Wave vs. Particle-Like Behavior

[f95toli]

I understand that the terminology I was using was perhaps not the best, but I have yet to hear anyone argue that the current QM formalisms are adequate at doing anything other than predicting the outcome of an observation - they seem to bear no resemblence to physical reality. Netwon's mechanics were once in the same position.

But how do you even know that there is such a thing as a "reality"? We can of course assume that there is such as thin which then would beindependent of observations but that has nothing to do with physics. Physics is- by definition- a science where we try to predict the outcome of experiments and the only way to observe the an outcome is of course by observation. Everything else is is just metaphysics.

Edit: Also, you should take a look at Gröblacher et al, Nature 446, 871-875 (2007). Their experiment seems to indicate that non-local realisic theories do not work unless they are modified. Hence, one interpretation is that the usual concept of a "reality" might have to be abandoned.

 

[jtbell]

I have yet to hear anyone argue that the current QM formalisms are adequate at doing anything other than predicting the outcome of an observation - they seem to bear no resemblence to physical reality.

How can we possibly know what physical reality is, other than by comparing the predictions of our theories with experimental observations? We have a mathematical formalism (QM) that agrees with experiment, so far. People have come up with a variety of interpretations (ideas of physical reality) for this formalism. All of these interpretations have "strange" features, in different ways. Since they are constructed to fit the formalism, they are apparently incapable of being distinguished by experiment. How are we to choose among them in an objective way?

 

[peter0302]

What recent entanglement experiments? As far as I can tell, the experiments themselves are showing that large classes of local realistic theory have been disproven by the experiment, and that the results are consistent with QM.

I was referring to DCQE (which I know Copenhagenists think is no big deal), and one of the recent Wheeler-style exerpiments:

http://arxiv.org/pdf/quant-ph/0610241

which I quote:

The quantum “mystery which cannot go away” (in Feynman’s words) of wave-particle duality is illustrated in a striking way by Wheeler’s delayed-choice GedankenExperiment.

Since they are constructed to fit the formalism, they are apparently incapable of being distinguished by experiment. How are we to choose among them in an objective way?

As (I believe) Feynman said, you need a hypothesis that fits all of the existing data but makes different predictions about experiments that haven't yet been tried, but can be. That's when you potentially learn something new about the nature of reality.

Edit: Also, you should take a look at Gröblacher et al, Nature 446, 871-875 (2007).

I read the abstract online - sounds fascinating. Any idea where I can get a copy of the whole article? Thanks!

 

[ZapperZ]

I was referring to DCQE (which I know Copenhagenists think is no big deal), and one of the recent Wheeler-style exerpiments:

http://arxiv.org/pdf/quant-ph/0610241

Yeah? And?

I'm not sure why this would somehow support what you've been arguing considering that this which-way experiment is more in line with what *I* have been saying. If you know about this, and you STILL asked me why can't everything be just "waves", then either you somehow missed your own contradiction, or something isn't kosher here.

Zz.

 

[peter0302]

Yeah? And?

AND these experiments show that the wave-particle duality is still real. All I'm asking is whether particle-like behavior can be explained via the interaction of two waves. Monish kindly answered that some phenomena could be, while others could not be at present. You, in another thread, said essentially the same thing, though you clearly place more weight in the particle model than he does. That's all the answer I was looking for. Thus, I don't understand your reaction to my question or how we got on a tangent about the meaning or existence of the duality.

or something isn't kosher here.

I assure you if I were trying to cause trouble this would not be my first choice of methods.

 

[ZapperZ]

AND these experiments show that the wave-particle duality is still real. All I'm asking is whether particle-like behavior can be explained via the interaction of two waves. Monish kindly answered that some phenomena could be, while others could not be at present. You, in another thread, said essentially the same thing, though you clearly place more weight in the particle model than he does. That's all the answer I was looking for. Thus, I don't understand your reaction to my question or how we got on a tangent about the meaning or existence of the duality.

No it isn't! It appears as if all you read was the abstract! Read the paper carefully and tell me afterwards that this is still what you think. Hint: They used single-photon sources, which was a reference I gave you way back in this thread. And read the final line of that paper:

"Once more, we find that Nature behaves in agreement with the predictions of Quantum Mechanics even in surprising situations where a tension with Relativity seems to appear."

So who here suggested that QM can be superceded?

I assure you if I were trying to cause trouble this would not be my first choice of methods.

Then it baffles me how, when you're in possession of such a paper, that you STILL had no qualms in making that statement.

Zz.

 

[peter0302]

I *quoted* the abstract because it is the clearest statement they make on the topic. However I (re)read the body of the paper and I see nothing inconsistent with that statement, nor my assertion that the photon self-interferes (wave-like) sometimes, and does not (particle-like) other times. The part you quote about a tension with relativity doesn't go to that at all.

You seem to be taking issue with my assertion that while QM provides (so far) accurate predictions of observations, it doesn't give us the whole picture of reality, i.e. it doesn't explain _why_ we observe what we observe. Far greater minds than mine believe this. Instead, all I asked was if a wave-interaction model could work. Clearly it cannot, and the answer to my question is 'no.' As far as I'm concerned I've received a satisfactory answer.

 

[CaptainQuasar]

I understand that the terminology I was using was perhaps not the best, but I have yet to hear anyone argue that the current QM formalisms are adequate at doing anything other than predicting the outcome of an observation - they seem to bear no resemblence to physical reality.

No, it doesn't resemble a reality we could feel or touch, or any "sensible" reality that Einstein could imagine. That's why it really frustrated Einstein and has frustrated lots of other people during the last eighty-odd years. Supposedly some scientists in the late 1920s literally became suicidal as it became clear that QM was the only option that could carry physics forward.

The "wave-particle" duality, or whatever you want to call it, as exemplified in the recent entanglement experiments, show us that QM has a long way to go before it can be said to be an accurate description of reality at the subatomic level.

I think you're making a mistake in interchanging the concepts "accurate" and "realistic". It's possible to come up with mechanisms that might place more "realistic" mechanisms behind the mathematical behavior we see but doing that requires coming up with crazy, complex structures and explanations that we have absolutely no experimental evidence of. Any deeper mechanisms that underlie the current QM may not be things that seem "real" to the human mind that only experiences things on the macro scale.

 

[peter0302]

I think you're making a mistake in interchanging the concepts "accurate" and "realistic". It's possible to come up with mechanisms that might place more "realistic" mechanisms behind the mathematical behavior we see but doing that requires coming up with crazy, complex structures and explanations that we have absolutely no experimental evidence of. Any deeper mechanisms that underlie the current QM may not be things that seem "real" to the human mind that only experiences things on the macro scale.

True enough, but I think the value in looking for "realistic mechanisms," that is to say any deeper understanding than the ability to predict, is more than superficial. There is, after all, potentially the ability to _influence_ quantum outcomes. Even if that's a pie in the sky hope, at least it seems a good motivation to keep looking deeper.

Or I suppose I might simply be going nuts like the scientists of the 1920s you mentioned. :)

 

[ZapperZ]

I *quoted* the abstract because it is the clearest statement they make on the topic. However I (re)read the body of the paper and I see nothing inconsistent with that statement, nor my assertion that the photon self-interferes (wave-like) sometimes, and does not (particle-like) other times. The part you quote about a tension with relativity doesn't go to that at all.

But think about what I said WAY in the beginning of this, that there is ONE, and ONLY ONE, single formulation to explain BOTH the so-called wave-like and particle-like behavior. ONE! That paper clearly demonstrates this! We went through such painful argument back and forth, when what I've been trying to illustrate all along has been in your very possession! And you don't realize this?

You seem to be taking issue with my assertion that while QM provides (so far) accurate predictions of observations, it doesn't give us the whole picture of reality, i.e. it doesn't explain _why_ we observe what we observe. Far greater minds than mine believe this. Instead, all I asked was if a wave-interaction model could work. Clearly it cannot, and the answer to my question is 'no.' As far as I'm concerned I've received a satisfactory answer.

Show me what area of physics actually "explains" anything at the very fundamental level. Until you can do that, what you have asked for has never existed and you're picking on QM the same way you're picking on photons without realizing that the rule applies to all of them.

Zz.

 

[CaptainQuasar]

True enough, but I think the value in looking for "realistic mechanisms," that is to say any deeper understanding than the ability to predict, is more than superficial.

Oh, definitely there's value! Especially because the clever attempts to do so have to be met with a serious response from science and hopefully are testable for some kind of experimental confirmation or disproof. And that keeps giving us more shots from more angles of what's actually going on, decade after decade. And with stuff this hard to grok it doesn't hurt to keep hearing "yes, it really, really works that way" if each explanation can clear up the picture a little bit.

 

[CaptainQuasar]

Hey, maybe a useful perspective is to go back even further. The fact that photons emitted by matter must be released in quantized amounts of energy, how Planck had to solve the ultraviolet catastrophe problem at the end of the 18th century, is what led to all this. That doesn't make any sense either, right? It doesn't have what we'd call realism. But trying to explain that, trying to make it realistic, is why we ended up with quantum mechanics.

So I think in not trying to be too aggressive in fitting the square pegs of QM into the round holes of realism physicists are operating on the "be careful what you wish for" principle, it's not that they're trying to be obstructive of anyone understanding physics.

 

[monish]

...all I asked was if a wave-interaction model could work. Clearly it cannot, and the answer to my question is 'no.' As far as I'm concerned I've received a satisfactory answer.

I don't think anyone has put forth conclusive arguments throughout this discussion to rule out the possibility of a wave-based model which would provide all the same conclusions as standard qm with its particles and "collapse of the wave-function". The paper you quoted claims to have ruled out all possible "semi-classical descriptions" of light. This is based on supposedly being able to generate one photon at a time and send it through an apparatus at will. I don't think this is as easy as it's made out to be.

Just because someone doesn't know how to explain something with waves doesn't mean it can't be done.

 

[ZapperZ]

Just because someone doesn't know how to explain something with waves doesn't mean it can't be done.

Luckily, this is physics, and not philosophy. We demand to be shown that something can be done first before we adopt it as valid.

Maybe you might want to consider the Philosophy forum, which might be more suitable for such speculation devoid of evidence.

Zz.

 

[CaptainQuasar]

So after considering the https://www.physicsforums.com/showthread.php?t=200819" than "wave" is for these purposes but specifically using "particle" is only confusing people between ballistic Newtonian particles, which include tennis balls and planets as well as elementary particles, and the QM entity that is referred to as a particle. And it totally doesn't help that people will say things like "well theoretically there's a vanishingly small likelyhood that a tennis ball could quantum tunnel through your racket too!"

For example, consider someone who never learned any classical physics at all, who started off in QM and got to fully understand it before anything else (which doesn't actually happen of course, this is hypothetical). When confronted with the phenomenon of light bending in a prism and how similar that is to the propagation of sound in air or other mediums, I don't think such an individual would question the existence of photons or start looking for a wave medium underneath QM, they would simply say "Oh wow, you're right! Photons passing through a prism is mathematically just like a wave analysis of vibration!"

Remember that the wave-like similarity between light and sound isn't obvious or realistic either, it's just as much an artifact of scientific learning as QM is. Biorhythms are waves too but biologists don't get any "but what about the heartbeat / chambered pump duality?!?" Physics is carrying around a ton of linguistic and historical baggage.

 

[CaptainQuasar]

Another abstract ontological problem that comes into this is that some hoity-toity philosophical academic types insist that the conclusion is equivalent to the sum of the dialectic, rather than the dialectic being something that may or may not provide perspective on and texture to the conclusion. So they really, really believe in teaching the dialectic, the entire history of physics at once, even at the cost of in some cases making the student unable to understand what science's best guess is now in the 21st century.

 

[Gordon Watson]

So after considering the https://www.physicsforums.com/showthread.php?t=200819" than "wave" is for these purposes but specifically using "particle" is only confusing people between ballistic Newtonian particles, which include tennis balls and planets as well as elementary particles, and the QM entity that is referred to as a particle. And it totally doesn't help that people will say things like "well theoretically there's a vanishingly small likelyhood that a tennis ball could quantum tunnel through your racket too!"

For example, consider someone who never learned any classical physics at all, who started off in QM and got to fully understand it before anything else (which doesn't actually happen of course, this is hypothetical). When confronted with the phenomenon of light bending in a prism and how similar that is to the propagation of sound in air or other mediums, I don't think such an individual would question the existence of photons or start looking for a wave medium underneath QM, they would simply say "Oh wow, you're right! Photons passing through a prism is mathematically just like a wave analysis of vibration!"

Remember that the wave-like similarity between light and sound isn't obvious or realistic either, it's just as much an artifact of scientific learning as QM is. Biorhythms are waves too but biologists don't get any "but what about the heartbeat / chambered pump duality?!?" Physics is carrying around a ton of linguistic and historical baggage.

How about:

WAVICLE? Google count = 25,300
PARTAVE? Google count = 147.

 

[Xeinstein]

So it's been said that the wavefunction has no physical meaning except to predict the presence of a particle at a particular space and time. Yet quanta seem to exhibit wave-like properties even in isolation (single-electron interference, for example). Further, quanta _NEVER_ exhibit particle-like behavior in the absence of an interaction (i.e. you can't see which slit the electron came through unless you interact with it while it's still close to the slit).

Doesn't the wave-particle duality mean simply that "particles" are nothing more than the result of two quanta (waves) interacting? And if so, is it really so peculiar? Isn't it possible that there is no such thing as a single free particle at all?

I think in modern view of quantum field theory, every particle has its associated fields.
Particles are just excitation of quantized field, like SHO is quantized

 

[CaptainQuasar]

How about:

WAVICLE? Google count = 25,300
PARTAVE? Google count = 147.

Heh heh, good one JenniT. But I think that those might not be so good either because they still allude to "particle" and "wave" and my impression is that these QM entities we're talking about aren't either, really - my point is that they're something different entirely than the things in classical physics they're analogized to.

Someone up above pointed out that electrons very rarely behave as particles but do they actually behave like waves in a medium that frequently, either? Sure they submit to analysis with trigonometric functions ("wave mechanics", the mathematics of how waves in a medium behave and its outgrowth) but so does everything, even heartbeats as I mentioned. But sine doesn't appear in the Schrödinger equation so frequently referred to as the "wave function" during the last eighty years. (Does it? Definitely not in the basic formulas but I haven't done a lot with the Schrödinger equation in a while, physics is not my profession.) (Mathematics and software engineering are, BTW.)

It seems as though it's probably terminology issues like this that prompted Gell-Mann to pull "quark" out of James Joyce for a name, specifically seeking out a nonsense word rather than even remotely attempt to be descriptive.

 

[peter0302]

Something is still bothering me about this discussion. Could someone explain _how_ the Schroedinger equation predicts the result of the Wheeler experiment? Specifically, how does the equation take into account the presence or absence of the second beam splitter?

The way I understand it, this is taken into account through the wave-function itself, into which the behavior of the photon in the presence or absence of the second beamsplitter is imported. But this is merely based on our experimental observations, is it not? So the logic seems rather circular. The reason the formalism predicts what we see is because we constructed the formalism based on what we saw.

But if I'm misunderstanding please let me know. Thanks!

 

[CaptainQuasar]

So the logic seems rather circular. The reason the formalism predicts what we see is because we constructed the formalism based on what we saw.

But isn't this true of any science, including classical wave mechanics and particle ballistics?

The formalism is a mathematical model we created that emulates and predicts the behavior observed in experiments. So yeah, for an experiment that was either used to construct the formalism or which confirms the formalism you've got that kind of circular relationship. It's when you come up with a new experiment that doesn't agree with the formalism that we say we're going to need a new formalism. The formalism isn't any more fundamentally connected to some underlying "reality" than is the raw experimental data or anything else in science.

 

[peter0302]

Well, one of the goals is to boil down all the observations into a small number of postulates or axioms, and then build a logical formalism on top of those that predicts everything else that is observed and hopefully more. That's how Newton and Einstein did it. But I don't see that with QM. Instead I see a lot of observations that are to this day not fully understood, and virtually no success in boiling them down to more fundamental axioms, so instead we have a formalism that is essentially drawing a circle around all of the experimental data and calling it a theory.

At the very least, it is rather surprising to me that as the subject matter becomes smaller, the "axioms" become much more complex and high in number. If we were to translate the laws of physics into a computer program, we'd see an inordinate amount of complexity at the quantum level. I'd have expected God to write more elegant code.

 

[CaptainQuasar]

Well, one of the goals is to boil down all the observations into a small number of postulates or axioms,

I know that's what Newton and Einstein did but I don't think that there's any assumption or goal like that in science. It's nice when it works out that way but it's not wrong or assumed to be out of touch with reality when it doesn't. I would like it if it worked out that way too but it hasn't so far.

I'd have expected God to write more elegant code.

You're God's quality assurance tester?

Actually, I'm a software engineer and you're reminding me here of managers who insist that there just has to be a really simple way to solve what's actually a very complicated problem. It's not to say that there aren't sometimes simple solutions to complicated problems but to just insist that there has to be a simple, elegant solution is often erroneous in my experience.

 

[peter0302]

Hah. Well I was a CS major in college, so I got indoctrinated with the "elegance" requirement pretty completely. I won't deny that it guides some of my biases about physics. But I cannot deny the elegance of the Theory of Relativity compared to Quantum Mechanics, and I cannot help but prefer the former approach.

Ultimately, any theory of everything that cannot be completely described in one sentence and is not understandable to a 10th grader is probably too complicated to be correct.

 

[CaptainQuasar]

Ultimately, any theory of everything that cannot be completely described in one sentence and is not understandable to a 10th grader is probably too complicated to be correct.

Well, I'm glad that you know so much about the way universes have to be made. I've got some Creationist friends you might get along with.

 

[peter0302]

On the contrary, that thought is anti-creationist. Only an intelligent being could design rules so complex and disjointed. The laws of a universe without a creator should flow as inevitably as 2 + 2 = 4. Otherwise, how can you not see a designer behind them?

 

[monish]

I was referring to DCQE (which I know Copenhagenists think is no big deal), and one of the recent Wheeler-style exerpiments:

http://arxiv.org/pdf/quant-ph/0610241

which I quote:

"The quantum “mystery which cannot go away” (in Feynman’s words) of wave-particle duality is illustrated in a striking way by Wheeler’s delayed-choice GedankenExperiment. "

I'm going to make an observation about the experiment quoted above. Bascially, it splits a beam into two separate paths, then recombines them and looks for interference. If you look for which path was followed, the interference goes away.

Up to this point the experiment would be consistent with a wave description. But then they do two things:

1. They contrive to operate the apparatus "one photon at a time".

2. They make the choice of whether to observe the path AFTER the photon beam has already been split.

It's a very impressive experiment but I have to point out one peculiar aspect which leaves room for a shadow of doubt about the intended conclusion: the interference pattern is NOT based on the path length difference. It's created AFTER the "beams" are recombined using something called a "KDP electro-optic modulator".

(edit:) My mistake: the EOM activates the second beam-splitter. Tilting the beam splitter varies the relative phase. Which takes place at the point of recombination.)

The whole point of the experiment is to show that the beams take one definite path if observed, and both paths simulataneously if not observed: "quantum wierdness". But wouldn't it be more convincing if the observed interference was caused by the path length difference? Yet that's not how they chose to do it.

 

[peter0302]

Isn't it because the beams always take both paths simultaneously until detection? Thus, the interference wouldn't be due to the path-length differene, but rather just due to the beamsplitters.

 

[monish]

Isn't it because the beams always take both paths simultaneously until detection? Thus, the interference wouldn't be due to the path-length differene, but rather just due to the beamsplitters.

Well, I think that's my point. Since the interference is due to something that happens at the beamsplitters, I don't think you can conclude that the photon took both paths simultaneously.

What I mean is that there is still a viable wave interpretation. Case 1, no recombination, the wave was split into path A or path B at the first beamsplitter. So there's no interference at the detectors. Case 2, with "recombination", you now have a device which sets up standing waves of multiple reflection between the second beamsplitter and the two detectors with the net effect that all the wave energy ends up in one of the detectors. I know that sounds farfetched, but it would seemingly have been easy to rule out such a scenario by letting the interference be governed by the path length difference. That's why I say it was peculiar that they didn't set up the experiment in this way. Because it leaves a loophole for the wave theory to survive.

That's what I mean by the wave-particle duality. For every particle explanation, there is a corresponding wave-based explanation which also works. The advantage of the wave-based explanations, where they exist, is that they provide a realistic mechanism for understanding what happens. It doesn't seem like the particle theories can ever do that.

 

[peter0302]

No, they can't, and if you read my other thread about the Afshar experiment, assuming I'm correct that the grid will never alter the detection rate, it literally means that the photons pass through the grid as though it wasn't there. That another area where the particle model breaks down completely. That's why I asked at the beginning of this thread whether we can consider particle "collision" as simply the interaction of two waves at a randomly chosen point in space.

 

[colorSpace]

What recent entanglement experiments? As far as I can tell, the experiments themselves are showing that large classes of local realistic theory have been disproven by the experiment, and that the results are consistent with QM. Read a thread in the General Physics forum on Recent Noteworthy Papers, especially on the most recent experiment testing the Leggett inequality.

So here, it appears that QM has a huge leg up on your speculation.

Zz.

I guess this is a typo and what you meant is that in addition to (all) local realistic theories, now also large classes of non-local realistic theories have been disproven.

 

[peter0302]

Maybe. I don't think FTL theories have been disproven, especially not if one allows backwards-in-time signaling. That would be non-local, non-temporal realism.

 

[colorSpace]

AFAIK, Bohmian Mechanics isn't in those classes which have been disproven, even though it is non-local realistic.

 

[peter0302]

Bohmian mechanics is basically the same as pilot wave, right? I think the pilot wave idea is probably pretty close to what I've been suggesting in this thread.

 

[CaptainQuasar]

On the contrary, that thought is anti-creationist. Only an intelligent being could design rules so complex and disjointed. The laws of a universe without a creator should flow as inevitably as 2 + 2 = 4. Otherwise, how can you not see a designer behind them?

If you don't regard things like that as theological statements I don't think I can express to you what I'm talking about. If you're confident that you know what the nature of the universe is and therefore what its governing rules must be like go ahead and run with that but don't use it to critique science. (Using a priori beliefs about the universe to critique science is the way I'm saying that you're similar to a creationist, I'm not saying that your doctrine is similar to theirs.)