Monday, July 3, 2017

Ambiguous causality in quantum mechanics

I have posted about important causality is to physics, and now I see an article claiming a quantum counterexample.

Philip Ball writes in Nature mag:
Causation has been a key issue in quantum mechanics since the mid-1930s, when Einstein challenged the apparent randomness that Niels Bohr and Werner Heisenberg had installed at the heart of the theory. Bohr and Heisenberg's Copenhagen interpretation insisted that the outcome of a quantum measurement — such as checking the orientation of a photon's plane of polarization — is determined at random, and only in the instant that the measurement is made. No reason can be adduced to explain that particular outcome. But in 1935, Einstein and his young colleagues Boris Podolsky and Nathan Rosen (now collectively denoted EPR) described a thought experiment that pushed Bohr's interpretation to a seemingly impossible conclusion. ...

Brukner's group in Vienna, Chiribella's team and others have been pioneering efforts to explore this ambiguous causality in quantum mechanics3, 4. They have devised ways to create related events A and B such that no one can say whether A preceded and led to (in a sense 'caused') B, or vice versa. ...

The trick they use involves creating a special type of quantum 'superposition'. ... The two observable states can be used as the binary states (1 and 0) of quantum bits, or qubits, which are the basic elements of quantum computers.

The researchers extend this concept by creating a causal superposition. In this case, the two states represent sequences of events: a particle goes first through gate A and then through gate B (so that A's output state determines B's input), or vice versa.
This research is somewhat interesting, but it is not what it appears.

They find an ambiguity in the path of the photon, but there are always such ambiguities in quantum mechanics. In a simple double-slit experiment, where a light source sends photons thru a slit A and slit B to a screen, the detector on the screen cannot tell you whether the photo went thru slit A or B. The preferred interpretation is that the light is some sort of quantum wave that goes thru both slits. The light is not really photons until they hit the detectors.

This experiment does not really violate causality as the term is usually understood. It is just another of many experiments that are hard to interpret if you think of light as Newtonian particles. Such experiments convinced physicists that light is a wave about 200 years ago. A century ago light was found to be a funny quantized wave, but not a particle in the way you normally think of particles.

I don't agree with calling light a particle, but I also don't agree with saying that it is random up until the instant of a measurement. We don't really know how to make sense out of such statements. Quantum mechanics is a theory about making predictions about observations, and I think Bohr and Heisenberg would say that it doesn't make any sense to talk about the path of the photon (such as going thru slit A or B, or going from A to B or B to A) unless you are actually measuring it.

9 comments:

  1. Yes!

    The article writes, “An indeterminate causal order lets researchers do things with quantum systems that are otherwise impossible.”

    Such as....

    How is this foundationally different from Wheeler's "delayed choice" experiments?

    https://en.wikipedia.org/wiki/Wheeler%27s_delayed_choice_experiment

    Or this:
    https://en.wikipedia.org/wiki/Delayed_choice_quantum_eraser

    Which, all in all, tell us little more than the double-lsit experiment does.

    :)

    Roger concludes: " I think Bohr and Heisenberg would say that it doesn't make any sense to talk about the path of the photon (such as going thru slit A or B, or going from A to B or B to A) unless you are actually measuring it."

    EXACTLY!

    This is the same statement that resolves the "which path taken?" of Wheeler's experiments.

    Unless one measures the photon, one cannot assign it a definitive path.

    So what is new in the Nature article? Anyone? Please do share!

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  2. Consider the photon.

    In its simplest case, it is described as a spherically-symmetric expansion of probability and nonlocality.

    Relativity tells us that the photon does not age. It experiences no time.

    According to the photon, the time that it is emitted and the time that it is detected are one and the same.

    Thus, in any experiment conducted within a sphere defined by the expansion of a photon's spherically-symmetric wavefront, which has its origin at the creation of the particles in the experiment, the past, present, and future may appear to be arbitrary, as in the photon's frame, they all happen at the "same time."

    Some folks are not happy with speaking of the photon's frame, as they state that it is undefined. Such folks wish to say that the photon does not exist, rather than thinking about the physics of the photon. Such folks are welcome to consider the frame of particles moving at .9999999999999999999c or far faster, so as to draw the same conclusion that in such frames, all events happen at essentially the same time.

    Dr. McGucken's Quantum Time/Causality Postulate: "Thus, in any experiment conducted within a sphere defined by the expansion of a photon's spherically-symmetric wavefront, which has its origin at the creation of the particles in the experiment, the past, present, and future may appear to be arbitrary, as after all, in the photon's frame, they all happen at the "same time.""

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    1. As an aside, I would like to see the following issue discussed in comprehensive and simpler terms somewhere:

      Why can't you (straight-forwardly) use Schrodinger's equation for photons? Why does the idea of a wavefunction of a photon itself does not make sense? Just because the photon number is not conserved? How?

      --Ajit

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  3. The final sentence in this article says it all:
    https://phys.org/news/2012-04-quantum-physics-mimics-spooky-action.html

    "According to the famous words of Albert Einstein, the effects of quantum entanglement appear as "spooky action at a distance". The recent experiment has gone one remarkable step further. "Within a naïve classical word view, quantum mechanics can even mimic an influence of future actions on past events", says Anton Zeilinger."

    Yes! "Within a naïve classical word view" = with an INCORRECT VIEW.

    Basically physics has become clickbait which relies on "incorrect views" about quantum mechanics to overhype the delayed-choice experiment over, and over, and over again.

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  4. Here is an excellent 2014 paper debunking all forms of "retrocausality" and "Ambiguous causality" in quantum mechanics.

    http://philsci-archive.pitt.edu/10216/1/SeparationFallacy-rev.pdf

    "Why Delayed Choice Experiments do NOT imply Retrocausality
    David Ellerman
    University of California/Riverside
    January 10, 2014
    Abstract
    There is a common fallacy, here called the separation fallacy, that is involved in the interpretation of quantum experiments involving a certain type of separation such as the: double-slit experiments, which-way interferometer experiments, polarization analyzer experiments, SternGerlach experiments, and quantum eraser experiments. It is the separation fallacy that leads not only to áawed textbook accounts of these experiments but to áawed inferences about retrocausality in the context of "delayed choice" versions of separation experiments."

    Long story short, Philip Ball and Nature mag do not make their money by clarifying concepts to the layman, but rather they obtain their income by generating clicks and using clickbait tactics to hype falsehoods and fool the laymen with intentional falsehoods.

    Likewise, the "researchers" secure their funding by hyping decades-old knowledge as something new, while denying and ignoring the correct, simple interpretation.

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    1. "áawed" textbook accounts should read "flawed textbook accounts"

      It is the separation fallacy that leads not only to flawed textbook accounts of these experiments but to flawed inferences about retrocausality in the context of "delayed choice" versions of separation experiments."

      Delete
  5. Wheeler actually lays this debate to rest in his original paper on the "delayed choice" experiment:

    https://books.google.com/books/about/Quantum_Theory_and_Measurement.html?id=L7r_AwAAQBAJ&printsec=frontcover&source=kp_read_button#v=onepage&q=delayed%20choice&f=false

    Wheeler writes, "In the delayed-choice version of the split-beam experiment, for example, we have no right to say what the photon is doing in all its long course from point of entry to point of detection."

    Brukner should have written, "In our version of the split-beam experiment, for example, we have no right to say what the photon is doing in all its long course from point of entry to point of detection."

    Problem solved!!

    But then how would he get funding?

    Philip Ball could have written, "In their version of the split-beam experiment, for example, we have no right to say what the photon is doing in all its long course from point of entry to point of detection."

    But then Nature would never pay him, as the simple Truth of quantum mechanics doesn't generate clicks nor "retrocausal" clickbait! :)



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  6. Interestingly, the "Many-Worlds" interpretation is also laid to rest by Wheeler et. al. on page 294 of Quantum Theory and Measurement (Princeton Leg… (Paperback) by John Archibald Wheeler, Wojciech Hubert Zurekin, in a paper by Wigner:

    “Many-world” theories are much more difficult to discuss than theories of hidden variables. They postulate—as mentioned earlier—that if a measurement with a probabilistic outcome is undertaken, the world splits into several worlds, and each possible outcome of the observations appears in the fraction of the new worlds given by the quantum-mechanical probability of that outcome. . . It is, of course, difficult to see the meaning of the statement that there are other worlds with which we never will have any contact, which have no influence on us, and which we cannot influence or perceive in any way. . . the statement that there are such worlds, and that they are constantly created in large numbers, is entirely meaningless. It can neither be confirmed nor refuted."

    From https://www.amazon.com/Quantum-Theory-Measurement-Princeton-Library/dp/0691613168/ref=sr_1_6?ie=UTF8&qid=1499118080&sr=8-6&keywords=quantum+theory+measurement

    Max Tegmark et al. have built entire "clickbait" careers out of denying what Wheeler, Wigner, Bohr and other founders of the working theories originally said and meant.

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  7. Yes, Elliot, you make excellent points. There are lots of these fancy quantum experiments, but the main strangeness is what was understood in the 1920s. And yes, Many-Worlds is silly for exactly the reasons that Wigner explains.

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