Why does "wave particle duality" not exist anymore?

[Dadface]

I think I've asked this question here before but I'm still not clear about the answers.

To reiterate, some experts on this forum seem to be of the opinion that wave particle duality doesn't exist anymore and this has been expressed in different threads and in different ways with certain explanations given. So far so good until, that is, one starts searching for more information.
Google comes up with page after page of articles on wave particle duality, some from very prestigious journals and organisations. So if wave particle duality doesn't exist why is there so much information about it?
I think the problem is that there is not a generally accepted definition of what wave particle duality means so please let me explain what I think duality is about. In a nutshell I think that duality simply expresses the experimentally observed facts that different situations can result in different observations. It's glaringly obvious.
Take, for example, an electron. Certain observations made in certain situations can be interpreted as electrons having some characteristics of particles. Observations made in certain other situations can be interpreted as electrons having some characteristics of waves. That's all there is to it and that along with some relevant experiments and the observations made is what is taught in UK schools. So what doesn't exist anymore?

 

[vanhees71]

There is a lot of inertia in teaching QT. Although more than 90 years old, many newly written textbooks just copy the heuristics from the very early days. They start with "wave-particle duality", claim that the photo effect a la Einstein proves the necessity to quantize the electromagnetic field, claim that photons have anything to do with classical particles, dwell on the Bohr-Sommerfeld quantization of the hydrogen atom, cementing utterly wrong qualitative pictures in the scholar's mind, which he or she has to unlearn just a moment later, when modern QT is finally taught, as it should have been taught from the very beginning.

In the quantum realm electrons are described neither as classical particles nor as classical waves but with modern quantum theory. All the intrinsic inconsistencies of old quantum theory are resolved by Born's probabilistic interpretation of the quantum state. In non-relativistic QT you can work in the "wave-mechanics picture", i.e., in the position representation, and the Schrödinger equation indeed leads to wave-like solutions. However that doesn't mean that particles have a wave-like nature, but the modulus squared wave function, evaluated by solving the Schrödinger equation is the probability distribution for the position of the electron. You always register each single electron at some "spot" (a point within the always finite resolution of the detector) not as a somehow smeared thing like a classical charge distribution. Born's interpretation, however, also implies that electrons are also not correctly described as classical particles. Particularly a description as a trajectory in phase space doesn't make sense due to the uncertainty relation $\Delta x \Delta p \geq \hbar/2$: if you localize the electron in a narrow region (small $\Delta x$) the uncertainty (standard deviation) of momentum is pretty large (large $\Delta p$).

So the only correct description of an electron is quantum theory itself, and according to quantum theory it's neither a classical particle nor a classical field (or wave).

 

[Dadface]

Thank you Vanhees but according to my understanding the concept of duality is related to observations namely that there are some situations where, for example, where electrons can be described as having particle attributes and there are other situations where electrons can be described as wave attributes. Any description of what an electron really is goes beyond the concept.

 

[Dadface]

I have done another search and found what seems to be an excellent blog article:

SCIENTIFIC EXPLORER...What is an Electron Really. It is 14 pages long and at the moment I have only scanned it. A few things jump out:

1. The electron is not a physical particle
2. Electrons are Fermions they are spin half particles
3. The electron like all other particles is a wave function

So it seems that electrons are not particles but they are particles
The last comment suggests the electron is a particle and a wave function. Does that mean it's a particle and a mathematical equation?

I hope I'm not criticising the article itself because it looks excellent and I'm going to read through it properly. I'm just pointing out an example of why it can be easy to get confused.

 

[SlowThinker]

An electron certainly is a particle, in the sense that you'll never see a half-electron. Let's call it a quantum particle.
Sometimes it behaves more like a classical particle, sometimes it behaves more like a wave, but really it behaves as a quantum particle all the time.
The important part is that it never behaves exactly like a classical particle or a wave.

 

[Dadface]

An electron certainly is a particle, in the sense that you'll never see a half-electron. Let's call it a quantum particle.
Sometimes it behaves more like a classical particle, sometimes it behaves more like a wave, but really it behaves as a quantum particle all the time.
The important part is that it never behaves exactly like a classical particle or a wave.

Thank you that's a good example of what I thought wave particle duality to be.

 

[vanhees71]

Thank you Vanhees but according to my understanding the concept of duality is related to observations namely that there are some situations where, for example, where electrons can be described as having particle attributes and there are other situations where electrons can be described as wave attributes. Any description of what an electron really is goes beyond the concept.

This is misleading. I'd rather say an electron is neither a classical particle nor a classical wave field, but described by quantum theory. Particularly if it comes to many-electron systems you cannot use the classical concepts anymore or only in specific circumstances as approximations.

 

[vanhees71]

I have done another search and found what seems to be an excellent blog article:

SCIENTIFIC EXPLORER...What is an Electron Really. It is 14 pages long and at the moment I have only scanned it. A few things jump out:

1. The electron is not a physical particle
2. Electrons are Fermions they are spin half particles
3. The electron like all other particles is a wave function

So it seems that electrons are not particles but they are particles
The last comment suggests the electron is a particle and a wave function. Does that mean it's a particle and a mathematical equation?

I hope I'm not criticising the article itself because it looks excellent and I'm going to read through it properly. I'm just pointing out an example of why it can be easy to get confused.

Here, I can only agree with 2. Indeed an electron is a spin-1/2 particle, described by a quantized Dirac field (or in non-relativistic approximation by a Pauli spinor). 1. is not true either without further explanation. For sure wrong is 3. since identifying the single electron with the wave function is clearly disproven by experiment. You detect an electron on a single spot rather than as a smeared-out charge distribution. What behaves like a wave are the probability amplitudes, which are represented by the wave function.

Do you have a link to the complete article?

 

[Dadface]

http://sciexplorer.blogspot.co.uk/2014/08/what-is-electron-really.html
This is the complete paper.

 

[Dadface]

This is misleading. I'd rather say an electron is neither a classical particle nor a classical wave field, but described by quantum theory. Particularly if it comes to many-electron systems you cannot use the classical concepts anymore or only in specific circumstances as approximations.

I think I agree with this but my understanding of duality relates not to theories about what entities such as electrons really are but to experiments and what can actually be observed. Electrons may not be wave fields but there are situations when they appear to have attributes that can be modeled as waves. For example do we not use the concept of de Broglie wavelength when calculating the resolving power of an electron microscope?

 

[phinds]

@Dadface, why do you have any problem with the starndard modern concept (the one that is NOT being carried forward from past errors) that quantum objects are exactly that. They are not particle and they are not waves and there is no wave-particle duality, there are just quantum objects. These quantum objects exhibit wave-like properties if you measure for wave like properties and they exhibit particle-like properties if you measure for particle-like properties, but neither of those facts makes them a wave or a particle, they are just quantum objects.

 

[vanhees71]

I think I agree with this but my understanding of duality relates not to theories about what entities such as electrons really are but to experiments and what can actually be observed. Electrons may not be wave fields but there are situations when they appear to have attributes that can be modeled as waves. For example do we not use the concept of de Broglie wavelength when calculating the resolving power of an electron microscope?

Can you quote an experiment, where one (and only one!) electron behaves like a classical wave or like a classical particle?

 

[Dadface]

@Dadface, why do you have any problem with the starndard modern concept (the one that is NOT being carried forward from past errors) that quantum objects are exactly that. They are not particle and they are not waves and there is no wave-particle duality, there are just quantum objects. These quantum objects exhibit wave-like properties if you measure for wave like properties and they exhibit particle-like properties if you measure for particle-like properties, but neither of those facts makes them a wave or a particle, they are just quantum objects.

Thank you for your comment phinds but please read my posts again. I have not said that I have problems with the standard modern concept. and I have not suggested that things are either waves or particles.

 

[phinds]

Thank you for your comment phinds but please read my posts again. I have not said that I have problems with the standard modern concept. and I have not suggested that things are either waves or particles.

OK, my mistake for reading your posts too quickly. Sorry.

 

[Dadface]

Can you quote an experiment, where one (and only one!) electron behaves like a classical wave or like a classical particle?

Why only one electron? Is the electron microscope not a good example of such an experiment? If you insist on just one electron would the following be acceptable:
Each electron in the collection of electrons needed to form the image in an electron microscope plays its part in the degradation of the image quality due to diffraction effects. That's may be a load of rubbish but I need time to think about it. Experiments that come immediately to mind are those which measure electron properties such as mass spectrometers to measure electron mass or Millikan type experiments to measure electron charge.

 

[vanhees71]

The point is that the "wave-like properties" of quantum "particles" always refer to many electrons. With one electron, you'd not be able to get a picture from an electron microscope. What's "waving" is the probability amplitude, i.e., $|\psi(\vec{x})|^2$ is the probability distribution for the position of the electron, not the electron in the sense of some classical-field description.

 

[Dadface]

The point is that the "wave-like properties" of quantum "particles" always refer to many electrons. With one electron, you'd not be able to get a picture from an electron microscope. What's "waving" is the probability amplitude, i.e., $|\psi(\vec{x})|^2$ is the probability distribution for the position of the electron, not the electron in the sense of some classical-field description.

I think we're talking at cross purposes here. I can agree that classical physics is incapable explaining the observations and quantum physics is brilliant at explaining the observations. But I see the concept of wave particle duality as being primarily about the observations themselves and not the explanations of the observations ... the fact that there are certain experimental set ups that seemingly demonstrate one type of behaviour and other experimental set ups that seemingly demonstrate a different type of behaviour.

 

[Blue Scallop]

The point is that the "wave-like properties" of quantum "particles" always refer to many electrons. With one electron, you'd not be able to get a picture from an electron microscope. What's "waving" is the probability amplitude, i.e., $|\psi(\vec{x})|^2$ is the probability distribution for the position of the electron, not the electron in the sense of some classical-field description.

In MWI, where there is no probability amplitude, what's waving?

In Bohmian, where the particle is localized.. can't we say particle is a wave?

Remember it is only in the orthodox where Born Rule is built-in.. hence what's waving is the probability amplitude.

 

[kith]

For me, thinking in terms of the wave-particle duality has some merits in certain situations but one needs to realize that it misses a lot.

First, it doesn't incorporate essential quantum phenomena like entanglement and indistinguishability. Second, it clouds the importance of probability. It is a basic fact of QM that for the outcomes of many experiments, we can only predict probabilities. This fact isn't challenged by interpretations like dBB or MWI.

In order to understand the different positions in this thread, maybe the analogy with acoustics is helpful. The wave-particle duality is related to the fact that in QM, position and momentum form a Fourier Transform pair. In acoustics, the corresponding quantities are time and frequency. The "wave" then corresponds to a pure tone (i.e. a sine wave of definite frequency and infinite duration) and the "particle" corresponds to an idealized acoustic impulse at a definite time and with zero duration (so something like an idealization of a hit on a drum).

Now real music is composed of notes of approximately constant frequencies but finite durations. So @vanhees71 can rightly object that the "wave"-"particle" duality misses all the music, while @Dadface may rightly point out that sometimes, using the "wave"-picture as an approximation for what happens during single notes may be a good enough approximation.

 

[vanhees71]

I think we're talking at cross purposes here. I can agree that classical physics is incapable explaining the observations and quantum physics is brilliant at explaining the observations. But I see the concept of wave particle duality as being primarily about the observations themselves and not the explanations of the observations ... the fact that there are certain experimental set ups that seemingly demonstrate one type of behaviour and other experimental set ups that seemingly demonstrate a different type of behaviour.

Then we disagree. For me the great breakthrough of modern QT in 1925/26 is that it got rid of a completely inconsistent concept (the so-called "old quantum theory"), particularly "wave-particle dualism", which is intrinsically contradictory in itself. The problem to learn QT is not so much the mathematics but to get rid of "classical prejudices", and if you are not willing to give up wave-particle dualism, you'll never get rid of your "classical prejudices".

 

[vanhees71]

In MWI, where there is no probability amplitude, what's waving?

In Bohmian, where the particle is localized.. can't we say particle is a wave?

Remember it is only in the orthodox where Born Rule is built-in.. hence what's waving is the probability amplitude.

I don't consider MWI and even less de Broglie Bohm as very helpful concepts to understand QT. I'm following the pure physics content, called "minimal statistical interpretation".

 

[Dadface]

Then we disagree. For me the great breakthrough of modern QT in 1925/26 is that it got rid of a completely inconsistent concept (the so-called "old quantum theory"), particularly "wave-particle dualism", which is intrinsically contradictory in itself. The problem to learn QT is not so much the mathematics but to get rid of "classical prejudices", and if you are not willing to give up wave-particle dualism, you'll never get rid of your "classical prejudices".

I've written several posts here vanhees71 and I think you still misunderstand the point I'm trying to make. I guess it's my fault and I'm probably not expressing myself clearly enough. I don't disagree with you at all...things move on theories can get replaced by better theories. I accept all that. Modern QT is better than old QT and I don't dispute that. I can't dispute it because I don't know enough about it yet to criticize it. And I certainly don't have "classical prejudices". Where did that idea come from?

I think any disagreement centres around the definition of what duality actually is. My understanding of duality relates to the experimental and to observational aspects not to the theoretical aspects. Please take another look at my posts. And please let me know what your definition of duality is.

Can I ask you to consider a few things:
1. Why do you think there's so much information out there on duality and mainly from prestigious organisations, not crackpot organisations?
2. What advice would you give to the UK examination boards who require that high school students be taught topics such as the photo electric effect along with Einsteins explanation of it?
3. Was de Broglie wrong and can we not use his equation in certain practical applications?

 

[Dadface]

OK, my mistake for reading your posts too quickly. Sorry.

I'm always doing that. I tend to scan rather than read properly.

 

[LeandroMdO]

You shouldn't think in terms of right or wrong. You should think in terms of what is fruitful for our understanding. Wave-particle duality used to be considered a useful thing to talk about, but it's fallen out of favor. Same for relativistic mass. Of course, "fruitful" is entirely subjective, but if you want to understand why physicists tend to avoid these concepts these days, that's the angle you should consider.

 

[vanhees71]

I've written several posts here vanhees71 and I think you still misunderstand the point I'm trying to make. I guess it's my fault and I'm probably not expressing myself clearly enough. I don't disagree with you at all...things move on theories can get replaced by better theories. I accept all that. Modern QT is better than old QT and I don't dispute that. I can't dispute it because I don't know enough about it yet to criticize it. And I certainly don't have "classical prejudices". Where did that idea come from?

I think any disagreement centres around the definition of what duality actually is. My understanding of duality relates to the experimental and to observational aspects not to the theoretical aspects. Please take another look at my posts. And please let me know what your definition of duality is.

Can I ask you to consider a few things:
1. Why do you think there's so much information out there on duality and mainly from prestigious organisations, not crackpot organisations?
2. What advice would you give to the UK examination boards who require that high school students be taught topics such as the photo electric effect along with Einsteins explanation of it?
3. Was de Broglie wrong and can we not use his equation in certain practical applications?

Again: The idea behind wave-particle duality was to explain quantum phenomena (observable facts!) with classical means. The very people who brought up these ideas (Einstein, Bohr, de Broglie, Sommerfeld et al) were very much aware of the fact that it was not a complete and consistent picture. There is no observed fact which in any way is consistent with wave-particle duality, and due to that modern QT has been discovered. To answer your questions:

1. I'm not sure which "organizations" you refer to. If you mean research institutes and universities, I guess you mean things they do for outreach to the public. As I said, physics didactics is plagued with a lot of inertia, and they tend to copy old textbooks and in the necessary attempt to simplify the complicated true picture for the public they use these old concepts rather than thinking about new ones.

2. That's a dilemma we also have in Germany: Here in the high schools they also teach the old concepts, and you cannot deviate from it, because the final exams ("Abitur") are centralized in most of the federal states (in Germany you have different curricula in each of the 16 federal states!). So as a teacher you cannot do anything, even if you want to, and have to teach your pupils these outdated stuff.

3. De Broglie's PhD thesis was a very important step forward to the discovery of modern QT, and I do not want to diminish has or any other's achievements in the discovery of modern QT, but it's the very nature of the natural sciences that any model or theory can get outdated by new discoveries. It may well be that one day something is discovered that makes modern QT as obsolete as old QT is considered today. Then we have to learn and teach the new stuff.

Of course, one should also be aware about "history of science" and the development of theory and experiment to gain an understanding about the most recent point of view, but one should not learn the old stuff first and then unlearn it again to substitute is by the new ideas. One should teach "old quantum theory" as "history of science" and not abuse it as "heuristics" for modern QT.

 

[atyy]

Alain Aspect - From Einstein to Wheeler: wave particle duality for a photon

 

[Dadface]

Again: The idea behind wave-particle duality was to explain quantum phenomena (observable facts!) with classical means. The very people who brought up these ideas (Einstein, Bohr, de Broglie, Sommerfeld et al) were very much aware of the fact that it was not a complete and consistent picture. There is no observed fact which in any way is consistent with wave-particle duality, and due to that modern QT has been discovered. To answer your questions:

1. I'm not sure which "organizations" you refer to. If you mean research institutes and universities, I guess you mean things they do for outreach to the public. As I said, physics didactics is plagued with a lot of inertia, and they tend to copy old textbooks and in the necessary attempt to simplify the complicated true picture for the public they use these old concepts rather than thinking about new ones.

2. That's a dilemma we also have in Germany: Here in the high schools they also teach the old concepts, and you cannot deviate from it, because the final exams ("Abitur") are centralized in most of the federal states (in Germany you have different curricula in each of the 16 federal states!). So as a teacher you cannot do anything, even if you want to, and have to teach your pupils these outdated stuff.

3. De Broglie's PhD thesis was a very important step forward to the discovery of modern QT, and I do not want to diminish has or any other's achievements in the discovery of modern QT, but it's the very nature of the natural sciences that any model or theory can get outdated by new discoveries. It may well be that one day something is discovered that makes modern QT as obsolete as old QT is considered today. Then we have to learn and teach the new stuff.

Of course, one should also be aware about "history of science" and the development of theory and experiment to gain an understanding about the most recent point of view, but one should not learn the old stuff first and then unlearn it again to substitute is by the new ideas. One should teach "old quantum theory" as "history of science" and not abuse it as "heuristics" for modern QT.

http://www.nature.com/search?q=wave...ld=subject|ujournal&sp-x-1=ujournal&submit=go

Thank you,
Your first paragraph seems to show where the "disagreement" is. My understanding of what is meant by duality refers to the observed phenomena but your understanding takes it a step further to include, history... the failed classical attempts to provide detailed explanations of those phenomena. It boils down to semantics and definitions.
Whatever physics we use, be it the new QM or any future more powerful theory that may replace it we will, according to my definition,still have duality because we can still make the same observations.
I think we still have wave particle duality because it is an expression that is so widely referred to. Try googling and see for yourself. Try the link I have included above and search wave particle duality. When you do so a list of 323 papers comes up several of which have wave particle duality in the title of the work.

 

[Dadface]

Alain Aspect - From Einstein to Wheeler: wave particle duality for a photon

Thank you very much Atty. I'm finding it tricky because I can't get the volume high enough and that, coupled with Aspect's accent, makes it very heavy going.

 

[Blue Scallop]

I know the common reason for wave particle duality no longer valid is due to the probability interpretation or squaring the amplitude.. I'll share this to a high school class.. but then someone can easily state Bohmian mechanics doesn't have the probability intepretation.. therefore what is the best reason to state why wave particle duality is no longer true.. like what is a best choice of words than saying "because of the probability interpretation".. like maybe "because the wave packet spreads"? Please suggest the best punchline words. Thank you!

 

[phinds]

I know the common reason for wave particle duality no longer valid is due to the probability interpretation or squaring the amplitude.. I'll share this to a high school class.. but then someone can easily state Bohmian mechanics doesn't have the probability intepretation.. therefore what is the best reason to state why wave particle duality is no longer true.. like what is a best choice of words than saying "because of the probability interpretation".. like maybe "because the wave packet spreads"? Please suggest the best punchline words. Thank you!

Because photons, for example, are NOT waves and they are NOT particles. They are quantum objects. That's really all there is to it.

 

[Blue Scallop]

Because photons, for example, are NOT waves and they are NOT particles. They are quantum objects. That's really all there is to it.

For photons with lots of energy and smaller wave length.. does it mean the energy is stored in a smaller region of probability? how is the energy stored?

 

[Mentz114]

For photons with lots of energy and smaller wave length.. does it mean the energy is stored in a smaller region of probability? how is the energy stored?

Photons are not localizable so there is no formal 'region of probability' for the position.

Photon detectors react to the electric field component $\vec{E}$ of the photon. The probability of detection is proportional to $E^2$.
This is a simplification, naturally.

 

[f95toli]

Again: The idea behind wave-particle duality was to explain quantum phenomena (observable facts!) with classical means. The very people who brought up these ideas (Einstein, Bohr, de Broglie, Sommerfeld et al) were very much aware of the fact that it was not a complete and consistent picture. There is no observed fact which in any way is consistent with wave-particle duality, and due to that modern QT has been discovered. To answer your questions:

I believe you are missing the point a bit. The reason why some people (especially experimentalists, including me) still use the "particle-wave duality concept" when describing and thinking about experiments is NOT because we think this somehow describes reality, but because it is often a good "handwavy" way of modelling/explaining results; a good example would say be experiments on electron quantum optics where we frequently think of single electrons as "waves".
To me the the duality is a bit like the Bloch sphere; it is a useful way of thinking of certain situations, but electrons are no more particles/waves than spin 1/2 systems (in my case qubits) are arrows in a sphere.

 

[vanhees71]

That's fine. You have some qualitative intuition how waves behave, and the Schrödinger equation is an equation leading to waves. You shouldn't however call this wave-particle duality anymore. The meaning of the waves is given by Born's rule, i.e., by the probabilistic interpretation.

For the Bloch sphere you don't run into such heuristical quibbles because spin 1/2 is specifically quantum theoretical without any reference to (false or at least inaccurate) classical heuristics!

The main problem in learning QT is not in the mathematical formalism but to develop an intuition for phenomena we have no every-day experience for.

 

[Dadface]

The assumption that the expression "wave particle duality" should not be used any more depends on ones definition of the expression. To me it seems that in the wider world of physics outside of this forum the expression is still widely used including, it seems, by giants in the world of quantum optics such as Zeilinger and Aspect. has anyone told them that they should not be using the expression?
I think one way to get everyone on the same page would be to reach a decision on how "wave particle duality" should be defined and perhaps a good start on that would be to clarify any definitions of "waves" and definitions of "particles".