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Why does this experiment seem to confuse many people.
Alex
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It is a simple experiment which at least strongly suggests that something non-local happens in QT. On the one hand, there seems to be a particle. As seen by the fact that the final picture is composed of a lot of small dots - the places where a particle has been observed.
On the other hand, the resulting picture depends on the fact that two slits are open. If only one slit is open, each of them half the time, the picture is different. This tells us that this particle, if it goes through one slit, somehow receives information (without being there) that the second slit is open too.
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No mystery here ... the simplest interpretation for 2-slit is standard QM. The probability amplitude wave passes through each slit and two wavelets emerge on the other side (by Huygen's principle). Then they interfere at the detector. The prob of the electron's position is given by the complex square, or norm, of the result. Where they reinforce each other the electron might appear, where they cancel each other, it doesn't.
If you place a detector in either slit, it destroys the coherent phases, and therefore the interference pattern. You get two "clumps" where the electron will hit. They are identical (almost anyway), it doesn't matter which slit had the detector.
That's all clear enough right?
But now, if you ask "which slit did the electron actually go through" you get into difficulties. Copenhagen says that's a non-question. There was no electron, only the wave, until it hit the detector. There's no "paradox" but many don't like the idea of an electron that (sort of) disappears in transit.
de Broglie - Bohm says there's a real electron; it passed through one slit or the other. The probability amplitude wave is interpreted as the "pilot wave" whose information guides the electron in such a way that it still produces the correct interference pattern. In fact you can just ignore the electron; the pilot wave winds up giving exactly the same result as Copenhagen's probability amplitude wave.
If, with dBB, you put detector in one slit, it destroys the interference and gives the same result. It's a more complicated picture, and the calculations are much more difficult if you want to track where the particle went. But the answer winds up the same. Doesn't matter if the particle went through slit with detector or not. With dBB 2-slit we normally wouldn't care about exact path of particle, just the resulting pattern on the detector. So we would wind up doing exactly the same calculations as with Copenhagen; but with a different picture of what went on in reality.
You might ask what good is dBB if it just gives the same answer, with a more complicated picture. For one thing it retains a classical electron which can be considered an advantage "philosophically". More important, it explains some more complicated QM "paradoxes" better than standard QM.
Anyway - there is no problem or paradox that I can see regarding "which slit" the electron went through, nor the behavior when a detector is placed in a slit, in either Copenhagen or dBB interpretation.
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The argument that the dBB picture is more complicate is misleading. The Copenhagen interpretation needs a subdivision of the world into a classical and a quantum part. In the classical part we have well-defined trajectories, in the quantum part the wave function. So, above things exist in the Copenhagen interpretation too. But somehow artificially restricted, by a classical/quantum border, which is not a border at all (no border control visible, not even a well-defined position of the border). Then, it has a Schrödinger equation, and another, non-Schrödinger evolution is necessary for the collapse, while dBB has only Schrödinger evolution for the wave function.
So, Copenhagen has to add complications: Restrict the domain where the trajectory exists (without necessity), split the world into classical and quantum part, and introduce some non-Schrödinger collapse evolution on their border.
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Schmelzer and secur,
I support (with my work) that QM became what it is because of the Copenhagen Interpretation. The fundamental error (few may actually notice it) of the Copenhagen Interpretation is that they have confused the cause behind the effect with the measurement problem. Quantum effects work all the time even if you do not measure. It is totally absurd when Copenhagenists claim that it is not a valid question "from where the particle passed through". This illusion (and more) comes from the used abstract maths that have nothing to do with reality.
The probability distribution that gives the interference pattern on the Double-Slit experiment is not associated with the cause but with the observation (screen). Since there is no experiment (because the interference pattern will be destroyed) that may justify from where the electron passed through then, the claim of simultaneously passing through both slits is not valid not to say absurd.
Although I do not speak about the Double-Slit on my work, I would propose the following view:
-Since slits dimensions are almost close (some orders of magnitude larger actually) to the charged particle then the slits plays the role of a bounded space as also the distance between them (small obstacle) where there it is expected (based on my work) the speed of light to decay (based on slit and particle dimensions as also the separation width)
-The electron beam used on such experiments have a specific profile given by a Probability distribution (like Gaussian). It means the electrons are distributed along the propagation axis
-Passing through one or the other will be determined by the position of the electron before and while passing through the slit
-Due to the decay of the speed of light (it will vary across the slits), the momentum of the electron will vary not in amplitude but in terms of direction (the Electrostatic Deflection experiment will prove exactly this)
-It cannot pass through both of them since it will violate the Energy and Momentum Conservation something that QM frequently ignores when comes a Wave like behavior of matter into play
In the same category falls the Quantum Tunneling that is actually a non-probabilistic effect according to my view.