Bohr's duality paradox 100 years later?

[skewzme]

TL;DR Summary

Although wave-particle duality has served physics well, is the physics community any closer to defining what that actually means 100 years since Bohr's declaration?

Bohr declared it the duality paradox. It works. But a paradox in science is an unresolved problem. Are we any closer to resolving that problem 100 years after Bohr's declaration, or has the physics community just grown numb to it?

 

[Demystifier]

In the meantime physicists developed several logically consistent interpretations of what that might mean, but there is no consensus which of those interpretations (if any) is the right one.

 

[Lord Jestocost]

At the end, there is nothing to resolve. As Paul Davies puts it in his introduction to Werner Heisenberg’s “Physics and Philosophy”:

The question ‘Is an electron a wave or a particle?’ has the same status as the question ‘Is Australia above or below Britain?’ The answer is ‘Neither and both.’ The electron possesses both wave-like and particle-like aspects, either of which can be manifested but neither of which has any meaning in the absence of a specific experimental context. And so the language of quantum mechanics employs familiar words, such as wave, particle, position, etc., but their meanings are severely circumscribed and often vague. Heisenberg warns us that: ‘When this vague and unsystematic use of language leads us into difficulties, the physicist has to withdraw into the mathematical scheme and its unambiguous correlation with experimental facts.’

This is really the bottom line of the argument, for quantum mechanics is, at its core, a mathematical scheme that relates the results of observations in a statistical fashion. And that is all. Any talk of what is ‘really’ going on is just an attempt to infuse the quantum world with a spurious concreteness for ease of imagination. [Emphasis added by LJ]

 

[Demystifier]

I'm surprised that so few physicists ask the following question: Is phonon (the quantum of sound) a wave or a particle?

 

[DrChinese]

1. Bohr declared it the duality paradox. It works. But a paradox in science is an unresolved problem.

2. Are we any closer to resolving that problem 100 years after Bohr's declaration, or has the physics community just grown numb to it?

1. The "paradox" can be better understood - in my opinion - as being a by-product of the Uncertainty Principle. You can constrain quantum particle behavior to be wave-like, particle-like, or a mixture of the two. This really isn't so much a paradox in the view of many.

2. Every physicist cannot work on the same deep questions all the time. Most have specialty areas where they feel their work can make a greater difference. Those that focus on these questions often end up aligning with a so-called interpretation, and attempt to find ways one interpretation might be discernible from another. For example, Demystifier writes frequently in the Bohmian camp.

If Campbell et al's work qualified as an interpretation, they would fit in that category. Around here, the generally accepted interpretations - there is no one source that dictates what those are - are discussed at length.

Given your interest in the area, I would recommend you study those and learn why they originated. Each answers a specific "mystery". And so far, none has added a single useful piece of physics to the equation. That after nearly 100 years of contemplation. So no, I wouldn't say people are "numb" about these subjects; but after an extended period of moving nowhere, you may notice a bloody wall where you have been beating your head.

 

[DrChinese]

I'm surprised that so few physicists ask the following question: Is phonon (the quantum of sound) a wave or a particle?

I was wondering: is it really turtles all the way down?

 

[Demystifier]

I was wondering: is it really turtles all the way down?

I'm not sure what's your question, but I'm sure it's answered in my "Bohmian mechanics for instrumentalists".

 

[DrChinese]

I'm not sure what's your question, but I'm sure it's answered in my "Bohmian mechanics for instrumentalists".

Another of the great scientific mysteries... https://en.wikipedia.org/wiki/Turtles_all_the_way_down

Stephen Hawking said:

A well-known scientist (some say it was Bertrand Russell) once gave a public lecture on astronomy. He described how the Earth orbits around the sun and how the sun, in turn, orbits around the center of a vast collection of stars called our galaxy. At the end of the lecture, a little old lady at the back of the room got up and said: "What you have told us is rubbish. The world is really a flat plate supported on the back of a giant tortoise." The scientist gave a superior smile before replying, "What is the tortoise standing on?" "You're very clever, young man, very clever," said the old lady. "But it's turtles all the way down!"

U.S. Supreme Court Justice Antonin Scalia (apparently a closet physicist as well as legal scholar) said:

In our favored version, an Eastern guru affirms that the Earth is supported on the back of a tiger. When asked what supports the tiger, he says it stands upon an elephant; and when asked what supports the elephant he says it is a giant turtle. When asked, finally, what supports the giant turtle, he is briefly taken aback, but quickly replies "Ah, after that it is turtles all the way down."

For this thread, we can answer that the quantum particle appears to behave as a wave or a particle at different times, and that may be a paradox to some and not a paradox to others. (I guess that forms a paradox too, eh?)

 

[skewzme]

At the end, there is nothing to resolve. As Paul Davies puts it in his introduction to Werner Heisenberg’s “Physics and Philosophy”:

The question ‘Is an electron a wave or a particle?’ has the same status as the question ‘Is Australia above or below Britain?’ The answer is ‘Neither and both.’ The electron possesses both wave-like and particle-like aspects, either of which can be manifested but neither of which has any meaning in the absence of a specific experimental context. And so the language of quantum mechanics employs familiar words, such as wave, particle, position, etc., but their meanings are severely circumscribed and often vague. Heisenberg warns us that: ‘When this vague and unsystematic use of language leads us into difficulties,

I am a fan Paul Davies works, but in this case I take issue with his analogy. Question of whether Britain is up or down from Australia can be answered by simply posing the correct question and that is, from what vantage point? Can a physicist answer the question of whether an electron is a wave or particle from a given vantage pointe? No. Therefore that analogy is not accurate. The purpose of science is to explain things. I submit vague answers simply means The physicist providing such answers has not solved the problem.

 

[skewzme]

Another of the great scientific mysteries... https://en.wikipedia.org/wiki/Turtles_all_the_way_down

Stephen Hawking said:

A well-known scientist (some say it was Bertrand Russell) once gave a public lecture on astronomy. He described how the Earth orbits around the sun and how the sun, in turn, orbits around the center of a vast collection of stars called our galaxy. At the end of the lecture, a little old lady at the back of the room got up and said: "What you have told us is rubbish. The world is really a flat plate supported on the back of a giant tortoise." The scientist gave a superior smile before replying, "What is the tortoise standing on?" "You're very clever, young man, very clever," said the old lady. "But it's turtles all the way down!"

U.S. Supreme Court Justice Antonin Scalia (apparently a closet physicist as well as legal scholar) said:

In our favored version, an Eastern guru affirms that the Earth is supported on the back of a tiger. When asked what supports the tiger, he says it stands upon an elephant; and when asked what supports the elephant he says it is a giant turtle. When asked, finally, what supports the giant turtle, he is briefly taken aback, but quickly replies "Ah, after that it is turtles all the way down."

For this thread, we can answer that the quantum particle appears to behave as a wave or a particle at different times, and that may be a paradox to some and not a paradox to others. (I guess that forms a paradox too, eh?)

I’m not sure I agree that this is a paradox in the truest sense of the word. But to say it is a paradox to some andnot to others, does not relinquish the mystery of the question, which is, do we live in materialistic world or not? Wave properties have been attributed to larger compound elements, and if I understand current scientific theory, or also attributed to full macroscopic objects but the waves are just too short to be measured at that size. Given that, It would seem to me we must acknowledge the true nature of our reality is not one of Newtonian or materialistic view. Is it accurate to say that is the consensus of the physics community in 2019? ( Note, I have edited this response several times because I misread it initially).

 

[phinds]

Although wave-particle duality has served physics well, is the physics community any closer to defining what that actually means 100 years since Bohr's declaration?

Personally, I think the whole thing is best addressed by just calling photons quantum objects(*) and moving on.

* Quantum object = something that is not a particle or a wave but that will show particle characteristics if you measure for particle characteristics and wave characteristics if you measure for wave characteristics.

 

[DrChinese]

Can a physicist answer the question of whether an electron is a wave or particle from a given vantage point? No.

Sure it can, and there are plenty of experiments that "prove" the electron is a particle... or a wave. And the result is dependent on the given vantage point. Just like the question about Australia.

That we don't know things should not be surprising. After thousands of years of pondering, we cannot answer the question of why there is something rather than nothing. Or the question of who/what created the universe.

But I, like many, do not think about it most days. I have more wine to appreciate.

 

[skewzme]

Personally, I think the whole thing is best addressed by just calling photons quantum objects(*) and moving on.

* Quantum object = something is not a particle or a wave but that will show particle characteristics if you measure for particle characteristics and wave characteristics if you measure for wave characteristics.

I kind of agree, and to some extent I think that’s already been done. But it avoids the fundamental question- what makes up the fundamental building blocks of our existence Material? Waves? Vibrations? Etc.

 

[phinds]

I kind of agree, and to some extent I think that’s already been done. But it avoids the fundamental question- what makes up the fundamental building blocks of our existence Material? Waves? Vibrations? Etc.

Why do you insist that it has to be something that is exactly ONE of those things?

 

[skewzme]

Sure it can, and there are plenty of experiments that "prove" the electron is a particle... or a wave. And the result is dependent on the given vantage point. Just like the question about Australia.

That we don't know things should not be surprising. After thousands of years of pondering, we cannot answer the question of why there is something rather than nothing. Or the question of who/what created the universe.

But I, like many, do not think about it most days. I have more wine to appreciate.

I enjoy wine, spirits and many things. One of those many things is questioning these issues, and immersing myself in some sense of wonder that I suspect most don’t appreciate. ;)

 

[skewzme]

Why do you insist that it has to be something that is exactly ONE of those things?

Not insisting anything. That’s why I put the “etc” there.

 

[PeterDonis]

Question of whether Britain is up or down from Australia can be answered by simply posing the correct question and that is, from what vantage point? Can a physicist answer the question of whether an electron is a wave or particle from a given vantage pointe?

Yes, if "vantage point" is taken to mean "experimental context", which seems like an appropriate way to look at the analogy.

 

[DrChinese]

I enjoy [...] and immersing myself in some sense of wonder that I suspect most don’t appreciate. ;)

That is a fantastical assumption, and certainly not characteristic of members here. You joined quite recently. The questions you think are "ignored" here are in fact well considered by PF members - who are also well acquainted with all sides of the physics. You don't think we have discussed these previously and in depth?

phinds, 14000+ posts.
demystifier, 9000+ posts.
I'm the junior guy, 7000+ posts.

You've come here for the physics. Let's talk about that. You want to know about the fundamental building blocks? Quantum Field Theory explains quantum objects as being "excitations" of quantum fields. (Of course, we could label them "quantum objects" or "turtles" rather than "excitations" and it wouldn't change much. )

No one say precisely why QFT is so useful, or why it seems to describe quantum phenomena. But that does not make it any less magical as a theory. Some of us have been fortunate enough to see the incredible revolutions in quantum physics which have occurred in recent decades.

PS PeterDonis weighed in while I was writing this. Sorry I missed your stats, Peter!

PeterDonis, 23000+ posts.

 

[skewzme]

That is a fantastical assumption, and certainly not characteristic of members here. You joined quite recently. The questions you think are "ignored" here are in fact well considered by PF members - who are also well acquainted with all sides of the physics. You don't think we have discussed these previously and in depth?

No assumptions made here. Let me be perfectly clear. I’m not referring to you or anyone on this forum I’m referring to friends and family, who would rather spend an evening watching reruns of Laverne and Shirley that reading a Paul Davies book or joining in a conversation on a forum like this. I recognize messages like this can be misinterpreted in context without the tone of the human voice, but I sense some anger from your post here and from some others who made assumptions that I was challenging anyone in someway. I am not. I am enjoying these conversations thus far. On the contrary, I am flattered that I am able to offer some arguments and points of view in the midsts of real physicists. Further, I am not a supporter or defender Thomas Campbell, for anyone who may have read my initial post and drawn that possible assumption. I have declared myself a skeptic. If I am coming across as someone who’s doing anything in a negative manner here it is being totally misinterpreted and please accept my apologies if I’ve presented anything as such.

 

[DrChinese]

...I sense some anger from your post ... please accept my apologies if I’ve presented anything as such.

No anger, and no apology needed.

You have some pretty experienced and knowledgeable folks here who want to help in any way possible. We love discussing these subjects. My specialty area is more around entanglement. I scan perhaps 1000 papers a year on that subject alone. The entirety of physics research is obviously many many times more, absolutely overwhelming. And it is very interesting: The sheer enormity of things to learn, and the breadth of things being researched at any time.

So my point is that you will find more time best spent familiarizing yourself with the basics so as to have a better foundation. Your commentary indicates you are lacking in that, trying to get to the big stuff before you have the foundation. That said, there is no one right or wrong way to acquire knowledge. But experience shows that some paths tend to be more effective than others. For example: Asking questions is a good way to expedite things; while expressing opinions without a suitable background is a good way to slow development (because you can end up defending positions that are not worthwhile).

 

[skewzme]

Yes, if "vantage point" is taken to mean "experimental context", which seems like an appropriate way to look at the analogy.

Interesting point, but I'm not sure "vantage point" and "experimental context" are one in the same in this analogy. Latitude and longitude, along with the four cardinal cardinal points of north, east, south, and west are prerequisites for determining up and down on a globe. I suppose we can say experimental context is a prerequisite for determining a wave or particle, but it still doesn't address why something can appear to be either, or why it would change based on "experimental context". Does that make sense?. The fundamentals of England's location with respect to Australia remains constant.

 

[skewzme]

Sure it can, and there are plenty of experiments that "prove" the electron is a particle... or a wave. And the result is dependent on the given vantage point. Just like the question about Australia.
/QUOTE]
Interesting. I need to digest that a bit. Thanks

 

[PeroK]

Interesting point, but I'm not sure "vantage point" and "experimental context" are one in the same in this analogy. Latitude and longitude, along with the four cardinal cardinal points of north, east, south, and west are prerequisites for determining up and down on a globe. I suppose we can say experimental context is a prerequisite for determining a wave or particle, but it still doesn't address why something can appear to be either, or why it would change based on "experimental context". Does that make sense?. The fundamentals of England's location with respect to Australia remains constant.

An electron doesn't change. An electron is an electron. We may choose to call one aspect of its behaviour "wave-like" and we may choose to call another aspect of its behaviour "particle-like".

Until the 1930's this was a puzzle because there was no theory that could explain both wave-like behaviour and particle like behaviour. Then Quantum Mechanics came along with the quantum model of an electron, which explained both wave-like and particle-like behaviour.

One of the most popular undergraduate textbooks on QM is by Griffiths. Wave-particle duality is mentioned once, as a historical footnote.

In the other QM textbook I have, by JJ Sakurai, it doesn't get a mention.

But, for some reason, the popular science writers and science journalists keep the mystique alive and give the impression that it's still a mystery.

That in many ways exemplifies the difference between physics as a academic subject as taught in universities and physics as popular science.

 

[PeterDonis]

I suppose we can say experimental context is a prerequisite for determining a wave or particle

Yes: quantum objects behave like waves in certain experimental contexts and like particles in others.

it still doesn't address why something can appear to be either, or why it would change based on "experimental context".

That's just because your intuition is not familiar with how quantum objects behave, whereas it is familiar with latitude and longitude and cardinal points on a globe. The fix for that is to retrain your intuition.

 

[PeterDonis]

for some reason, the popular science writers and science journalists keep the mystique alive and give the impression that it's still a mystery

It's a lot easier to sell books and attract readers to articles by saying something is a mystery, than by saying it's all just ordinary humdrum physics.

 

[Nugatory]

Question of whether Britain is up or down from Australia can be answered by simply posing the correct question and that is, from what vantage point? Can a physicist answer the question of whether an electron is a wave or particle from a given vantage pointe? I submit vague answers simply means The physicist providing such answers has not solved the problem.

The vague answer is because the question not posed correctly, just as in the Australia/Britain up/down case.

The words “particle” and “wave” were introduced many years before the theory that we now call quantum mechanics was hammered out in the late 1920s, while it was still mistakenly (although understandably) assumed that those concepts would explain the surprising non-classical phenomena that were being observed at the beginning of the 20th century.

So there’s no clear answer to the question about whether an electron is a particle or a wave for the same reason that there’s no clear answer to the question of whether a sheep is more like a pillow (because it is soft and fuzzy) or a table (because it has four legs). If you want a clear answer about sheep you should ask what a sheep is instead of investigating the “paradox” of a sheep’s table-like and pillow-like behavior, and if you want a clear answer about electrons you should ask what the theory says about electrons instead of demanding that they be explained in terms of waves and particles. The difference is that we’re all familiar with sheep while many fewer people have put in the (seriously non-trivial) intellectual effort required to understand the behavior of electrons without recourse to familiar notions of waves and particles.

Two posts above to which you should pay particular attention:
1) @PeroK #23 where he points out that wave-particle duality doesn’t appear in modern textbooks (and the same is true of the QM texts I studied 40 years ago, so this is not exactly a new development). This should suggest to you that your understanding of what QM really says about electrons is not based on completely reliable sources.
2) @Demystifier #4 where he points out that we feel no need to ask whether phonons are waves or particles. There’s a serious question there for you: why do YOU believe that discussing electrons as waves or particles is necessary, but not phonons? There’s a historical reason why we do: electrons were known and discussed before the wave-particle duality notion was abandoned whereas phonons were not; but that tends to suggest that the wave-particle question is indeed badly posed.

 

[Demystifier]

There’s a serious question there for you: why do YOU believe that discussing electrons as waves or particles is necessary, but not phonons?

Well, I think that both should be discussed on an equal footing. See the paper linked in my signature below.

 

[skewzme]

Well, I think that both should be discussed on an equal footing. See the paper linked in my signature below.

And if the question of waves or particles with respect to electrons has been settled for decades ( which seems to be the consensus I've gathered here ), why would it need to be discussed for phonons?

 

[Demystifier]

And if the question of waves or particles with respect to electrons has been settled for decades ( which seems to be the consensus I've gathered here ), why would it need to be discussed for phonons?

Because, given the existence of Bohmian interpretation of quantum mechanics, the question has not been settled.

 

[skewzme]

Let me clarify: With respect to phonons only?

 

[Demystifier]

Let me clarify: With respect to phonons only?

What's your question?

 

[skewzme]

My original question was if the question of waves or particles with respect to electrons has been settled, why would it need to be discussed for phonons? You replied that it has NOT been settled.
I am asking you now to clarify if you mean that specific to phonons only. Others here seem to be saying it HAS been settled with respect to electrons.

 

[Demystifier]

My original question was if the question of waves or particles with respect to electrons has been settled, why would it need to be discussed for phonons? You replied that it has NOT been settled.
I am asking you now to clarify if you mean that specific to phonons only. Others here seem to be saying it HAS been settled with respect to electrons.

I say that it is equally unsettled for electrons and phonons.

 

[DrChinese]

My original question was if the question of waves or particles with respect to electrons has been settled, why would it need to be discussed for phonons? You replied that it has NOT been settled.
I am asking you now to clarify if you mean that specific to phonons only. Others here seem to be saying it HAS been settled with respect to electrons.

Again, different interpretations make different statements about fundamental elements of QM. From Demystifier's Bohmian viewpoint, an electron is a quantum particle with definite position at all times. Most other interpretations won't agree on that.

 

[skewzme]

Thank you for clarifying.
Why then (if I may continue this line of questioning), is this unsettled subject only a footnote in the academic community and it's textbooks? I am not interested in the woo woo propagations of the many who push it to sell books. But as a lay person interested in the subject, it does seem to me a VERY significant point if it is unanswered or still not understood. It goes to the very heart of whether our world is materialistic in nature. Or something else. Is that a fair assertion?