# B Bohr's duality paradox 100 years later?

#### PeroK

Homework Helper
Gold Member
2018 Award
I do understand that nature is inherently probabilistic, and that measurable quantities do not have well-defined values all the time.
Having said that, from my amateur point of view ( and I don't mean that facetiously), how does your reply address the fundamental question of whether elementary particles are physical, material objects or waves?
Let me finish with one example to make you analyse what you mean by physical material object.

Take an electron and a proton. Each has a certain mass. Let's assume that they really are physical material objects and the mass, of course, represents the amount of physical material in each.

Now you put the two together to form a hydrogen atom. The mass of the hydrogen atom must be the sum of its parts.

But, actually, it's not. It is less than the mass of the particles that make it up. It is in fact less by the amount of binding energy in the atom.

Now this doesn't prove that particles aren't particles. But it does show that a simplistic requirement that particles be well-defined material things is misplaced.

It forces you to reconsider the concept of particles, mass and the nature of physical quantities.

How much of your mass is the particles that make you up and how much is binding energy or other nuclear energy? Is nuclear energy material too?

The irony of one of your previous questions is that 20tb century scientists did not sweep anything aside. Instead, they looked in every last nook and cranny to establish as wide and deep an understanding of nature as possible. They discovered more things than were ever imagined in any discussion about wave-particle duality.

#### zonde

Gold Member
Take an electron and a proton. Each has a certain mass. Let's assume that they really are physical material objects and the mass, of course, represents the amount of physical material in each.

Now you put the two together to form a hydrogen atom. The mass of the hydrogen atom must be the sum of its parts.

But, actually, it's not. It is less than the mass of the particles that make it up. It is in fact less by the amount of binding energy in the atom.

Now this doesn't prove that particles aren't particles. But it does show that a simplistic requirement that particles be well-defined material things is misplaced.

It forces you to reconsider the concept of particles, mass and the nature of physical quantities.

How much of your mass is the particles that make you up and how much is binding energy or other nuclear energy? Is nuclear energy material too?
I think that you just described one thing that actually is swept under the carpet - that particles do not seem to "contain" the mass.

#### skewzme

As I understand Bohr with his complementarity principle meant that reality is somehow fundamentally fuzzy and this should be accepted as inevitable property of reality. But then it seems there is nothing much to discuss:
- either you accept this Bohr's philosophical position that reality is fuzzy and then it means you believe that wave-particle duality paradox can not be resolved
- or you do not accept it and in this case you look for some not-so-fuzzy description of reality and there definitely are some option to chose from.
So I would say that wave-particle duality is not really a paradox (it is a paradox only if you believe it is a paradox).
As I said previously, I do not consider the question a paradox in the truest sense of the word.
With respect to the not-so-fuzzy options to chose from, is there any kind of consensus or majority accepted option at this point in time? Or, is a fuzzy description the more agreed upon option at this stage in our scientific understandings?

#### Nugatory

Mentor
I think that you just described one thing that actually is swept under the carpet - that particles do not seem to "contain" the mass.
How is that being swept under the carpet? If we had a dollar for every time that we pointed out that total energy is a property of a system as a whole and not attached to the individual components Greg would be able to pay us mentors.

There is a big difference between "not fully covered in elementary texts and totally misdescribed by popularizations" and "unsolved problem that's being ignored because it's under the carpet".

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#### Nugatory

Mentor
With respect to the not-so-fuzzy options to chose from, is there any kind of consensus or majority accepted option at this point in time?
Quantum mechanics, as covered at the level of for example Ballentine, can reasonably be considered majority-accepted for purposes of a B-level thread. That's not saying that Ballentine is a B-level reference - it's not - nor that it is free of controversy, just that you need that level of understanding to take on the stuff that's not yet settled.

#### skewzme

Quantum mechanics, as covered at the level of for example Ballentine, can reasonably be considered majority-accepted for purposes of a B-level thread. That's not saying that Ballentine is a B-level reference - it's not - nor that it is free of controversy, just that you need that level of understanding to take on the stuff that's not yet settled.
Appreciate that reference. I intend to read it.

#### skewzme

Why did the electron cross the slit undetected?

So he could "wave" from the other side . . .

#### A. Neumaier

I'm surprised that so few physicists ask the following question: Is phonon (the quantum of sound) a wave or a particle?
Because they know that it is a quasiparticle.

#### Demystifier

2018 Award
Because they know that it is a quasiparticle.
But photon is also a "quasiparticle", in the sense that photon is not a fundamental object in QED, but a collective excitation of EM field at all spatial points. The similarity between phonon and photon is particularly manifest if one considers QED on a lattice.

Moreover, in principle one could do a two-slit experiment with a single phonon followed by the phonon position detection (such experiment has not yet been done, but it is possible in principle), in which case the question whether the phonon is wave or particle would make perfect sense.

#### A. Neumaier

But photon is also a "quasiparticle", in the sense that photon is not a fundamental object in QED
A free (asymptotic) photon, unlike a phonon, is an asymptotic bound state of a QFT (here of QED), in addition to being an elementary excitation of the electromagnetic field.

Thus in solid state physics, a phonon is not a fundamental object while in QED, a photon is fundamental.

Only its particle status is questionable. Unlike free photons, photons in glass are quasiparticles traveling at a speed much lower than the vacuum speed of light (something impossible for a QED photon). Photons in air (with which standard experiments are done) are, strictly speaking, also quasiparticles, but with properties are very close to those of a free photon. When light passes through a prism placed in a vacuum, the nature of the photon changes upon entering and leaving the prism from being a true asymptotic particle to being a quasiparticle to being again a true particle.

This shows that the particleness of an excitation is to 100% a convenient fiction, to be taken seriously only in a figurative sense.

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#### Demystifier

2018 Award
A free (asymptotic) photon, unlike a phonon, is an asymptotic bound state of a QFT (here of QED), hence an elementary excitation of the electromagnetic field.

Thus in solid state physics, a phonon is not a fundamental object while in QED, a photon is fundamental.
As long as photon is an elementary excitation of the EM field, a phonon is an elementary excitation of a crystal lattice. See https://www.amazon.com/dp/0738201154/?tag=pfamazon01-20

#### zonde

Gold Member
How is that being swept under the carpet? If we had a dollar for every time that we pointed out that total energy is a property of a system as a whole and not attached to the individual components Greg would be able to pay us mentors.

There is a big difference between "not fully covered in elementary texts and totally misdescribed by popularizations" and "unsolved problem that's being ignored because it's under the carpet".
So total energy is property of the system. So what? The problem is inertia of the system vs inertia of all the parts of the system (plus kinetic energy within the system) - they do not add up to the same value, it comes out bigger (if it would be smaller we could speculate that we missed some invisible part of the system, but it's bigger).
And you can't claim that parts of the system don't have inertia. Otherwise it would mean for example that you have no meaningful inertia because you are gravitationally bound to earth.

#### vanhees71

Gold Member
Anything containing energy, pressure, and stress has "inertia" (and thus within general relativity is a source of gravity, aka space-time curvature). That's the true meaning of the most misexplained formula of physics, $E=m c^2$. Einstein, of course, got it right right away in 1905. Then the theory got deformed by well-meaning popularizers (well-meaning usually is sloppy).

#### bhobba

Mentor
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?
There is actually no such things as the wave particle duality, at least in the way it usually presented. Sometimes it acts like a particle, and sometimes like wave but really it's neither and most of the time it acts like neither. What is the real foundation of QM - read:

Formally we know exactly what it is - its what is known as a generalized probability model:

Exactly what makes QM special - some think its entanglement:

There are issues such as since QM is a theory about observations that occur here in the classical world, and since QM is meant to explain that world its a bit tricky to resolve the apparent dichotomy. But great progress has been made and there is hope it will eventually be resolved:

Research is ongoing - Gell-Mann gives a good account of his approach here:

The point to note is its generally not what popularization's espouse and the early ideas of Bohr etc such as wave-particle duality have been well and truly superseded. Here is a paper on some common myths:

Thanks
Bill

#### julcab12

Qoute "..they are neither waves nor particles, but something new that has properties of both; it is only the case that in many cases, only their particle-like or wave-like behavior happens to be relevant to their behavior, and so we treat them as such. So while the phrase "wave-particle duality" might make it seem as if particles can become waves, what actually exists is a strange sort of object that always has properties of both particles and waves, only one of which may be easily observable in some cases."

Wave particle duality doesn't really say that waves are particles. It says that "particles" aren't really particles, nor are they really waves, they're just little objects/something that have some depicted properties of waves/ripples and some properties of particles(lumpy wavelets), and there are certain situations where one is more visible than the other. I've heard it said (in a very rough sense) that subatomic objects travel like waves, and interact like particles. Again, this is a huge simplification, but there's an important intuition, which is that these objects are always a little like waves and a little like particles. We can describe their position by a function that tells you the probability that the object will be at a particle point in space at a particular time; this function takes the mathematical form of a wave, so we call it a wavefunction, and this is the sense in which particles are like waves. When these objects interact, however, we tend to see them more as particles, like little depiction from classical objects like marbles.

The double-slit experiment is a good example of this. Once more, I emphasize that this is a very big simplification, but just for the purposes of giving you a bit of context, we can imagine that as the electron travels through the slits, its wavelike character is more obvious, and so there are noticeable behaviors we normally attribute to classical waves, like interference. When it collides with the backboard, however, its particle-like character is more obvious, and so we see a single point where the electron collided with the wall. But at all times, the electron had both wave and particle characteristics, and that's the essence of wave-particle duality. Qoute EtaZetaTheta

#### bhobba

Mentor
The double-slit experiment is a good example of this
Not really - it's a good example of the principle of superposition and the uncertainty principle:

Note the above is not the definitive analysis of the double slit - merely better than the one in beginning texts.

It is not s bit wave and a bit particle - quantum is simply quantum -dialectics like it contains a bit wave and a bit particle is not much use.

Thanks
Bill

#### RUTA

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.
We answer that question in our book "Beyond the Dynamical Universe." Therein we argue for "adynamical" or "constraint-based" explanation over "dynamical" or "time-evolved" explanation. You can see immediately why dynamical explanation leads to that question/problem, i.e., you need to explain initial conditions for dynamical explanation to be complete. As Wilczek wrote

The account it [dynamical explanation] gives -- things are what they are because they were what they were -- raises the question, Why were things that way and not any other?
Crowther sums it up nicely in her review

From this [dynamical] perspective, even if we discover some fundamental laws, or a 'theory of everything', not only would we be left asking, 'why these laws rather than some other ones?', but we would also be beleaguered by the initial conditions of the universe at the Big Bang, defying dynamical explanation in terms of any 'prior state'. Instead, from the [adynamical] perspective, 'there is nothing particularly mysterious or sacred about the initial conditions at the Big Bang [...] because the conditions at any point in spacetime globally constrain the conditions at the other points of spacetime' (p. 102). The character of the explanation thus shifts and can be captured by the slogan 'everything is the way it is because everything is the way it is', in accordance with the adynamical global constraint.
The reason I bring it up here is because all the mysteries of QM disappear when you use adynamical explanation, i.e., we don't need to 'fix' QM, we just need to 'fix' how physicists explain physical reality.

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#### victorneto

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.’ ..

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.

I agree with you. This is my interpretation for a long time. And, also, is it true that we can not simultaneously determine the position of the momentum of a particle? But this, by itself, is already a form of terminism! So, if you want another interpretation, it's wanting to invent the wheel.

#### Henryk

Gold Member
Particle/wave duality is a very old concept, something like over 100 years old. The origin of that was an attempt to describe atomic-scale world with concept that are directly perceived by humans. One idea is a wave - everyone has seen waves, all you have to do is to drop a stone into a pond of water and you see circular waves propagating from the point of impact. Same with particles; there are big particles like stones, smaller ones like grains of sands, even smaller dust particles. You know what it is because you've seen it. At the end of the 19'th century everything could be explained by using a concept of either particle or a wave. This world came crushing down when physicists started probing at atomic scale. neither concept worked, electrons did have a fixed charge, fixed mass (like a particle) but also could show an interference pattern (like a wave).
The solution to that dilemma came early in the 20's, forget about particles and wave. Electron (and other elementary particles) can be represented as vectors in Hilbert space. Neither particle, nor waves!. And with that, you could actually model and calculate everything you needed.

#### vanhees71

Gold Member
The reason I bring it up here is because all the mysteries of QM disappear when you use adynamical explanation, i.e., we don't need to 'fix' QM, we just need to 'fix' how physicists explain physical reality.
That's interesting! I think to the contrary many of the apparent, imho not existing, problems with QM come from the fact that the dynamics is played down in many textbooks. Everything is usually fixed to evaluate eigenstates of the Hamiltonian, i.a., the stationary states. That's nice to evaluate atomic spectra, but that's it more or less.

Physics is about dynamics, i.e., to understand how things change with time given an appropriate initial condition and the dynamical laws (i.e., in QT the Hamiltonian).

E.g., all the mysteries concerning the preparation of spin in a Stern-Gerlach experiment go away immediately if you think dynamically about what's happening when the particles run through the magnetic field: The dynamics provides (to a very good approximation) an entanglement between the spin component in direction of the field and the position of the particle, and thus when looking only at one of the partial beams you get (to very good approximation) eigenstates of the measured spin component.

#### RUTA

That's interesting! I think to the contrary many of the apparent, imho not existing, problems with QM come from the fact that the dynamics is played down in many textbooks. Everything is usually fixed to evaluate eigenstates of the Hamiltonian, i.a., the stationary states. That's nice to evaluate atomic spectra, but that's it more or less.

Physics is about dynamics, i.e., to understand how things change with time given an appropriate initial condition and the dynamical laws (i.e., in QT the Hamiltonian).

E.g., all the mysteries concerning the preparation of spin in a Stern-Gerlach experiment go away immediately if you think dynamically about what's happening when the particles run through the magnetic field: The dynamics provides (to a very good approximation) an entanglement between the spin component in direction of the field and the position of the particle, and thus when looking only at one of the partial beams you get (to very good approximation) eigenstates of the measured spin component.
But that doesn't explain the Mermin device for the spin singlet state dynamically, unless you want to violate locality or measurement independence (conditions for Bell inequality). Using adynamical/constraint-based explanation of the Mermin device you end up with a very transparent conservation principle (conservation per no preferred reference frame) with no dynamical counterpart, i.e., no causal mechanism or hidden variables. And, it's totally in accord with special relativity in a very fundamental sense (see this talk I'm giving in Växjö next month).

"Bohr's duality paradox 100 years later?"

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