Double-Slit Diffraction Re-visited

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Double-Slit Diffraction Re-visited

Postby Pivot on July 12th, 2018, 12:36 am 

Double-slit diffraction is a corner stone of quantum mechanics. It illustrates key features of quantum mechanics: interference and the particle-wave duality of matter

There are many U-tube videos and descriptions in printed and electronic literature showing the interference-styled patterns from double slit experiments that are used to assert interference and the particle-wave duality of particle-wave of light, electrons and even atoms.

The similarity between the appearance of interference patterns and those created from intersecting wavefronts is interpreted as evidence to wave-like characteristics of particles in support for the particle-wave duality concept. The pattern similarity was originally put forward as evidence of the wave nature of light, and later for electrons and atomic particle streams. However such similarity represents pretty thin evidence, and other possibilities that might explain the phenomena need to be thoroughly examined, tested and eliminated before the particle-wave duality explanation can be considered to be the only possible explanation thus validating the particle-wave duality concept.

One of the best documented single and 2-slit experiments is the paper titled ‘Controlled double-slit electron diffraction’, by Roger Bach, Damian Pope, Sy-Hwang Liou and Herman Batelaan (New Journal of Physics Volume 15, March 2013; downloadable at ... 5/3/033018). The paper provides an excellent historical overview of the most significant experimental evidence on the subject since Richard Feynman’s thought experiment concept and is one of the few single/double slit experiments that provide full details (although the backstop distance is missing) of the setup for electron streams and for single electron-by-electron accumulation together with good presentation of the results.

An alternative explanation for the 2-slit interference patterns for electrons involves the electromagnetic edge effect. Assuming the orbital model for the structure of atoms, the positive charges of the nuclei of edge-boundary atoms (a 2nm layer of Gold in the referenced article) deflect the negatively charged electrons as shown in figures 1 and 2.

Figure 1 represents the single-slit setup (P1 or P2 in the referenced paper) showing how low strength electromagnetic fields around the inner edges of the slit can deflect those electrons travelling close to the side of the slits. Those electrons closest to the slit edge, and thus the inner corner, are deflected most and those further away less, with sweet-point deflection groupings (not dissimilar to the quantum-like orbitals around a nucleus in the orbital model of atomic structure based upon speed, charge and spin) overlapping slightly. The insert of figure 1 shows more the proportion of the electron beam corner deflection zone which has been scale-distorted for clarity in the main diagram.

The result for the single-slit is a convex lens shaped concentration made up of the overlapping symmetrical central strip and skewed distributed deflection bands, as represented by the dashed green composite plot and experimentally determined concentrated distribution shown rightmost.

Figure 2 represents the 2-slit setup (P12 in the referenced paper). It shows how the skewed electron distributions from the two slits overlap and re-inforce each other to produce zones of higher intensity that emphasise the lower intensity gaps in between, creating the interference-like patterns associated with 2-slit experiments. The modal sweet-point paths of deflected and unaffected electrons are shown as the blue (upper slit) and maroon (lower) lines, and the distribution in each deflection zone is shown for each. As for figure 1, the dashed green plot shows the combined frequencies which correspond pretty well to the experimentally determined P12 frequency distribution (rightmost in figure 2).

Thus the experimental results for electrons, and similarly for atoms streams, can be explained by electromagnetic deflection without having to rely upon any assumed wave-like characteristics of the particles.

In the ‘Double Slit Experiment Explained!’ video at Jim Al-Khalili claims that when the deflected particles (here atoms) are counted, the deflections do not take place, suggesting that the act of counting or observing the particles magically stops the deflection process. Could it just be that the particles are counted by detecting them as they pass through a magnetic field, and that magnetic field subsumes and cancels out the low strength electromagnetic fields around the inner edges of the slits, thus eliminating the cause of the deflection? The lack of detail regarding the ‘atom counting’ device is a critical omission.

The crisp ‘Professor Dave Explains’ series of u-tube presentations provide an insightful explanation of the particle-wave duality concept for electrons, with energy waves of various frequencies forming around a central energy core. However he fails to explain how these energy waves cause interference patterns for the single and 2-slit experiments. 

For light the alternative deflection mechanism is different to that for electrons as light is not significantly deflected by electromagnetic fields: instead they just cause a rotation of polarisation angle (the Faraday Effect) within light. It can be argued that the 2-slit interference patterns produced by spatially coherent monochrome light results from a combination of surface (slit side) reflection and polarisation and edge differential refraction (birefringence).

The 2-slit experiments should not be considered to represent strong experimental evidence of the wavelike nature of photons (i.e. the full electromagnetic radiation spectrum including visible light), electrons and atoms without first eliminating other alternative feasible explanations for the observed interference patterns. To do so means that the wave explanation remains disputable, and is at risk of being embarrassingly proven incorrect further down the track.
2 slit Figure 2.JPG
figure 2
2 slit Figure 1.JPG
figure 1
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Re: Wave Theory Still Holds Water

Postby Faradave on July 12th, 2018, 12:45 pm 

Water waves and sound waves follow wave theory as they pass through slits in a barrier, yet they can ultimately be resolved into the motions of the particles of their respective mediums. That does not invalidate the wave description.

Particle beams obey wave theory with respect to wavelength, slit width slit separation (if more than one and distances from source to barrier and on to screen. They hold precisely for a variety of interferometers, where slits are not involved at all and respond accordingly when beams are rotated specific amounts.

That there is agreement between wave and particle constructs (though each excludes the other for a given measurement) is the basis of wave-particle duality. Depending largely on scale, wave theory can be seen emerging from particle theory or vice versa.
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