The dual slit experiment seems like...well...BS!

Discussion in 'Physics & Math' started by rustyw, Mar 15, 2016.

  1. Billy T Use Sugar Cane Alcohol car Fuel Valued Senior Member

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    Yes that unlike "time," space has two observable, measurable properties even with no matter there..
     
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  3. The God Valued Senior Member

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    So the space has at least two observable properties...any experiment which is conducted at Quantum Level, should consider these.....the outcome may be different.....
     
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  5. danshawen Valued Senior Member

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    The double slit experiment is all about time and memory. The screen on which the interference pattern emerges freezes an instant of time the same way any photograph does. Without that element, "observation" of any kind is not even possible, much less the sort of observation that Dr Superquark claims changes the outcome of the double slit experiment for a stream of electrons. Our retinas are memory devices for photons also. There is nothing magic or spooky about the process of observing, other than for relativity, in which two observers in relative motion can always be relied upon to report different effects. It works like that on quantum scales as well, no surprise. Including and especially relative time dilations. Make a case that it doesn't.

    'Collapsing' a wave function seems to be nothing more physically exotic than compressing the time domain of a train of propagating bound energy or waves to an instant of time instead of just a negligibly short duration time interval. If you do that to the bound energy of an electron, of course you see the same pattern you do for photons. The effective wavelengths expressed as light travel time are different. That's all.

    No one ever explained 'collapsing a wave function' as the same as physically collapsing an umbrella (from a plane to a line), but it does make sense.

    'Space' is light travel time. The screen of the double slit experiment collapses or compresses time, not space. Only a two dimensional space remains for a description of any kind; interference fringes that are also separated by light travel time in an orthogonal direction of propagation.

    It all makes perfect sense unless E=mc^2 still somehow mystifies you. What if the screen were replaced by a mirror, and the image spread to image on a bigger screen? Would this be tantamount to observing the process at multiple instants? Would it collapse the wave function multiple times, or is this manner of terminology/thinking just muddled quantum mystic gobbledegook?

    Send Dr SuperQuark back down his rabbit hole where his cousin the mad hatter is still having a tea party, which you must admit, has really caught on politically as well.
     
    Last edited: Mar 25, 2016
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  7. Confused2 Registered Senior Member

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  8. danshawen Valued Senior Member

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    The double slit doesn't cause total cancelation of the energy of any the photons. Instead it affects a change (rotation) in the direction of propagation (and the plane perpendicular to it as well). If you are not very circumspect about how it is you try to observe any interfering photons, you will no doubt cause another rotation from the propagating direction or fringes/maxima, and as a result, no interference fringe patterns other than the unrotated central maxima will be viewable at all.

    A phased array means of observation might work without the destruction of the fringe patterns, but something as simple as the one suggested in the previous post simply will not work.
     
  9. The God Valued Senior Member

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    Let me clarify further......

    virtual particle popping in and popping out is convenienty used by QM guys...

    In vacuum where this test should be conducted, we have

    1. Photon or electron (say some particle).
    2. Slits.
    3. Vacuum (media)
    4. Detector (optional)

    No explanation takes care of any possible impact or perturbations due to presence of so called 'non materialistic yet with physical properties' media called vacuum. I am sure if we wish we can find a more common sensical explanation to this observation....the explanation is quite weird when we jut in observer/detector.

    A single photon cannot move as wave front...but if this single photon can cause some perturbation in vacuum then that perturbation can be theorized to move as wave front, thus giving the interference patterns ?

    I feel, mainstream is too afraid to discuss vacuum, as there is tremendous fear of Ether popping out again....
     
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  10. James R Just this guy, you know? Staff Member

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    rustyw:

    It's more general than that. If you do anything that will let you know which slit the photon went through, then the interference pattern vanishes. And believe me, people have tried lots of different schemes to try to sneakily detect which slit the photon went through without "disturbing" the system. Yet the statement still holds.

    Yes, but remember what we're doing with the 2 slit experiment. We want to detect that the photon went through one slit or the other. Can you think of any way to do that which would not disturb the photon in its travels?

    If you block the observation, you don't know what happened. Same thing if you look away. This is a tree falling in the forest situation.
     
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  11. danshawen Valued Senior Member

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    The interference patterns observed in the double slit experiment are a 2D array, the dark bands representing a LACK of photons. As the previous post mentioned, how would anyone expect to observe a non-photon, or a photon that has left, or never entered the area? When did it happen or not happen? This idea is problematic all by itself. A lack of photons, other than virtual ones, is virtually everywhere, whether it is practical to find a means of reliably observing them or not.

    How does one even distinguish a lack of photons in one location vs. a lack of photons in another? A lack of observation is not the same thing as an observation, is it really? This doesn't really blow anyone's mind other than Dr. SuperQuantum. It's like he's two dimensional, made up of photons (or a lack of them, or a subtle phase difference between photons propagating in different directions) himself.

    I'm done berating obsessions with the double slit experiment. For a short while, it was all the rage in physics. I still don't get exactly why. I don't think it is usually possible to make a determination from which direction a single photon has arrived once it has been detected, is it? You would need to know details about how it was created just to determine the energy it had with respect to the frame in which it was created, much less where it came from, or in the case of the double slit experiment, whatever process or photon from another direction it was that changed its path and caused appearance of interference fringes. It's all relative. It is a question of relativity long before quantum dynamics and any other hidden variables come into play.
     
    Last edited: Mar 26, 2016
  12. brucep Valued Senior Member

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    Sounds like he's into magic
    I don't take much advantage of YouTube. My bad. Check this one out on Youngs experiment. They also list one for single photon.
     
  13. Layman Totally Internally Reflected Valued Senior Member

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    They use electrons in the two slit experiment, not photons. If you could detect what slit the electron went through without "disturbing" it, it would allow for information to travel backwards in time. The electron can act as if it already knows if someone will observe it in the future, and the wave function will collapse before it even goes through the two slit. In other words, it doesn't matter where the detector is at in the experiment. It could be set in the very back of the experiment right before it would hit the black board in the back, and the electron will choose only one path before that time. If an observation of any kind could take place at the slits which wouldn't result in a collapse of the wavefunction, someone could know if the detector was on or off before it even detected the electron. The wave function appears to collapse faster than light and through time. Then information cannot be sent back in time, because the observer at the two slits would collapse the wavefunction. Then the wavefunction would always be collapsed rather the detector at the end of the experiment was off or on, and it would just appear to be more like a particle every time.
     
  14. origin In a democracy you deserve the leaders you elect. Valued Senior Member

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    You can use either photons or electrons (or any other number of other particles) for the 2 slit experiment.
     
  15. QuarkHead Remedial Math Student Valued Senior Member

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    Please explain to a non-physicist like me what precisely you mean by "wavefunction collapse"
     
  16. Layman Totally Internally Reflected Valued Senior Member

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    What I mean by that is making an observation of a property of a particle, so the degree of quantum uncertainty is lower for the property measured. Simply, it starts looking and acting like a particle instead of a wave. Then other properties can begin to exhibit more wavelike behavior, so the collapse of the wavefunction is just for the property I am describing. Also, the degree of quantum uncertainty never actually becomes zero. It actually becomes so small at macroscopic scales that it just becomes negligible and unnoticeable, as far as anyone really knows. It also has not been proven to be able to go away completely outside of the microscopic realm of quantum mechanics, even though larger objects don't behave the same way.
     
    Last edited: Mar 31, 2016
  17. Layman Totally Internally Reflected Valued Senior Member

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    Never heard of photons being used in this experiment. Could you give an example of this? Or reference?
     
  18. origin In a democracy you deserve the leaders you elect. Valued Senior Member

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  19. exchemist Valued Senior Member

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    This discussion brings to mind an old limerick by Mgr. Ronald Knox, dating I think from his time at Trinity College Oxford. It is apparently cast in the form of one letter, supposedly written to a philosophical journal apropos Bishop Berkeley's philosophical views, and another letter received by the same journal in response:

    There was a young man who said, "God
    Must find it exceedingly odd,
    When he finds that this tree
    Just ceases to be
    When there's no one about in the quad.".

    Dear Sir, your astonishment's odd.
    I am always about in the quad.
    And that's why this tree
    Continues to be,
    Since observed by,

    Yours Faithfully,

    GOD.

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  20. Farsight

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    I don't think there's any odd about the dual slit experiment. Take a look at the physicsworld breakthroughs article from 2011, and you can see Jeff Lundeen mentioned in second place. When you look at his web site you see things like this:

    "With weak measurements, it’s possible to learn something about the wavefunction without completely destroying it. As the measurement becomes very weak, you learn very little about the wavefunction, but leave it largely unchanged. This is the technique that we’ve used in our experiment. We have developed a methodology for measuring the wavefunction directly, by repeating many weak measurements on a group of systems that have been prepared with identical wavefunctions. By repeating the measurements, the knowledge of the wavefunction accumulates to the point where high precision can be restored. So what does this mean? We hope that the scientific community can now improve upon the Copenhagen Interpretation, and redefine the wavefunction so that it is no longer just a mathematical tool, but rather something that can be directly measured in the laboratory".

    He's saying wavefunction is something real that can be measured in a laboratory. Something wavelike. And it doesn't really matter whether you're talking about photons or electrons, because you make the latter from the former in pair production anyway, and you typically detect the photon with an electron. Both are comprised of wavefunction, which Lundeen says is real, and which interact in some fashion. As for how, I think it's interesting to take a look at the Optical Fourier transform as per Steven Lehar's web page:

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    Think of the many-paths photon coming in from the left. This would go through both slits. Then think of a "detector" electron as the lens. It converts the photon into a dot or dots on the screen. So you think the photon is something pointlike, and then you're scratching your head wondering how something pointlike can go through two slits at once. But it isn't pointlike, a photon has an E=hf wave nature, it's as pointlike as a seismic wave. Hence it goes through both slits. But when you detect it at the screen something akin to an Optical Fourier transform occurs, the photon gets converted into something pointlike, and you see a dot on the screen. In similar vein if you detect it at one of the slits, something akin to an Optical Fourier transform occurs, the photon gets converted into something pointlike, so it goes through that slit only. So you lose the interference pattern.

    There's no need for any magic or mystery, and there's definitely no need for a many-worlds multiverse.
     
  21. PhysBang Valued Senior Member

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    Sure, if one ignores enough aspects of the experiment and the nature of physics, then there doesn't seem to be a problem. A great example of cherry-picking taken beyond textual analysis and into a more direct approach with science.
     
  22. exchemist Valued Senior Member

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    Actually I find this interesting. I suspect a lot of us put up with a degree of cognitive dissonance when we use QM concepts in science. As someone trained in chemistry, I am very comfortable with treating electrons in atoms and molecules as standing "wave" patterns, when we consider chemical bonding, spectroscopy etc. So in those contexts I tend naturally to think of the wavefunction as a thing that pervades space, as permitted by the various constraining potentials etc it experiences. But equally, it is something that can only interact - and thus be be measured - in quantised lumps, each corresponding to a whole electron "particle". So when we deal with reactions, electrons move from one atom to another, forming covalent bonds or ions or what have you - and in such contexts they are implicitly treated as particles.

    It has always seemed to me one can think of the double slit situation quite naturally if one thinks of the wave as the primary thing, but that detection (i.e. interaction) can only occur in discrete lumps. What that actually means, though, is where it is still a bit of a mystery.
     
  23. Farsight

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    Weak measurement is a demonstration of an interaction that doesn't occur in discrete lumps. As Lundeen said, as the measurement becomes very weak, you learn very little about the wavefunction, but leave it largely unchanged.
     

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