Where Physics and Mysticism don't meet

The double-slit experiment: problems with particles and waves

The actual physical items needed to perform this experiment are very simple. They consists of a photon �gun� � which will shoot a single photon at a screen with two slits in it. After passing through the slits, the photon will be registered on a photographic plate. If we shut one of the slits and fire our photon gun at the screen, (lets say at one photon per minute) over time as each individual photon hit the photographic plate the resulting pattern will look something like the image below. This seems to imply that photons are particles, like bullets.

If we were to open both slits and repeat the process, we would expect to see two similar patterns on the photographic plate � one for each open slit. This, however, is not what happens! The resulting pattern will appear similar to the image below.

The pattern is what would be expected if the photon were a wave. Waves in water behave this way when passing through a barrier. Waves will interfere with each other � creating a higher peak where two peaks meet, a trough where two troughs intersect and cancelling each other out, where a peak and a trough coincide.

So depending upon how the experiment is set up, photons appear to be either a particle or a wave. Which one is true? Contrary to what is expected quantum mechanics says that they are both simultaneously. And even worse for our tired heads, it states that all �particles� are in fact wave / particles.

Revolving Electrons

Going back to our simple hydrogen atom again, we recall that it is composed of one proton and an electron. Earlier we learned that the electron �revolves� around the nucleus of the hydrogen atom, the proton. This, as well, isn�t quite correct.

One way of visualising the electron is to imagine it as a cloud moving around the nucleus. The �thickness� of the cloud represents the probability that the electron could be found in that location � thicker cloud means a higher probability, thinner, lower probability. The image below shows two different ways of representing an electron cloud orbiting the nucleus of an atom.

So, we now roughly understand the problem that Heisenberg was facing. What can we know about a thing which is both wave and particle and with a location that appears to be randomly determined based on probability?

Other than imagining the electron as a cloud, another way is to describe it as a wave packet. A wave packet contains a number of wave curves as shown in the animation below.

Shaping an atomic electron wave packet - Michael w. Noel & C. R. Stroud Jr. - 1997

This model helps us to understand and 'visualise' a manner in which an electron can appear to be both a particle and a wave. Returning to Heisenberg, he still has to resolve the issue on how to locate an electron.

What does Heisenberg tell us with his Uncertainty Principle?