On starting out our discussion of the double slit experiment we stated that it was one of the simplest yet most profound demonstrations of the quantum nature of matter.
Simple we see ... but profound?
It may seem tenable that electrons and photons interact and interfere with each other as they travel from the double slit to the screen. Something happens in between the slits and the screen to effect the way they distribute themselves on the screen ... but what if we were to arrange the experiment so that only one electron or one photon at a time were to travel through the slits towards the screen? ... Logic would tell us that being one particle, photon or electron, it would necessarily pass through only one slit and as such could not create an interference pattern since the path through the second slit is unoccupied at the time of travel.
The physical implementation of this experiment became feasible when low level electron and photon detectors became available. The light/electron level was simply set low enough that it could be determined that only one particle at a time would travel the path towards the screen. The result was as you might of guessed was that the interference pattern remained.
If a particle is travelling by itself what is it that it could possibily interfere with? And being one particle how could the presence of a second slit effect the path of the particle if the the other slit was unutilized, or equivalent to being closed at the time of travel?
What happens if we watch one slit?
In an effort to understand exactly what is happening, experiments with electrons have been carried out where a detector has been place on one slit to measure which slit each electron passed ... the result ... the interference pattern disappears and only a gaussian scatter distribution remains. Seemingly the interference of particle waves only works if we don't look!
What is really happening?
There is no consensus as to how to explain these phenomena. However this author believes that the inescapable truth is that the unobserved quantum particle travels through both slits. However a particle cannot be a particle if it is in two places at once. If we take Shrodingers equation at face value it tells us that a probability wave exists that travels through both slits, interferes with itself and then collapses to a point when the particle is detected at the screen. Observing the electron at the slit, causes the probability wave to collapse at the slit, thus eliminating the wave interference after the slit.
In effect this experiment tells us that Shrodinger's probability wave equation may be more than simply a measure of our uncertainty about a particle's postion but it may in fact reflect the physical manifestation of that particle during the period prior to the particle's detection.