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Murshid Salam

Program Controller, Ericsson India Global Services Pvt. Ltd.

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In double slit experiment of Thomas Young, why do electrons (particles) behave like waves when we aren't observing them?

In double slit experiment of Thomas Young, electrons (particle) behaves like a particle when they are under observation. But how they suddenly behave like a wave when we do not observe them.


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  • s greco

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    Jul 20 2012: I honestly think anyone who, right now with what we know, tells you they have a definitive answer is either woefully wrong or ..hmm fudging the truth.

    There's the "observation makes the wave function collapse" camp and the "the detector causes interference that alters the outcome" camp. The truth is, there's no solid evidence either way. Its one of the most curious and perplexing puzzles in physics.

    Given what we know about electrons and their penchant for being in multiple places at the same time, I personally find the "the detector causes interference that alters the outcome" to be too closed minded.

    Who knows? Maybe when we do find the answer it will trump any of its predecessors.
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      Jul 20 2012: Check out the link below. They have managed to detect the path of photons without destroying the interference pattern by using entangled photon pairs. Now your choice in paragraph 2 is unnecessary.
      • Jul 20 2012: A very interesting approach, although I wonder what the ramifications are ultimately since the photons are ostensibly split before reaching the slit. But I found another article on the subject and offer this excerpt:

        'Steinberg stresses that his group's work does not challenge the uncertainty principle, pointing out that the results could, in principle, be predicted with standard quantum mechanics. But, he says, "it is not necessary to interpret the uncertainty principle as rigidly as we are often taught to do", arguing that other interpretations of quantum mechanics, such as the pilot-wave theory, might "help us to think in new ways".
        David Deutsch of the University of Oxford, UK, is not convinced that the experiment has told us anything new about how the universe works. He says that although "it's quite cool to see strange predictions verified", the results could have been obtained simply by "calculating them using a computer and the equations of quantum mechanics".
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      Jul 29 2012: I believe there is solid evidence:
      In around the year 2000 several physicists put together a Young's slit experiment called a “Delayed Choice Quantum Eraser” which positively excludes any other possibility than it being the actual observation by a conscious observer which causes “wave function collapse”. The experiment was conducted by Y.H. Shiti, Ho Kim, Ri Yu, S.P. Kulick, Marlan O-Scully, details may be looked up on the internet.

      In all these experiments there is the common ground of probability waves and wave-function collapse. This interpretation of reality is not necessary, another interpretation will emerge which will explain all experiments to date without the need of recourse to statistics. This is the subject of my theory yet to be published.

      Nevertheless the significance of consciousness remains. There is a definite and proven connection between quantum experiments and conscious observation of them.
      • Jul 29 2012: "consciousness" has nothing to do with this experiment.. See paper here:


        The word "conscious" does not appear in the paper.

        The experiment uses detectors which gather data and store that data. That stored data would be the same whether a conscious being looked at it or not. Your conclusions are NOT supported by this experiment.

        Addition: Carl, since your reply has no reply button, I reply here.

        I stand by my original assertion, it is still just an arrangement of detectors and there is absolutely no 'proof' that consciousness is involved. That is an interpretation, not an observation. And that is why the conclusion of the paper does not make that conclusion.
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          Jul 30 2012: Dear Barry, (part 1)
          You are right that the word “conscious” does not appear in the paper. There is a history to the Young's slit experiment which I understand as follows:
          1. Young conducts the classical light wave projecting through two slits onto a screen experiment. The screen displays an interference pattern and hence proves that light is a wave.
          2. Through the photoelectric effect Einstein discovers that light has a discrete property and therefore must be a particle.
          3. Discrete photons are then projected through the slits to discover which slit they went through. Yet it is still discovered that the individual photons interfere with themselves and produce a diffraction pattern on the screen.
          4. In an attempt to find out which slit each photon goes through, detectors are placed AT THE SLITS. The results on the detector screen show a discrete pattern thus indicating a particulate nature of light which appears when 'which slit' information is detected.

          Thus it seemed that the act of detection had 'forced' the photon to decide which particular slit it was going to pass through. This experiment was the juncture at which Schrodinger had his famous “Schrodinger's cat” thought experiment. It seemed like the observation of the results of the experiment had collapsed the probability wave function and made the light particulate.
          Since this experiment there have been many who have thought that it was not CONSCIOUS observation of the photons at the slits which brought about wave-function collapse but it was mechanical or energetic contact with the wave at the slits which brought it about.
          It has been impossible to definitively prove whether it was mechanical or conscious observation until this elegant experiment was devised and conducted.

          The question to be answered by this experiment is as follows:
          Does the availability of 'which slit' information to the observer of the results of the experiment cause the light to become particulate?
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          Jul 30 2012: (part 2)
          In order to conduct the experiment no detection is made at the slits. Photons are split into entangled pairs AFTER the slits. One of these pairs is sent to detector 0. Detector 0 therefore provides positive detection for every photon through both slits, but it provides NO information about which slit it passed through.

          Through the use of 50% transmission/reflection mirrors, length of transmission paths, subsequent detectors and coincidence of detection circuitry, the paths of the other entangled halves of the photons is traced into one of four possibilities:
          1. A photon is detected having gone through through either slit A or B.
          2. A photon is detected having gone through slit A and not through B
          3. A photon is detected having gone through B and not through A
          4. A photon is detected having gone through either A or B, but the equipment set-up erased the information subsequent to obtaining it. Thus although having been made energetically this information is NOT AVAILABLE to the observer of the experiment.

          It is the logical detection path 4 which is crucial to the experiment. The set-up is devised such that 1 of two types of information can be retrieved from the experiment: 1. A photon passed through slit A or B and was detected as having passed through, or a photon passed through, was detected as having passed through, but information as to the specific slit it passed through was erased.

          The results clearly show that WHEN the 'which-slit' information is available to the experimental conductor - the (fig 5) photons exhibit particle like behaviour. When the information is collected BUT SUBSEQUENTLY destroyed (randomly by the clever use of 50% mirrors) (figs 3 & 4) then the photons exhibit wave-like interference patterns.

          Therefore I think that the experiment clearly shows that it is ONLY WHEN information is made available to a CONSCIOUS OBSERVER that the light becomes a particle.
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          Jul 30 2012: And that it is NOT the physical act of detection which causes the wave-function collapse.
      • Jul 30 2012: Carl,

        The experiment you reference gathers and deletes "information". Perhaps what the paper refers to as "information" is more than just something that we perceive. Perhaps this "information" has physical effects; can anyone definitively rule this out?. In science there is no agreed upon definition of "information" and no one can explain to me the basic nature and structure of information. So I can conclude that when this paper refers to "information" they do not completely understand "information" and what effects it might or might not have.

        Stephen Hawking once proposed a scenario where black holes could destroy information, and apparently there is a law of science which says this is impossible. Apparently physicists have equations dealing with information. I have no idea if the word "information" as used in the experiment is the same kind of "information" that they have in their equations.

        There is no logical or other explicit or implicit connection that permits the conclusion that physical effects caused by gathering or deleting information involves consciousness. Many physicists refer to these results as a mystery, because that is exactly what they are.
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        Jul 31 2012: The physical detection IS the act that causes the wave-particle duality collapse.

        The method was refined earlier this year, and photons could be detected without interfering with their wave-like properties. Refinement of tools, no consciousness involved.

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          Aug 2 2012: Hi Marius,
          The article you refer to is interesting, but it doesn't seem to accurately reflect the experiment as conducted. I found the original document on which the article was based here:

          Their experiment is actually two experiments (lets call them exp A and B). In experiment A the particle is tested and detected as a particle. In experiment B it is passed through the slits, interferes, and performs as a wave. Since it is two different particles they are looking at, they do not even attempt to address the issue of "observation of the results affecting the outcome of the experiment". They understand this and in conclusion in their paper they state:
          " However, the EPR
          discussion (3) or the delayed-choice experiment (22) have taught us that quantum objects do not have a reality independent of a measurement as summarized (29) by the phrase “one cannot consider quantum properties as being ’real,’ in the sense of ’objective
          John A. Wheeler (3), paraphrasing Bohr, expressed this fact most vividly by saying “No elementary quantum phenomenon is a phenomenon until it is a recorded phenomenon brought to a close by an irreversible act of amplification.” In this sense our experiment can also be interpreted as another confirmation of the nonobjectifiability of quantum mechanics or, as stated by Torny Segerstedt, “Reality is theory.” "

          There is another recently conducted test using the same/similar materials in the detection process (http://arxiv.org/abs/quant-ph/9903047). The arrangement and protocol of this one is such that ONLY the same particle is tested. The results confirm & reinforce the currently held understanding that availability of the results of the experiment affect its outcome.

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