through the double-slit

In many conversations, mostly after panel discussions or at social gatherings in salon talks, as well as very often on the occasion of my exhibitions and the many questions arising there, I am asked about inexplicable phenomena in quantum physics. I will now refer to an often mentioned experiment: the double-slit experiment:

Characteristic for this experiment is not only its simplicity, but also its radicalness in the result. Hardly any other experiment shows in such a clear way a phenomenon which seems to be inexplicable for us in a reproducible way. It can be reproduced today with two razor blades and a light source as a student experiment.

After dealing with this experiment, our view of the world will no longer be what it was before. It changes permanently and – if one continues to engage with the subject – it reshapes our reality. Often, unfortunately, its consequence is not taken into account.

Another peculiarity of this experiment is that it was developed 200 years ago and, moreover, by a doctor and amateur physicist, the Englishman Thomas Young. In everyone there is the ability to move great things.

To understand the experiment, one should know how the world view was at that time, which still prevails in many people today:

The course of the flight and impact of a pistol bullet is, in our view, always predictable by Newton’s laws of mechanics. The course of a wave, e.g. by throwing a stone into the middle of a pond, is also predictable, even if it is crossed by other waves and “interferes”, i.e. interacts with other waves. In the time around Newton, light was seen as a stream of particles, i.e. composed of many particles, which could not be shown so clearly by Thomas Young’s experiment.

Thomas Young designed a simple experimental setup to check this for light: he arranged two very narrow slits parallel to each other and let light shine through. To see if the light passed through the slits, he placed a wall surface at a distance behind them on which the light impinged.

Now, one could anticipate the result: behind the slits, two streaks of light appear on the surface, which can be explained by the fact that some of the light particles passed through one slit, the others through the other slit, and hit the wall behind them.

But it is not like that. Surprisingly, on the wall many stripes appear next to each other, their intensity decreases to the left and to the right, between the stripes there is no trace of the light. Even the intensity of the middle stripe is the highest, where actually no particles could land, because the center is covered by the area between the slits.

How is this possible? It can be explained only if one assumes that light is not a stream of particles, but a wave. This contradicts the particle nature of light postulated by Newton, but so be it: If you send this wave out and it hits the slits, it passes through both of them, becomes a ring-shaped propagating wave behind each slit and both of them unite again shortly afterwards on their way to the wall.

This creates an interference pattern as wave crests and troughs overlap, doubling or cancelling in intensity. (Cancellation is used today in active headphones, which attenuate environmental noise by analyzing it and generating a counter wave that cancels the sound wave). This pattern of cancellation and enhancement manifests itself in the observed streaks on the wall.

All well and good, so this is the proof that light is a wave, and this is what Maxwell finally confirmed around 1870 by developing the principle of the electromagnetic wave.

Two times no:

First no: we observe also if one shoots particles (electrons) through the two slits a resulting wave pattern, as if one had sent waves and no particles.

Second no, because Thomas Young checked now what happens if he closed one of the two slits. One would assume that now a wave goes through the remaining, open slit and indeed no more interference with the other wave arises, but this wave hits the wall and maps there an equally broad brightness course, only weaker. And here there is now a break in our reality perceived so far: Only a vertical line is imaged, as assumed at the beginning, when we assumed that light is a stream of particles.

But how can light behave once as a wave and another time as a particle? (One must understand, a divided wave consists also after division of individual waves, which do not become thereby particles).

Erwin Schrödinger assumed, the particle smears in the flight to a wave and Max Born came finally on the idea to regard this wave not as an actual wave, but as a probability compression. The particle is on its way as a big pile of possibilities in the form of a probability wave. Whew…you will say: how to imagine this?

Let’s leave this topic detail and just accept for the moment that “something” can be particle as well as wave, depending on the need or situation.

After around 1900 Max Planck proved the quantization of the light, i.e. the packets of electromagnetic waves = quanta and these were called “photons”, the double-slit experiment was repeated, now with single photons. If one sends many single photons one after the other through the double slit, then the typical wave interference pattern at the impinging wall is created. But this is not possible, because the single bombardments by particles (photons) were waiting and thus only a line behind the slit should have been created.

But as soon as one wanted to determine by detectors through which slit the respective photon had come, the wave pattern changed to a particle pattern, namely only two light stripes. The photon apparently moves – still not intuitively comprehensible for us – through both slits at the same time and gets only by the measurement behind the slit a single, fixed and plausibly random (Gaussian distributed) scattered location. So the measurement led to the fact that the reality changed, from the wave became particles by measurement. The wave collapsed and formed particles.

This made the foundation of the physical experiment shake, the objectivity of an observation was at stake, and, to anticipate, the previous postulate of an independent observation of an experiment tipped over the cliff and has been history ever since.

As a result, this means that one cannot observe without changing what is observed. Put more abstractly, we shape the world through our observation.

If you think that this is possible “only” with massless light particles, then your knowledge can be enriched:

The experiment has now also been successfully performed with atoms and fullerenes (60 carbon atoms) in vacuum without any interaction.

The double-slit experiment and its result-rich consequence stimulates to ponder, so we read in the Copenhagen interpretation of Nils Bohr and Werner Heisenberg:

“In a way incomprehensible to us, our consciousness is a part of matter and matter carries a part of consciousness.” (around 1927)