A common misunderstanding about wave-particle duality

https://www.chemistryworld.com/opinion/a-common-misunderstanding-about-wave-particle-duality/4019585.article

by mathgenius • 2 years ago

79 103 2 years ago

Jun8• 2 years ago

Good article. If you want to delve in more have a look at the answers to this Physics SE question: https://physics.stackexchange.com/questions/46237/is-the-wav....

One unfortunate effect of the presenting it as "wave-particle duality" to the laymen audience is that it's as if physicists don't have a good understanding of what's going on and are puzzled by the behavior, whereas QFT is well-established.

gipp2• 2 years ago

Tl;Dr: "Wave-particle duality" is not the notion that matter is "sometimes" a particle and "sometimes" a wave. It is, at all times, its own separate category of thing, for which "particle" and "wave" are just metaphors that approximate its behavior.

One metaphor usually comes closer than the other depending on what system you're looking at, but it's never changing back and forth between some "particle state" and "wave state".

qsdf381003• 2 years ago

I would say, they move like waves but they interact like particles.

I don’t recall from where I’m stealing it, but I thought it captures the "paradox" quite well.

oneshtein6• 2 years ago

It's particle with it pilot wave.

andoando• 2 years ago

My understanding is that what we consider as particles can be described as waves of smaller particles.

For example, atoms are fluctuating electrons, neutrons and protons, all of which are fluctuating subatomic particles and so on. And what we describe as particles are essentially the maxima of these fluctuations.

ajkjk3• 2 years ago

Not really. Electrons are as far as we know not composed of smaller particles, and there's good reason to think they are elementary. Basically because smashing them with things reveals no smaller structure. Whereas when you smash things into a proton it is very clear that there are smaller localized objects inside it (there is a great blog post with pictures of what a proton looks like at different energies that I can't seem to find now...)

ajkjk3• 2 years ago

This article seems to get it backwards?

This doesn’t mean that the atoms themselves are smeared out like waves; rather, what spreads is the probability distribution of them being found subsequently in a given location

No, it really does mean they're smeared out like waves. Prior to the measurement they are in superposition, relative to you. When your experiment images the atoms, that's a measurement that entangles you with the atoms.

If the observable value takes values in (A,B) then when you get entangled you end up in a state (A, measured A) + (B, measured B), each of which perceives a definite value of the measurement. The whole system would still be (to an outside, non-entangled observer, if you could pull such a thing off) in a superposition which could continue to interfere with itself, but the observer who's inside the superposition will never be able to tell.

Afaik this is the standard interpretation nowadays. Particles are waves (well, in the sense that what we call a particle is usually a momentum eigenstate that evolves in space like a wave), but the measurement process that entangles us to the causes them to come in quantized packets which we call particles.

Maybe I'm missing the point of the article somehow though...

notfed4• 2 years ago

Afaik this is the standard interpretation nowadays

You've described the Everett interpretation. It should be the standard. There's still a surprising amount of resistance for some reason.

ajkjk3• 2 years ago

I feel like there isn't really, not anymore, and most of the semblance of a debate is people reporting on historical debates. Although hard to be sure without, like, a survey.

At least in what I read it seems like the remaining debate is over how literally to interpret the words "many worlds", rather than whether the basic idea is correct. Ideas like entangling with the experiment and the decoherence of quantum states are very well-established.

pdonis5• 2 years ago

There's still a surprising amount of resistance for some reason.

It's not surprising at all; it's a perfectly rational unwillingness to treat our current quantum mechanics (and not even relativistic quantum field theory, but non-relativistic QM, which is already known to be just an approximation) as though it were an exactly complete fundamental theory.

l33tman6• 2 years ago

There is no real resistance I think, it's just that the various interpretations are not very relevant in practice. All of engineering that depends on quantum mechanics make do with the standard treatment that has been taught since 100 years now.

The MWI doesn't offer any other way to calculate anything (as it's an interpretation, not a theory). It's a good mental image of what happens though.

I guess at some point there will be some experiment that can be performed that requires a more advanced theoretical method than what current QM provides and then maybe the MWI (or other interpretations) get a chance.

pdonis5• 2 years ago

what we call a particle is usually a momentum eigenstate

Not really. In non-relativistic QM, it's a wave packet. True momentum eigenstates are not physically realizable.

In quantum field theory, "particle" is just a shorthand name for certain quantum field states. Quantum fields aren't waves or particles, they're quantum fields.

dahart4• 2 years ago

Isn’t superposition the quantum physics word for probability distribution?

marcosdumay2• 2 years ago

It's more complicated than a probability distribution (e.g. the wave can have a negative amplitude), but it's not very far from it.

mrkeen• 2 years ago

To me 'probability distribution' could be interpreted as a bunch of classical particles whose emergent behaviour resembles a wave in some way. Or a series of observations of a classical system whose errors add up to some wave-like phenomenon.

But others in this thread have mentioned that you can interfere a single electron with itself (which to my mind rules out classical-phenonema-which-appear-quantum.). And 'superposition' seems like a better word for that.

Jensson2• 2 years ago

No, the superposition is a real physical thing and not just a probability distribution, the particle is in all parts of the superposition and not just randomly at one of the spots, we can see in experiments that it is in every spot. If you measure it then the superposition squared is the probability distribution of where you will find something, but it isn't just a probability.

This is what makes quantum mechanics hard, if it was just a probability distribution nobody would find that vexing its just a normal probability.

csours• 2 years ago

To me, Wave/Particle means that we have observations consistent with waves and observations consistent with particles.

I don't think you really have to go beyond that - except that is not how things behave in the macro world. The problem is trying to map Wave/Particles to the macro scale. Just don't do it.

wry_discontent• 2 years ago

This is exactly what I learned in a philosophy of science course in college.

Not this specific topic, but that approach. Science is a model, and as soon as you're generalizing past experiments and observations, you're outside of where you should be.

lijok• 2 years ago

What I always wondered as a layman with no background in physics is - couldn’t the observed particle-like properties of waves be an emergent property?

burnished3• 2 years ago

You can observe the effect even when there is only one particle moving at a time.

marcosdumay2• 2 years ago

All the observations are consistent with waves... that change in behavior when you poke them in any way.

Macroscopic waves also change in behavior when you poke them, this is not mysterious. But the relative energy of the poking to the wave on quantum mechanics is huge for out macroscopic standards.

pjs_• 2 years ago

Why is the bicycle self stable?

https://www.science.org/doi/10.1126/science.1201959

We can't say, in any satisfying way. The mathematics is uncontroversial, but all of the simple natural-language explanations fail under scrutiny.

Where is the electron in the double slit experiment? Is it a particle or a wave?

Similarly, we can't say. We don't have a good way of talking about this by analogy, or using natural language. As with the bicycle, the mathematics is bulletproof and boring.

This is not to say that quantum mechanics is unmysterious - I think it is very mysterious. However, the bicycle example shows how this characteristic, frustrating elusiveness of good natural-language descriptions is not limited to exotic quantum systems.

TheBlight• 2 years ago

I think a significant aspect of this specific problem is that a notion of a "field" is very difficult to translate to everyday language no matter how easy it is for a physicist to conceptualize and model with math. Similar to coming up with a good analogy for a hash function in computer science world. These are largely very foreign concepts for everyday human life experiencers.

amelius• 2 years ago

Would it be possible to remote-control a bicycle like you can remote-control a drone?

dahart4• 2 years ago

Absolutely! This is on my bucket list of home robotics projects. A stretch goal is to get steering during a wheelie, but I think this will be extra difficult. I’m guessing a well tuned PID controller will handle counter-steering just fine, but we’ll see.

itishappy7• 2 years ago

Just strap a drone to it! Only somewhat facetious. :)

dahart4• 2 years ago

The link doesn’t support the notion that we can’t describe why a bike is self-stable. On the contrary, it simply contradicts one popular explanation and offers another one. In fact the story is pretty easy to describe: there are oscillating forces acting on a bike that cause it to right itself. The oscillating forces just aren’t accurately described as gyroscopic, which was one of the previous proposals.

We don’t know that there is no “natural language” story and no analogies for either bikes or quanta, what we know is we have some analogies that don’t work. History is full of instances of things we had incorrect stories for, and then found better ones. We have no reason to believe that won’t happen for photons and electrons too.

layer83• 2 years ago

The article completely ignores quantum field theory, which forms the basis of the standard model of particle physics, and in which particles are emergent (rather than fundamental) features of the respective underlying field, which is described by wave equations.

gipp2• 2 years ago

I'd say it's more or less consistent with QFT -- it's saying essentially the same thing, just not in that exact language

layer83• 2 years ago

The article says that particles are neither waves nor particles, which is not wrong (depending on your definition of “wave”), but it omits to mention what the actual fundamental ontology is in modern particle physics.

criddell3• 2 years ago

If a particle is an excitation of a quantum field, what is a wave?

itishappy7• 2 years ago

The same thing. A wave is just something that obeys the wave equation by wiggling in a certain way.

oneshtein6• 2 years ago

In some mathematical models, a particle is an excitation of a field, e.g. a mountain is an excitation of a height field. It makes calculations much easier. In those models, waves can be represented in many different ways.

In the real world, a particle can be a lot of different things, but usually we use this word to describe small parts of larger objects. There is no much physical difference between a small grain of sand and a planet, but we use different words for them, while we use the same word for small things, which can be different by many orders of magnitude.

In the real world, a wave, is just a form of group behavior of particles, i.e. many particles are doing the same thing at once because of an interaction between them and some energy in a system. For example, merging of black holes makes waves in a galaxy, so we can watch that in super slow motion.

breck4• 2 years ago

Any chance you could provide some analogies to someone with deep computer expertise who wants to learn quantum physics?

Is QFT analogous to binary? Everything builds up from QFT?

layer83• 2 years ago

There are no good analogies I know of. I wouldn't say it's like binary in any way, but I also wouldn't say that that everything builds up from binary in CS. There is really no particular relation to (or benefits of having) computer expertise.

Two recent popular books about QFT are:

Matt Strassler’s Waves in an Impossible Sea (no math, just hand-wavy analogies): https://www.amazon.com/dp/B0CB3LZXR9/

Sean Carroll’s Quanta and Fields (tries to give you a basic understanding of the actual math): https://www.amazon.com/dp/0593186605/

l33tman6• 2 years ago

All of the complications and complexity in QM/QFT are from doing calculations that map to the extremely coarse-grained boring low-energy reality we spend all of our lives in. If you ignore that, the underlying math is exceedingly trivially formulated. It's like 2 powerpoint slides to show what CUDA arrays and ops are needed to underpin a quantum field theoretic calculation of quarks and gluons inside a proton for example.

Unfortunately you're probably not going to be satisfied with that, at some point you need to connect these simulations to real measurable parameters and this is exceedingly non-trivial :/

Starting from the other end, pure old-school undergrad quantum mechanics is really the easiest way to connect to measurable quantities and this is why it's used in all of engineering, quantum optics etc. It just provides the right amount of detail.

DavidHull2• 2 years ago

As an interested amateur, I recommend the book "Something Deeply Hidden: Quantum Worlds and the Emergence of Spacetime" by Sean Carroll as a good overview of quantum fundamentals. The book discusses several interpretations of the reality of matter at the quantum level. Dr. Carroll himself believes that everything is waves/fields at the lowest level, and a many-worlds interpretation of why matter appears to be particles when we observe it, but also discusses de Broglie–Bohm pilot wave theory and spontaneous collapse theory.

https://en.wikipedia.org/wiki/Many-worlds_interpretation https://en.wikipedia.org/wiki/Interpretations_of_quantum_mec...

mech4227• 2 years ago

seems to be my day for stupid questions...

Do quantum effects make solar sails work ? My High School physics said F=MA and photons have zero mass.. so where does the force come from ?

Thanks!

bandrami• 2 years ago

Zero rest mass

AnimalMuppet• 2 years ago

Things that move at the speed of light carry momentum, even though they have zero mass. For something moving at the speed of light, the momentum is the energy divided by c. (You can't have something with mass moving at the speed of light.)

red75prime2• 2 years ago

Photons have momentum, while not having mass. Interestingly, Newton has formulated his second law in terms of momentum: F=dp/dt.

Maxatar4• 2 years ago

Technically the formula for force is F = dp/dt, or the derivative of momentum with respect to time.

For particles with mass the momentum of such a particle is p = mv, and so you can use that to yield F = ma. However for a massless particle like a photon, its momentum is p = E / c. If you use that momentum to describe a beam of photons being absorbed by a material, then you get F = n * E / c, where E is the average energy of the photons, n is the rate of photons per second, and c is the speed of light.

zokier• 2 years ago

This whole article and discussion reminds me of the key idea that physics is a model and map is not the territory. Reality is just its own thing, to me it seems pointless to debate if something is really wave or particle or quubaz, the question should be what insights and predictions you can get. Conversely just because you can model things with waves/particles/x doesn't mean that they are waves/particles/x.

EncomLab• 2 years ago

Hence the point that Norbert Weiner made that "when modeling a cat, the best model is a cat, and if possible the same cat."

dr_dshiv2• 2 years ago

“The map is not the territory” — but maybe the territory is maps all the way down.

tetris112• 2 years ago

If you fire a single particle between two slits repeatedly, the cumulative places it hits form a wave-like interference pattern, of course.

If you fire a single particle between two slits just once and it still forms a wave-like interference pattern, then surely what is being observed is more than just a probability distribution?

breck4• 2 years ago

If you fire a single particle between two slits just once

Can you ever be sure you only fired a single particle?

itishappy7• 2 years ago

Yup! Photons are tough, it's a lot easier with electrons.

dahart4• 2 years ago

Where did you read about a single particle forming an interference pattern?

“The atoms themselves are only ever observed in a given experimental run as particles – just as quantum mechanics says they should be. The wavelike behaviour – which is to say, the smeared-out probability distribution – is reconstructed from many particle-like observations.”

epgui• 2 years ago

A single particle is never observed as a wave afaik (not my area of expertise).

crazygringo• 2 years ago

just once and it still forms a wave-like interference pattern

That's not a thing. There's no pattern to observe from a single particle.

The situation you're describing doesn't exist.

Wave behavior and interference patterns can only be observed in statistically significant collections of particle interactions. Not single interactions.

itishappy7• 2 years ago

Somewhat reductive. Waves behave differently than classical particles, and we can measure this on the level of individual particles even though we can't image the full pattern with a single measurement. For example: a single particle can end up in the middle of a shadow, but this shouldn't be possible classically.

https://en.wikipedia.org/wiki/Arago_spot

l33tman6• 2 years ago

You do have to assume at some point that if you fire individual electrons through the dual-slits and record their individual results, that the results are independent. If you can assume this, you're drawn to the current model which is that the electron is in a superposition (i.e. "interferes with itself") that when resolved gives the observed statistical outcome of your independent results. This seems also to be the simplest model (there has been 100 years of trying to find simpler models that reproduce these results).

qsdf381003• 2 years ago

If you fire a single particle just once, you don’t get an interference pattern, right? You get a single impact.

JoeAltmaier• 2 years ago

What's the difference between that, and firing a singleton particles periodically? Shouldn't that just be a collection of 'single impacts'? Yet it isn't.

itishappy7• 2 years ago

You get both. You get a single impact in a location determined by the interference pattern.

breck4• 2 years ago

A question for any physicists on here: is Wave/Particle duality analogous to Lisp/Binary duality? In other words, 2 different languages/models but 1 underlying reality that neither perfectly represents?

scotty79• 2 years ago

there is no reason to say that quantum entities are ever really waves

I'm on the completely opposite end of the spectrum. I see no reason to say that quantum entities are ever really pointlike particles.

I'd rather see them as smeared wave-like entities that occasionally rapidly reshape while exchanging energy and momentum through fields as if the were two small billiard balls bouncing.

There's really nothing particle-like about those quantum objects apart from this momentum and energy exchanges (and even that is weird because it's quantized) and that evolution of their center of mass in time seems like a thing flying in space according to Newtonian dynamics.

We draw our simplest intuitions from macroscopic objects that are built of huge number of actual elemental material objects so tightly bound with one another that they are barely smeared.

It's not an accident that macroscopic object obeys the same equations that a tight quantum objects obey.

But it's a huge mistake to think that those equations that we wrote for this very bizarre state of matter that are macroscopic objects is anything primary just because the math describing them is as simple as it goes.

Take a look at ideal gas equations about pressure volume and temperature. They are childishly simple when compared to the math you'd need to accurately describe what actually happens in a gas.

Framing quantum mechanics in terms of "observation" instead of "instance of momentum and energy exchange" might be a very computationally convenient interpretation of what happens but I don't think it's real in any sense of the word.

In broader context we have very many interpretations in physics l that are the simplest possible interpretations of the mathematical abstractions of our models. With complete disregard for how sensible they seem. Even though there are completely reasonable alternative interpretations of the same math available.

Physics educators seem to delight in the quirkiness of the interpretations that theoretical physicists love because they are just their equations narrated, nothing more, nothing less, instead of exploring more reasonable interpretations or even mention that they exist.

In absence of new math, bringing new insights to our fundamental knowledge, one of the goals of physics should be to get real. New, or old but rekindled, more plausible interpretations might inspire new generations of young physicists to visit avenues less explored. Because abstract narratives we globally adopted failed to do that for many decades already.

jiggawatts• 2 years ago

Something that I've been wondering about is whether the original development of quantum mechanics involved a simple "mixup" due to the duality of the mathematics involved in wave mechanics.

Imagine implementing a QED simulator: some EM source emitting billions of photons, each with a vector clock rotating to indicate the wavelength. You could code this up as an array tracking each photon.

Alternatively, at very large numbers of photons, you'd notice that each pixel on your screen would have so many (maybe millions!) that you could just simulate the aggregate behaviour of each little square patch of space instead of individual particles.

Ta-da... it's a continuum. No particles.

You can simulate waves in space either using a Monte Carlo particle simulation or by subdividing the space into finite elements and tracking exchanges over their boundaries.

Superficially the maths looks different, but the result is the same, and the finite element method has locality and makes the speed of light limit manifest.

Why do we keep insisting on covering only the particle model in text books?

lupire• 2 years ago

The article clouds more than it illuminates. I don't think the author knows what he is trying to say. This is common in strawman "mythbusting" articles, but even more fraught when the topic is quantum mechanics.

Saying that an electron "isn't a wave" when it is in motion, because the wave is probability, not the electron, is equivalent to saying the electron doesn't exist between emissions and absorption. This is a valid interpretation, but even more conunterintuitive to novice, and raises more questions. Ultimately, arguing over vocabulary as interpretation is a distraction. What the thing does is what the thing is. Interpretations are intuitive guides.

A common misunderstanding about wave-particle duality

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