Double-slit experiment: Alternating results

[mk9898]

How can the results of a double-slit experiment change just by adding a detector at the point of entrance of the split? Is it really so that if there is a detector, we will only see two lines and if there is no detector we will see interference?

 

[vanhees71]

It depends on the setup. Of course, a detector interacts with the objects running through the double slit and thus disturbe the interference of the partial waves running through each slit and thus alter the interference pattern observed at the screen.

 

[mk9898]

a detector interacts with the objects running through the double slit

So it's not necessarily the act of observing but rather the interaction between a detector and the particles? Also, why is it that electrons are shot through the double split and not photons? Electrons are in fact particles and are not light. I thought it would make sense to shoot photons since they are electromagnetic waves and particles.

thus alter the interference pattern observed at the screen.

The interference pattern is only altered or is it in fact two lines without any other lines?

 

[Lord Jestocost]

Electrons are in fact particles and are not light.

What we term "electron“ as a matter of convenience is neither a wave or a particle. It is not true that an “electron” sometimes behaves like a wave and sometimes like a particle. It always behaves like itself, but we sometimes choose to measure one property, sometimes another.

As respects the point of view of the “Copenhagens”, Paul Davies summarizes the essentials in the following way (in his introduction to Heisenberg’s Physics and Philosophy): “What, then, is an electron, according to this point of view? It is not so much a physical thing as an abstract encodement of a set of potentialities or possible outcomes of measurements. It is a shorthand way of referring to a means of connecting different observations via the quantum mechanical formalism. But the reality is in the observations, not in the electron.”

 

[vanhees71]

Electrons are massive spin-1/2 quanta and thus have some particle properties, e.g., they are localizable to a certain extent. Of course as elementary particles they are neither classical particles nor classical fields but described by a quantized Dirac field. Under some circumstances the probability distribution for finding an electron at a given point on a screen shows the same pattern as classical wave intensities predict, but the interpretation is completely different: Quantum theory is inherently probabilistic, i.e., the electron cannot be interpreted as an extended object like a classical (e.g., electromagnetic) field, but its wave function (which makes sense in the non-relativistic limit) provides the probability distribution for its position, i.e., letting an electron run through a double slit will always yield in its detection in one point-like region (of finite extent however!), but the very position each individual electron ends up cannot be predicted better than with the probabilities provided by the wave function, which can be calculated with help of Schrödinger's Equation. Letting run very many electrons through the slits, always prepared in the same way, yields a distribution on the screen which looks like the interference pattern of corresponding waves, but note again, modern quantum theory has been discovered, because the old-fashioned quantum theory a la Planck, Einstein, Bohr, and Sommerfeld was inconsistent in itself and also quantitatively wrong in even quite simple cases (like for the predicted spectra of all atoms except the hydrogen atom). Thanks to Born, Jordan, and Heisenberg, Schrödinger, and particularly Dirac in 1925/26 we have modern quantum theory today, which is the most successful theory ever, and there's no wave-particle dualism and likewise self-contradictory features of the old theory.

Now let's briefly come to photons. It's unfortunate that most popular books on QT start with photons, providing wrong pictures about them, even more than 80 years after the correct description of them by Dirac. Even introductory university textbooks provide still wrong pictures in their introductory chapters. Photons are, however, among the more difficult subjects to explain and cannot be understood without the full use of relativistic quantum field theory. Forget about any particle picture you might have been suggested by such bad popular-science writing! It's very probable to be wrong or at least misleading. First of all it's a mathematical fact that photons, as we understand them within the most accurate theory ever, the Standard Model of elementary particles, are not localizable in any sense. It's not even possible to define what the position observable of a photon might be. The only thing you can define and measure are detection probabilities for a given experimental setup. In the case of the double-slit experiment with single photons the description of these probabilities is not much different from that for electrons, and you get probability distributions as predicted by classical electrodynamics, but again particularly what's nowadays understood as a "single photon" is far from being anything classical. It's not a bullet-like particle since it's not even posssible to define its position nor is it in any sense localizable, but it's also not like a classical electromagnetic wave field either. E.g., the expectation value for the lectromagnetic field $(\vec{E},\vec{B})$ of the single-photon state vanishes. On the other hand a photon carries energy, momentum, and angular momentum. As I said before, you cannot understand what a single-photon state is without studying relativistic quantum field theory and quantum electrodynamics.

 

[vanhees71]

As respects the point of view of the “Copenhagens”, Paul Davies summarizes the essentials in the following way (in his introduction to Heisenberg’s Physics and Philosophy): “What, then, is an electron, according to this point of view? It is not so much a physical thing as an abstract encodement of a set of potentialities or possible outcomes of measurements. It is a shorthand way of referring to a means of connecting different observations via the quantum mechanical formalism. But the reality is in the observations, not in the electron.”

Well, obviously the introduction by Davies is far better than Heisenberg's book itself (I guess it's the translation of the German original "Der Teil und das Ganze"). Anyway, if you want understand quantum theory, don't read writings by Heisenberg and Bohr. They are those of the founding fathers who confused the subject as much as possible. Don't get me wrong, both were geniusses in creating the theory, but they were not able to write about it in a clear way, because everything is buried in philosophical erudition rather than in a clear mathematical exposition of the content, and mathematics is the only language you can use to really formulate the content of quantum theory as concise as you have to to make it a consistent physical theory. If you want to read original literature, I rather suggest to read the writings by Dirac, Pauli, Schrödinger, Born, Jordan, and of course Sommerfeld than Heisenberg or Bohr. Somewhat in between is von Neumann, who on the one hand gave the first fully rigorous mathematical formulation of the theory in terms of Hilbert space theory but on the other hand was even worse than Bohr and Heisenberg concerning its physical interpretation (leading to the rather solipsistic "Princeton Interpretation", which is a particularly weird flavor of the many interpretations often subsumed as "Copenhagen Interpretation").

 

[Nugatory]

Also, why is it that electrons are shot through the double split and not photons? Electrons are in fact particles and are not light. I thought it would make sense to shoot photons since they are electromagnetic waves and particles.

The experiment has been done both with electrons and photons. Both are quantum particles (that is, neither particles nor waves as these words are usually understood), and both show interference as predicted by quantum theory.

 

[PeroK]

So it's not necessarily the act of observing but rather the interaction between a detector and the particles? Also, why is it that electrons are shot through the double split and not photons? Electrons are in fact particles and are not light. I thought it would make sense to shoot photons since they are electromagnetic waves and particles.

The double slit is an experiment. It makes sense to fire whatever you like and to see what happens.

 

[Lord Jestocost]

Well, obviously the introduction by Davies is far better than Heisenberg's book itself (I guess it's the translation of the German original "Der Teil und das Ganze").

“Physics and Philosophy: The Revolution in Modern Science” by Werner Heisenberg is an outgrowth of his Gifford Lectures at St Andrews University.

 

[mk9898]

Thanks for the posts. What type of observation does it take to change the pattern on the surface from interference to two lines? I mean I could put a video camera there viewing the double-slit or I myself can be there looking at it and that wouldn't change the results - it would of course be an interference pattern since I cannot see the quantum particles. There has to be something that triggers the change.

 

[DrChinese]

Thanks for the posts. What type of observation does it take to change the pattern on the surface from interference to two lines? I mean I could put a video camera there viewing the double-slit or I myself can be there looking at it and that wouldn't change the results - it would of course be an interference pattern since I cannot see the quantum particles. There has to be something that triggers the change.

The best example (for photon interference) is to place polarizers at each slit. When aligned parallel, there IS interference. When aligned perpendicular, there is NO interference.

 

[Nugatory]

Thanks for the posts. What type of observation does it take to change the pattern on the surface from interference to two lines?

Just about interaction that makes paths through one or the other slits physically impossible will do the trick. The easy way to do this (you'll see this most often in experiments) is to just block one of the slits so there are no contributions from that path to the probability of the particle landing at a particular point on the screen, hence no interference from those paths.
A more subtle approach is to put polarizers behind each slit. If one polarizer is oriented horizontally and the other vertically, then any photon that reaches the screen will be either vertically polarized and have passed through one slit or horizontally polarized and have passed through the other slit - no interference pattern. On the other hand, if we orient both polarizers vertically then all the photons arriving at the screen will be vertically polarized, and paths through both slits will be contributing to probability of a photon landing at any given point on the screen - these paths will interfere and there will be an interference pattern.

I mean I could put a video camera there viewing the double-slit or I myself can be there looking at it and that wouldn't change the results - it would of course be an interference pattern since I cannot see the quantum particles.

Whether you see it or not is completely irrelevant. If the particle interacts with anything at the slit, (a measuring device, a camera, a brick that you've placed in front of the slit, a stray speck of dust, anything) in such a way that any arbitrarily delicate and precise lab equipment could even in principle detect the effect and hence which slit the particle went through... there will be no interference. It doesn't matter whether you or anyone else actually know about it, just that the interaction happened

There has to be something that triggers the change.

There is. It's whatever interaction at the slit changes the system in such way that the particle can in principle be shown to have gone through one slit instead of the other.

Be aware that this talk about the particle "going through" either or both slits is somewhat misleading, and we do it only because it's really hard to describe what's really going on using ordinary English. "Going through" isn't part of the math. Instead we consider all the paths between source and screen that were available to the particle; each one makes a contribution, either additive or subtractive, to the probability of the particle landing at any given point on the screen. When two slits are possible, the contributions add in such a way that areas of high probability alternate with areas of low probability to produce an interference pattern. Feynman's book "QED: The strange theory of light and matter" is a good layman-friendly explanation.