clock then the temporal relationship between the events
    is less problematic.

    For example, wrt the 'twin paradox' of special relativity,
    if a 'global' observer were to time the traveller's journey by
    using, say, revolutions of the earth, then he would observe
    that the journey took a certain number of revolutions -- which
    would be the same for the earthbound twin as for the
    travelling twin.

    Nonlocality refers to spatially separated events that are parts
    of a single behavioral system. Nonlocality is evident in nature.
    There is a hierarchy of systems, or observational contexts.
    The scale of behavior/observation doesn't change the basic
    meaning of 'time', or contradict the standard notion of
    local causality.

    The origin of the universe will remain a matter of untestable
    speculation for a long time I think.

    Anyway, cause and effect thinking isn't paradoxical. Causes
    can't happen after the effects that they cause, by definition.
    Given any, causally related, chicken-egg duo, either the egg
    was laid by the chicken or the chicken hatched out of the
    egg. If they 'sprang' into existence at the same time, then
    they're not causally related to each other -- but they might
    be nonlocally related as parts of a system that encompasses
    them both.

    There's nothing to "get out of."

    Two events are causally related if there is an invariant,
    sequential relationship wrt their occurance. Which one is
    called the cause and which one the effect depends on their
    relative placement in the temporal indexing of the sequence
    of events.

    What remains to be understood is the deep, qualitative nature of
    reality. We can describe/predict the gravitational behavior of
    macroscopic objects pretty accurately, but don't know what
    causes it. We can predict rates of coincidental detection in
    Bell tests pretty accurately, but don't know what's happening
    at the level of paired emissions. We can predict detection
    patterns in photon/electron two-slit interference experiments,
    but don't know what's happening at the level of the emissions
    interacting with the two-slit and detection devices.

    It's not a matter of reinventing or redefining causality or
    time. There just isn't enough data.
  10. Aug 1, 2005 #9
    This kind of mutual causality can arise in the most unlikely places - for example, Newton's third law. For every action an equal and opposite reaction. I throw a ball and the ball "throws" me. Both action and reaction spring up at precisely the same moment. And even though you might still want to say one led to the other, put the situation into deep space - say two masses colliding - then you really don't know which exerted the action, which responded with a reaction.

    Think then about why this strange law was necessary. Newton made the world mechanical cause and effect with the first two laws. Then had to add back in the deeper mutuality between figure and ground, event and context, as a fictitious symmetric force of reaction.

    You don't need to get into QM or relativity to find causal weirdness in mechanical logic - cause and effect thinking.

    Again, mechanical logic is a very effective tool in modelling. No surprise that it is first choice when pragmatism rules. But that does not close the door on broader causal models that may capture more of the truth of reality.

    I'm not sure I see the logic in your position here. The data tells us our causal models are inadequate. So rather than reconsider our causal models, let's gather more data.

    Even if we disregard the evidence that science is theory-led rather than data-led on the whole (ie: what looks like data, what you feel is worth measuring, is determined by what you believe is probably happening), you seem to want to put unnecessary limits on enquiry.

    And to return to the specific issue of temporal sequence, are you saying that delayed choice twin slit experiments don't seem to put the cause of a choice of path after the apparent effect, the actual choice of a path?

    Cheers - John McCrone.
  11. Aug 1, 2005 #10
    The delayed choice twin-slit experiments illustrate how what
    you see depends on how you look at something. With both
    slits open, if you only have two places where a localized photon or
    electron detection can appear, then it will appear in one of the
    two places. If you use a continuous screen, then eventually
    you'll get an interference pattern. In either case, or even if
    one slit is closed, there's never any path information, per se.

    Must go now, but will discuss more of your comments later
  12. Aug 3, 2005 #11
    Did the ball leave your hand before your arm moved?
    If so, then the ball leaving your hand would be called the action or
    cause and the the movement of your arm would be called the
    reaction or effect.

    In the case of two bodies colliding in deep space, the collision
    event can be said to cause the events that can be
    immediately, locally related to it -- such as the fragmenting of,
    or changing the trajectories of the colliding bodies.

    I think Newton probably played a lot of croquet, and observed
    that not only did a stationary ball move when hit by a moving
    one, but the moving ball slowed down or stopped following
    the collision -- and that this behavior (due to interaction) could
    be related, quantitatively, in a general way.

    There's nothing causally weird about mechanical logic.
    It's just a way of talking about things that facilitates

    I'm not sure what you mean by "broader causal models that
    capture more the truth of reality". Nonlocal contexts aren't
    causal, they're correlational.

    If you mean, eg., Bell tests, then it's not that "our causal models
    are inadequate" in general or anything like that. It's that Bell test
    setups are not causal contexts. They're correlational contexts.
    Events at A and B are correlated wrt variations in a global variable.
    You can trace the chain of events in such experiments and see that
    local causality isn't violated (even if there are ftl signals travelling from
    A to B or vice versa). There's simply the open question of
    how are the actual physical disturbances that are travelling from
    emitter to polarizers to detectors related (if they are related) to each
    other, and when/where is the relationship (that might be relevant for
    the predictable correlations via the global variable) created.
    This is an empirical question, I think.

    What unnecessary limits?

    The temporal sequence is 1. emission 2. choose detector 3. record

    There isn't any path information. There's just two different
    detection methods, one of which is in place *before* the
    disturbance or disturbances transmitted by the twin-slit reaches
    the detector. With both slits open there's just no way to tell
    if something went through both slits or only one.
  13. Aug 3, 2005 #12
    You're missing the point. Of course the third law is about locality. But even in the classical modelling of locality there is an example of chicken and egg paradox. Newton smuggled in a reaction that arises instantly (not before, not afterwards). Einstein then went on to exploit the in principle impossibility of saying one part of the system did the moving, the other part stayed still.

    You could equally well say that locality is a model and therefore all that is observed are correlations with the model. If there was no problem created by nonlocality, then people (like Einstein) wouldn't have expended so much effort on hidden variable explanations.

    It is easy to agree that you don't need to "explain" nonlocality if your concerns are only practical. Modelling the world in a local way captures enough truth for most human purposes. But still, nonlocality exists and is not - by general agreement - explainable by a local logic.

    Locality is violated in QM. But that fact isn't visible to a local observer. The question for the modelling of nonlocality is does it make sense to talk of global observers?

    Cheers - John McCrone.
    Last edited: Aug 3, 2005
  14. Aug 11, 2005 #13
    I could be missing the point. Maybe we're using the word,
    "nonlocality", differently. I'm thinking of it as just referring
    to context. The predictable variable results gotten wrt nonlocal
    observational contexts are understood as being due to
    looking at spatially separated parts of the same system,
    or looking at spatially separated objects that have interacted
    or have a common origin.

    Of course it makes sense to talk of global observers -- but, I'm
    not sure what it means to say that "locality is violated in QM."
    I mean, if the observational context is a combined, rather than
    an individual, one, then is locality 'violated'?
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