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Is DCQE erasing information or resetting the system?

  1. Aug 2, 2015 #1

    I want to warn readers (non-physicists) of my blog that it is wrong to claim that erasing or not detecting "which-path" information in DS and DCQE experiments is the same as turning off your recording device or deleting a data file on your PC, and it is wrong to assume that the particle creates diffraction pattern just because the Universe "knows" that you don't have "which-path" information recorded anywhere (although detector might have detected it earlier). But to get this to my readers I need a simple, dumbed down analogy.

    Long story:

    I know that there have been many discussions about DS and DCQE experiments. But I would like to have some simplified, dumbed down explanation which would prove why exactly the people who claim that DCQE changes the past or that it proves that observing the results affects the experiment are wrong.

    I am working together with a friend on an article in our native language (Latvian) where we'll try to explain the basics to an average technically thinking reader who might have no quantum physics knowledge at all. I myself am not physicist nor mathematician. I have a degree in programming, but I selected Beginner level for this thread because I feel like a beginner in quantum physics. That's why I hope on your help to explain it all in as simple words and analogies as possible.

    This all started with a "crack-pottery" case discussed in this thread:
    I saw the video mentioned there, so I did a web search for keywords "double slit tape recording", and that thread was the best one but too technical and full of physics details.

    Essentially, this guy, Thomas Campbell in that video (and also in some later videos in Calgary) claims that in double slit experiments it is possible to observe particles without any interaction with particles, and thus it is wrong to say that particles are collapsed to wave functions just because detectors have somehow "damaged" the particles. And he says that not detecting or erasing "which-path" information is the same as turning off the recording device or deleting recorded data files while the experiment is being run as usual, with detectors being on, but no-one consciously observing the experiment real-time.

    But when reading about these experiments, I've got impression that detecting a particle implies some interaction with it because it is not possible to "observe" a particle in the same sense as we observe macroscopic objects. We can look at a ball and the ball won't be affected by our vision at all. But we cannot look at a photon or electron in the same way, we have to interact with it to detect its position. Am I right this far?

    And in that case in DCQE experiments they are not just merely erasing "which-path" information. They are first "damaging" the particle to obtain that information, but at that point the information is not stored anywhere. We could say, that at the moment of detection, the "which-path" information is not real information, it is potential information and we simply cannot record it on some media (tape or computer RAM) before the measured particle hits the screen because then our recording system must be capable of recording bits of information faster than the time required for a particle to travel from slits to the screen. The following sentence from Wikipedia seems to prove my point:
    "Experiments observe nothing whatsoever between the time of emission of the particle and its arrival at the detection screen."

    And then later in DCQE experiments when they are "erasing" the information, they are "undoing" the damage done by the detector to a particle, at the same time losing the "which-path" information because it was not stored anywhere, anyway. This means that you cannot make an analogy of erasing a recording on a tape (or a file on computer's disk or in RAM) or running detectors without recording the data.

    Or maybe I'm wrong and it is actually possible to record "which-path" information on a computer before a particle reaches the erasing stage of DCQE and keep the "which-path" information recorded on a computer even after it has been erased in DCQE itself? I highly doubt that, but I'm not sure.

    In case if I'm right and erasing information in DCQE is not the same thing as erasing data already recorded, the claim "Scientists show future events decide what happens in the past"
    also seems somewhat exaggerated crack-pottery with wrong conclusions.

    Still I'm curious, has there been any experiments done which would prove those "crack-pot" guys to be wrong?
    It would be actually easy to implement. Just take a classical DS experiment and make it fully automated black box, so no conscious being can observe the results real-time. Record the detector data and screen data as two separate sets of information (e.g. data files). Repeat the experiment twice. Now you have two sets of pairs detection+screen.
    But do not look at contents of the data files yet. Erase the "which-path" file from your second data set. And only then take a look at all the data you have. If "crack-pots" are right, then you'll find diffraction pattern in the set where you have deleted the "which-path" data, and no diffraction pattern in the set where both files are present.
    But if you won't get such results, then it will prove "crack-pots" wrong once and for all.
    I guess, such experiments haven't been done because they sound too crazy and do not make much sense for physicists. But still, it would be great to be able to prove that it does not work that way.

    And there is one technical thing which confuses me. Wikipedia claims that such experiments have been done also with molecules. But is it possible to do DCQE with molecules? That would negate quantum weirdness to some extent because it should be easier to detect a molecule and then delete that "which-path" information, and it would be closer to our macroscopic world.

    I'm hoping for you, physicists, to confirm my conclusions or explain where I'm wrong, so I can then pass that to readers of my blog.

    Thank you for your time reading this.
  2. jcsd
  3. Aug 2, 2015 #2


    Staff: Mentor

    If that's what he is saying he is wrong. You can't detect anything without interacting with it in some way.

    First of all collapse is an interpretation thing - the QM formalism doesn't have it, and a number of interpretations don't either. Detectors may or may not destroy particles - damaging them - that sounds a bit silly to me.

    If you want to understand the eraser experiment best to study the proper literature:

    See Chapter 20 - but if you don't know too much about the formalism of QM you will need to go through the previous chapters.

  4. Aug 2, 2015 #3
    Ok, thank you.
    So essentially the results (seeing / not seeing diffraction pattern) do not depend on whether we are recording (to a tape or a disk or RAM) the data from the detectors or not, but depend only on the detectors detecting (and hence interacting) with the particles or the detectors being turned off.
    And we won't get diffraction pattern if we don't record the data or delete it while the detectors are still being on, hence all these claims about conscious observer influencing the results are false (although no-one has actually performed a peer reviewed experiment to prove that deleting data won't affect the results in any way).
  5. Aug 2, 2015 #4


    Staff: Mentor

    I had a look at the video.

    It's new age rot of a similar type you see in junk like What The Bleep Do We Know Anyway. For example that conciousness has anything to do with QM is a very very fringe idea. If you want to know the history behind it start another thread and me and others will be only too happy to explain it - but of relevance to this thread is its basically rubbish that it must be involved.

    Here is a much better explanation of the double slit:

    In modern times QM is IMHO best viewed as an extension of probability theory:

    You may also find the following that looks at common myths of QM useful:

    Last edited: Aug 2, 2015
  6. Aug 2, 2015 #5


    Staff: Mentor

    That's correct.

    Its got nothing to do whether you record the data or not.


    You don't perform experiments checking something whose answer is obvious. QM is a theory about observations that occur here in an assumed common sense classical world. Once an observation is actually recorded what you do with the results has nothing to do with anything.

    Again if you want to pursue it start a new thread, but what's going on in the eraser experiment isn't all that weird - its basically in simple cases decoherence can be reversed.

  7. Aug 2, 2015 #6
    Thanks again, I don't see a point to start a new thread, now when my doubts were confirmed :smile:

    Just one final thought, more in a philosophical and social context. It's sad that there are so few scientists playing mediator role between science and "average people" out there. This is one of the reasons why this empty space is being filled with pseudo science and false assertions from journalists. I guess, the problem is that real scientists are too busy doing their work. And when scientists talk to journalists, they fail to explain things in as simple words as possible. I know that QM is hard to explain, but good scientists should at least think a step ahead and warn journalists and readers from deriving some specific conclusions which are expected to come to one's mind (these two about time going backwards and consciousness effects).

    When scientists explain their point of view, they refer to articles and scientific papers full of specific math and theory. This causes major confusion for uneducated readers. And then scientists look at all that "crack pottery" and laugh, not recognizing that all of that is actually happening because scientists do not spend more time explaining things and eliminating those ridiculous assertions.

    I myself am programmer and I happen to be a "tech support guy" for my relatives and neighbors. I often have heard them complaining that they already have had some experience with some technical person looking at the PC and fixing things, but this person did everything too fast and did not explain what and why should be done next to prevent problems from happening again. As a result, these people have their PC fixed, but they still have some weird assertions about how their PC works, and these assertions make them do stupid mistakes which would make PC gurus laugh. I usually try to fill that empty space between being a "technical PC guy" and being a teacher for "average uneducated user" because I know - if I explain things in simple words and they really understand me, then they will have less problems in the future, and there will be less reasons for them to trouble me with "support requests" (yeah, maybe I'm just being selfish and trying to work less :smile:)

    In short - we miss highly skilled science journalists who are willing to explain complicated things to "average John and Joe".
  8. Aug 2, 2015 #7


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    Gold Member

    The short version is that it is not wrong. It is what is called "interpretation dependent". Answering questions such as "can the future affect the past" or "is the moon there when no one is looking" get different answers according to which of several assumptions you make. Regardless of interpretation, quantum mechanics predicts the same outcomes.

    1. Although DCQE experiments appear to have the future affect the past, they are not the only ones with that feature. I personally push "delayed choice entanglement swapping" (I guess that would be DCES) as a better example - it seems easier to visualize:

    a. It is possible to entangle 2 photons (call them Alice and Bob) via entanglement swapping; and Alice and Bob will have never existed in a common light cone.
    b. Alice and Bob will exhibit perfect correlations at any common polarization angle setting, an attribute of entangled photons. So far, so good.
    c3. The entanglement swapping can be made to occur AFTER Alice and Bob no longer exist (i.e. after they are detected).
    d. They will NOT exhibit perfect correlations if you do not perform the entanglement swapping after they are detected.
    e. You can change whether Alice and Bob are perfectly correlated - something that already occurred - by your later decision to swap the entanglement or not.

    http://arxiv.org/pdf/quant-ph/0201134v1.pdf (see page 5)

    "Such a delayed-choice experiment was performed by including two 10 m optical fiber delays for both outputs of the BSA. In this case photons 1 and 2 hit the detectors delayed by about 50 ns. As shown in Fig. 3, the observed fidelity of the entanglement of photon 0 and photon 3 matches the fidelity in the non-delayed case within experimental errors. Therefore, this result indicate [sic] that the time ordering of the detection events has no influence on the results and strengthens the argument of A. Peres [4]: this paradox does not arise if the correctness of quantum mechanics is firmly believed."

    2. Bell's Theorem attacks the idea that quantum particles have properties when not observed. Of course, the Heisenberg Uncertainty Principle says as much.
  9. Aug 2, 2015 #8


    Staff: Mentor

    I am not a scientist - I am just like you - a programmer. I however did a combined degree many moons ago in math and computer science and in fact the area of math I studied, Hilbert spaces, is exactly what you need for QM. So I learnt it myself. You too can do the same.

    The sad truth is some things can be explained without math but most stuff cant, and that is especially true of the important deep things like decoherence. When scientists write for the lay audience they often unfortunately step into areas that really belong to math and end up speaking in half truths such as particles are in two places at once etc. These get picked up by, for want of a better word, 'shysters' and confusion ends up all over the place.

    All I can suggest is if you want the truth - come here - you will get it - warts and all. Even better if you study it yourself - but that will take time.

    Last edited: Aug 2, 2015
  10. Aug 2, 2015 #9
    Good point about interpretations and all of them make sense, as long as everyone is informed that these are just possibilities but not "The scientists have proven that...", as often claimed by journalists.

    I myself, being deeply interested in game engines with their level-of-detail and lazy-loading techniques,sometimes think that QM could be interpreted as a simulation on demand with the last LOD level being the QM world, but then we get to the question "but who is demanding the simulation" and then all the hell gets loose with those New Age ideas and whatnot.

    In this particular case I was confused by claims from Thomas Campbell and some other physicists (not sure, if they are physicists at all) who said that "quantum-erasing" the "which-path" data is the same as not recording data to a tape or deleting a file. In one of his Calgary videos Thomas Campbell even makes an analogy with burning a paper letter with "which-path" information after it has been printed by automated DS experiment. And, no matter what QM interpretations might be, such an analogy seems completely out of place because DCQE erasing does not have the same meaning as deleting a file.

    Still it would be great to see someone reproducing the experiment Thomas Campbell's way, just for fun. I myself have done illogical programming experiments on purpose - just to step through the code with debugger and see exactly what, where and, most importantly, why goes wrong. And sometimes I learn something new in the process. I guess, physics labs are much more restricted - expensive devices and lab time, therefore lab superviser or manager would laugh at you if you tried to explain that you are going to try out such an experiment.
  11. Aug 2, 2015 #10


    Staff: Mentor

    Yes - Dr Chinese is one of our resident experts on this sort of stuff.

    As usual his answer is excellent.

  12. Aug 3, 2015 #11


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    Thanks Bill. I would have been thrilled with "mildly useful". :smile:

    Always love your posts in case I hadn't mentioned. Always well thought out and timely. Allows me to draft off your coattails.
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