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I'm reading a book of philosophy (Less Than Nothing by Slavoj Žižek) and a chapter in it (called The Ontology of Quantum Mechanics) frequently references the double-slit phenomenon. At one point it says the following three points:

(1) Even if we shoot the electrons individually, one after the other, they will, if we do not measure their path, form a wave pattern―but how can they? With what does each individual electron interact? (With itself.)

(2) Even if we measure (or not) the path after the electrons have already passed through the slits, the pattern still depends on our measurement―but how can it, when the measurement takes place after the passage through the slit? It seems as though we can retroactively change the past.

(3) Even if we do not enact measurement at all, the mere fact that the measurement apparatus (and, with it, the possibility of measurement) is there makes the electron behave as a particle―but how can it, when it was in no way affected by the measurement apparatus?

While I'm studying biophysics and I've done a course on elementary atomic physics and QM, I'm far from being qualified to tell whether these claims are problematic or not, and if they are indeed problematic, in what ways. Of course I'm not looking for an in-depth analysis, I'd just be glad if someone pointed out if there are fundamental and obvious misunderstandings contained in the quoted passage.

I'm especially suspicious about the third point - from my studies I would think that the mere presence of a measuring apparatus is irrelevant, what matters is the physical act of measurement and the perturbations it involve.

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    $\begingroup$ Is it even remotely correct? No, it's not even remotely correct. Electrons aren't particles but quanta of a quantum field. What you are reading there are the desperate attempts to make sense of a particle picture in the 1920s. By 1929 it was understood in broad terms why it's not necessary and by the mid 1930s the smarter physicists had moved on to sorting out relativistic quantum field theory. That is the correct framework to deal with all of these phenomena in a self-consistent way and it basically puts away with particles completely. It's turtles riding quantum waves all the way down. $\endgroup$ – CuriousOne Mar 15 '16 at 10:18
  • $\begingroup$ I am curious as to how many others on this site have also given up on the particle theory and rely only on quantum Field theory for answers. Are you saying that electrons are not physically there? I have also heard you make similar statements about photons. Could you please explain this a little clearer. If so many smart physicists have really given up on particles then maybe that's why many other physicist think we have been at a dead in for a long time. $\endgroup$ – Bill Alsept Mar 15 '16 at 21:19
  • $\begingroup$ @BillAlsept: An electron is physically "there", if you mean that "it's there where you measure it". From that it does NOT automatically follow that it's a physical thing that is also in a defined position at all other times when you do not measure it. That is exactly the big lesson to be learned that differentiates quantum mechanics from classical mechanics. In CM we can make that step, in QM we can't. And that's OK, because in QM "the field" is still always there. We do have an entity that has an existence ontology. It's just everywhere all the time instead of being "here some of the time". $\endgroup$ – CuriousOne Mar 16 '16 at 4:30
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This is an answer by an experimental physicist.

Quantum mechanics started with the wave equation of Schrodinger and then the relativistic covariant ones of Dirac and Klein-Gordon. These describe single particles in potential wells and the solutions are called wave functions and their complex conjugate square gives the probability of observation/interaction/decay for given boundary conditions. This is the basic quantum mechanical framework, usually called first quantization. In this framework electrons, atoms and molecules are quantum mechanical entities, described by the wavefunction. When measured they have particle type properties, i.e. an individual (x,y,z,t) location . Collectively they display the wave nature of the wavefunction.

Lets take the statements one at a time:

(1) Even if we shoot the electrons individually, one after the other, they will, if we do not measure their path, form a wave pattern―but how can they? With what does each individual electron interact? (With itself.)

The electron does not interact with itself, it is described by the boundary conditions set up by the slit which will define the wavefunction and thus the probability wave that describes its behavior. Here is a single electron at a time:

doubleslit

Electron buildup over time

Each individual electron ends up as a spot on the screen, in its particle like aspect. The accumulation of electrons with the same slits ( boundary conditions) shows the probability distributions which has a wave pattern.

(2) Even if we measure (or not) the path after the electrons have already passed through the slits, the pattern still depends on our measurement―but how can it, when the measurement takes place after the passage through the slit? It seems as though we can retroactively change the past.

This is confusing . The screen can be up to the slits , the screen is a measurement and the pattern's widths will depend on the location of the screen , not the interference pattern which will be there. Only if a measurement apparatus is placed at the slits or on the way it will change the boundary conditions and thus the solution, and the pattern will be destroyed ( see 3).

(3) Even if we do not enact measurement at all, the mere fact that the measurement apparatus (and, with it, the possibility of measurement) is there makes the electron behave as a particle―but how can it, when it was in no way affected by the measurement apparatus?

This is confusing, it might be discussing a which slit passage detection. This by construction changes the boundary conditions of the problem . Changed boundary conditions will give different results and randomize the pattern. It is no longer the same problem. Also he is using "particle" pattern as classical, billiard ball particle which is not really true

This recent paper clears this type of confusion:

Overall, the results suggest that the type of scattering an electron undergoes determines the mark it leaves on the back wall, and that a detector at one of the slits can change the type of scattering. The physicists concluded that, while elastically scattered electrons can cause an interference pattern, the inelastically scattered electrons do not contribute to the interference process.

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  • $\begingroup$ I have a problem with the linked paper you cite - the presence of a detector at a single slit is enough to destroy interference, even if the detector is not triggered; i.e. the subset of marks on the screen not correlated with a slit detection click does not produce fringes. No click at the slit = no interaction, meaning the electron should still be able to elastically scatter and contribute to interference according to their interpretation, which is not what we see experimentally. $\endgroup$ – JPattarini Mar 15 '16 at 14:46
  • $\begingroup$ I may be missing how this differs from the "no click" detector subset, I'll re-read the paper again but it seems analogous $\endgroup$ – JPattarini Mar 15 '16 at 16:25
  • $\begingroup$ I built an ultrafast photo-electron diffractometer during my doctoral research; I did many, many electron diffraction experiments, and can note the following: when detected, there is always one spot; if you let the system accumulate, a diffraction pattern appears. When the electrons are generated, the photo-electric effect emits electrons one-by-one. In some runs the electron bunches were very small, statistically there was one electron at a time passing through the ultrathin gold film. Particle-wave-particle is how it appears to the experimentalist. $\endgroup$ – Peter Diehr Mar 15 '16 at 17:09
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    $\begingroup$ @PeterDiehr: Do you know Mott's 1929 paper "The wave mechanics of α-ray tracks"? It explains why "particles" seem to appear in wave mechanics. Today it can be reproduced in general with weak measurement theory and decoherence. Particles are of no use and of no need in quantum mechanics and we understood this very early, it's just one of those flukes of science history that "the news" never made it into the classroom. $\endgroup$ – CuriousOne Mar 15 '16 at 20:39
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    $\begingroup$ @CuriousOne Many people have the illusion that once a successful mathematical model is built, it is reality. History of physics shows that this is not true up to now and there is no reason to believe that the present most advanced model is really really reality. Not to forget also that at the heart of this "particles are no use" model, the Lagrangians, point particles with masses are used to get the basis of QFT. My position is that the appropriate , mathematically correct, model should be used for the appropriate framework under study/question, and in answers, the clearest for the OP. $\endgroup$ – anna v Mar 16 '16 at 4:19
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(1) Even if we shoot the electrons individually, one after the other, they will, if we do not measure their path, form a wave pattern―but how can they? With what does each individual electron interact? (With itself.)

The way to work out what's happening is to try to guess what's happening and then look for ways to rule out guesses. Suppose a single electron goes through just one slit and there's nothing else going on. At any given point on the screen, if you open up another slit, then that point should see more electrons because you've let in electrons that otherwise would not have come into the experiment. But you don't see that: rather, you can have fewer electrons at a particular point when the slit is open. So every time you detect an electron there was something going through both slits. If you put a detector in front of both slits, only one will go off at any given time, so the electron doesn't split into two parts, each of which goes through one slit. Nevertheless whatever is going through the slits behaves just like an electron: it can be deflected by magnets etc. So there are two versions of the same electron and they interfere with one another. See "The Fabric of Reality" by David Deutsch, Chapter 2 for a clearer explanation.

(2) Even if we measure (or not) the path after the electrons have already passed through the slits, the pattern still depends on our measurement―but how can it, when the measurement takes place after the passage through the slit? It seems as though we can retroactively change the past.

(3) Even if we do not enact measurement at all, the mere fact that the measurement apparatus (and, with it, the possibility of measurement) is there makes the electron behave as a particle―but how can it, when it was in no way affected by the measurement apparatus?

No. There are two versions of the electron, one going through each slit. Both versions contribute to the outcome of the experiment. Now, when the electron version going through the other slit comes to the place where you put the detector, it interacts with something that acts exactly like the detector. For example, if the detector blocked some particular subset of the electron's possible paths, that set would no longer contribute to the interference. But you don't see the detector go off, so there is another version of the detector that went off in response to the presence of the electron.

See

http://arxiv.org/abs/hep-th/9305002.

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  • $\begingroup$ Electrons are not particles. You really have to let go of this illusion. It's like asking the question "How do I make epicycles work better?" eighty years after Kepler had found the correct solution. Really. $\endgroup$ – CuriousOne Mar 15 '16 at 20:34

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