Comments about the article in Nature: Particle, wave, both or neither? The experiment that challenges all we know about reality

Following is a discussion about this article in Nature Vol 618 15 June 2023, by Anil Ananthaswamy
To study the full text select this link: In the last paragraph I explain my own opinion.




In a simple, modern form, Young's 'double-slit' experiment involves shining light of a single frequency (say, from a red laser) through two fine, parallel openings in an opaque sheet, onto a screen beyond.
But that's not what happens. Instead, you see many bands of light and dark, strung out in stripes like a barcode: an interference pattern (see 'Wave-particle weirdness').
This, by itself, is a very complicated experiment. To understand you should perform three experiments.
  1. First you should open only the left slit. What you will see is a pattern like this ..fedcbabcdef.......
  2. Next you should open only the right slit. What you will see is a pattern like this ......fedcbabcdef...
  3. Next you should open both slits. What you will see is a pattern like this ...........fedcbabababcdef...
Experiment #3 is a physical combination of the Experiments #1 and #2.

Wave-Particle weirdness

When quantum objects such as electrons are fired one by one through a pair of closely spaced slits, they behave like particles: each one hits a screen placed on the far side at exactly one point.
Two things are very important:
1) that they are fired one by one (for example they are generated at a 1 second interval) and 2) that each hit the screen at one point.
Also it is important that the experiment should be performed in 3 flavours: The left slit open, the right slit open, both open.
The patterns observed in case 1 and 2 should be identical. In case 3 an interference pattern will be observed.
In all these experiments in 1 minute exactly 60 electrons should be generated and 60 should be detected.
But they also behave like waves: successive hits build up a banded exactly like that generated by a wave passing through two slits.
That is the situation described as flavour #3.
The wave-particle duality is described by a mathematical tool known as a wavefunction.
The wave nature is described as a wavefunction, but does not explain the interference pattern
This interference pattern, which is only observed when both slits are open, indicates that what we call one electron, in fact goes through both slits simultaneous.
With the emergence of quantum mechanics, the idea of light as a wave faced a challenge. But it wasn't as simple as going back to the particle view. Further tests of quantum theory using the double-slit experiment only deepened the mystery. And it hasn't been solved yet.
Why call it a mystery? What means solved?
The only correct path to go is to perform more detailed experiments.

1. Singularly quantum

Classical intuition says each photon can go through only one slit or the other.
Intuition is not a guidance to perform science. Experimental results should be the origin of this opinion. More detail is required.
Yet the mathematics of quantum theory implied that the interference pattern would persist.
As mentioned before, mathematics has nothing to do which the behaviour of elementary particles. Each interference pattern is the result of physical behaviour, influenced and emanating from two sources. The behaviour can be described with mathematics, but mathematics is not the cause.
Aspect won a share of the 2022 Nobel prize in physics for his contribution to confirming the predictions of quantum mechanics through experiment.
Aspect won a share of the 2022 Nobel prize in physics for his contribution through experiments that lay the groundwork for the behaviour of elementary particles.
There exists as far as I know, no proper definition of what quantum mechanics is. As such it is not clear what these predictions are.
It is for example difficult to predict in advance, without any explanation and knowledge, what will happen when two perpendicular water wave patterns meet each other. But: if someone asks you this question, first explaining what water waves are, without performing any experiment, there exists a high chance that you can predict the correct answer.
But such experiments leave matters of interpretation wide open. There is simply no way to comprehend what's happening with minds attuned to the classical world of everyday objects.
To understand the behaviour of elementary particles more experiments are useful. These experiments describe a different reality of the world surrounding us, but whatever the details, no magic is involved and it does not make sense to speak of a classical world. All processes, large or small influence each other in due time.
When it comes to the double-slit experiment, quantum mechanics does tell a form of story.
This raises an important question: what exactly is quantum mechanics. Roughly speaking it is the branch of physics which studies elementary particles. It starts with the periodic table and the fact that there are protons, neutrons electrons and quarks. But is it more.
It says that a photon's position is described by a mathematical abstraction called the wavefunction - which, as the name suggests, behaves like a wave.
This immediate raises an important question: How is the position of a photon measured? IMO that is impossible. This is more or less what can be as expected. The wave function of a physical water wave can also not be measured.
The value of the wavefunction at any location on the photographic plate lets you calculate the probability of finding the photon there.
That is not the order how science is done. First you measure the position of photon and using many positions you calculate the parameters of your model i.e., the wave function.
As mentioned this is very difficult.
The probability is very high in regions of constructive interference, and very low in regions of destructive interference.
That is mathematical correct, but physical difficult to demonstrate.
In a sense, then, a photon or any other quantum object acts like both a particle and a wave.
This sentence is not clear. What means act? What exactly is a quantum object? The problem is that if the pattern of the individual positions of electrons behind the screen is different, if one slit is open or two, then in some way or another the trajectory of each photon, when both are open is influenced by both. This has nothing to do with the concept of a wave function. It is physics.
This 'wave-particle duality' embodies many of the central conceptual mysteries of quantum mechanics that are unresolved to this day.
The problem is that the physical behaviour has nothing to do with this duality.
Even if you could know everything about a photon's initial state, there's no way to tell exactly where it'll land on the detector.
Because of the same physical reasons, it is impossible to calculate the wave function.
Before the measurement — in this case, detection by the photographic plate — the mathematics says the particle exists in a superposition of states: in a sense, it has taken both paths, through the right slit and the left.
Mathematics has nothing to do with the behaviour of photons.
Measurements in general only give information what happened before the measurement was done.
If this pattern is different if one slit or two slits are open then that is the only reason involved.
To claim that the photon exist in a superposition state, then first a clear definition is required what that physical means.
Standard quantum mechanics says that the wavefunction 'collapses' when measured, and that the act of observation in some way precipitates that collapse.
This sentence is not clear.
When a photon hits your skin, this particular photon, as a photon, ceases to exist. That is all.
Before this, the photon has a finite probability of being found in many different regions, but on measurement, the wavefunction peaks at the location in which the photon appears (the probability there equals 1) and is nullified everywhere else (probability equals 0).
This text is not very helpful to understand what is physical involved.
It gets even odder. If you can determine which path the photon took on its way to the detector, it acts like a particle that does indeed go through one slit or the other: the interference pattern disappears.
When both slits are open you cannot. You will observe an interference pattern as demonstrated by experiment #3.
When one slit is open the answer is simple as demonstrated by experiment #1 and #2.

2. What's a wavefunction?

But to generate interference, something has to go through — or at least interact in some way with — both slits.
That is correct. More and better experiments should be able to discover more details.
In the mathematics, the wavefunction does the job.
But the link from mathematics is difficult.
Some physicists would say that the wavefunction simply represents information about the quantum system and is not real — in which case it's hard to explain what interacts with both slits at once.
To understand the physical reality by means of mathematics, starts from the wrong footing. The rest of the text more or less demonstrates this wrong approach.
1. One can avoid this with interpretations of quantum theory 2. that don't collapse the wavefunction, 3. but that opens other cans of worms.
This sentence consists of three parts that each are not clear. It gives a bad feeling.
Perhaps the most notorious is the many-worlds interpretation, the brainchild of US physicist Hugh Everett in the 1950s.
The many-worlds interpretation does not explain how anything in the world operates.
The de Broglie-Bohm theory, named after quantum pioneers Louis de Broglie and David Bohm, provides another alternative. It says that particles exist with definite positions and momenta, but are guided by an all-encompassing, invisible 'pilot' wave, and it's this wave that goes through both slits.
You cannot explain something any behaviour by introducing a new concept that is also not clear.
Most notably, John Wheeler at the University of Texas at Austin designed the 'delayed choice' thought experiment.
The same comment as the previous comment. The next sentence: idem.
Imagine a double-slit set-up that gives the option of gathering or ignoring information about which way the particle went.
The problem is that the answer of this abstract experiment cannot be used to explain the outcome of a real experiment.
Did the photon travel back in time and come back through the two slits as a wave?
The concept: 'travel back in time' is not realistic. To describe the behaviour of a photon, in a real experiment, as going forward in time does not make sense.
To avoid such nonsensical explanations, Wheeler argued that the only way to make sense of the experiment was to say that the photon has no reality — it's neither wave nor particle — until it's detected.
What means: until the photon is detected? The photon is detected, when the photon hits your skin.
In reality, there only exists a photon before the photon is detected.
Back in the 1980s, Marlan Scully, then at the University of New Mexico in Albuquerque, and his colleagues came up with a similarly befuddling thought experiment.
Thought experiments have a very limited practical value.
They imagined collecting the which-way information about a photon by using a second photon 'entangled' with the first — a situation in which measuring the quantum state of one tells you about the quantum state of the other.
In principle two photons can be correlated. It is very important that this correlation has to be established by means of 100 experiments, and what it physical means.
As long as the which-way information can in principle be extracted, the first photon should act like a particle.
More text is required to explain what is involved.
But if you erase the information in the entangled partner, the mathematics showed, the first photon goes back to behaving like a wave.
It is not clear how mathematics, all by itself, can explain this behaviour. See also Reflection 4 - The tools we should not use to understand the reality.
Surprisingly - or unsurprisingly, by this stage - intuition was once again defeated and quantum weirdness reigned supreme.
This sentence is not clear. Intuition is an ambiguous concept.

3. Larger and still larger

Again, the mathematics predicts exactly this behaviour, so physicists aren't surprised.
Mathematics cannot be used to predict almost any behaviour.
But it is more proof that the double-slit experiment highlights the lacunae in our understanding of reality, a quarter of a millennium after the birth of the man who devised it.
Our understanding of the reality can only come by performing different experiments.

Reflection 1 - Understanding the physical reality

The article "Double trouble: two slits, many questions" is a very important article because it raises many philosophical questions. The main questions are around the subject: When can we claim that we understand something, when can we claim that we understand the physical reality.
One answer is that we can describe the present reality in an unambiguous way. What is happening and why.
A different answer is that we should be able to predict what is going to happen.
The main tool we use is one set of concepts which should be clear to all people involved.

The main issue of the article is that there is no agreement what these concepts are related to the behaviour of elementary particles and photons. A typical case is the text below the picture at page 454: "The double-slit experiment's interference patterns suggest: something is in two places at once." What does it mean that something is in two places at once? What is this: something? In this case, it not an object. To understand the issues involved, we first have to define: something what we call a water wave. A water wave is a local disturbance of for example the water level in a lake. This can happen when you drop a stone in a lake. Around the point of contact with the water, circles, waves will appear which will spread out in even larger circles. They will slowly disappear until the water level becomes flat again. It is the water level at each point, locally, that moves up and down. (Also, sideways). A water wave is a combination of these points.

When you drop two stones the circles that each stone causes, after they hit the water surface, will influence each other. You can claim that each water particle in influenced from two sources.
To claim that in this experiment: 'something is in two places at once' is tricky to clarify. No water particle is in two places at once.

Reflection 2 - God does not play dice. Article by Vasant Natarajan This statement comes from Albert Einstein and raises two questions:
1. Did God ever played dice? and 2. Did Einstein ever played dice?
When you play dice and you predict that the outcome is always a 1, then in 1 out of 6 throws the outcome is correct. That means the chance is 16.66. This also implies that there is a probability issue.
Einstein knew that. He used this statement to express, that God, being the creator of the Universe, never used a dice to perform this task. The consequence is that there is no probability issue.
That is 'strange' because the outcome (including all the parameters involved) of every real performed experiment, cannot be predicted with 100% accuracy. Only experiments in mathematical sense, i.e., computer simulations, can be predicted with 100% accuracy. The most important issue is, that in many computer simulations not all parameters that influence the outcome are considered. The model used, of the reality, is too simple.
This is true for large astronomical simulations, including the behaviour of galaxies, for a roulette as for the behaviour of elementary particles, including photons and quarks.

Reflection 3 - The tools we use to understand the reality.

  1. The first and most important tool is physical observation. That means to make observations with our eyes and all our senses.
    The result is curiosity. This does not happen immediate but when we continue with observations, we slowly see what is normal and what is special. For example, the first time we observe a rainbow. When we see: snow. This raises the question: what is this. When does it happens. What are the specifics?
    Physical observations is the primary starting tool, to understand the evolution of the universe including all the different 'objects' it contains. Including galaxies, stars, black holes and planets. This evolution is a physical process.
  2. The second important tool is experiments.
    Experiments are the only tool to investigate the details of any process.
    Each experiment consists always of three steps: 1) You start simple 2) you repeat the same experiment. The result should be the same. 3) You make one modification etc.
    It is very important to keep a logbook of the experiment.
    This logbook should contain a detailed of the whole experiment. The main emphasis which parameters and how they are measured. The main purpose is that others should be able to repeat the experiment.
    The logbook specific should contain the results of each experiment. The final chapter should contain an explanation of the results. This can also be an assumption which require additional experiments.
  3. The third tool is improvements of everything that is related to experiments, specific to improve accuracy.
    This can be a better microscope or whatever.
    An other not mentioned tool are scale models. Air plane models are used to estimate or calculate the parameters of actual airplanes. As a side benefit these models can be used to train future pilots.
  4. The fourth and maybe the most controversial tool is clear and unambiguous definitions of concepts and whole sentences.
    Sentences (and definitions) can only be clear when all concepts used are clear. This smells like a bottom-up approach.
    Concepts that should be clear (if used) are physical time, mathematical time and clock time. Absolute and relative. Special Relativity, General Relativity, Quantum mechanics and classical mechanics
    If these definitions are not clearly defined and agreed upon, nothing can be discussed and no science can take place.
  5. The fifth is integer arithmatic. This is primarily used to count objects.

Reflection 4 - The tools we should not use to understand the reality.

The most important tool that primarily should not be used is to understand the physical reality and the evolution of physical processes is: mathematics. It is the other way around. First you should try to understand the physical reality using the steps explained in Reflection 3, and when that is accepted you can improve your understanding by using mathematics. This includes mathematical models, laws, thought experiments and theories.
I expect that many readers will reach their eyebrows. Let me explain:
One of the most important laws is Newton's Law. Which are mathematical equations which describe the trajectories of objects. How ever, and that is the issue, the most important part are accurate observations, measurements, physical assumptions and explanations, which are the basis of these equations and require extra tests if the predictions are correct.
For General Relativity the same applies. If you want to predict the position of the planets, the observations and the measurements, in both cases, initial are the same. However, in GR the equations used to predict the future are different.
A typical example of why mathematics and laws are not important is, when you want to understand an airplane, a glider, a swan, a kite, a hot air balloon or a balloon
In fact if you want to understand why does an airplane fly, you start with a balloon and build one. Blow the balloon up with air, loose the balloon and observe what happens. No mathematics is required. You go up in the list and finally you can explain why an airplane can fly and why initially you need a horizontal speed. In order to understand you don't use laws. In fact when you go up in the list you will see similarities between the different examples, which is the basis for our laws.
When you understand this, you can build a mathematical model of your airplane and measure all the parameters of your airplanes and others involved. First you should test if the actual behaviour of the airplane corresponds with the simulation. In a next phase you can change some parameters, observe the changes in the simulation and see if there is an improvement in performance. If there is, you have a reason to modify your airplane and observe if this actual the case in open air.

Related to waves: It is very difficult to describe actual waves in a mathematical sense. For example, if there are two tsunamis happening in the Atlantic Ocean at the same time. The main problem is that each tsunami is unique.

What this shows is that physical understanding comes first and mathematics second.
In that sense 'para phrasing reflection 2': God created the physical universe. Mankind created mathematics.

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Created: 11 July 2023

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