# Picturing wave/particle duality

I need some help understanding the nature of the wave in terms of QM. Many articles and papers seem to use the metaphor of a wave in water, though admiting its inadequacies. And I've seen models from victorian times that showed the wave in terms of a leading edge following a helix like pattern. This latter is obviously inadequate to explain youngs double slit results in terms of achieving interference patterns. So the water one is more useful but it does have the problem that its medium is kind of two dimensional (removing time for the time being), and the wave function itself provides the extra dimension, whereas the reality being described has, from 'our' perspective, a three dimensional medium, and the wave itself creates the fourth dimension we call EM. So we can adapt our imagination and see waves emanating from a point almost as if that point is a planet 'radiating' its atmosphere away in a pattern thats almost like breathing. Of course breathing requires an inward breath and so thats only really a good visualisation for something like the Transactional Interpretation.

But lets forget about dimension for now, after all the maths seems to suggest there are more than space and time, and focus on that old victorian idea of a particle being the leading edge of the wave. We are imediately confronted with a problem. The particle does not go through both slits. When you see the sea, its not uncommon for waves to "break" along a lateral line, it always seems to be right to left but thats probably just the beaches I've been to. But a wave that is breaking has far more effect on a body it meets than one that is just a swell. So in a detector placed at the slits, if the wave in water analogy holds this far when ignoring dimensions, then the detector would only detect the part of the wave that is "breaking".

I realise these analogies are simplistic, in fact I'm working backwards from the fact light doesn't experience time and the concept of extra dimensions, so simplistic is the best I can do in terms of a way to think about these things. It really gets exciting when you consider Neutrinos that travel as the speed of light and so don't experience time, yet still change as they move.

Of course there is something about symetry that seems plain wrong with the idea of a rolling wave front. But if you accept the possibility of extra dimensions, and how crude the visualisations presented even in the best papers are on the actual mechanics of whats happening in something as simple as the double slit experiment, then I'd prefer to think that the macro is full of clues about the micro.

Now we must bring time back in. For EM of any type it doesn't exist. So why do all QM interpretation rely on it so much ? To be fair the Transactional Interpretation is bold enough to push time's arrow out of the picture in considering the whole concept of advanced and retarded waves. And Bohm didn't seem to concerned with making his theories relativistically invariant. But surely a theory on something that doesn't experience time should start at that point, rather than our own perspective where time exists ? Then we can work towards a way of seeing things that do experience time in a fuller way.

We know that the maths, the quantum formalism, is good. And we know that all the interpretations have no real explanatory power in terms that einstein could explain to his waitress. Why are people so attached to the copenhagen interpretation that has spawned so many "New Age" misinterpretations, when those misinterpretations are mainly due to the fact it doesn't explain what happens in the double slit experiment. Observation in terms of physical measurement is an input into the system - of course. But maybe even when the "particle" is detected at one of the slits, its 'wake' also travelled through the other slit ?

Simon

You can't picture it, ever. They have all the properties of waves until you analyze it and then it behaves as a particle. The closest you'll ever come is to visualise it as a packet of waves spread over a finite volume. The point is the wave/particle duality means it either a particle OR a wave. It can change between them but it's not both at the sametime.

No wonder you support Fulham :)

i must chime in and say i think it is extremely difficult (especially for the western mind) to imagine an object that holds two contrasting properties simultaneously. it is much easier for us to view things as 'either, or', not both.
i think this is key when trying to visualize electromagnetic waves.
the problem i have when trying to do this is i see the waves only projecting from a 2d level.
ex. imagine my body is a transmitter and i hold my arm out at 90degrees and this is where the waves emanate. i know this is not the case, and in fact (i suppose) the waves are projected at all angles into space .
its very tricky

Hi Et

That was why I refered to Einsteins waitress. Surely its mainly "tricky" because we don't understand it ?

Simon

i think I must agree with Fulhamfan, but I would like to add that it is in the state that you want to observe it in. This doesn't mean that it's both, it just means that it is whatever you are looking for. If the problem is seeing it as both, don't try.

Quid pro quo

Hey you physics people are getting really deep and cosmic sounding man!

If the problem is trying to avoid understanding things, then don't try.

If the problem is trying to avoid understanding things, then don't try.

It's not that we're trying to avoid it. It's that we do understand that we don't understand.

You could take the view of string theory where all particles are vibrating strings of pure energy. That would explain alot about why a partcle would diffract but doesn't explain to me about why the other slit would affect.

Or there's another view where it's interacting with particles in another universe. I think I heard that Stephen Hawking takes this view seriously.

Then there's feynman's interpretation. where a particle not only goes through both slits but also take an infinite number of other paths. Meaning a particle going through one slot is as plausible as a particle going to the moon and back. All paths cancel out except for the one you observed for the experiment.

I think the most plausible is feynman's where if you think of it as a wave that propagates everywhere, all paths cancel out except one.

hi SimonA

SimonA said:
It really gets exciting when you consider neutrinos that travel at the speed of light and so don't experience time, yet still change as they move.

Neutrinos don't travel at the speed of light. They have mass.

SimonA said:
Of course there is something about symmetry that seems plain wrong with the idea of a rolling wave front. But if you accept the possibility of extra dimensions, and how crude the visualisations presented even in the best papers are on the actual mechanics of what's happening in something as simple as the double slit experiment, then I'd prefer to think that the macro is full of clues about the micro.

Now we must bring time back in. For EM of any type it doesn't
exist. So why do all QM interpretation rely on it so much?

It is in the nature of a localised particle that it travels at a
speed less than $c$. As you know, noncommutativity of
observables is fundamental to QM. For example, between
$\mathbf{x}$ (static) and $\mathbf{p}$
(dynamic). It is therefore quite tricky to describe this without
putting time in.....

....but as it happens, there is a way to interpret things where
quantum information 'generates its own spacetime'. Consider the
qubit: this is simply $\mathbb{C}^{2}$ as a Hilbert
space with basis $| 0 \right\rangle$ and $| 1 \right\rangle$. According to:

A modular functor which is universal for quantum computation
Michael Freedman, Michael Larsen, Zhenghan Wang
http://arxiv.org/abs/quant-ph/0001108

one can associate to the state space of a quantum computer built
on k qubits, given by $$( \mathbb{C}^{2} )^{\otimes k}$$, a
disc with 3k marked points, on which there is naturally an action
of the braid group $B_{3k}$.

In other words, one can use Jones polynomials....the same knot
invariants that get used to describe so-called pure quantum
gravity in three dimensions (that's one time and two spatial
dimensions).

Now recall that the 'wave' is to be interpreted as a probability
density. If we can remove the space on which the wavefunction is
specified, then the probability density does not rely on the
background a priori, but defines the geometry that we observe.

If we have two slits, we should expect to see evidence that the
geometry is 'wavelike' because that is a constraint that we have
put on our observation.

Don't know if this helps.

Kea

Kea

You are a gentleman (or woman), and a scholar :)

Now recall that the 'wave' is to be interpreted as a probability
density. If we can remove the space on which the wavefunction is
specified, then the probability density does not rely on the
background a priori, but defines the geometry that we observe.

This is great. I hope to understand it better soon!

Thanks

Simon

Hi FulhamFan3

Does "weak measurement" change your fundamental adherence to CI at all ? And where exactly is the observer relevant except in terms of the detector made of matter ?

Simon

I don't think the wavefunction of a particle and the physical wave of a particle is the same thing, right?

What is "the physical wave of a particle " ?

What do you mean by "weak measurement"? If the experiment is setup as particle source aimed at 2 slits then the observer is at a screen past the slits taking note of where those particles land.

Do you want another interpretation because you think you can predict the result otherwise?

Ever seen this ?

Although it is not necessary to have a picture or model for everything in physics, still it is better to have one. It only helps in its progress. Noharm in trying to visualize things. And there's nothing that can't be visulaized.

spacetime
www.geocities.com/physics_all/

waveparticles

Hi All:

What helped me to obtain a satisfactory mental image of this conundrum was remembering Einstein's remark that matter is just lumpy bits of spacetime, the lessons in Feynman's book, " The Strange Theory of Light and Matter", the concepts of loop quantum gravity and the shapes of the "curled up" dimensions of M Theor and Born's interpretation of the Schroedinger/Heisenberg equations ( the wave packet is the particle). Bearing in mind from the teachings of field theory that the wave equation's expanse is infinite, albeit asymtotically diminishing, that a particle can "smell" the area surrounding it (Feynman) becomes more comprehensible. This view also provides a model for understanding non-locality events. Of course, this all requires that we abondon the zero dimensional concept of a point, but nothing ventured, nothing gained!

Dennis Plews
Sarasota Fl

The "Wavefunction Colapses" when we do a measurement.. what exactly is a measurement?

If in the double slit experimet we just turn off the Wich way detectors at the slits, but we leave them there, the interference patterns disapear too??

another question, inside of a living cell, what dominates? quantum or clasical physiscs???????

Thanks Mr Plews for your thoughts.

Burnsys - "a measurement" is what is required to detect a property of the "particle(s)"

On your second point, apart from "weak measurement" which I believe is still controversial, detectors are matter which will absorb the "particle" whether or not they are "turned off". Its a good point though in terms of some of the interpretation of CI you get.

On your last point, both quantum and classical processes are at work. And in the middle are molecules such as proteins that move around a cell like the way cars move around a city. We have no idea really what drives them to do so.

Someone recently introduced be to Shroedingers Negentropy -> http://www.i-sis.org.uk/negentr.php

'Negentropy', as stored mobilizable energy in a space-time structured (organized) system, can be intuitively understood as follows. In an equilibrium system, energy is fixed, which in turn fixes the population of energy levels characteristic of the temperature of the system. In a nonequilibrium system such as the organism, energy is stored over all space-time domains. For a given temperature, the energy stored is no longer fixed, but on account of efficient coupling, becomes transferred to ever larger space-time domains (starting from the photon trapped in photosynthesis, or the energy in food) until all characteristic domains are equally populated. This implies that the organism itself has no preferred levels, its activities spanning the 'quantum' to 'classical', from the 'microscopic' through 'mesoscopic' to the ' macroscopic' in a quasi-continuum of self-similar patterns.

Last edited:
ZapperZ
Staff Emeritus
Dennis J. Plews said:
Hi All:

What helped me to obtain a satisfactory mental image of this conundrum was remembering Einstein's remark that matter is just lumpy bits of spacetime, the lessons in Feynman's book, " The Strange Theory of Light and Matter", the concepts of loop quantum gravity and the shapes of the "curled up" dimensions of M Theor and Born's interpretation of the Schroedinger/Heisenberg equations ( the wave packet is the particle). Bearing in mind from the teachings of field theory that the wave equation's expanse is infinite, albeit asymtotically diminishing, that a particle can "smell" the area surrounding it (Feynman) becomes more comprehensible. This view also provides a model for understanding non-locality events. Of course, this all requires that we abondon the zero dimensional concept of a point, but nothing ventured, nothing gained!

Dennis Plews
Sarasota Fl

OK, I just can't help but ask this: Aren't you the least bit uncomfortable or even weary, that you are basing your confidence of your ability to explain away this problem on an area that (i) are still FAR from being in an accepted stage (ii) are still not verified empirically (iii) are still undergoing major changes that is still nowhere near a final, coherent form? I mean, REALLY! It's one thing to read about loop quantum gravity and M-theory and "curled-up" dimensions, but to actually use that to comfortably explain away what is essentially a non-problem? That's astounding!

There is no such thing as "duality" of light in QM. Don't believe me? Open a QM text and find where light is described in two different formulations. This duality arises due to our inherent NEED to catagorize something either as a wave, or as a particle, base on the classical wave and particle behavior. You are trying to fit a square object into a round hole. When it doesn't fit, you blame the hole, rather than look at the fact that what you're holding is not MEANT to fit into that hole. Once we all realize this, then we automatically see that there's no "duality" or any problem here!

Zz.

ZapperZ
Staff Emeritus
SimonA said:
Thanks Mr Plews for your thoughts.

Burnsys - "a measurement" is what is required to detect a property of the "particle(s)"

On your second point, apart from "weak measurement" which I believe is still controversial, detectors are matter which will absorb the "particle" whether or not they are "turned off". Its a good point though in terms of some of the interpretation of CI you get.

This is wrong. If a particle such as an electron passes through a loop of wire, it doesn't get absorbed by it. Yet I can still detect its presence via the current in the loop. Unless you do not consider a loop of wire a "detector".

Someone recently introduced be to Shroedingers Negentropy -> http://www.i-sis.org.uk/negentr.php

What you quote is pure quackery. For example:

For a given temperature, the energy stored is no longer fixed, but on account of efficient coupling, becomes transferred to ever larger space-time domains (starting from the photon trapped in photosynthesis, or the energy in food) until all characteristic domains are equally populated.

Other than the fact that this is nothing but goobleygook (the usage of physics words but in very awkward manner), a photon doesn't get "trapped" in anything, other than supercold Cs atom under laser-induced transparency![1] The energy of a photon is absorbed. It goes away! Photon number is not conserved! Saying it is "trapped" implies that its coherency (phase) is preserved! This is bogus!

Again, I suggest that this is all moot if one properly studies QM.

[1] C. Liu et al. Nature v.409, p.490 (2001).

Zz

I have read a few of your posts and so know you are one of the better informed here. But I'm still curious if you could explain Youngs slit experiment in terms of why the "wave" only appears at one slit and/or why the "particle" goes through both slits.

But more pertinantly, when is it that shroedingers cat becomes dead (and smelly ?) or a cute puring little kitty hungry for some fish :)

ZapperZ
Staff Emeritus
SimonA said:
Zz

I have read a few of your posts and so know you are one of the better informed here. But I'm still curious if you could explain Youngs slit experiment in terms of why the "wave" only appears at one slit and/or why the "particle" goes through both slits.

But more pertinantly, when is it that shroedingers cat becomes dead (and smelly ?) or a cute puring little kitty hungry for some fish :)

The wave only appears at ONE slit? Huh?

How about reading this: The interference pattern in the result of, NOT the photon (or any particle you wish you insert in there), but rather the SUPERPOSITION OF PATHS!

People are reading Feynman and citing him left and right, yet, it appears that most of you ignore the most important contribution he did, his path integral method! But if you do not study QM properly, how could you know this? All you care about are the stuff about him that's written in pop-sci books!

Again, READ the Marcella paper that I've cited a gazillion times on here to see how one can painfully derive the 2-slit interference patterns from photons using standard QM! There is no mystery here, especially when one strip away the very stubborn insistence of imposing a classical particle or wave behavior onto a "photon" or any other quantum entity.

Zz.

Who said I was imposing classical behaviour onto quantum entities ? Where ? This is a lazy excuse to lie back and say "ahh its too complicated to understand - just learn the maths". And that exactly why you avoided any reference to the cat.

And why do you get do funny about the idea that the "wave" only appears at one slit ? Do you accept the idea of the wave form collapsing ?

I'm not questioning that the formalism predicts the results of youngs interpretation. I'm asking questions about whats actually happening. Thats what I was hoping for in my first question. The second one I'm still curious about your opinion on.

To ZapperZ

I missed this post where you said this;

This is wrong. If a particle such as an electron passes through a loop of wire, it doesn't get absorbed by it. Yet I can still detect its presence via the current in the loop. Unless you do not consider a loop of wire a "detector".

I was wrong there. But I was answering a question I didn't think I needed to think much about :)

Simon