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Argument about inteference experiments : help me win!

  1. Jun 13, 2008 #1
    Ok, I've been taught this stuff in school about the particle/wave duality. And what's been always bugging me is that it doesn't really tell me anything : I want a theory that is cohesive without compromises. SO...I was shown an experiment similar to Young's 2 slit experiment.

    It has a half silvered mirror and a laser.
    The "/" is the half silvered mirror.

    (single photons)................../........|

    Basically, you think...well the photons come out of the laser, hit the mirror, and either go North to one Mirror or East to the other. They bounce off, there's some slight angles, and head for the detector.

    The thing is, even though only 50% of the photons go to each mirror, if you move EITHER mirror it changes the time it takes for ALL of the photons to get to the detector.

    Well, ok, so that is tough to understand. Obviously, every photon must take BOTH paths.

    If you change the two mirrors to detectors, then any given photon only is DETECTED by one detector. Yet, when the light wave is at the mirror, it can't know a detector is waiting for it. So the laws of physics must be the same : the light does NOT pick a path because it is GOING to be observed, it STILL goes both ways.

    Just, when it gets to a detector, the photon has been split, and so has half the electromagnetic field strength. It randomly "chooses" whether to give all of it's energy to the detector, with the probability varying by the light's field strength.

    This theory requires instantaneous communication along individual photons : every piece of the spread out photon must "know" that it is being absorbed somewhere. However, you cannot use this instantaneous communication for practical purposes.

    I talked to some TAs, and they said I should write up my "theory" in mathematical terms. Ha!, whatever. I don't have time in my lifespan to go to Caltech for 5 years to learn the math needed for quantum mechanics directly. Plus, the math is bullgarbage : it's like string theory. The blackboard full of equations is just a model of what is going on, not what is physically happening.
  2. jcsd
  3. Jun 13, 2008 #2


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    I guess you are asking for an explanation that "makes sense".
    Well, that won't happen. Quantum mechanics does not make sense to us. It doesnt matter how long you study QM, it never stops being weird; but after a few years you get used to that.
    In fact, the reason why we need math is that if we were to just use "common sense" we would get results that do not agree with the experimental data. Hence, the math is neccesary in order to "compensate" for the fact that our brains are only wired to understand the classical -non QM- world.
  4. Jun 13, 2008 #3


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    No it's not like string theory. QM makes predictions that can be tested, and have been verified many many times, to many decimal places of precision. As far as I know, string theory has yet to make unambiguous predictions that can be tested experimentally.
  5. Jun 14, 2008 #4


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    i) Is there an a priori reason for why math is not the best way do describe the physical world? Why is our daily life language better? And if our daily life language is better, why do we stick with the math at all?...

    ii) if you don't have time to learn french, don't visit france. Mastering the language of what you want to learn and understand is essential, and the language of physical science is math.

    iii) What is really happening in the physical world is beyond our reach (classic philosophy), so we MUST do models in order to describe the world around us, and it seems that the best way to do so is by using math.
  6. Jun 15, 2008 #5
    Reasons I hate the math : first of all, mathematical representations are a language, all right. One thousands of years old, based upon a set of ancient assumptions and structures. As such, it is a very inefficient language sometimes, and you need to use many terms to describe something.

    Even the best of us cannot instinctively understand what is going on. It can be difficult to impossible to see how a complicated mathematical relationship relates to physical reality.

    Mathematical equations are also NOT as efficient as they could be : "wormholes", "dark matter", and other things that more than likely cannot exist are artifacts of the equations we have for dealing with relativity.

    Einstein didn't use math, originally, for formulating his theory. He thought of what was going on, first, and THEN wrote up a bunch of equations to rigorously describe it.

    The only explanation for a couple of physics experiments involves the protein "stretching out" over a probability distribution, simultaneously in every place along that distribution. I can see the distribution just fine in my head, and could draw a picture. Of what use is a bunch of equations only 0.1% of the population can read?
  7. Jun 15, 2008 #6


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    No the math we use today is only 250yeras old (invented by Newton and Leibniz), then it have devolped by guys like Hilbert (early 1900) and so on, math is under research today as well.

    Must nature and mathematical relationships a priori be understandable instincitively by everyone?

    All things that come up from our physical theories exists in the real world, that is true - that is why physics is also an experimental science. If you think you can do better, then give it a try.

    Einstein had several theories, please name the one you are referring to. I think you are referring to General Theory of relativity, and he basically starded in the same way as Newton did. Newton got an apple in his head, and Einstein imagined how physics would behave in a falling elevator (Free fall), and then he basically just started to elaborate. When he learned differential geometry, he got the tool come to formulate the theories.

    This is what theorists do all the time, one starts from an idea, then trying to forumulate a coherent theory that can explain all known things and make predictions of new results.

    Special theory of relativity is based on two axioms, imposed by einstein. Axioms can't be proven right nor wrong, but the theory that comes form these two axioms are one of the biggest triumphs of physical science, since it has so many experimental verifications.

    What if it is not meant for the whole population to understand everything? Is there an a priori reason for why they have to?

    What is the use for a language that only 0.1% of the population can read? 'Everybody' CAN learn math, everybody can learn chinease and so on.
  8. Jun 16, 2008 #7


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    If you do the bookkeeping of a small shop, what do you think is easiest ? Counting over and over again the money in the cash register, and the goods in the store, or just using some arithmetic (= a mathematical structure) ?

    Well, the mathematics used in physical theories is similar. Only, the structures are more sophisticated. But it is still easier to do the math, than to try to find an equivalent of "counting the money".
  9. Jun 16, 2008 #8
    Then you have a long time to wait for one. :smile: Quantum theory has always reeked havoc with common sense.
    I don't understand. Either the photons are deflected to the left or they go straight through moving in the downwards direction in the diagram. They certaintly don't move to the right.
    To be precise you'd have to move both mirror and source since if you move the mirror too far the laser light may just miss the mirror or the reflected beam may miss the detector.
    Huh? Where did you get that idea?
    I don't understand what that means.
    You're using classical reasoning for a quantum mechanical system. The path that the photons takes cannot be spoken of until the photon is detected. Trying to make a statement about which path it goes through will lead to the wrong answer.
    Photons don't get split. What happens is that there is a 50% chance that the photon will be defected and a 50% chance that it will simply move through the mirror and straight down after it passes through the mirror.
    It is the intensity that drops, not the field strength. The intensity of the light is proportional to the square of the intensity of the field. And I don't think its meaningful to speak of the strength of the electric field of a photon.
    The math isn't that hard. If you know calculus then it wouldn't take more than a several months to have a good understanding of it. However your TAs are wrong in that there is nothing mysterious about this problem.
    The math will give you the correct answers if you resolve to think in quantum mechanical terms and stop asking what the photon is doing when nobody is observing it. Such questions have no answers in quantum mechanics in that such questions are meaningless.

    Regarding math: Math is merely a language in which one can describe nature precisely. There's nothing more to it than that. Trying to describe nature without math is like trying to have a conversation about programming using machine language. While it might be possible I know I certainly wouldn't want to try! One can speak about the physics by translating the math into english but while it might take one line on a piece of paper to describe it mathematically it might take many pages to describe it using, say, English. I'm not even sure that such a thing is possible!

    Last edited: Jun 16, 2008
  10. Jun 16, 2008 #9


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    By "with common sense", you of course mean "with the intuition I have developed about classical physics". :tongue:
  11. Jun 16, 2008 #10
    Absolutely. :smile:

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