Wave-particle duality revisited

[Royston]

This follows on from the thread 'what makes an electron orbit', but it's ten pages long (and counting) and gets a bit awry half way through. So I decided to start a new thread, and include a poll, where you can cast your vote, and include your reasoning to back it up. I've kept the description simple to just waves and particles, so it allows anyone to explain what they mean by (for instance) a particle.

 

 

Note, that I have not studied QFT, so I'll only provide a basic description from what I've been told, and then I'll provide a few examples to the whole wave-particle duality conundrum. It would be nice to hear a few expert opinions, as I know there's a few on here who have studied QFT. It would be doubly nice if they could provide some information, that could bridge the gap of understanding, for anyone who has not studied QFT.

 

 

Classically, a particle could generally be considered as a zero dimensional point, it can have properties such as mass, but point-like keeps the maths simple, and is ample for describing macroscopic behaviour.

 

 

In basic QM, the state of a system is described by a wave-function, which is the most complete description of that system. The wave function is not measurable, so is acted on by an operator, an observable is associated with that operator. The outcomes of the observable are dictated by it's eigenvalues. If the observable is position, then the act of measurement collapses the wave function, and what is detected is a particle. Taylor's diffraction experiment, or Young's double slit experiment, are good examples of this.

 

 

In basic QFT, (sticking to QED) particles can be considered as quantized excitations of a field. The number of particles are not fixed, and the use of operators (lowering and raising) create and destroy particles in that field. Applying QED to say a Mach Zehnder interferometer, still requires a wave and particle description.

 

 

The above definitions are probably far from perfect. However, some experiments still require a wave-particle duality interpretation, where as others do not, perhaps say a bubble chamber experiment. Are there more abstract definitions of a particle that could do away with the wave-particle interpretation in some experiments ?

 

 

Any corrections are welcome, but I'd be very interested to hear your thoughts on the matter (no pun intended).

Edited May 9, 2012 by Royston

[ajb]

Quantum field theory seems to be the best framework for nature we have. Really quantum field theory is a theory of quantum fields not particles, which are rather a derived notion and quite a special one particular to "flat-like" space-times. Mathematically one often deals with the theory rather formally, but that is okay.

 

Even more abstractly, one school of though is that the fields are not the primary objects, but rather the algebra of observables. This gets mathematically tough very quickly and the framework cannot cope with realistic theories.

[Royston]

Really quantum field theory is a theory of quantum fields not particles, which are rather a derived notion and quite a special one particular to "flat-like" space-times.

 

That's absolutely fascinating, thanks.

 

One of our tutors said pretty much the same thing, he then went on to use QED as an example, he had to keep it basic as we're studying an undergrad course. I guess quantum field theory can be reduced back down to more basic formalisms depending on the circumstances, analogous to the correspondence principle. So I guess the outcome of certain experiments can be viewed as a special case of QFT. I realise there are a number of quantum field theories, that seem to deal with particular problems, so I may be over simplifying here, but I'm hoping I'm on the right track.

 

Even more abstractly, one school of though is that the fields are not the primary objects, but rather the algebra of observables. This gets mathematically tough very quickly and the framework cannot cope with realistic theories.

 

Blimey, well I'll stick with basic QM for the time being, I'm actually just studying this out of interest i.e I have enough credits to obtain my degree already, but I'm absolutely hooked on the subject.

 

I'm going to vote waves and particles, because that's as far as my understanding goes for the time being (regardless of whether it's correct). It seems unfair to vote 'something more abstract', when I don't understand that level of physics.

Edited May 9, 2012 by Royston

[juanrga]

Classically, a particle could generally be considered as a zero dimensional point, it can have properties such as mass, but point-like keeps the maths simple, and is ample for describing macroscopic behaviour.

 

Where in the chapter "1.1 Mechanics of a Particle" of Goldstein's classic textbook you find that a classical particle must be zero-dimensional?

 

I do not find necessary to introduce such requirement in the concept of particle.

 

Precisely the concept of "classical electron radius" was obtained in classical physics. Of course, I am not saying that this classical model was realistic.

 

The next is not clear for me. Do you consider a massless object a particle?

 

In basic QM, the state of a system is described by a wave-function, which is the most complete description of that system. The wave function is not measurable, so is acted on by an operator, an observable is associated with that operator. The outcomes of the observable are dictated by it's eigenvalues. If the observable is position, then the act of measurement collapses the wave function, and what is detected is a particle. Taylor's diffraction experiment, or Young's double slit experiment, are good examples of this.

 

This is not true. As Cohen-Tannoudji states in his textbook on QM:

 

Actually, the introduction of state vectors and the state space does more than merely simplifying the formalism. It also permits a generalization of the formalism. Indeed, there exists physical systems whose quantum descriptions cannot be given by a wave function: we shall see in chapters IV and IX that this is the case when the spin degrees of freedom are taken into account, even for a single particle.

 

There are many more examples of quantum systems whose state is not described by a wavefunction.

 

In the double slit experiment, what is measured is the position of the particles (plural), the particles are always detected (we know that the particles are there). Next is the typical sequence of such one experiment. (a) is the detector when only eleven electrons were used. (e) is the detector when the experiment is repeated with thousand of electrons:

 

 

In basic QFT, (sticking to QED) particles can be considered as quantized excitations of a field. The number of particles are not fixed, and the use of operators (lowering and raising) create and destroy particles in that field. Applying QED to say a Mach Zehnder interferometer, still requires a wave and particle description.

 

There exists an old interpretation of QFT, where particles are excitations of a field. But there is a problem or two... First, a field is not observable, by definition. It makes no sense to select as basic building blocks of nature non-observable stuff. What one detects in experiments as those made at CERN are particles never fields.

 

Second, the model of fields is based in many approximations (infinite N, harmonics, free fields...). Third, it is possible to explain all the phenomena associated to QED without fields. This is the no-fields approach to QED, sometimes named the action-at-a-distance-approach. This is my favourite approach.

 

Today field theory is considered an effective theory, not a fundamental theory. Therefore, another reason for which is not reliable to associate the concept of particle to the field is that a field is not fundamental.

 

I do not know what do you mean by "still requires a wave and particle description".

 

The above definitions are probably far from perfect. However, some experiments still require a wave-particle duality interpretation, where as others do not, perhaps say a bubble chamber experiment. Are there more abstract definitions of a particle that could do away with the wave-particle interpretation in some experiments ?

 

I do not know a single experiment in physics, chemistry, biology, astronomy, cosmology... which cannot be explained by the known theories of particles. In fact this fact is the reason for which S. Weinberg and other Nobel winners use the Wigner definition of particle. As the CERN emphasizes in its own website:

 

The theories and discoveries of thousands of physicists over the past century have resulted in a remarkable insight into the fundamental structure of matter: everything in the Universe is found to be made from twelve basic building blocks called fundamental particles

 

By "everything" they mean everything known up to now.

Edited May 10, 2012 by juanrga

[questionposter]

Quantum field theory seems to be the best framework for nature we have. Really quantum field theory is a theory of quantum fields not particles, which are rather a derived notion and quite a special one particular to "flat-like" space-times. Mathematically one often deals with the theory rather formally, but that is okay.

 

Even more abstractly, one school of though is that the fields are not the primary objects, but rather the algebra of observables. This gets mathematically tough very quickly and the framework cannot cope with realistic theories.

 

Don't those fields have planar-wave solutions though?

Also, I would say right now that "wave-particle-duality" is the best description, but that doesn't mean particles can't have properties of waves or have wave-mechanics describe them. They either are the oscillation itself or they are the whole of an oscillating field.

Not only that, but the only distinction of particles that I have seen so far as that one is a little solid sphere that has mass and spin, I haven't seen anything to suggest they can account for superposition and field cancellation without considering that a particle has oscillatory properties.

Edited May 10, 2012 by questionposter

[Royston]

blah, blah, blah

 

You've completed glossed over terms such as basic, and could be considered, there's only so much you can cover in a few sentences. I was setting the scene for a discussion. juangra, personally I couldn't care less what you think, you come across as an educated crank, and your attitude sucks. You've already been banned from the physics forums, and people like you make it hard for others to learn. I may get a mod warning for that, but it needed to be said. I've stuck you on my ignore list...along with questionposter, who ONCE AGAIN, has the temerity to lecture an expert with nonsensical drivel.

 

Sorry for the outburst, but I really wanted to have a decent discussion about the subject, not have it polluted with this guff.

[hypervalent_iodine]

!

Moderator Note

In fact, Royston, staff tend to agree with you on this (although in future, please use our reports feature so we can deal with it).

To add an official note to Royston's post, juangra, your attitude leaves something to be desired. Your posts are framed in such a way that it seems you are trying to win some sort of imaginary competition, which is not how a discussion works. Please stop acting so condescending towards other members and start being more cooperative or your time here will be brief (which is not something we want).

Questionposter, this is not an official warning, more of a bit of advice. For the benefit of the people who take the time to respond to you, could you please take some time to properly think through your posts and the posts of others to avoid the circular and (at times) frustrating debates encountered in other threads.

[ajb]

Don't those fields have planar-wave solutions though?

 

The free non-interacting solutions do. In the Dirac picture (which is mathematically ill) you take the in and out states to be plane waves.

[juanrga]

You've completed glossed over terms such as basic, and could be considered, there's only so much you can cover in a few sentences. I was setting the scene for a discussion. juangra, personally I couldn't care less what you think, you come across as an educated crank, and your attitude sucks. You've already been banned from the physics forums, and people like you make it hard for others to learn. I may get a mod warning for that, but it needed to be said. I've stuck you on my ignore list...along with questionposter, who ONCE AGAIN, has the temerity to lecture an expert with nonsensical drivel.

 

Sorry for the outburst, but I really wanted to have a decent discussion about the subject, not have it polluted with this guff.

 

The wave-particle myth arose in during the development of QM, when the subject was being developed and was not still understood. That myth has been completely eliminated from any modern and rigorous treatment of the subject. Although it persists in some circles.

 

An electron is not sometimes a particle sometimes a wave as some believe.

 

Chemists and physicists define the electron as a particle not as "sometimes a particle sometimes a wave". Links to the CERN and to the official definition of electron by the American Chemical Society were given, quotes from textbooks were given...

 

Those are the facts and those facts will not disappear by you ignoring them and insulting to others.

 

Regarding physics forums, I kindly asked one moderator (in a personal message) to close the account. And I am considering to do the same here because it is a waste of time to debate with people who do not know the most elementary stuff but still pretend to be experts: Questionposter is an excellent example of this, but he is not the only.

 

!

Moderator Note

In fact, Royston, staff tend to agree with you on this (although in future, please use our reports feature so we can deal with it).

 

To add an official note to Royston's post, juangra, your attitude leaves something to be desired. Your posts are framed in such a way that it seems you are trying to win some sort of imaginary competition, which is not how a discussion works. Please stop acting so condescending towards other members and start being more cooperative or your time here will be brief (which is not something we want).

 

Questionposter, this is not an official warning, more of a bit of advice. For the benefit of the people who take the time to respond to you, could you please take some time to properly think through your posts and the posts of others to avoid the circular and (at times) frustrating debates encountered in other threads.

 

 

My posts usually include reasoning, advanced details, citations to academic references, quotes and links. And I agree that this seems to be uncommon among posters here.

 

It is curious that you appeal to "some sort of imaginary competition", because this is exactly the impression I get when I read the replies of other posters. Some of them systematically change what others wrote and then reply just to try to submit some post giving them the impression that they are winning that "imaginary competition". Whereas other posters pick some unrelated stuff from Wikipedia and post it in their reply, giving them the impression that they are not still out of the "imaginary competition"...

 

Several people have noticed, in public way and several times, the silly behaviour of those other posters, but this seems to be the first time that you write something to one of them.

 

Moreover, this is the second time that 'Royston uses ad hominem and insults against me. If my memory does not fail he was warned by a moderator in another forum.

 

Although I was warmly received when I joined the forum, I am seriously considering if I will continue here or not.

 

Regards.

Edited May 10, 2012 by juanrga

[studiot]

I have held off this thread until now in the hope that it could develop in better spirit that in the previous one.

 

Juan, everyone can be wrong.

 

Even you can be wrong, as you undoubtedly are in this piece of basic mathematics, which I did not respond to before as I put down it to a language issue.

 

psi is not a variable

 

If psi is not a variable it is either precisely zero or constant. There are no other available alternatives. A constant or zero psi is a pretty useless property to work with.

 

Psi is a varaible, correctly known as the dependent variable

 

[math]\Psi (x,t)[/math]

 

of the independent variables x and t.

 

You undoubtedly know a great deal but have you heard this old (English) saying?

 

"Wise men suffer fools gladly"

 

From your posts I gather you are some kind of lecturer or teacher (presumably at a university).

I have never been a formal teacher, however I have often found that attempting to explain in simple terms to someone else (perhaps an assistant or non technical person) has improved my own knowledge of a subject.

 

Rather than leave or shun humanity, join in and be welcome. Several here have offered pointers.

Edited May 10, 2012 by studiot

[hypervalent_iodine]

!

Moderator Note

For the record, I don't give a damn about the posting history or staff warnings given to members on other forums. That is their business. Staff here base moderator actions on the posts you make on SFN and SFN only.

juanrga, if you wish to question something that staff have said to you in-thread, it's usually better to do so via PM or by using the report feature. Replying to mod notes isn't strictly against the rules, though it does lend to off topic posting and it's usually a much simpler process when dealt with outside of the rest of the conversation.

[granpa]

http://arxiv.org/abs/1002.3880

 

the electron conducts itself not as re-radiated by the edge wave and not as a "probability amplitude wave". It behaves as deterministic non-local micro-object with a spatial density distribution. From the experimental observations of the angles of preferred scattering we may infer that this density

distribution has the shape of concentric shells of different hardness, the interval between the shells being of the order of de Broglie length

 

 

conclusions:

 

In the series of experiments above described I disproved the generally accepted since 1930s opinion that it is allegedly impossible without destroying interference pattern to observe the place, say, a slit, where the electron passed.

 

using the semiconductor sensors xed in the slit's edges enabled me to observe electron's fly not destroying the interference pattern.

 

 

Semiconductor sensors perceptible to nearby electron were so efficient that appeared to be capable to determine not only the slit where the electron passed through but even the proximity of the track to the left or right edge of each aperture.

 

When encountering an obstacle the electron does not bend around it, as EMWS do, but bounce o it as is appropriate to classical elastic macro-objects.

 

In the scattering pattern of electrons the left side peaks is formed by the ricochet from the right edge of the slit, and the right side peak is formed due to ricochet of electrons from the left edge of the slit, no interference of the electron's flows taking place

[Royston]

juangra, and questionposter, firstly, apologies for the rude post, it does nothing to bolster my arguments, and I really should know better than to post when I'm cranky. There's nothing worse than a hypocrite.

 

juangra, I guess we'll have to agree to disagree, that reality is solely described as particles. I only chose to vote waves and particles, because that's as far as my understanding takes me, and I realise it's far more complicated than that. I think the correct answer is that it is something more abstract, and I've already checked this elsewhere, with other people in the field (I guess you could argue that's an appeal to authority) but I feel they are trustworthy sources. I don't really wish to discuss the matter with you any further, thanks.

 

Regarding physics forums' date=' I kindly asked one moderator (in a personal message) to close the account. And I am considering to do the same here because it is a waste of time to debate with people who do not know the most elementary stuff but still pretend to be experts: Questionposter is an excellent example of this, but he is not the only.[/quote']

 

My bad, also remember you can use the ignore feature, if somebody is getting on your nerves. I hope I don't come across as somebody masquerading as an expert, I'm very careful to include my level of knowledge when I try to tackle more technical queries.

 

Moreover' date=' this is the second time that 'Royston uses ad hominem and insults against me. If my memory does not fail he was warned by a moderator in another forum.

 

Although I was warmly received when I joined the forum, I am seriously considering if I will continue here or not.

 

Regards.

[/quote']

 

I try very hard not to use logical fallacies in my posts, so if I did use an ad hominen, again my bad, but I don't recall this. Also, unless you're a member of my Uni forums, which I very much doubt, then this is the only forum I use. I am a member of the physics forums, but I've never posted on there, just a lurker.

 

Lets get back to discussing this very interesting subject.

Edited May 10, 2012 by Royston

[derek w]

In the standard model,there are 4 forces gravity,weak,strong and electromagnetic.An oscillator requires a driving force,but there is no mention of such a force.

My question is which force is driving the oscillations of a wave function,is it the weak force.

[juanrga]

I have held off this thread until now in the hope that it could develop in better spirit that in the previous one.

 

Juan, everyone can be wrong.

 

Even you can be wrong, as you undoubtedly are in this piece of basic mathematics, which I did not respond to before as I put down it to a language issue.

 

 

 

If psi is not a variable it is either precisely zero or constant. There are no other available alternatives. A constant or zero psi is a pretty useless property to work with.

 

Psi is a varaible, correctly known as the dependent variable

 

[math]\Psi (x,t)[/math]

 

of the independent variables x and t.

 

You undoubtedly know a great deal but have you heard this old (English) saying?

 

"Wise men suffer fools gladly"

 

From your posts I gather you are some kind of lecturer or teacher (presumably at a university).

I have never been a formal teacher, however I have often found that attempting to explain in simple terms to someone else (perhaps an assistant or non technical person) has improved my own knowledge of a subject.

 

Rather than leave or shun humanity, join in and be welcome. Several here have offered pointers.

 

Thank you for taking a quote from a closed thread and use it for sharing your wise point that [math]\Psi[/math] is a variable. Although I will ignore you and continue to consider that [math]\Psi[/math] is a function. But thanks again.

 

!

Moderator Note

For the record, I don't give a damn about the posting history or staff warnings given to members on other forums. That is their business. Staff here base moderator actions on the posts you make on SFN and SFN only.

 

juanrga, if you wish to question something that staff have said to you in-thread, it's usually better to do so via PM or by using the report feature. Replying to mod notes isn't strictly against the rules, though it does lend to off topic posting and it's usually a much simpler process when dealt with outside of the rest of the conversation.

 

When I wrote that he was warned in "another forum" I did mean one of those in the menu Forums (above between the Homepage link and the Members link). Evidently, I did not mean that you as moderator would eliminate insults and nasty behaviour in forums outside SFN, but in any case thank you by the clarification. It was very helpful!

 

juangra, and questionposter, firstly, apologies for the rude post, it does nothing to bolster my arguments, and I really should know better than to post when I'm cranky. There's nothing worse than a hypocrite.

 

juangra, I guess we'll have to agree to disagree, that reality is solely described as particles. I only chose to vote waves and particles, because that's as far as my understanding takes me, and I realise it's far more complicated than that. I think the correct answer is that it is something more abstract, and I've already checked this elsewhere, with other people in the field (I guess you could argue that's an appeal to authority) but I feel they are trustworthy sources. I don't really wish to discuss the matter with you any further, thanks.

 

 

 

My bad, also remember you can use the ignore feature, if somebody is getting on your nerves. I hope I don't come across as somebody masquerading as an expert, I'm very careful to include my level of knowledge when I try to tackle more technical queries.

 

 

 

I try very hard not to use logical fallacies in my posts, so if I did use an ad hominen, again my bad, but I don't recall this. Also, unless you're a member of my Uni forums, which I very much doubt, then this is the only forum I use. I am a member of the physics forums, but I've never posted on there, just a lurker.

 

Lets get back to discussing this very interesting subject.

 

Thank you for your very informative post and the "appeal to authority", although your sources do not represent any real authority.

 

As you wrote in the OP, those sources say you that the state of a quantum system is given by a wavefunction [math]\Psi(x,t)[/math].

 

In more advanced and rigorous literature we rewrite this as [math]\Psi(x;t)[/math], and next we generalize this formalism

[math]\Psi(x;t) \rigtharrow \hat{\sigma}(x;t)[/math]

to a broad kind of quantum systems beyond those considered in your "trustworthy sources".

 

Following your explicit desire I will not explain why advanced and rigorous references write [math]\Psi(x;t)[/math] instead of the outdated [math]\Psi(x,t)[/math] that you can still find in many sources. I will not give the references (are not at the undergrad level evidently) and I will not explain the differences between both functions, neither why the outdated form is mathematically incorrect and physically nonsensical.

 

I have just deleted my vote from your poll giving you and other participants in this forum more freedom to share your thoughts. Thank you again.

 

In the standard model,there are 4 forces gravity,weak,strong and electromagnetic.An oscillator requires a driving force,but there is no mention of such a force.

My question is which force is driving the oscillations of a wave function,is it the weak force.

 

After reading this, I have stopped from watching this thread.

Edited May 11, 2012 by juanrga

[qsa]

@Royston

 

I will not debate with juanrga, But I will share few things with you and I will elaborate when I have the time. My point of view is that the debate as to what is more fundamental is ongoing. There are 4 or 5 main views with a lot of other variations.

 

1. Particle physicists view of particle being fundamental.

 

 

2. field/wave functional ( which is nothing but the extension of normal Non relativistic schrodinger wave equation) supported by Weinberg as I showed in the other thread.

 

3. The question is unscientific,we only care about the formalism.

 

 

4. it is both like in this reference(bohemian picture) see page 38

 

http://xxx.lanl.gov/...-ph/0609163.pdf

 

 

 

also this

 

http://www.mat.univi...hysics-faq.html

 

and particularly this

 

http://www.mat.univi...opics/pointlike

 

so the question is more involved and depends on the prespective.

Edited May 12, 2012 by qsa

[michel123456]

bohmian, not bohemian

[qsa]

bohmian, not bohemian

 

Thanks for the correction and the link.

[derek w]

Emergence is the way complex systems and patterns arise out of a multiplicity of relatively simple interactions.

My vote goes with this idea.

I think the wave/particle duality arises because of some undiscovered more fundamental interactions.

[studiot]

Hello Derek,

 

"It takes two to tango"

 

Interactions require at least two particles or whatever.

 

This thread is about an isolated electron or whatever.

 

Depsite his unwillingness to accept comments by others, juanrga made many good points that should not be lightly dismissed.

[Greatbear]

Dears all,

A recent formalism treating  a system exhibiting this duality is mathematically exposed in the article

link removed by moderator

GreatBear

 

[Phi for All]

1 hour ago, Greatbear said:

A recent formalism treating  a system exhibiting this duality is mathematically exposed in the article

!

Moderator Note

Can you post the relevant information here, please? We have no way of knowing how other sites might treat our members, so the rules require anything the members need to know in order to discuss the topic be posted here. Thanks for understanding!

 

[TheVat]

3 hours ago, Greatbear said:

Dears all,

A recent formalism treating  a system exhibiting this duality is mathematically exposed in the article

link removed by moderator

GreatBear

 

Were you at the now defunct sciencechatforum?  Henriette, or some similar name?  This seems very familiar.  Anyway, looking forward to more info.