Double slit experiment

Discussion in 'Physics & Math' started by Xmo1, Nov 15, 2016.

  1. James R Just this guy, you know? Staff Member

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    Write4U:

    We don't expect photons to strike the exact same spot each time. Each individual photon has a calculable probability of landing at different points on the screen/detector. But there's no way to predict where individual photons will hit.
     
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  3. James R Just this guy, you know? Staff Member

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    exchemist:

    Actually, in the quantum description, the wave function of the photons is exactly the same kind of probabilistic function as you would write down for electrons (for example).

    The description of light as "a true wave in the electromagnetic field" is a classical description of light as an electromagnetic wave, which is conceptually different from the quantum description of light. In quantum field theory, photons are excitations of the electromagnetic field, so in a rough sense it may be ok to refer to the waves as "waves in the electromagnetic field". Mathematically, the kinds of "wave functions" you write down for photons passing through a double-slit arrangement are similar to the ones you write down when you're describing light as a classical electromagnetic wave - as they of course must be to produce the same interference pattern. But in one case, the "fundamental" mathematical function is describing a complex-valued probability amplitude, while in the other case the "fundamental" mathematical function is describing a real-valued amplitude of the electric (or magnetic) field.

    In the classical case, we square the modulus of the electric field magnitude to find the classical intensity of light. In the quantum case we take the squared modulus of the probability amplitude to find the probability of a photon being found at a particular point in space. Experimentally, both of these approaches predict the same observed results, in this particular experiment.
     
    Last edited: Nov 18, 2016
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  5. James R Just this guy, you know? Staff Member

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    It is your prerogative to reject whatever you like. A discussion of whether the "many worlds" idea is scientific or not is probably one that is better had in the Philosophy forums. Your objection applies equally well to every other interpretation of "wave-function collapse", including the Born (Copenhagen) interpretation.

    I don't know who these "others" would be. A theory that nobody can understand would not be a very useful theory.

    In quantum field theory, everything is a field. But we don't need to go that far to deal with the 2-slit experiment. Simple wave mechanics is sufficient for that.

    The 2-slit experiment does not require a lens.

    You're glossing over the "magic" involved in how, exactly, that "conversion into something pointlike" occurs. That is what the many-worlds interpretation seeks to explain, and what the Copenhagen interpretation almost explicitly avoids addressing. This is known as the "measurement problem" in quantum physics.
     
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  7. Write4U Valued Senior Member

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    Is this what David Bohm was saying when he used the term "potential" as a fundamental probabilistic aspect of expression in reality at quantum level? The Implicate order?
     
    Last edited: Nov 18, 2016
  8. exchemist Valued Senior Member

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    Thanks for this James, Indeed, what I meant by a true wave in the EM field was a reference to an excitation of it, as in QFT, though I do not pretend any great familiarity with QFT.

    We are saying, I think, that de Broglie matter waves and light waves are different things, but square moduli of both are probability functions. Is that fair? I think in fact your explanation serves to reinforce my point that this is a subtlety not always brought out in treatment of QM such as I had at university.
     
  9. Xmo1 Registered Senior Member

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    Could there be an analysis of data points that would use derivatives to predict the number of dimensions at play in the experiment? If the data points were mapped or embedded would the results produce a form, shape, or object? I'm just thinking what could be done with the data that might produce more information. Just for fun maybe the angle of the gun with respect to the screen, or the time to impact could be added. What happens when the gun is angled arbitrarily? Are there any differences in the resulting data points? Have any spherical (or other) matrices of the target screen been constructed? I mean, this experiment is producing results. Why not expand it, or is it fully understood?
     
    Last edited: Nov 18, 2016
  10. exchemist Valued Senior Member

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    So far as I know there are no surprises at all in the findings, for those who can apply relatively straightforward wave mechanics to the set-up. It all behaves just as if a wave is being diffracted through the slits, with the proviso that if one fires the wave-particles through one by one, the resulting interference pattern builds up point by point, statistically. But that is exactly what QM tells you to expect, because QM says the wavefunction is that function which, when its square modulus is calculated, gives you a probability density. So I don't think any more information would be obtained by trying different angles etc. Simple knowledge of diffraction would tell you what to expect.

    The intriguing thing about the double slit experiment is not that we don't understand why we get these results: QM models that successfully. It is rather that it vividly demonstrates the apparent truth of the paradox of the wave-particle duality of matter, which QM proposes and which has had people scratching their heads since the 1920s.
     
  11. Confused2 Registered Senior Member

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    Last edited: Nov 18, 2016
  12. rpenner Fully Wired Staff Member

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    Thank you,
    Post #11 has a link to a similar apparatus and Flash videos of it's operation.
     
  13. Xmo1 Registered Senior Member

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    Among other things I've seen a youtube here: Double Slit Experiment explained! by Jim Al-Khalili, where 'you put a detector' and it changes the results to compress the waveform. Unplug the detector, and the waveform is uncompressed. So I don't see why the observer (the experimenter) without the detector does not cause the waveform to compress. Why is a detector needed? What is it about the detector that causes the compression? If the observer put a yardstick (a ruler) over the box would the waveform compress?

    Another experiment is setup in a 2,000 pound box, and all kinds of stabilizers to ensure accuracy. People call in to an internet site to visualize the slits in their minds, and supposedly there are positive results. I wonder if a simulator using the data to churn mathematical functions in multiple dimensions would yield something new. What if the laser scanned through a hologram before it arrived at the slit screen. Could there be a multidimensional mapping with unforeseen results? It's been great to review this experiment in a little more detail here. I've got a lot to look at as it is, and certainly another resource is welcome. So thanks for that.
     
  14. Xmo1 Registered Senior Member

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    Replying to my own post. Is there a difference in the results - sure light energy has a waveform, and (I think) the experiment sets up something that polarizes the wave, but I'm wondering about the observer effect that I see in some explanations. That is really the thing holding my interest. (Schrödinger's old cat and the like). Sorry for my ignorance. I don't study full time, and find myself going back to relearn things that get my attention, sort of like reading a book page twice.
     
  15. Confused2 Registered Senior Member

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    Staying with the Teachspin sample data
    From this result:-
    http://nebula.wsimg.com/0317f0d9b14...45B40D076900BE550&disposition=0&alloworigin=1
    We see that the total number of photons detected is doubled when both slits are open (compared to a single slit) but where they are most likely to be detected is profoundly changed by the effect known as interference which has absolutely nothing to do with compression.
    From the description of the apparatus ( http://www.teachspin.com/two-slit-interference--one-photon-at-a-time.html ) we could work out the wavelength used to obtain the sample data (if we wanted to).
     
  16. Confused2 Registered Senior Member

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    While I am certainly a fan of flash animations I do maintain there is no substitute for a physics practical session with real apparatus. Nothing saps the will to live in quite the same way.
     
  17. paddoboy Valued Senior Member

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    That's OK, we know he is a quack.....


    In the mean time, here's a more reputable account by Professor Jim Al-Khalili who is a Iraqi British theoretical physicist, author and broadcaster. He is currently Professor of Theoretical Physics and Chair in the Public Engagement in Science at the University of Surrey. A short 9 minute video.....



    and of course here is the account by that great Professor Feynman.....
     
  18. exchemist Valued Senior Member

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    Nobody should be required to watch YouTube videos on a science discussion forum. It is a time-wasting format and prevents readers who have not seen the video from participating. That is not forum discussion.

    Participants should be expected to support their claims with arguments in written words.

    In this particular case, considering that:-
    a) this Lapoint person has zero credentials,
    b) we know YouTube - and the internet generally - is full of shit, and
    c) the sole recommendation to watch this comes from someone who has a reputation for not understanding science and for promoting nonsense,

    then the course of action for readers seems clear: ignore it.

    Personally, I would excise the entire exchange on Lapoint and put it in the Fringe section. But that's up to the Mods.
     
    Last edited: Nov 23, 2016
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  19. paddoboy Valued Senior Member

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    https://en.wikipedia.org/wiki/Galileo_Galilei

    Galileo Galilei
    (Italian pronunciation: [ɡaliˈlɛːo ɡaliˈlɛi]; 15 February 1564[3] – 8 January 1642) was an Italianpolymath: astronomer, physicist, engineer, philosopher, and mathematician, he played a major role in the scientific revolutionof the seventeenth century.

    He has been called the "father of observational astronomy",[4] the "father of modern physics",[5][6]the "father of scientific method",[7] and the "father of science".[8][9]

    His contributions to observational astronomy include the telescopic confirmation of the phases of Venus, the discovery of the four largest satellites of Jupiter (named the Galilean moons in his honour), and the observation and analysis of sunspots. Galileo also worked in applied science and technology, inventing an improved military compass and other instruments.

    Galileo's championing of heliocentrism and Copernicanism was controversial during his lifetime, when most subscribed to either geocentrism or the Tychonic system.[10] He met with opposition from astronomers, who doubted heliocentrism because of the absence of an observed stellar parallax.[10] The matter was investigated by the Roman Inquisition in 1615, which concluded that heliocentrism was "foolish and absurd in philosophy, and formally heretical since it explicitly contradicts in many places the sense of Holy Scripture."[10][11][12]Galileo later defended his views in Dialogue Concerning the Two Chief World Systems, which appeared to attack Pope Urban VIIIand thus alienated him and the Jesuits, who had both supported Galileo up until this point.[10] He was tried by the Inquisition, found "vehemently suspect of heresy", and forced to recant. He spent the rest of his life under house arrest.[13][14] While under house arrest, he wrote one of his best-known works, Two New Sciences, in which he summarized work he had done some forty years earlier on the two sciences now called kinematics and strength of materials.[15][16]
     
  20. Write4U Valued Senior Member

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    Yes, it seems we are gonna need to define "common sense". The double slit experiment seems to defy common sense, unless you know something about the science of the wave function and wave-interference, which we can observe in water by common sense, but requires knowledge of the behavior of photons. which can be represented by scientific symbolization, because it is a recurring pattern, not only in water but as a universal constant, which requires much greater knowledge (science) than what the term "common sense" can possibly describe. describe.

    Common sense is the result of what we can observe with our senses. But that does not necessarily mean we understand what causes the observed phenomena. Thus the gods were invented as a result of common sense prevailing in the days just after we stopped swinging in the trees and became tool users.
     
    Last edited: Nov 24, 2016
  21. Write4U Valued Senior Member

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    The telescope allowed him to see what common senses are not . He proved something that common sense had never questioned.
    Even Aristotle had it wrong, from a "common sense" POV when he proposed that heavier objects fall faster than lighter objects,. until Galileo proved him wrong.
     
    Last edited: Nov 24, 2016
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  22. Write4U Valued Senior Member

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    That does not follow. Here you are compounding sloppy reading and incomplete understanding, with a conclusion which is completely misplaced.

    The intent of this thread is to explore the possibility that there may exist some universal constants which we have not yet discovered or understand as yet There may be some unknown processes and functions, but that does not make them "alien science", but "as yet unknown".

    I have posted this before but please do check it out. It is concise and simple but reveals some very interesting perspectives on universal mathematically recurring functions or patterns taken from different perspectives.
    http://www.ted.com/talks/roger_antonsen_math_is_the_hidden_secret_to_understanding_the_world?.
     
  23. rpenner Fully Wired Staff Member

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