The double slit experiment
Richard Feynman was fond of saying that all of quantum mechanics can be gleaned from carefully thinking through the implications of this single experiment, so it's well worth discussing.
Feynman’s recommendation is indeed an excellent one and we will follow his advise. You don’t need to have a knack for maths, just be able to think logically. That in itself is quite difficult for many people of course (read Thinking Fast and Slow by Daniel Kahnemann) but certainly not beyond the reach of people with average intelligence if they take the trouble.
Pretty much everything I have to say about quantum physics and its interpretation is ultimately based on the essential and simple double slit experiment. Many of the most current and interesting quantum experiments are based on this as well. It invariably concerns quantum waves that split in two and meet themselves – after having shortly traveled different paths – in order to interfere. On meeting the two split waves cancel each other out or reinforce each other in different locations in a geometric pattern. Other interesting interference experiments are the Mach-Zehnder experiments, but I won’t go into these here because they don’t add new insights here.
An infinite wave of possibilities
The most common interpretation of the quantum wave, also called a state wave, is that it is a wave that contains all the potential material manifestations of the to be observed object (superpositions) and that it expands dynamically in space and time until it hits a physical detecting instrument on its course. Because it is a wave of potentialities it should be considered immaterial.
At that meeting the object manifests itself, possibility becomes physical presence, and the wave dissolves at the same time. You could also say that only one single element of the infinite spectrum of possible material manifestations contained in the wave remains in existence and thus manifests. The rest of it disappears into thin air without a trace. The wave has then become the particle.
That abrupt transition of potentiality into material existence is the so-called quantum collapse for which more hypotheses have been devised than there are in the case of the eight blind monks and the elephant.
Double and single slit patterns
With a double slit we get an interference pattern, with a single slit also, but that looks very different. In the center of the single slit pattern we see a spot with a maximum brightness in the center that decreases to zero towards the edges and then some small maxima to the left and right of it. Those small maxima are the result of effects the wave is subjected to at the edges of the single slit. That’s diffraction. The wave deflects from its main central direction at the edges. The waves arriving from the left and right sides of the slit interfere.
So there is a very clear difference between the patterns that are observed behind a single slit and behind a double slit. Both patterns are the result of wave behavior and both show interference.
What happens when we observe the slits to see what is happening in the slits?
I hope it is becoming already obvious that in these double-slit experiments information playes an important role. In any double-slit experiment with particles, be it photons, electrons, buckeyballs or even larger objects like small viruses, it happens that, when we arrange the experiment in such a way that it also makes information available about the chosen slit, the characteristic double-slit interference patterns, those dark and light fringes, do not appear. Instead, the result is a spread-out spot that is brightest right behind the middle of the slits and decreases in brightness outwards. That pattern is the superimposed result of two slightly offset single slit projections. See Figure 7.
The most obvious and simple explanation of the above pattern – figure 6 and 7 – is that the quantum wave for each observed object passed through only one of the two slits. So it does not have to be the case that the object actually manifested itself in the slit. The odds of finding it in the slit there were 100% at a certain point in time, but the philosophically interesting question is whether that equates to a material presence.
Measuring the influence of information
An important experiment in this regard was conducted at Korea’s Institute for Basic Sciences (IBS) in 2021. They measured the gradual effect on the interference of the degree of information about the chosen slit. In the article in Physicsworld I see the frequently committed fallacy that the experiment demonstrated the wave-particle complementatity as Niels Bohr called it, the impossibility to see quantum wave behavior and particle behavior at the same time. In my opinion this is a wrong description of Bohr’s idea because in all cases – always afterwards – we suppose wave behavior that we don’t really observe until that ultimately results in a really observed particle at the detector. So there is always a wave that itself is not observed but is assumed in order to explain the phenomena, and there is always ultimately a particle that is observed. If you think that’s a subtle difference, then you’re right, but it’s important nonetheless in the interpretation I want to give below.
The effect in the Korean experiment is that the interference pattern – see Figure 2 – that we see when we can’t get information about the passed slit gradually changes into a pattern like in Figure 7 above, two superimposed single slit projections. Gradually, as more information is made available about the passed slit, the pattern also gradually starts to resemble Figure 7 more closely. The experimenters were therefore able to increase or decrease that information in a controlled gradual manner.
Conclusion: The information that the experiment can provide plays a crucial role in how the quantum wave moves through the slits. The more the information, the more the wave goes through one of the slits and the more the single slit pattern is shown. In the experiment, a mathematical relationship was even established between the available information and the distribution of the quantum wave between the slits. It looks like this: V (interference pattern clearly two slits) and P (slit information) are related according to the mathematical expression P2 + V2 = constant. Pythagoras once again looks over our shoulders as so often happens in quantum physics.
Decoherence of the quantum wave
I see it this way. When the wave only goes through one slit, the quantum wave has lost half of the possibilities that the wave has for the manifestation of the particle. It’s reduced. That is nothing more or less than decoherence, albeit a partial one. Many phycisist prefer the term ‘The reduction of the quantum wave’ over ‘Decoherence’. So decoherence and information are positively correlated, the more information, the more decoherence. Viewed in this way, the quantum wave is reduced in such a way that we can infer that the particle passes through one slit. And then the usually drawn conclusion is that the particle was materially in the slit when, strictly speaking, we could only say that the wave went only through one of the slits. In the Korean experiment, when adhering to the idea of real particles going through the slits, we should have to say that each particle passed one slit a little bit more materially than the other, which is an absurd image of reality.
Decoherence is usually attributed to the molecular turmoil of the detection instrument. Here we are clearly dealing with a different interpretation of the cause of quantum decoherence. A hypothesis that von Neumann had already protested against and that Schrödinger indirectly argued for with his cat-in-a-box thought experiment.
But what about the full decoherence of the quantum wave at the detector? Is it also caused by the information we could retrieve from the measurement? That is indeed a good defensible position. The information we posses already beforehand is also important. We posses information that the detector always forms a physical barrier to the particle. I hope you agree. So, that physical barrier information is 100% correlated with the full decoherence of the quantum wave at the detector.
We can also explain – with this information decoherence interpretation – the exceptions that had to be made to what causes molecular unrest decoherence. These exceptions are all the optical parts, such as lenses and mirrors. They do not cause decoherence while they are large and molecularly restless enough. The fact that we can drop these exceptions is of course an additional strong argument for this information-decoherence interpretation. Just think about that. Perhaps good news for the builders of quantum computers where the decoherence of their entangled qubits is the big problem.
Caveat: Correlation is not proof of a causal relation. It is indicative.
The role of the observer
Coming back to those eight blind monks and their elephant, I think that the parts of their experimental set-ups that many physicists seem to have a blind spot for are precisely those indispensable parts for which information plays a major role: the observers. The observer is the elephant in the room.