Introduction

If photons are directed at a screen with a hole or slit in it, an interference pattern can be produced and seen on a detection screen. If the intensity of the photons is so low that they can be thought of as travelling one at a time, and the detection screen is replaced by a photographic film, the same interference pattern builds up over time.  If we detect which slit the photons have gone through the interference pattern disappears.

I’ll discuss the slit version of this experiment using the phot model for a photon. Then I’ll discuss the double slit experiment.

Explanation in terms of Phots

The phot explanation is simple.  Since the phot has spatial extent sideways, it interacts with the whole slit. It is absorbed and reborn in one edge of the slit or the other.  It sets off again, again in the form of a flat electromagnetic “frozen pizza”, travelling face first, with a well defined path and a definite central line of travel.  But something special has happened to it while it was being reborn. A part of the original phot interacted with the other side of the slit. This energy then became part of the new child phot, but it a special way. It reunited with the rest of the child phot in discrete steps related to the energy level of the phot (which is manifested in the frequency of the electromagnetic disturbance created when a phot is absorbed). It forces a synchronisation with the rest of the phot.  The result is that the recombined child phot has to head off at a discrete set of directions. Each direction corresponds to one of the synchronisation modes. The child phots then hit the s detection screen and build up the interference pattern.

The traditional explanation of radiating spherical wave fronts with peaks and troughs reinforcing works, but if you think about it closely, it does not stand up to even elementary skeptical scrutiny.

Now let us move on to the famous/notorious double slit experiment.  This works best with monochromatic light.  Note that the experiment can also be done with electrons, or even atoms and larger particles, but that is another story.

Note that the two slits have to be a distance apart that is comparable to “wavelength” of the light. But phots do not have a wavelength. In phot language the first sentence in this paragraph should be replaced by a sentence that says “make sure that every phot can splat into both slits”.

The explanation is very similar to the single slit explanation. Every individual phot interacts with both slits. It is reborn in one slit or the other but with a specialized contribution from the other slit which forces the reborn “child” phot to head off in a discrete set of paths that builds up into the resultant interference pattern.

Underneath the main interference pattern it might be possible to discern two faint diffraction patterns arising from each of the slits individually.

The enduring perplexity of double slit experiments comes from clinging to the wrong mental model of a photon as a discrete particle that can only go through one slit or the other. This also leads some teachers to say that a photon cannot interfere with itself.

A phot is a discrete object, but it can go through both slits and it can and does interfere with itself. Once you admit this interpretation the experiments become a lot less perplexing. The magician’s trick is no longer so mysterious and magical (see my very first blog in this series).

It should also be now apparent that trying to determine which slit the light has gone through is a misguided exercise. It is the wrong question.

Conceptually a valid question is “which slit is the principle origin of the child phot and which slit provided the steering component”, but answering this will be tricky. You cannot observe a phot in flight. You can only observe it once and to observe it is to destroy it. Nevertheless it might be fun to experiment with one of the slits and not the other to see what happens. For example, heat up one of the slits, or influence it with strong electric or magnetic fields.

I think Erwin Schrodinger got close to an interpretation of the double slit exercise which is similar to the one provided here using the phot model. Schrodinger replaced the photon by a probability wave function and allowed some of the probability to percolate through one slit and the rest through the other. This allows photons to interfere with each other. The whole approach works well mathematically, but to be honest, I think the interpretation adds a lot of unnecessary mystery to quantum physics and eventually causes problems.

Schrodinger (added and abetted by Einstein) spent a lot of time worrying about the fate of a cat exposed to quantum fluctuations and the Complementarity Principle school of thought (see Schrodinger’s cat thought experiment). According to the standard quantum theory approach, the act of observing the cat resolved whether it was alive or not. Einstein is reported to have said “God does not play dice”.

There are a lot of versions of the double slit experiment and it will be interesting to apply the phot approach to all of them. However, for the sake of any readers, and because I have some other and equally interesting destinations in mind, I will move on to discuss the Michelson-Morley experiment in terms of phots and then I want to discuss general relativity, the physics of spiral galaxies, the origins of inertia and the big bang theory.  I hope to give a description of double slit experiments using electrons and atoms in some later blogs.

Conclusion