# B Is wave particle duality really wrong?

1. Oct 2, 2016

### fanieh

Can all seasoned physicists please confirm if it is true that wave particle duality is really wrong as when Bill stated in the decoherence branches thread "This is a little different than what you will find in a beginner text becauise it doesnt use the wave particle duality which is wrong - but that is another thread."?

2. Oct 2, 2016

### DaveC426913

Would need to understand the context of 'wrong'.

Light does behave like a particle or a wave, depending on how you choose to observe it.

At a guess, I would say he's intimating that explanations are over-simplified. Which could be 'wrong', or 'not wrong' depending on your viewpoint.

3. Oct 2, 2016

### Simon Bridge

I'll sometimes use the analogy of blind men examining an elephant.
One gets the front and experiences the trunk and concludes that an elephant is a fat snake.
The other gets the tail and says "no no the elephant is a taught rope."
As they repeat each others experiments they realise they are somehow both right and declare to the World that elephants display rope-snake duality.
This is, of course, true - insofar as it goes. Later experiments show that there is more to the elephant than that.

But anyone saying "an elephant is both a rope and a snake at the same time" has got it wrong.

Here, Feynman explains what we mean by "particles" these days.
http://www.vega.org.uk/video/subseries/8

Aside: there is a similar story of the blind elephant examining a man - it concludes that men are flat creatures...

4. Oct 2, 2016

### fanieh

or let's be more specific...

Bill shared the following paper: https://arxiv.org/ftp/quant-ph/papers/0703/0703126.pdf

"3. Concluding remarks
In this presentation, the Born postulate is used to obtain the interference pattern for
particles scattered from a system of slits without referring, a priori, to classical wave
theory."

Is the paper controversial or all physicists can agree to it that the wave aspect can be ignored or redundant to the more direct mechanism of "uncertainty principle and superposition principle" as Bill put it. But isn't it there is wave aspect in the superposition principle? Can you have superposition principle without wave??

5. Oct 2, 2016

### Simon Bridge

... the quote comes from Marcella M. (2007) Quantum interference at slits
The paper is not controversial at all ... unfortunately Marcella made a mistake when he wrote that: turns out that was not what he had done.

See: Rothman T. & Boughn S. (2010) "Quantum interference at slits" revisited.
In a nutshell, Marcella's treatment implicitly uses freshman approximations to the classical wave approach... he missed it because it's just hidden in the notation.

The neat thing about science is that it is a process: an ongoing discussion. When you find an interesting paper that is a few years old, check to see what papers cite it. If it were significant there will be quite a few. See what those authors think.

But isn't there a rope-aspect to the elephant?
Yes you can.

Please watch the Feynman lectures ... it will save the rest of us a lot of typing.
Feynman is demonstrating what Bill means.

6. Oct 3, 2016

### vanhees71

Let's not discuss this superficial Marcella paper again. It's not completely wrong but inaccurate and misleading in many points. I never understood why bhobba likes to cite this

Concerning "wave-particle duality" it's just an outdated not very accurately defined notion of the precursor theories of modern quantum theory. Since 1925/26 we have a consistent and very well working quantum theory, where wave-particle duality is not necessary anymore.

7. Oct 3, 2016

### Simon Phoenix

You'll have to include a bit more context - what question or comment is Bill referring to? It's extremely unlikely that Bill is incorrect (unless he's made a typo), but are you certain you've understood all the nuances implicit here?

The basic principle is that properties, or things we can measure, have to be described by operators in the QM formalism. That's a bit different to the situation we learn in basic classical physics where we think of things like position, or momentum, or energy (etc) as just numbers to describe the 'amount' of the relevant property.

But describing things in terms of operators has important consequences. Loosely speaking we can think of an operation as an 'action' - so we could have the operation, or action, of putting our socks on, or the operation of putting our shoes on. The thing to appreciate here is that when we talk about operations we have to pay careful attention to the order. Putting our shoes on then putting our socks on has a quite different outcome to putting our socks on followed by our shoes. In these cases, where the order of operation matters, we say that the operators do not commute. Classical properties or observables, which are just numbers, do not care about order so that 2x3 = 3x2 for example.

In QM this non-commutation means that for any quantum state there is a relation between the variances of measurement results for pairs of non-commuting observables - this is called an uncertainty relation. Broadly and loosely speaking it means that if we have 2 observables A and B that do not commute then if we do a precise measurement of A we 'lose the capability to gain information' about the B property. That's a rather imprecise way of stating it (and without further qualification of what I mean is actually incorrect) but phrased in this intuitive way we can draw a link to so-called wave-particle duality.

In wave-particle duality we have a similar property - measurement of particle-like properties obliterates any chance we have of observing wave-like properties, and vice versa. So gaining information about the particle like properties means we can't gain information about the wave like properties. In the context of the 2 slit experiment we would describe being able to determine the slit the 'particle' went through as a measurement of a particle-like property, and the measurement of the interference pattern as measurement of a wave-like property.

But it's all wondrously vague and imprecise at this level of waves and particles - which are only classical pictures we impose to try to understand things. We have operators to describe energy, polarization, spin (etc), but there's no corresponding operator for 'wave'. The fundamental things are the operators and whether they commute or not.

So it would be wrong to say that QM is about wave-particle duality. It would be correct, I think, to say that there are certain experiments we can do, which when we try to interpret them in terms of waves or particles, exhibit this wave-particle duality.

So I wouldn't say wave-particle duality is 'wrong', as such, more of a special and incomplete (and ultimately unhelpful) way of looking at certain special situations in QM.

8. Oct 3, 2016

### vanhees71

As I said above wave-particle duality is an old-fashioned precursor model to quantum theory. It's a pretty vague concept with almost self-contradictory elements, and it was never to the satisfaction of its own inventors, including Einstein. Modern quantum theory has overcome this intrinsic contradictions by providing a consistent theory in terms of a probabilistic description (Born's rule). It's well-known that many of the people discovering it, including Einstein, de Broglie, and Schrödinger were still dissatisfied with the theory, mostly due to the implications of this fundamental indeterminism which is at the heart of resolving the intrinsic inconsistencies of old-fashioned quantum theory. However, today we still do not have any other theory to accurately describe an amazing amount of observations. Except for gravity QT describes all aspects of our knowledge about nature!

9. Oct 3, 2016

### fanieh

This is the context of Bill Hobba. In message number 12 of https://www.physicsforums.com/threads/decoherence-branches.887285/ he stated regarding the double slit experiment "For example wavefunction collapse has nothing to do with it - its from the uncertainty principle and superposition principle. Its got nothing to do with interactions other than the slits between siource and screen. The reason detecting at the slits destroys interference is the superposition principle no longer applies."

Please comment how much you agree or not with Bill. Thank you.

10. Oct 3, 2016

### Simon Phoenix

Why would I disagree with this? It's spot on. Do you disagree?

11. Oct 3, 2016

### fanieh

I don't have opinions. I just follow the leaders. So I guess where Bill got it not so correct was saying wave-duality was wrong as in the following he stated in message number 25 of https://www.physicsforums.com/threads/decoherence-branches.887285/page-2:
"For completeness I will give you the beginner explanation. We will consider electrons in a hard vacuum. There is no environment just a detection screen. Just behind the slit the electron has an exact position. By the uncertainty principle it has an unknown momentum, but because its kinetic energy is known this means it has an unknown direction. it can hit the screen anywhere - the exact place being determined by decoherence. But we have two slits. That means we superimpose the two states (ie apply the principle of superposition) and when you chug through the math you find the wavefunction has interference like peaks and troughs.

It hits the screen and where it has a peak it has a greater chance of, via decoherence, of registering a position. Where it has a trough it has no chance so none will be found there. This is a little dfifferent than what you will find in a beginner text becauise it doesnt use the wave particle duality which is wrong - but that is another thread."

12. Oct 3, 2016

### Staff: Mentor

It was proven wrong by Dirac when he came up with his transformation theory, at least by then but probably before:

What is the wave function of two entangled particles? What is its dimension?
Its a myth we tell beginning students because we have to start somewhere and the history is as good as any. I however prefer the following:
http://www.scottaaronson.com/democritus/lec9.html

Thanks
Boll

13. Oct 3, 2016

### Simon Phoenix

Well I'll let Bill answer for himself as to what he was getting at, but I can think of a few ways of interpreting the word 'wrong' as used by Bill here - none of which would make Bill wrong

Basically as Bill and Vanhees have said - the notion of wave-particle duality is really part of the early days when physicists were trying to figure out what it all meant. It's not 'wrong' in itself - but it means something very specific - it means that in certain cases where we can classically attribute observed properties to 'waves' and observed properties to 'particles' - then if we try to think in these terms then there is a duality between these implied wave and particle properties. So not really very helpful. Bill's answer in terms of uncertainty and decoherence has a lot more explanatory power don't you think?

14. Oct 3, 2016

### fanieh

Most Pop-Sci books on quantum mechanics talked about wave particle duality. I have read over 15 books on it. Is there any Pop-Sci quantum books that describe the post wave-duality quantum world in terms of uncertainty and decoherence? Would it be proper for new pop-sci books to state that wave-duality is an old idea already and describe what Bill has given?

15. Oct 3, 2016

### Simon Phoenix

As has already been mentioned it's something that tends to crop up in introductory material - and certainly in popular accounts. Most people are familiar with the idea of a 'particle' and people are familiar with the notion of 'waves' - so it's not a bad idea to get across some of the ideas (as far as possible) using these familiar notions. But popular accounts of QM are ferociously difficult to write and so the authors can be forgiven, I think, for being a tiny bit economical with the truth - or at least applying a major coat of gloss on the technical details.

Johnjoe McFadden's book "Quantum Evolution" is a popular science book that has some account of decoherence in there. I like it but it runs into the same difficulty as every other popular account in that so many technical details have to be glossed over (and if they weren't glossed over it wouldn't be a book written for the layman - but a textbook)

16. Oct 3, 2016

### fanieh

I have read it too. I have read almost all pop-sci quantum books already.
About what produced one outcome or the problem of outcome or how improper mixed state becomes proper mixed state. Maybe the whole idea of Bohmian Mechanics and Many Worlds and Objective Collapse is still clinging to classical viewpoints (just like wave-duality)? Can we say the wave function is real yet it is neither Bohmian nor Many worlds nor Objective Collapse nor even Copenhagen but just simply information?? This makes more sense is it not.

17. Oct 3, 2016

### Staff: Mentor

Simon is being kind. The Marcella paper is wrong. The issue is you are a beginner - the Rothmanan paper is unfortunately from an advanced standpoint but also wrong:
An introductory course in modern physics contains the typical misconceptions some of the PF users are always trying to fight. It teaches students the obsolete model of Bohr, the obsolete idea of wave-particle duality, emphasizes incorrectly that you need a particle description of light to explain the photoelectric effect and describes the Compton effect as a billiards problem. Then if students get lucky to get a good graduate course from their uni, they should have the chance of washing off all these misconceptions. Or not.

You have admitted you are a beginner but for some reason want to delve into very very deep waters. Don't do that. Accept the issues and as you progress they will be fixed, I know from bitter experience with the issue of Rigged Hilbert Spaces's.

What it really is is 'knots' of a quantum field:
http://www.physics.usu.edu/torre/3700_Spring_2015/What_is_a_photon.pdf

Thanks
Bill

18. Oct 3, 2016

### Staff: Mentor

Last edited by a moderator: May 8, 2017
19. Oct 3, 2016

### Simon Phoenix

I have a personal liking for pop-sci books - I really admire these guys for making the effort despite all the corners that are cut to make them accessible. To be honest - having tried on forums to explain QM to beginners, and seeing just how many knots I tie myself in, I don't really see how many of these pop-sci books could be improved on. Sure they contain 'misconceptions' for the sake of getting a flavour across - but overall I can live with that if it gets more people fascinated by physics. Hardest work I ever did was to try to explain cryptography to a class of 7-10 year olds at a local primary school. Great fun though.

However, Bill is right - I think you really need to get serious now if you want to get your questions answered properly. That will take time and effort. Some of the questions you've asked don't have easy answers - if any answers at all - and some, like the 'reality' of the wavefunction are still hotly debated by physicists. Basically you have to get to the point where, at the least, if someone says to you something like "a quantum state is represented by a positive semi-definite operator" you don't freak out. Ideally that technical shorthand should also be ringing all sorts of bells and linking to all sorts of related topics and ideas.

But to get to that point requires a lot of work - and the book by Susskind that Bill recommended is an excellent place to start.

20. Oct 3, 2016

### mikeyork

Wave-particle duality, is a concept embedded in a space-time framework. All we actually have are energy (and a few other quantum numbers) eigenstates. It is becoming more common to accept that space-time is a projection of observers who find a space-time context helps them understand some aspects of reality in a limited way. So wave-particle duality suffers that limitation.