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Question on observer of double-split experiment

  1. Nov 10, 2011 #1
    Question on "observer" of double-split experiment

    From what I understand when we detect from which of two splits each electron passes we see two lines on the board. But, the moment we don't electrons become self-interfering waves. A puzzling result.

    My question is, can you explain me how we detect the trajectory of each electron? We light a beam on it or via magnets?

    Also, in what way does our measurement act differ from interactions of the electron with the environment? If it doesn't then the only difference would be that our measurement has a direct "purpose" on interacting with the electron?

    So, if observation is the same as any interaction how does the electron know which is which?
    Last edited: Nov 10, 2011
  2. jcsd
  3. Nov 10, 2011 #2
  4. Nov 12, 2011 #3
    Re: Question on "observer" of double-split experiment

    So is the bottom line that interaction with photons collapses the wave-function and causes the experimented particle to manifest as a particle?

    What if the slits contains magnets for detecting where did the electron pass?

    If interaction between particles/quanta causes particle-like behaviours how come we see any wave-like behaviour? So are electrons acting as waves only in isolated, dark vacuum?

    Why is it even neccessary to bring consciousness on if the above are not resolved?

    I also read this http://thefacultypublishinggroup.com/Physics/Double-slit%20Explanation(Ragazas).pdf" [Broken]that challenges the silent assumption that the electron fired is the electron detected.

    One last question. I read a phycisist who said the math are successsful but the explanations vary. But, I wonder, why don't the formulas devised to explain the phenomena give the answer themselves? Are there hidden variables? Are the math so abstract and complex no one can tract them?

    UPDATE: I read "Physicists are fully aware that you cannot use photons to observe particles like electrons because photons are larger than electrons. They use other techniques to observe such as observing the changes in the background radiation. This technique does not interfere with the electrons behavior."

    Is the above true? If so, then this truly is a mystery that could justify Penrose's consciousness theory.
    Last edited by a moderator: May 5, 2017
  5. Nov 12, 2011 #4
    Re: Question on "observer" of double-split experiment

    Re-view the video. You missed a few points.

    Any experiment devised to test which slit an electron went through, be it with photons, magnets, or an kaons as the leaned professor stated will result is a non-interference pattern. Subsequently the trajectory of an electron is unknown - you cannot determine ahead of time or look into the future and state that the electron will go through slit #1 or slit #2. so even the concept of "hidden" values is moot. One can only state the probabilty of an event to happen.

    You are getting ahead of yourself if you are asking about the wave function collapse at this point.

    Pls re-view the video.
  6. Nov 12, 2011 #5
    Re: Question on "observer" of double-split experiment

    I carefully watched the video, but still I can't seem to understand:

    1) Only the interaction with the measuring device collapses the wave-function? What about interaction with the environment?

    2) What about measuring methods that only register the presence of the electron only passively like "using other techniques to observe such as observing the changes in the background radiation. This technique does not interfere with the electrons behavior."
  7. Nov 12, 2011 #6
    Re: Question on "observer" of double-split experiment

    The double slit experiment shows that photons have a wave nature and a particle nature. If the slit size and distance between slits is comparable to the frequency of the light, then with only one slit opened, the phtons will behave as a particle and the "detector" will show a pattern similar to any other particle shot through one slit such as a bullet. With the two slits open, the photon will show an interference pattern on the detector. This pattern will be similar to that obtained from a wave such as ripples of water such as the professor explained. So, in one instance the photon will act as if is a particle, and in another instance the photon will act as if it is a wave.

    Now, does the one photon interfer with itself, go through both slits at the same time, go through one slit and then loop around and go through the other, half go through one slit and the other half go through the other slit, or whatever? As explained in the video it is impossible to tell.

    Why, you ask. Because, if you put any kind of contraption at slit one or slit 2 to detect whether the photon has gone through slit one or 2, and when your slit detector has gone off then you have already detected the photon at the slit detector. Being already detected the photon's interference pattern is broken and will not show up on the second screen.

    As for wave function collapse, that is a way of explaining what may have happened. You can also look up the many paths explanation. Mathematics can describe the situation but it does not explain what really does happen ie . we are used to the behavior of macroscopic objects, but when things get really tiny other features show up and the wave-particle duality of the tiny is one of them.

    An electron will behave the same way under certain conditions. but as you move up to larger and larger objects such as molecules and baseballs, then the experiment cannot be performed. a baseball would have a hard time travelling through a narrow slit.

    An intelligent observer is not a prerequesit for photon, electron behavior in thsi regard. Humans are as much a part of nature as anything else. There is nothing special about a human that dictates how a photon or electron will behave. What humans can do though is devise experiments to study in a controlled fashion repeatable events such as thousands or hundreds of thousands of photons or electrons to pass through 2 slits so that we can then study that many events and see the data, do some mathematical processing, and then state that small particles act as particles and waves.

    As for the paper you cite, the author talks about energy accumulation at the detector, to the point where there is enough energy to make a "pop" which may be a little bit on, as they say here, the "cranky" side of science.

    Background radiation. Interesting point. thermal energy doesn't seem to have an effect since everthing above absolute zero emits thermal energy. Make one slit hot and the other cold and see what that would do to the interference pattern. Maybe it would skew the pattern to one side or no effect at all. But then, you still would not be able to tell which slit an individual electron went through.
  8. Nov 12, 2011 #7
    Re: Question on "observer" of double-split experiment

    When there is interaction, or over a level of interaction, the "wavicle" behaves like a particle, otherwise as a wave?

    If so, where is the mystery?

    Otherwise, interaction set by humans for the purpose of observing causes the particle behaviour as opposed to "random" interaction, ceteris paribus?

    Is that the bottom line? If so this is the most amazing thing.

    I quote Usaf Moji, of this forum, "I heard that when the display screen is off (i.e. the thing that the observers look at) but the detectors are still in place and everything else is the same, the interference pattern remains. When they turn the screen on again (and change nothing else), it collapses."

    Very mysterious indeed.
    Last edited: Nov 12, 2011
  9. Nov 13, 2011 #8
    Re: Question on "observer" of double-split experiment

    Is it conclusive that the interference pattern vanished ONLY if the slit passage information is KNOWN to humans? I read many sources and they all point to that.

    What about animal observers? I googled and found no results for this.

    For example, put a cat into a dark room and have a green light turn on when the electron passes through slit A, or have a blue light turn on when the electron passes through slit B.

    If we have the same wave-function collapse as with humans KNOWING about the slit passage information we can deduct:
    -The cat senses the lights but does not know the source of this. Thus just indirectly "sensing" quantum phenomena collapses the wave-function. There is no need for direct cognition as with humans.

    I tend to believe this will happen, through Occam's razor, because otherwise there are many complexities as to what level of cognition marks the level after which the observer interacts with the electron? An oblivious child watching the screen would suffice to collapse it? And so on.

    Lastly, the Delayed Choice Quantum Eraser experiment version demonstrated that past events are dependent upon current knowledge! Without shutting down the sensors. So, did the electrons even reach the screen until we decide to know or not know?
  10. Nov 14, 2011 #9
    Re: Question on "observer" of double-split experiment

    No, it has nothing to do with a consciousness, if you performed the experiment with your back turned there would be no reason for things to be different other than you'd have no idea what it was that happened.
  11. Nov 14, 2011 #10
    Re: Question on "observer" of double-split experiment

    Are you sure? At first I thought too that observer meant measurement, but it turns out the results don't change once the sensors run, but when someone KNOWS the slit passage information! (See Delayed Choice experiment)
  12. Nov 14, 2011 #11


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    Re: Question on "observer" of double-split experiment

    Although there are some crackpot sites out there claiming the contrary, what you say is just not true. There is no backward causation or outcomes depending on a human observer in the delayed choice experiment. Try a forum search on that topic. there are at least a dozen threads on that topic.
  13. Nov 14, 2011 #12
    Re: Question on "observer" of double-split experiment

    ^Reading from this forum I got that mere interaction with the electron will cause it to behave as a particle.

    If the variable is interaction with the test subject why the measuring device is even mentioned? And bring the already confusing term "observing"? Wouldn't a dark chamber result in interference pattern and a lighted chamber result in two stripes?

    So many people say so many different theories that seem to make leaps above strict experimental data. What kind and level of interaction causes the electron wave-function to collapse?

    Is it a lie that the variable of human cognition of the electron path causes the subsequent course of the electron as either a wave or a particle? It is either true or untrue. Is it difficult to test that and give a final yes or no?
  14. Nov 14, 2011 #13


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    Re: Question on "observer" of double-split experiment

    Any irreversible interaction causes collapse. Experiments like the quantum eraser rely on reversible interaction. There you bring the system into some state that would give you which-way information if you performed a measurement on that state. However, the system is brought into a state where there is no which-way information left and then the measurement is performed and an interference pattern can be seen. Typical examples of such manipulations that are reversible are the usage of waveplates that rotate the polarization of light.

    There is no experimental result that gives any evidence that human cognition plays any role. However, to fully rule out the influence of humans, you would have to perform an experiment that no human being ever becomes aware of. Such experiments are of course unpractical as you never get to know the results.
  15. Nov 14, 2011 #14
    Re: Question on "observer" of double-split experiment

    If so where is the justification of all the meta-physical interpretations and consciousness relatedness. Because I see nil.

    Has the harm been made because of the unfortunate description about an observer? As far as I know an observer is a living thing.

    So, we have here a misconception between an interaction (detection) and observation?

    What about Feyman, why he calls it the only mystery in Physics if it just comes down to interaction = particle manifestation, if that is what it is?
  16. Nov 14, 2011 #15


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    Re: Question on "observer" of double-split experiment

    There is none. I would also like to point out that I am not aware of any serious scientific community supporting consciousness relatedness or claiming that real experiments show it.

    I think it is a problem of how the media or others try to simplify things for the "mass market". Like many words "observing" can have a different meaning in science than it has in layman usage. However that is only my opinion.

    I think Feynman was not only referring to what is happening, but also to the why and how one can get a sensible ontology consistent with it. The double slit experiment contains not only collapse and wave/particle behavior, but also the statistical nature of qm. These are iondeed already many of the mysteries of qm.
  17. Nov 14, 2011 #16
    Re: Question on "observer" of double-split experiment

    Consciousness has nothing to do with it, the point is that observing the path of quantum particles involves doing something which will collapse the wave function, whether or not you care to pay attention to the results has nothing to do with it, merely the fact that any possible method of measurement will disturb these particles enough that they no longer do what they were going to do.
    Say you want to measure the velocity of a ball, you might set up a series of lasers so that when the first light is broken a timer starts and when the second light is broken the timer stops, this is fine for a large ball because the laser light does not affect its path in any observable way; however if you try the same setup with an electron instead of a ball you will find that the light whacks the electron pretty hard and disturbs its path appreciably.
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