# Quantum myth 1. wave-particle duality

1. Apr 17, 2008

### pellman

Demystifier has a paper available entitled "Quantum mechanics: myths and facts". http://xxx.lanl.gov/abs/quant-ph/0609163 This is a fine overview of a lot of stuff which I would like to understand better. Please join me in discussing.

There are 9 myth categories. By myth the author means widely repeated statements which, true or false, are not something we can validly assert given our current understanding.

myth category 1. Wave particle duality (see section 2 of the paper)

The gist of section 2.1 is that based on the usual interpretation of QM, there is only the wave. What we call particle is merely the special case of a localized wave packet--of finite width and only ideally a delta function in the limit of $$\Delta x \rightarrow 0$$.

I have two tentative objections.

(1) The single particle wave function $$\psi (x,t)$$ can be misleading. It suggests something like an EM wave or a fluid wave. That is, a field, a value at each point in physical space at a given time. But this is wrong of course. For two particles we don't have $$\psi_1(x,t)$$ and $$\psi_2(x,t)$$ (although that may be approximately true if they don't interact) but rather $$\psi (x_1, x_2; t)$$. The wave function lives in configuration space not physical space. Hence, it is not physically real but instead only a calculational tool.

That is, unless we are willing to grant configuration space a sort of platonic reality that is more real than the physical world that we experience with our senses. A logical possibility of course. The world of normal space time of our experience may merely be the result of how our brains+senses fit in as part of the universal wave function. In that case one could say that the wave function is the reality and it is the world of our experience which is the "calculational tool" provided to us by evolution for survival.

But this is a position of which I think we should be very suspicious. The most glaring problem with it in my opinion is the "measurement problem" and the fact that we don't observe superposition of macro objects, a fact for which the wave function alone has no answer. And for other reasons which will come up as we discuss the "myths".

(2) Particle position plays such a central role in actual measurements. I have heard it said that all measurements ultimately reduce to positions measurements. (But is this one more myth?)

This suggests to me that the usual formulation of QM is only an approximation and that a better theory would be a particle theory. And indeed, in Sec 2.2 Demystifier discusses such a possibility in the Bohmian interpretation.

But what do we even mean by "particle"? I have to admit that at this level I have better mental grasp of the wave function than what a "particle" is or might be.

2. Apr 17, 2008

### Demystifier

Could be. But I don't know any counterexample. Does anybody?

3. Apr 17, 2008

### Moridin

There is no duality, and things such as light is neither a "particle" or a "wave". Quantum mechanics provide a singular and consistent description.

4. Apr 17, 2008

### Haelfix

calorimeter?

5. Apr 17, 2008

### nrqed

Wow...where did you hear that? I personally have always thought this to be the case and I even said so in a recent thread in the GR forum (I even said that, as far as I can tell, even all time measurements actually reduce to position measurements...) and one of the forum monitors closed down the thread basically calling me a crackpot!

interesting thread by the way. I have always wanted to discuss that very interesting paper by Demystifier

6. Apr 17, 2008

### reilly

Re $$\psi (x_1, x_2; t)$$..A similar form for E and B will hold in classical E&M for example, for two charged particles interacting and radiating with the E&M field. And there's always a mapping from configuration space -- as you define it -- to physical space. Think, for example, also about Hamilton-Jacobi Theory and how it is formulated.
Regards,
Reilly Atkinson

7. Apr 17, 2008

### pellman

I don't know. I am sure I have heard and read it a number of times. That is, ... er ..I was sure ...

Let's see if I understand. A calorimeter is a thing in which an energetic particle is directed and its energy ultimately transferred to a dense material. From the change in temperature of the material we infer the energy of the original particle. Right?

Now that I think about it, the claim about all measurements reducing to position measurements is probably in the sense that the measurement is made by checking the position of something, not necessarily a quantum particle. In the calorimeter case, the change in length or volume of the material associated with its change in temperature? (I don't know how the temp change is measured.) Something like that.

But this position measurement would be a quantum variable. There would be an uncertainty in the observation. In this case it would likely have to do with energy/time uncertainty. When do we note the temperature change? When can we safely say all the energy has dissipated throughout the material?

Clearly, I haven't thought it through carefully. But that is the gist of it, I suppose.

The time/energy uncertainty relation is handled in a later section of the paper, btw.

8. Apr 17, 2008

### RandallB

I can understand “consistent description” limited to detail no finer than allowed by HUP.
But what might you mean by “singular description” ?

Seems to me there are a multitude of descriptions, all consistent within the limits of QM.
BM, MWI, oQM, Strings, etc.

This seems to fall more in the category of something we currently cannot validly assert, or a myth as defined in the OP.

9. Apr 17, 2008

### Moridin

RandallB, I made a poor choice of words.

I meant that there wasn't really a particle/wave duality. Perhaps I should have used single (in contrast with dualist) instead.

10. Apr 17, 2008

### strangerep

In QFT, the standard meaning of "particle" is "unitary irreducible representation (unirrep)
of the Poincare group", which I think is a good meaning - as far as it goes.
Multi-particle/composite states are then constructed as tensor products of these
basic unirreps. But this runs into trouble when interactions are introduced, and controversy
when we try to define localization operators. There's more problems when we try to do
QFT beyond the confines of special relativity and the Poincare group.

11. Apr 17, 2008

### peter0302

I really don't think wave/particle duality should be called a myth. Maybe "misnomer" is more appropriate. The apparant paradox between wave-like and particle-like behavior remains the most fundamental mystery of QM and is hardly a non-issue.

12. Apr 17, 2008

### Hurkyl

Staff Emeritus
It doesn't look very mysterious to me.

13. Apr 17, 2008

### Hans de Vries

What is wrong with the wave function as a physical reality?

Is the EM field not a physical reality? What about the success of all the modeling
theories which use the electron field in a similar way as the EM field, as a continuous
charge and spin distribution?

1,660,000 hits for: "Density Functional Theory"
_,_75,000 hits for: "Copenhagen Interpretation"
_,_45,000 hits for: "Bohmian"

The hits are there because it's a proven reality, an industrial success.

Nobody has ever detected a point-like particle.....

A flash on a detector screen is easily 10^15 times larger as the minimum size
for which some aspects of Quantum Field Theory have proven to be still valid.

Regards, Hans

14. Apr 17, 2008

### peter0302

Guess you're just smarter than me.

15. Apr 18, 2008

### Demystifier

If the wave function collapses due to a change of information available about the system (which the classical electromagnetic field does not do), then it seems that the wave function does not represent reality, but only our information about reality. Unless, of course, you are an extreme positivist who identifies information about reality with reality itself.

16. Apr 18, 2008

### Demystifier

Does the wave function collapse look mysterious to you?

17. Apr 18, 2008

### Demystifier

Or maybe you are just smarter than him. Feynman said something like "If you are not confused with quantum mechanics, than you do not understand it."

18. Apr 18, 2008

### Demystifier

That is true. But it is also true that nobody ever detected a particle as a wave. For example, in famous double slit experiments that demonstrate the wave nature of particles, in every measurement of a single particle you actually observe this small flash.

19. Apr 18, 2008

### Demystifier

422,000,000 hits for: "God"
14,200,000 hits for: "Santa Claus"
I guess it is because these are even better proven realities. :rofl:

Last edited: Apr 18, 2008
20. Apr 18, 2008

### Fra

IMHO, the real practical as well as fundamental problem, is that anything we learn about anything, needs to be acquired. There seems to be an information process through which knowledge is acquired, and I think that the nature of this process is a non-trivial way acutally has profound impacts on reality itself. Becaseu this isn't the isolated problem of a human scientist, I think all elements of the universe, including particle systems are bound to face the analog problem. A particle can hardly predict it's environment completely. And this should have consequences when you look statistically on group dynamics and complex systems.

So no matter what you think nature really is, I focus on exactl how do you conclude that, and how confident are you in the conclusion itself?

I can not even imagine the meaning of trying to picture that nature "really is" in a way that bypasses the learning/acquisition process? Such classical realism IMO seems to release itself from observability ideals, that IMO releases itself from my idea of the scientific method.

I consider the information a given observer have about it's environment is as real as anything possibly gets. It's an "image of reality" but this image is all he has, wether he wants is or not. But of course this images is in constant motion. The dynamics of this image is what I think interactions is about. Clearly the actions of this observer will depend on this image as well, which is thus also dynamical.

But I agree that QM as it currently stands doesn't reflect this to the full extent. One problem IMO is that QM is still too deterministic. The concept of deterministic probabilities makes no sense to me in this sense. I think the deterministic evolution is emergent in special cases. It's the limiting case where our incomplete guesses, happens to be in agreement with the expectations of the environment, then our guesses will appear to be like deterministic predictions. But thinking they are fundamentally deterministic, and taking that to be a principle of the foundations, that can be used for reasoing and extending the theory is IMO a mistake.

I think one confusion conclusive criticts that realists have on this, is that it would limit our ability to learn and make progress. And that accepting QM weirdness is like throwing in the towel. IMO that is a totally twisted conclusion, because IMO the fact that I don't know, does NOT mean I can't learn in the future. My personal poitn is that it is not possible at the present to determine the dice of tomorrow with certainty! So what is rated with probability zero today, might happen tomorrow. That is part of the indeterminism of the probabilities. But this is not realised in current QM formalism, which is why I still think that we need a fundamental revision of QM.

But I still maintain strong observability ideals and information perspectives. There is no contradiction here. I think the key is the physical basis of probability, this needs more attention in physics. The physical basis of the axioms of probability in relation to something observalbe it IMO very vague in current models.

/Fredrik