This kind of quotes do keep popping up in reports about quantum phenomena: “Depending on the way in which it is measured, the quantum object manifests itself as a particle or as a wave.” No, no, and again no, that is not the true image of quantum reality in my opinion. In fact it is severely misleading en confusing.
Such statements create the impression of an object that deliberately adapts to the measurement methods used and then decides whether it shows itself as a wave or as a particle. No wonder many people decide that the quantum world is utterly weird and incomprehensible and stop thinking about it.
This false image, this misunderstanding, has its origins in the image of the world that we received from our earliest memories on. An image of a world existing independently of us and in which we fulfill merely the role of spectator, an accidental bystander who might as well not have been there. We are used to imagining something, every physical thing, as something that simply IS and has always been there. We tend to stick to that way of looking at reality even when, depending on the way we look at it, its properties suddenly appear completely different and extremely ambiguous, like the quantum object mentioned above.
It is rather unusual to think that things are there BECAUSE we perceive them, that they did not exist before our observation and are no longer there after our observation. If we would opt for that way of thought, things would attain properties that we usually attribute to dreams and thoughts and not to ‘real’ things. This way of thinking about reality is not in keeping with the common perception of the permanence of our world. Yet the quantum world teaches us that our idea of an objective permanent world is most likely false.
Looking at the double slit experiment
The double slit experiment is a crucial experiment in quantum physics able to provide a lot of insight. So let’s take a look at it
When we fire a large number of particles, photons, electrons or even large molecules, through a double slit, an interference pattern will be created on the screen after the slits. We see a pattern of light and dark bands. That pattern also arises when we fire particle by particle. Even after a long period of firing single particles, certain areas on the screen appear to be hardly hit, which are the light bands in the picture above.
Such an interference pattern is the result of wave behavior. It occurs because waves reinforce or extinguish each other in certain places depending on their synchronous concurrent or opposite motion, respectively. Watch this YouTube video for a very enlightening demonstration of double slit interference.
There is a mathematical relationship between the spacing of the bands of the interference pattern, the spacing between the slits, the distance from the slits to the screen, and the wavelength, but we don’t need to go into that to understand the meaning of this experiment.
Such an interference pattern of dark and light bands only arises when the originating waves have the same frequency and wavelength. It happens when two wave sources vibrate synchronously. The two slits here function as wave sources vibrating in phase. The rather amazing conclusion drawn from this interference pattern is: “Every particle exhibited wave behavior and must therefore also have been a wave.” This also applies to electrons and even to large molecules of more than 800 atoms.
Catching the particle in the slit
When we adjust the experiment in a way so we can determine for each particle which slit it has gone through, the interference pattern disappears and we get a pattern that you can interpret as two single slit patterns that are projected over each other and therefore are actually indistinguishable from a single slit pattern. Each of the two slits now produces a single slit pattern, which is a single light spot with the highest intensity in the center, in much the same location on the screen.
The correct conclusion is that the waves passing through the slits no longer interfere with each other. The relationship between these two waves running from the slits, which let them extinguish or strengthen each other in fixed predictable places, has disappeared. The often drawn conclusion is that we now see particle behavior instead of wave behavior, which actually makes no sense. A single slit pattern is still for 100% the result of wave behavior, only we no longer observe interference such as occurs with two synchronous wave sources. It seems more like as if every wave, connected to each particle, is now originating from only one of the slits and no longer from both. And that’s exactly what’s going on here.
How we see the world as a collection of things
“.. we can determine for each particle which slit it went through …“. Notice how this sentence is formulated. The implicit assumption here is that there is a particle that travels along a path and that shoots through one of the slits. That is an image that stems from the way we got to know the world around us from childhood. And apparently we find it extremely difficult to let that premise go. Ask yourself: Did the fired bullet travel every part of the path to the target? Or didn’t it?
The simple hypothesis: observation manifests the particle
Now, if only for a moment, try to let go of that premise, set it aside. Imagine now that, there is no particle traveling a path, there only is a wave. A wave that appears to be particularly intimately connected to our perception of the particle. (I will postpone here the effort of trying to understand how this connection works.) A wave that will end when we make an observation. An observation thus means that we seem to manifest the particle at that time and in that location. Immediately after our observation has been made, the particle is no longer there, but the wave is there again starting from where we last observed the particle. Now look again, assuming this hypothesis is right, at the version of that double slit experiment where we could determine which slit the particle passed through. Are we now perhaps able to understand this enigmatic disappearing act of the interference bands somewhat better?
Therefore, try to follow the following five logical steps:
- According to this hypothesis, it is the observation, in this case through which slit the particle passed, that made the particle to appear in one of the slits.
- Its appearance in the slit implicitly means the end of the wave.
- Only at the moment the observation information tells you, the particle manifested and existed in the slit.
- Immediately afterwards there is no particle and a new wave leaves the slit eventually ending up on the screen behind the slit.
- Since the particle did not appear in both slits – at least let’s assume that there is no magical particle multiplying – we now have only one single wave source.
- So there is indeed a wave – between the double slit and the screen – but now there is no more interference, because you need two synchronous vibrating wave sources for it to observe.
This hypothesis – observation manifests the particle – gives thus a complete and logical explanation of the disappearance of the interference when we observe the particle at the slit.
Two time-consecutive manifestations of the particle in a single experiment
Where the wave hits the screen, we do observe a bright little spot. In principle, that is also an observation. So when we set up the measurement in such a way that we can observe in which slit the particle appeared, we create a measurement setup with two consecutive locations for observations – and thus, manifestations. One in the slit and the other on the screen behind the slits. That dual observation is the crucial aspect in an experiment where we do observe the particle at the slit.
So it is confusing to say that the observed object behaves like a wave or a particle depending on the way of observing. In both setups, it is consistently true that there is a wave that results in the manifestation of a particle through an observation. In the setup where we look in which slit the particle appeared, we simply make two consecutive observations, whereby a wave manifests itself twice as a particle. The measurement directly influences the measured object and doing two consecutive measurements at two locations within the setup therefore logically should arrive at a result different from a single measurement done only at the screen. As if you gave during billiards the already rolling ball an extra kick and then got surprised that it influenced the outcome. We really don’t have to assume an intelligent ball for that.
Not a particle and wave at the same time, it’s a probability wave
If we look at it that way, then there is no longer a particle that adapts magically in terms of properties to our way of measuring. The whole process is clear and extremely predictable. As long as we don’t measure the object we want to measure it is a wave. As soon as we measure where and when the object was , we will find the object to have been there. The measurement and manifestation of the object thus become identical! This is a very important and deep conclusion.
Now the question of what that wave is and what it consists of becomes an important one. The answer to that question was first proposed by the physicist Max Born in the early 20th century. In his proposal, the quantum wave is a wave that, when interpreted correctly, gives you the probability per location and time, where and when, to find the object during a measurement. Thus, the quantum wave gives us a prediction of reality but not an exact one. It is a statistical prediction, just like when rolling a dice, the probability of exactly getting a six coming up is 1/6 and that the average outcome of a roll is 3.5. Incidentally, Max Born still assumed that the particle was somehow ‘guided’ by the wave which means that the particle traveled a path, albeit unpredictable. That interpretation was later abandoned by most physicists.
Quantum mechanics is statistics
Statistics is the way in which quantum mechanics accurately predicts the results of experiments. With the enormous numbers of particles that play a role in objects larger than a few micrometres, the outcome of a physical event can be predicted with great precision. Just as the average outcome of a hundred billion throws with an ideal die will be exactly 3.5 with a deviation that we will find only after the 8th decimal place. Many quantum physicists do accept the idea that the particle only manifests itself during measurement, but they disagree about how the measurement achieves this, given the large number of different interpretations. Most interpretations attempt to save the objective permanence of the world but until now these fail to do so convincingly. That there is not a winner since more than 100 years could be an indication of wrong underlying and deeply hidden assumptions. In technical applications, quantum physicists simply use the statistical calculation methods – shut up and calculate – and leave the interpretation to the disputing theorists.
The simplest explanation is usually the best
As I wrote at the beginning, assuming that the ‘thing’ aspect of reality only appears because we are looking and that it does not exist physically when we are not observing, means that the reality we perceive has the same quality as thoughts and dreams. If that is the assumption that provides us with the simplest unambiguous explanation of the double slit experiment, the idea that observing manifests reality might now have become not as strange as it probably sounded to you at first. Applying this hypothesis we are able to visualize every part in the double slit experiment without having to try to imagine something that is simultaneously a particle and a wave, which is impossible. This could mean that our belief that the world is permanently out there, regardless of our presence in it, is a very persistent misunderstanding. That is anyhow my deeply felt opinion. The world is there because we create it when observing it. This also applies to something dramatically destructive like the Covid-19 virus in the end. Such a message should raise of course a number of rather hard questions. For some answers on these have a look at another page on this website.
Paul J. van Leeuwen graduated in applied physics in Delft TU in 1974. There was little attention to the significance of quantum physics for the view on reality at that time. However, much later in his life he discovered that there is an important and clear connection between quantum physics and consciousness.
What he learned between then and today resulted in a post academic course in quantum physics for non-physicists. A little bit later he decided to put the contents of that course, and more, in a book published in Dutch: Kwantumfysica, Informatie en Bewustzijn – and started a website on the subject. He translated the Dutch version of his book in English, titled: ‘Quantum Physics is NOT Weird’.