# I Why does "wave particle duality" not exist anymore?

1. Apr 8, 2017

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?

2. Apr 8, 2017

### 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).

3. Apr 8, 2017

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.

4. Apr 8, 2017

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.

5. Apr 8, 2017

### SlowThinker

An electron certainly is a particle, in the sense that you'll never see a half-electron. Lets 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.

6. Apr 8, 2017

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

7. Apr 8, 2017

### vanhees71

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.

8. Apr 8, 2017

### vanhees71

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?

9. Apr 8, 2017

10. Apr 8, 2017

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 modelled as waves. For example do we not use the concept of de Broglie wavelength when calculating the resolving power of an electron microscope?

11. Apr 8, 2017

### 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.

12. Apr 8, 2017

### vanhees71

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

13. Apr 8, 2017

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.

14. Apr 8, 2017

### phinds

15. Apr 8, 2017

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.

16. Apr 8, 2017

### vanhees71

The point is that the "wave-like properties" of quantum "particals" 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.

17. Apr 8, 2017

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.

18. Apr 8, 2017

### Blue Scallop

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.

19. Apr 9, 2017

### 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.

Last edited: Apr 9, 2017
20. Apr 9, 2017

### vanhees71

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".