retrocausality

Standard memberhumy
Science 08 Jul '17 10:16
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    08 Jul '17 10:16
    I find this a weird concept I never heard of before;

    https://phys.org/news/2017-07-physicists-retrocausal-quantum-theory-future.html
    "...First, to clarify what retrocausality is and isn't: It does not mean that signals can be communicated from the future to the past—such signaling would be forbidden even in a retrocausal theory due to thermodynamic reasons. Instead, retrocausality means that, when an experimenter chooses the measurement setting with which to measure a particle, that decision can influence the properties of that particle (or another particle) in the past, even before the experimenter made their choice. In other words, a decision made in the present can influence something in the past.
    ..."
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    08 Jul '17 12:465 edits
    There is a wiki page on this;

    https://en.wikipedia.org/wiki/Retrocausality

    I find it strange this is the first time I have heard of this concept.
    I think it highly unlikely it has any relevance to what I am currently researching (mainly probability) but I guess I should, like with many other concepts, still study it just in case it has some kind of subtle relevance I haven't yet figured out.
  3. Cape Town
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    08 Jul '17 12:58
    Causation is a tricky subject (as is apparent from several recent threads on the subject).
    Most of physics does not specify a time direction so in almost all cases, one can claim causation runs both ways through time. The key difference in directions is the level of determinism and not the lack of causation.

    As for your OP, I generally dislike it when physicist (or others) give special place to the experimenter as it often leads to people drawing the erroneous (and usually quasi religious) conclusion that consciousness is somehow 'special' in quantum dynamics.
    Far better to pose the description in terms of fundamental particles on a small scale.
  4. Cape Town
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    08 Jul '17 13:07
    Originally posted by humy
    "Instead, retrocausality means that, when an experimenter chooses the measurement setting with which to measure a particle, that decision can influence the properties of that particle (or another particle) in the past, even before the experimenter made their choice. In other words, a decision made in the present can influence something in the past.
    ..."
    Lets take the two slit experiment. Set up a plane with two slits and allow starlight to shine through it. You will get an interference pattern. One could interpret that as the photons going through both slits. Now close one slit. No interference pattern. Photons are going through only one slit.
    Now remember that these photons left a star maybe millions of years ago. Did they know which slit to go through when they left the star?
    One can interpret this as retrocausality, but I believe it is largely a case of trying to interpret quantum mechanics in classical terms and getting erroneous results. Quantum mechanics is inherently weird when it comes to location and time. When a photon leaves a star, its path to its destination is not determined until it interacts, possibly billions of years later.
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    08 Jul '17 13:072 edits
    Originally posted by twhitehead

    I generally dislike it when physicist (or others) give special place to the experimenter as it often leads to people drawing the erroneous (and usually quasi religious) conclusion that consciousness is somehow 'special' in quantum dynamics.
    Likewise.
  6. Subscribersonhouse
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    08 Jul '17 18:39
    Originally posted by twhitehead
    Lets take the two slit experiment. Set up a plane with two slits and allow starlight to shine through it. You will get an interference pattern. One could interpret that as the photons going through both slits. Now close one slit. No interference pattern. Photons are going through only one slit.
    Now remember that these photons left a star maybe millions o ...[text shortened]... path to its destination is not determined until it interacts, possibly billions of years later.
    Could we modify the experiment like this: Use some proven random event, whatever scientists can say for sure is true randomness, like the timing of radioactive decay products or something. Then use some randomly picked number to tell a robot arm to block either slit A or slit B with no human looking but recording devices recording the results.

    Would the results be the same in that case or could it statistically happen that closing one or the other slit would change the no interference pattern somehow?
  7. Cape Town
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    09 Jul '17 07:52
    Originally posted by sonhouse
    Could we modify the experiment like this: Use some proven random event, whatever scientists can say for sure is true randomness, like the timing of radioactive decay products or something.
    It is impossible to prove something is truly random, that's a related topic as it also deals with causation or lack of causation.

    Then use some randomly picked number to tell a robot arm to block either slit A or slit B with no human looking but recording devices recording the results.

    Would the results be the same in that case or could it statistically happen that closing one or the other slit would change the no interference pattern somehow?

    The results would be identical. As I mention above, human consciousness is irrelevant to the actual physics.
  8. Cape Town
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    09 Jul '17 08:06
    Originally posted by humy
    "Instead, retrocausality means that, when an experimenter chooses the measurement setting with which to measure a particle, that decision can influence the properties of that particle (or another particle) in the past, even before the experimenter made their choice."
    Giving this some more thought.
    The current state of the universe predicts or determines the past to a much greater accuracy than it predicts or determines the future.
    So, once could argue that causality more accurately describes the reverse of the traditional timeline.

    When an atom emits a photon, its direction is apparently random. But, when it is detected, we know which direction it went. Hence the detection causes the emission direction and NOT the reverse.

    The confusion in the OP is that is sets up another event that puts the detector in a specific spot. So it says, if the detector were moved somewhere else, that would influence where the photon is detected and hence the emission direction. Then it adds the question of causality to the location of the detector ie if the detectors position is dependent on an event prior the experiment. I personally think that what at first looks paradoxical is not actually so. We just have multiple states of the universe in a series that are bound by certain laws of consistency across the states.
    In one direction of time, fundamental events appear to be utterly random within the constraints of the laws. In the other direction, they are not so random, and lead to an inexplicable orderliness which is predictable hence our illusion of a fixed past and a less than fixed future.
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