Wave-particle duality of atoms and molecules

[DesertFox]

Hello ladies and gentlemen,

On the website of Encyclopedia Brittanicca I read the article about wave - particle duality. The article says that the wave - particle duality is experimentally established for light, electrons and protons. However, i found other internet sources which say that the duality is experimentally proven even for atoms and molecules: as an example, the alleged double-slit experiment with Carbon-60 molecules was performed by Markus Arndt, Anton Zeilinger and co-workers at the University of Vienna in Austria.

However, the article in Encyclopaedia Britannica does not mention wave-particle duality for atoms and molecules. So here it is my question. Are the double-slit experiments with atom/molecules considered (by the majority of scientists) as a conclusive evidence for the wave-particle duality of atoms and molecules?

I guess that the wave-particle duality of atom and molecules is not firmly experimentally established fact. Am I right about that?

I will be very thankful for replies...

 

[phinds]

There is no such thing as wave particle duality. That was deprecated about 100 years ago. Quantum objects are just that --- quantum objects. They are not waves and they are not particles (in the common sense of that term)

Pillows are fuzzy, and tables have four legs, but when you encounter a sheep (which is fuzzy like a pillow and has four legs like a table) you aren't going to find the concept of "table/pillow duality" very helpful.

 

[DesertFox]

There is no such thing as wave particle duality. That was deprecated about 100 years ago. Quantum objects are just that --- quantum objects. They are not waves and they are not particles (in the common sense of that term)

So, in the context of your answer... What is the threshold between quantum objects and classical objects? Or, there is no sharp threshold, just a gradual shift from quantum to classical? Or, the classical objects are considered as a special case (specialization) of quantum objects?

 

[CoolMint]

So, in the context of your answer... What is the threshold between quantum objects and classical objects? Or, there is no sharp threshold, just a gradual shift from quantum to classical? Or, the classical objects are considered as a special case (specialization) of quantum objects?

Classical objects are quantum objects. There are no classical particles. Interpretations of quantum mechanics belong in the interpretations sub-forum.

 

[phinds]

So, in the context of your answer... What is the threshold between quantum objects and classical objects? Or, there is no sharp threshold, just a gradual shift from quantum to classical? Or, the classical objects are considered as a special case (specialization) of quantum objects?

Classical objects are quantum objects. There are no classical particles. Interpretations of quantum mechanics belong in the interpretations sub-forum.

Classical physics is usually, at the macro level, extremely useful and accurate but that does not change the underlying fact that the classical world has been supplanted by the quantum world in the same way that Newtonian mechanics have been supplanted by General Relativity.

 

[DesertFox]

Classical objects are quantum objects. There are no classical particles. Interpretations of quantum mechanics belong in the interpretations sub-forum.

Ok, no interpretations, and let's consider everything as quantum objects... So, tell me please, when (under what conditions) the quantum objects does not show quantum diffraction pattern? Obviously, this is not a matter of interpretation, it is a matter of empirical proof. And obviously, electrons do show such pattern. But tennis ball (considered as quantum objects) do not show such pattern. So again, where is the threshold? Or it is gradual shift from quantum diffraction pattern to no-quantum-diffraction pattern?

 

[CoolMint]

Ok, no interpretations, and let's consider everything as quantum objects... So, tell me please, when (under what conditions) the quantum objects does not show quantum diffraction pattern? Obviously, this is not a matter of interpretation, it is a matter of empirical proof. And obviously, electrons do show such pattern. But tennis ball (considered as quantum objects) do not show such pattern. So again, where is the threshold? Or it is gradual shift from quantum diffraction pattern to no-quantum-diffraction pattern?

That would be dependent on the precision of your measurements. You could, at least in principle, get infinitely sharp values on observables.

 

[CoolMint]

Your eyes are too crude to notice quantum artefacts and behavior(tennis balls, etc) but with high precision instruments, the quantum nature of objects is easier to see.

 

[PeroK]

Ok, no interpretations, and let's consider everything as quantum objects... So, tell me please, when (under what conditions) the quantum objects does not show quantum diffraction pattern? Obviously, this is not a matter of interpretation, it is a matter of empirical proof. And obviously, electrons do show such pattern. But tennis ball (considered as quantum objects) do not show such pattern. So again, where is the threshold? Or it is gradual shift from quantum diffraction pattern to no-quantum-diffraction pattern?

Waves and particles can be defined classically with specific and incompatible experimental behaviour. Nature, however, does not conform to that model of behaviour. In particular, both classical wave-like and classical particle-like behaviour can be exhibited by the same object - whether photon, electron, atom or molecule.

That's why QM replaces classical mechanics to explain the behaviour of microscopic objects - which are generally called particles, rather than invent a new word.

The question is why wave-like behaviour gradually diminishes as the system involves more and more elementary particles. Until eventually at the macroscopic level it is no longer observable and we adopt the almost exact approximation of classical mechanical behaviour.

There are a number of reasons for this. The two main ones being statistical (averaging out of large numbers) and decoherence (which is a subject in itself).

 

[anuttarasammyak]

Matter is particle not wave in the sense that coordinate measurement or where it is, results a dot. Nothing like waves that expands in space is measured in a single measurement. Further no ( classical ) entities of transmitting waves, e.g. air for sound, water for Tsunami, is identified for elementary matter, e.g. electron,
photon, lepton.
Matter is wave not particle in the sense that during two measurements classical motion of a particle, i.e. single trajectory of inertial constant speed or under some potential, cannot be applied. Multiple paths as if it is waves going toward everywhere and their superposition decide the next position measurement result introducing probability.

 

[Delta2]

The wave particle duality is part of the old quantum theory, circa 1900-1950.

Since then Quantum Field Theory has fully unified the concept of a particle and the concept of wave.

Particles in quantum field theory are nothing more than the energy quanta of fields, which fields obey a wave equation. There is a separate field for each type of elementary particle, i.e. the electromagnetic field for photon, the electron field for electron, the quark field for quark e.t.c.

 

[DesertFox]

Your eyes are too crude to notice quantum artefacts and behavior(tennis balls, etc) but with high precision instruments, the quantum nature of objects is easier to see.

But again the core of my question remains untouched. If we consider quantum objects (of course, using high precision instruments but not with our crude eyes) and if (in our consideration) we move from really tiny quantum objects (electrons, protons) to bigger and bigger quantum objects (atoms, molecules, tennis balls etc.): Is there a sharp threshold? Or it is gradual shift from quantum diffraction pattern to no-quantum-diffraction pattern?
In other word: does the quantum diffraction suddenly disappear as an effect at some level? Or this pattern gradually "fades away" when we consider bigger and bigger objects (under the appropriate conditions and with the appropriate instruments, set ups and so on)

 

[PeterDonis]

Is there a sharp threshold?

We don't know. We can't do the same sorts of experiments on larger objects that we can do on small objects. For example, the largest object for which a double slit experiment has been done is a buckyball (carbon-60), with 60 atoms. A typical macroscopic object that exhibits classical behavior without any observable trace of quantum effects, say a tennis ball, might have $10^{25}$ atoms. There is a huge range in between that we have limited ability to explore as far as testing possible quantum effects goes.

 

[DesertFox]

The wave particle duality is part of the old quantum theory, circa 1900-1950.

Since then Quantum Field Theory has fully unified the concept of a particle and the concept of wave.

Particles in quantum field theory are nothing more than the energy quanta of fields, which fields obey a wave equation. There is a separate field for each type of elementary particle, i.e. the electromagnetic field for photon, the electron field for electron, the quark field for quark e.t.c.

Let's rule out the old wave-particle duality concept. But, as it was mentioned, that does not actually touch the core of my question.

Lets suppose that we consider quantum objects (of course, using high precision instruments but not with our crude eyes) and (in our consideration) we move from really tiny quantum objects (electrons, protons) to bigger and bigger quantum objects (atoms, molecules, tennis balls etc.): Is there a sharp threshold? Or it is gradual shift from quantum diffraction pattern to no-quantum-diffraction pattern?

In other word: does the quantum diffraction pattern suddenly dissappear at some level? Or this pattern gradually "fades away" when we consider bigger and bigger objects (under the appropriate conditions and with the appropriate instruments, set ups and so on)?

 

[anuttarasammyak]

In other word: does the quantum diffraction pattern suddenly dissappear at some level? Or this pattern gradually "fades away" when we consider bigger and bigger objects (under the appropriate conditions and with the appropriate instruments, set ups and so on)?

I find no reason of threshold that makes sudden appearance/disappearance of quantum effect.

 

[Delta2]

In other word: does the quantum diffraction pattern suddenly dissappear at some level? Or this pattern gradually "fades away" when we consider bigger and bigger objects (under the appropriate conditions and with the appropriate instruments, set ups and so on)?

I think it gradually fades away. But there is no way to detect the diffraction pattern for say a tennis ball, or for Earth moving around the sun cause it is very very small. @PeterDonis and @PeroK are better than me in this area and I think they gave some insight, check posts #9 and #13.

 

[DesertFox]

I think it gradually fades away. But there is no way to detect the diffraction pattern for say a tennis ball, or for Earth moving around the sun cause it is very very small. @PeterDonis and @PeroK are better than me in this area and I think they gave some insight, check posts #9 and #13.

Yes, i know that the observation of quantum diffraction pattern becomes harder and harder for bigger objects. At some level it even becomes impossible, so of course we should rule out objects with such size from our consideration of the pattern.

 

[PeterDonis]

i know that the observation of quantum diffraction pattern becomes harder and harder for bigger objects. At some level it even becomes impossible

As I pointed out in post #13, for the double slit experiment, at least, the "level" at which this becomes impossible, at the present state of our experimental capabilities, is anything larger than a buckyball, carbon-60.

 

[DesertFox]

the largest object for which a double slit experiment has been done is a buckyball (carbon-60), with 60

I read about that experiment. It is really fascinating. But do you think that the conclusion of the experiment is widely accepted? Many official sources (for example Brittanicca Encyclopedia), which i consider trustful sources, don't even bother to mention the quantum diffraction for atoms and molecules. They state it as a fact but only for electrons, protons and light. That is what strikes me a lot. And that's why i think MAYBE the quantum diffraction pattern of atom and molecules is not firmly experimentally established fact, despite the alleged concluions of the team which performed the carbon-60 experiment...

 

[PeterDonis]

do you think that the conclusion of the experiment is widely accepted?

Do you mean the double slit experiment with buckyballs? I am not aware of any physicists who dispute the result.

Many official sources (for example Brittanicca Encyclopedia), which i consider trustful sources, don't even bother to mention the quantum diffraction for atoms and molecules.

If you really want to know the current state of experimental knowledge in physics, you need to be reading the actual physics literature. No encyclopedia is going to give you the same completeness of coverage. Not even most physics textbooks will give you the same completeness of coverage, since textbooks are usually a fair bit behind the current experimental state of the art, because of the unavoidable time it takes to get a textbook written and published. Peer-reviewed papers by experimentalists are the best way to keep up with current findings.

thats why i think MAYBE the quantum diffraction pattern of atom and molecules is not firmly experimentally established fact

Your thinking is incorrect because you are relying on the wrong sources. See above.

 

[DrClaude]

As I pointed out in post #13, for the double slit experiment, at least, the "level" at which this becomes impossible, at the present state of our experimental capabilities, is anything larger than a buckyball, carbon-60.

Let me be pedantic: it has been done with larger molecules.

https://doi.org/10.1103/PhysRevLett.91.090408

​

 

[Cthugha]

About 15 years later the same group even went up to 2000 atoms and masses of 25 kDa. However, they needed to use a different kind of interferometer to show interference.

Original publication (behind paywall)

As has been pointed out before, there is nothing mysterious about the size of whatever you use in the experiment. It is all about the phase of the wavefunction of the system you send through the double slit. If the phase difference between the two possible paths is well defined, you will see an interference pattern. If the phase randomizes completely along the way, you will not see an interference pattern. If the relative phase only randomizes a bit, you will see an interference pattern with reduced visibility. Every kind of interaction - reversible or irreversible - that happens along the path will change this phase slightly. If you repeat the experiment several times and in each run these random phase changes are different enough to result in complete randomization of the relative phase, the ensemble avarage integrate over many experimental runs will not show an interference pattern.

There is nothing intrinsic about size that influences the visibility of the interference pattern besides the obvious fact that it is comparably easy to protect an individual atom from any interaction, while it is very hard to protect a full ensemble of 2000 atoms from interactions with the outside world.

 

[vanhees71]

The wave particle duality is part of the old quantum theory, circa 1900-1950.

It's 1900-1925 at most. In 1925 Heisenberg had an ingenious idea that was then worked out by Born, Jordan, and himself and called "matrix mechanics". In 1926 Schrödinger came up with "wave mechanics", which very soon he proved to be equivalent with "matrix mechanics". Finally Dirac found the general representation-free formulation, then called "transformation theory" of the same theory. We can thus safely say, the inconsistent "old quantum theory" was replaced with the still valid "new quantum theory" of Heisenberg, Born, Jordan, and Schrödinger, and Dirac in 1926.

Since then Quantum Field Theory has fully unified the concept of a particle and the concept of wave.

Particles in quantum field theory are nothing more than the energy quanta of fields, which fields obey a wave equation. There is a separate field for each type of elementary particle, i.e. the electromagnetic field for photon, the electron field for electron, the quark field for quark e.t.c.

More accurately particles in quantum field theory are asymptotic free one-particle Fock states of their corresponding quantized field.

 

[PeterDonis]

it has been done with larger molecules.

About 15 years later the same group even went up to 2000 atoms and masses of 25 kDa.

Ok, so we're up somewhat from where I said before. But this is still a very, very small size.

 

[DesertFox]

Gentlemen, i am so thankful to all of you for the replies and the comments. And please excuse me if i sound harsh in my post: dialectics is my method when it comes to understanding the things and finding knowledge on my own. And i am just an amateur who is very interested and fascinated by the quantum world. Your posts were really illuminating for me and very informative and educating. THANK YOU! This forum is wonderful place!

 

[PeterDonis]

THANK YOU! This forum is wonderful place!

You're welcome!

 

[vanhees71]

Ok, so we're up somewhat from where I said before. But this is still a very, very small size.

Nevertheless there is no known physical reason that QT becomes invalid for "sufficiently large" systems. Of coarse due to decoherence it's difficult to observe non-classical behavior, but that's just a technical challenge not a fundamental limit beyond which QT might get invalid.

 

[PeterDonis]

there is no known physical reason that QT becomes invalid for "sufficiently large" systems.

Theoretically, yes, that's true (modulo some concerns with QM interpretation that are out of scope for this thread and would be properly discussed in the interpretations subforum). I am talking about the limits of our actual testing of this by experiment.

 

[vanhees71]

There have been experiments with pretty large macroscopic objects like

https://www.nature.com/articles/d41586-021-01223-4

 

[PeroK]

Nevertheless there is no known physical reason that QT becomes invalid for "sufficiently large" systems. Of coarse due to decoherence it's difficult to observe non-classical behavior, but that's just a technical challenge not a fundamental limit beyond which QT might get invalid.

Sufficiently large systems may no longer be well-defined in terms of a fixed set of elementary particles; or, at least in terms that QT could make sense of them completely. For example, trying to establish the UP for a car would require defining and monitoring the precise set of particles that constitute the car to a level of detail that even theoretically may make no sense. It's not a time-independent set of particles the way a specific molecule is. That seems to me to be more than a technical challenge. It's not clear to me what an experiment to detect an interference pattern for Formula One cars would even look like.

 

[votingmachine]

This seems analogous to asking at what low speed relativity is not valid. There may be no way to measure the deviation from Newtonian calculations at a low speed but that doesn’t make relativity invalid.

The wavelength of a large slow object may be immeasurable, but that doesn’t prove QM does not apply. Or that at some threshold the rules change. At some point between small fast things and large slow things it is reasonable to ignore QM. That is different than the theory fading away.

 

[vanhees71]

Sufficiently large systems may no longer be well-defined in terms of a fixed set of elementary particles; or, at least in terms that QT could make sense of them completely. For example, trying to establish the UP for a car would require defining and monitoring the precise set of particles that constitute the car to a level of detail that even theoretically may make no sense. It's not a time-independent set of particles the way a specific molecule is. That seems to me to be more than a technical challenge. It's not clear to me what an experiment to detect an interference pattern for Formula One cars would even look like.

Of course, and that's why the classical approximation is usually sufficient to describe a car. The relevant macroscopic observables are coarse-graining over all these minute microscopic details that are of course neither describable nor relevant for the description of the car, but that doesn't make QT invalid only because all the particles are forming a car, i.e., a macroscopic object.

 

[DesertFox]

that's just a technical challenge not a fundamental limit beyond which QT might get invalid.

The "technical challenge" sets the limit beyond which we can't say meaningfully if Quantum Theory is still valid or gets invalid.

Wittgenstein: “Whereof one cannot speak, thereof one must be silent.”

 

[DesertFox]

The wavelength of a large slow object may be immeasurable, but that doesn’t prove QM does not apply. Or that at some threshold the rules change. At some point between small fast things and large slow things it is reasonable to ignore QM. That is different than the theory fading away.

Excuse me for the off-topic parallel, but you sound a little bit like a theologian: "God is transcendent (supernatural), so by definition you can't empirically prove his existence. Now, atheists, prove that God does NOT exist!"

Some more food for thought:

1) Of course, "reasonable to ignore" and "the theory fading away" are different. They are just different choices of linguistic framework. Nothing more.

2) You said "At some point"... And what is that point? Where is it? It is not "proven" threshold? OK

 

[CoolMint]

2) You said "At some point"... And what is that point? Where is it? It is not "proven" threshold? OK

You seem to want a number. Your number is 7 nm. This is the size where quantum effects start to dominate. How do I know? Ask the microprocessor industry. They have dealt with these issues for a decade now with gates leaking via quantum tunneling below this threshold.

So how big is 7 nm? About a few dozen water molecules across.
For comparision's sake - most viruses are 70-200 nm in size.