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Arcane
2010-Dec-21, 05:52 AM
This is news to me, but apparently Protons and Electrons are just like Photons in their Particle/wave properties?

In a double slit experiment a proton will act just like a Photon and go thru both holes?

I am confused by this article. Can somone make sense of this? Please, and thank you.


http://hyperphysics.phy-astr.gsu.edu/hbase/mod1.html

Ken G
2010-Dec-21, 06:24 AM
Yes, all particles will exhibit that type of behavior. The history of waves is interesting-- at first, it was thought that waves were an excitation of a medium, like sound waves in air or water waves on water, and so waves were something totally different from particles. But then with relativity, it was discovered that light had wave properties without the need for a medium (there was no need for an aether), and it also had particle properties (like the photoelectric effect). That was the birth of wave-particle "duality." Shortly after that came an even bigger surprise-- all particles are governed by wave mechanics, and none of them need a medium. So now, we can view sound waves and water waves as merely an analog for this vastly more general type of wave, and waves and particles are not two different things: waves tell particles where they are allowed to go.

TobiasTheViking
2010-Dec-21, 10:29 AM
It actually goes to the extreme that you, me, the earth and the moon, all have wave/particle duality.

But as the mass increases the superposition decreases.

Thus, for a very small and very light particle the superposition can be very spread out. But for something of a common world size, the superposition is so small that it is for all intents and purposes, not something that has any measurable effect.

Staticman
2010-Dec-21, 02:01 PM
Yes, all particles will exhibit that type of behavior. The history of waves is interesting-- at first, it was thought that waves were an excitation of a medium, like sound waves in air or water waves on water, and so waves were something totally different from particles. But then with relativity, it was discovered that light had wave properties without the need for a medium (there was no need for an aether), and it also had particle properties (like the photoelectric effect). That was the birth of wave-particle "duality." Shortly after that came an even bigger surprise-- all particles are governed by wave mechanics, and none of them need a medium.

I think this answer is appropriate for a first contact with this kind of puzzling questions, which seems to fit to the OP question level. But I wonder if it could be a little misleading in the light of the modern view of these questions we get from QFT.
It is accepted among quantum and condensed matter physicists that the vacuum can be considered a kind of substance or matter phase with a whole lot of properties that confirm this nature of the vacuum, and in this sense it can be considered a medium for waves.
The fact that this view is not very popular among some physicists and that no conclusions are drawn from this understanding is IMO that the conclusions might be controversial and there is huge resistance to such changes, another reason explained in the book I cite below is the possible confusion with the taboo concept of ether of the 19th century. But concepts like effective-action, vacuum expectation values, vacuum state (zero-point energy). etc have been in use for many decades.
Probably the clearest account of these issues in popular science books are in the works of two brilliant recent Nobel prize winners in physics:
Bob Laughling's:A different universe, mainly in the chapter "spacetime structure", and Frank Wilczek "The lightness of being" in its chapter 8.
And in technical terms there is:"Probing the quantum vacuum: perturbative effective action aproach in quantum electrodynamics and applications" by Dittrich and Gies.

Cougar
2010-Dec-21, 02:08 PM
Here is an interesting quote:



If [wave-particle/double slit] seems very mysterious, you are not alone. Understanding what is going on here is in some sense equivalent to understanding Quantum Mechanics.... Feynman admitted that he never understood Quantum Mechanics. It may be true that nobody can understand Quantum Mechanics in the usual meaning of the word "understand."

This webpage appears to be a more comprehensive, helpful explanation: The Feynman Double Slit (http://www.upscale.utoronto.ca/PVB/Harrison/DoubleSlit/DoubleSlit.html)

Ken G
2010-Dec-21, 02:19 PM
I think this answer is appropriate for a first contact with this kind of puzzling questions, which seems to fit to the OP question level. But I wonder if it could be a little misleading in the light of the modern view of these questions we get from QFT.
It is accepted among quantum and condensed matter physicists that the vacuum can be considered a kind of substance or matter phase with a whole lot of properties that confirm this nature of the vacuum, and in this sense it can be considered a medium for waves. No, the vacuum is not thought of that way in QFT. It is thought of as a kind of seething sea of virtual particles, true enough, but the virtual particles can hardly be thought of as a medium for waves, as they were not even dreamed of in the day when people thought waves needed a medium-- they are a new idea intended to replace the concept of a medium. Granted, if you are inclined to force the modern thinking into the old language, you might be tempted to call it some kind of "generalized medium", but I see no value in the concept-- better to simply move on and leave the medium concept behind altogether. The key point is that the older type of waves that require a medium should be relegated to a kind of second-class wave status-- those never really were fundamentally what waves are, they were just the way we encountered the concept, nothing more than a kind of ponderable analog to the more fundamental version that comes up in quantum theories. I don't think it would be very accurate to say that a sound wave is to air molecules like a light wave is to virtual photons, the way the two come up in the mathematics of wave propagation is completely different.


The fact that this view is not very popular among some physicists and that no conclusions are drawn from this understanding is IMO that the conclusions might be controversial and there is huge resistance to such changes, another reason explained in the book I cite below is the possible confusion with the taboo concept of ether of the 19th century. The problem with the aether is not that it is taboo, it is simply that it is useless. There is no need to postulate an aether, unless one clings to the old and discredited idea that waves require a medium. Indeed, if you forget about light for a moment, and just think about particles with rest mass, the need for an aether becomes even more clearly superfluous.


But concepts like effective-action, vacuum expectation values, vacuum state (zero-point energy). etc have been in use for many decades.Certainly. And none of those have anything to do with waves propagating in a medium. All you are saying is that vacuum is not considered to be nothing, which is a completely orthogonal issue to waves in media. Instead, it means there are several things going on in a vacuum-- wave propagation, and virtual-particle effects. One might even say that the virtual particles propagate in the vacuum.


Probably the clearest account of these issues in popular science books are in the works of two brilliant recent Nobel prize winners in physics:
Bob Laughling's:A different universe, mainly in the chapter "spacetime structure", and Frank Wilczek "The lightness of being" in its chapter 8.Again, both about the surprising properties of vacuum, not about waves needing a medium like sound in air.


And in technical terms there is:"Probing the quantum vacuum: perturbative effective action aproach in quantum electrodynamics and applications" by Dittrich and Gies.Effective action also has nothing to do with a medium to "carry" the wave function. Indeed, the wave function is not even considered something real that would need to be "carried" by anything, it is now viewed as an abstract mathematical concept. What medium does one need to propagate an abstract mathematical concept? The vacuum has an influence on the propagation of a mediumless abstract mathematical construct-- that's the step forward, not the step back into saying that waves require a ponderable medium. The main issue is that a medium has a rest frame, and the vacuum does not.

Cougar
2010-Dec-21, 02:30 PM
Probably the clearest account of these issues in popular science books are in the works of two brilliant recent Nobel prize winners in physics:
Bob Laughling's:A different universe, mainly in the chapter "spacetime structure", and Frank Wilczek "The lightness of being" in its chapter 8.

Yes and yes. Thanks for the update, Staticman, which one might add is still in progress.

Staticman
2010-Dec-21, 06:48 PM
No, the vacuum is not thought of that way in QFT. It is thought of as a kind of seething sea of virtual particles, true enough, but the virtual particles can hardly be thought of as a medium for waves, as they were not even dreamed of in the day when people thought waves needed a medium-- they are a new idea intended to replace the concept of a medium. Granted, if you are inclined to force the modern thinking into the old language, you might be tempted to call it some kind of "generalized medium", but I see no value in the concept-- better to simply move on and leave the medium concept behind altogether. The key point is that the older type of waves that require a medium should be relegated to a kind of second-class wave status-- those never really were fundamentally what waves are, they were just the way we encountered the concept, nothing more than a kind of ponderable analog to the more fundamental version that comes up in quantum theories. I don't think it would be very accurate to say that a sound wave is to air molecules like a light wave is to virtual photons, the way the two come up in the mathematics of wave propagation is completely different.
The problem with the aether is not that it is taboo, it is simply that it is useless. There is no need to postulate an aether, unless one clings to the old and discredited idea that waves require a medium. Indeed, if you forget about light for a moment, and just think about particles with rest mass, the need for an aether becomes even more clearly superfluous.

Certainly. And none of those have anything to do with waves propagating in a medium. All you are saying is that vacuum is not considered to be nothing, which is a completely orthogonal issue to waves in media. Instead, it means there are several things going on in a vacuum-- wave propagation, and virtual-particle effects. One might even say that the virtual particles propagate in the vacuum.
Again, both about the surprising properties of vacuum, not about waves needing a medium like sound in air.
Effective action also has nothing to do with a medium to "carry" the wave function. Indeed, the wave function is not even considered something real that would need to be "carried" by anything, it is now viewed as an abstract mathematical concept. What medium does one need to propagate an abstract mathematical concept? The vacuum has an influence on the propagation of a mediumless abstract mathematical construct-- that's the step forward, not the step back into saying that waves require a ponderable medium. The main issue is that a medium has a rest frame, and the vacuum does not.

Well, I have to refer you to the chapters I cited above, I understand your arguments but they are responding to a number of things I didn't say, and therefore I can't debate you those since we are basically in agreement. In this case you might have some preconceived ideas about what I mean by vacuum as a form of matter, but I'm basically quoting the words of 2 highly respected scientists, this is not old physics, is more of a prejudice-less account of current physics.
For instance when Wilczek remarks thru his book several times that the vacuum field (or the Grid" as he calls it) weighs, how would you interpret it and what conclusions could you draw from it? This is not old physics at all.
If you think I'm misinterpreting, please say it, but I don't think so. Perhaps it all comes down once again to semantics. How would you define "medium"? I think a medium in physics can be understood as some material substance, accordin to wikipedia is :" any material substance which can propagate waves or energy", well we know the vacuum field carries energy, now you have to define matter and then you realize that is a tricky definition, according to wikipedia again:" A common way of defining matter is as anything that has mass and occupies volume.[3] In practice however there is no single correct scientific meaning of "matter," as different fields use the term in different and sometimes incompatible ways." But if the vacuum weighs and certainly occupies volume, you tell me why should Robert Laughling for instance be wrong to call the vacuum a form of matter (certainly different from fermionic matter).
I thnk it all comes down to decide if the vacuum fulfills the required properties to be called a medium, I haven't made up my mind yet, But the arguments of the Nobel winners are very good, if you opine differently or think I'm misunderstanding, I'll be delighted to read your arguments. But remember we are not talking here so much about hte mathematical abstract wave function representation but about physical(measurable) properties, not so much about the non-relativistic Schrodinger wave function as about fields of QFT, and whether the vacuum can be considered a medium in the physical sense.

Staticman
2010-Dec-21, 07:08 PM
The main issue is that a medium has a rest frame, and the vacuum does not.

Hmm, wouldn't you call the CMB blackbody radiation a sort of vacuum rest frame?

In case someone thinks this goes against relativity, here is from the FAQ of a Brituish Columbia university page about physics:

Quote
"How come we can tell what motion we have with respect to the CMB? Doesn't this mean there's an absolute frame of reference?
The theory of special relativity is based on the principle that there are no preferred reference frames. In other words, the whole of Einstein's theory rests on the assumption that physics works the same irrespective of what speed and direction you have. So the fact that there is a frame of reference in which there is no motion through the CMB would appear to violate special relativity!
However, the crucial assumption of Einstein's theory is not that there are no special frames, but that there are no special frames where the laws of physics are different. There clearly is a frame where the CMB is at rest, and so this is, in some sense, the rest frame of the Universe. But for doing any physics experiment, any other frame is as good as this one. So the only difference is that in the CMB rest frame you measure no velocity with respect to the CMB photons, but that does not imply any fundamental difference in the laws of physics." Unquote

forrest noble
2010-Dec-21, 07:09 PM
This is news to me, but apparently Protons and Electrons are just like Photons in their Particle/wave properties?
In a double slit experiment a proton will act just like a Photon and go thru both holes?
I am confused by this article. Can somone make sense of this? Please, and thank you.

http://hyperphysics.phy-astr.gsu.edu/hbase/mod1.html

These waves that particles produce are called De Broglie waves, named after Louis De Broglie. De Broglie himself believed these waves that matter particles produced were physical waves. His related statement was:

"When in 1923-1924 I had my first ideas about Wave Mechanics I was looking for a truly concrete physical image, valid for all particles, of the wave and particle coexistence discovered by Albert Einstein in his "Theory of light quanta". I had no doubt whatsoever about the physical reality of waves and particles."

"In my view, the wave is a physical one..."

"For me, the particle, precisely located in space at every instant, forms on the v wave a small region of high energy concentration, which may be likened in a first approximation, to a moving singularity."

"I called this relation, which determines the particle's motion in the wave, the guidance formula. It may easily be generalized to the case of an external field acting on the particle."

Using De Broglie's interpretation the double slit and other related experiments are very easy to understand. The wave enters all available slits. The 'particle' enters a single slit. Upon exiting the slits, the waves create interference which 'acts upon the particle' exiting a single slit, causing the direction the particle travels to be altered.

De Broglie's interpretation was and is contrary to most present interpretations of quantum mechanics so his views of particle waves as being physical, created by the particle, and coexisting with the particle, have always been a minority view since his first discovery of these waves and explanations of them. Some theoretical physicists still favor his explanations and some are just re-discovering them as mentioned by Staticman above.

http://www.natscience.com/Uwe/Forum.aspx/relativity/15559/de-Broglie-s-statement-and-the-double-slit-experiment

For the presently preferred quantum mechanical interpretations/ explanation of particle waves see Ken G's posting.

Ken G
2010-Dec-21, 07:31 PM
Hmm, wouldn't you call the CMB blackbody radiation a sort of vacuum rest frame?
No. The CMB does not appear to interface with the laws of physics in any way at all-- they appear to be nothing but an arbitrary boundary condition. Remove it tomorrow and no laws of physics change in any way, so far as we know. An actual rest frame for the vacuum must appear in the laws of physics, not in an arbitary boundary condition-- that's the difference between a true "rest frame of the vacuum", and simply a coordinate of convenience. It might someday turn out that the CMB is more than that, but that day has not yet come.

Staticman
2010-Dec-21, 08:38 PM
No. The CMB does not appear to interface with the laws of physics in any way at all-- they appear to be nothing but an arbitrary boundary condition. Remove it tomorrow and no laws of physics change in any way, so far as we know. An actual rest frame for the vacuum must appear in the laws of physics, not in an arbitary boundary condition-- that's the difference between a true "rest frame of the vacuum", and simply a coordinate of convenience. It might someday turn out that the CMB is more than that, but that day has not yet come.

Oh, but interface with the laws of physics it does, at least with those of optics, if you care to read section 5.2 of the abovementioned book by Dittrich and Gies, especially pages 187 and 188,(it can be seen in amazon.com free view) they show that photons are affected by the CMB blackbody radiation for Hubble length magnitude paths. Wich means that at that scale the vacuum is optically thick. Please correct me if that is not implied there, I'm sure you'd know better than me.

Ken G
2010-Dec-21, 08:52 PM
You don't understand. The presence of the CMB is certainly physically real, that's why it shows up in our telescopes. But that does not mean it is part of the laws of physics. The CMB is just a boundary condition-- that's how physics works, you take the laws, which in some sense "have to" be true, and then you plug in some particular boundary condition relevant to the problem at hand. So far our view is that the laws of physics are something independent from the particulars of the CMB, and that's why the CMB does not define a rest frame for the vacuum that is anything but a convenient coordinate choice for treating that particular boundary condition. Someday a theory might be generated where there is an actual rest frame for the vacuum, and it may or may not correspond to the CMB frame-- that's all idle speculation at this point.

Ken G
2010-Dec-21, 08:59 PM
Perhaps it all comes down once again to semantics. How would you define "medium"? I think a medium in physics can be understood as some material substance, accordin to wikipedia is :" any material substance which can propagate waves or energy", well we know the vacuum field carries energy, now you have to define matter and then you realize that is a tricky definition, according to wikipedia again:" A common way of defining matter is as anything that has mass and occupies volume.[3] Yes, I agree it is an issue in semantics. I am happy with the Wiki language-- a medium is matter, and virtual particles are not matter. However, one can certainly imagine that virtual particles cause the vacuum to be a kind of "generalized" medium, I think one may imagine these things in many ways. I'm just saying that our idea of "what is a wave" has changed a lot rather recently, pursuant to the OP question.

Staticman
2010-Dec-21, 09:03 PM
You don't understand. The presence of the CMB is certainly physically real, that's why it shows up in our telescopes. But that does not mean it is part of the laws of physics. The CMB is just a boundary condition-- that's how physics works, you take the laws, which in some sense "have to" be true, and then you plug in some particular boundary condition relevant to the problem at hand. So far our view is that the laws of physics are something independent from the particulars of the CMB, and that's why the CMB does not define a rest frame for the vacuum that is anything but a convenient coordinate choice for treating that particular boundary condition. Someday a theory might be generated where there is an actual rest frame for the vacuum, and it may or may not correspond to the CMB frame-- that's all idle speculation at this point.
I'm sure there is a lot I don't understand, what you have answered now I think I understood the first time you said it in the previous post. But there are other sound sources besides your opinion, concede me that, that is why I referred you to them for this specific point , I would really appreciate it if you took a look at it and correct me if I err, or tell me exactly why this type of interaction descrribed in the text would not be part of the laws of physics.
Thanks

Staticman
2010-Dec-21, 09:09 PM
Yes, I agree it is an issue in semantics. I am happy with the Wiki language-- a medium is matter, and virtual particles are not matter. However, one can certainly imagine that virtual particles cause the vacuum to be a kind of "generalized" medium, I think one may imagine these things in many ways. I'm just saying that our idea of "what is a wave" has changed a lot rather recently, pursuant to the OP question.

I agree.

Ken G
2010-Dec-21, 10:04 PM
I'm sure there is a lot I don't understand, what you have answered now I think I understood the first time you said it in the previous post. But there are other sound sources besides your opinion, concede me that, that is why I referred you to them for this specific point , I would really appreciate it if you took a look at it and correct me if I err, or tell me exactly why this type of interaction descrribed in the text would not be part of the laws of physics.

Any interaction the CMB has with anything else is going to be governed by the laws of physics, but that does not mean that the CMB, or its reference frame, appears explicitly in the laws of physics. Anything that happens to you or I in our lifetimes is part of the laws of physics too (speaking generally about the "laws", not intending the full-scale philosophical ramifications of that claim), but we don't appear in those laws either, and neither does our reference frame.

Cougar
2010-Dec-22, 01:28 AM
The CMB does not appear to interface with the laws of physics in any way at all-- they appear to be nothing but an arbitrary boundary condition. Remove it tomorrow and no laws of physics change in any way, so far as we know.

Well, Isn't that what Staticman said in quoting from that British Columbia university page about physics:



However, the crucial assumption of Einstein's theory is not that there are no special frames, but that there are no special frames where the laws of physics are different. There clearly is a frame where the CMB is at rest, and so this is, in some sense, the rest frame of the Universe.

And how is the CMB "arbitrary"? It would seem to be a universal comparator.

Ken G
2010-Dec-22, 01:56 AM
Actually, I never disputed anything in that quote. Rather, I disputed Staticman's claims that modern views of the vacuum constitute a medium, with a rest frame in the laws, through which quantum-field-theory waves propagate. Instead, I claimed the modern view of waves has gotten away from the need to have a medium in the first place. None of that says that there is not some frames that are more convenient than others, or that spawn a more "natural" language to talk about what is happening in the universe. The British Columbia physicist is either being loose with his language, or is being misinterpreted. To say that there is "in some sense a rest frame" of the universe can easily be intepreted, and indeed should be interpreted, as simply a statement that there are some coordinates that seem a lot more natural than others. We do that in physics all the time-- do we not say that gravity points "down"? Is that a law of physics? No, it is merely a convenient language for talking about the completely arbitrary coordinate choices that give a sense of direction to gravity. So it is with the rest frame of the CMB.

What is "arbitrary" about the CMB is that it is an artifact of history, like the clothes you are wearing right now, rather than anything that is embedded in the laws of physics as we currently understand them. If the CMB were not there, the universe would need a different history, but not different laws.

Cougar
2010-Dec-22, 02:50 AM
Well, here is Wilczek's description for lay folk:






The model Nature gives us for making force-carrying particles heavy is superconductivity. For inside superconductors, photons become heavy! ... Photons... are moving disturbances in electric and magnetic fields. In a superconductor, electrons respond vigorously to electric and magnetic fields. The electrons' attempt to restore equilibrium is so vigorous that they exert a kind of drag on the fields' motion. Instead of moving at the usual speed of light, therefore, inside a superconductor photons move more slowly. It's as if they've acquired inertia. When you study the equations, you find that the slowed-down photons inside a superconductor obey the same equatjions of motion as would particles with real mass.

If you happened to be a life-form whose natural habitat was the interior of a superconductor, you'd simply perceive the photon as a massive particle.

Now let's turn the logic around. Humans are a life-form that observes, in its natural habitat, photon-like particles, the W and Z bosons, that are massive. [Earlier Wilczek pointed out "Left to themselves, according to the equations that define them, [the W and Z bosons] would be massless, like the photon and the color gluons. Reality's script, however, calls for them to be heavy.] So perhaps we humans should suspect that we live inside a superconductor. Not, of course, a superconductor in the ordinary sense, that's supergood at conducting the (electric) charge that photons care about, but rather a superconductor for the charges that W and Z bosons care about... Thus we come to suspect that the entity we call empty space is an exotic kind of superconductor.

Where you have superconductivity, there's got to be a material that does the conducting. Our exotic superconductivity works everywhere, so the job requires a space-filling material ether.

Big Question: What is that material, concretely? What is it that, for the cosmic superconductor, plays the role that electrons play in ordinary superconductors?

Well, Wilczek goes on to point out that no presently known form of matter has the right properties. But he says we have a name for it: the Higgs condensate. Obviously, this is not the luminiferous ether of old....

Ken G
2010-Dec-22, 07:13 AM
In my opinion, what Wilczek is doing there (and thanks for the illuminating quotes) is providing a material analog to help us understand how particles acquire mass in the Higgs model (that the LHC is even now attempting to verify). Thus he is retracing the advancement of our understanding of waves, which is a kind of bridging between the "old style" of fundamental wave (waves in material media) to the "new style" of fundamental wave (abstract mathematical behavor reminiscent of waves in material media, without the material media). Understanding always involves bringing some abstract notion into contact with something much closer to our own experience, and that may be what he is doing here, though I admit some of his words suggest he takes the picture rather literally, or at least he favors the image that this "condensate" is a kind of material medium. His word choices that make this unclear are when he said that the "entity" of empty space is an "exotic" form of superconductor-- when is an "exotic" material exotic enough to be different from a material? At this point, I'd say we are deeply into pedagogy, rather than physical theory-- we are talking about the pictures we use to understand something, rather than the attributes of a theory we can test.

So I'm happy to leave it that if one is inclined to want to imagine that empty space is an exotic kind of material medium, for its descriptive power, then one is welcome to do that. But one can also simply take the theory at face value, without those crutches that bring it into contact with our own experience, and just say that the vacuum incorporates fields and virtual particles that have the effect of dressing the propagation of matter with the concept of mass. No reference to any material medium is then required to propagate the waves, just that waves can propagate without a medium, but the way they propagate will be affected by virtual particles and fields. What isn't clear is if the analogy with propagation in a superconductor is figurative or literal-- in other words, if the equations are exactly the same, or if they just show reminiscent behavior. I really don't know-- if the mathematics is identical, I could count that as a strong reason to attribute propagation in a vacuum as being like propagation through a material medium, and just count superconductors as "exotic." But in a superconductor, we can identify the material, we can dissect it into its parts and talk about how they interplay-- so if the mathematics is not identical, I'd say we have a simple case of a descriptive picture of a simpler system fueling our intuition about a more complicated one.

Staticman
2010-Dec-22, 11:35 AM
The only reason Wilczek seems to have to make this "descriptive picture" as he explains it is that the mathematics of QFT, specifically of QCD and outcomes of experiments in high energy physics to check QCD, forces him to do it, what other reason could he have? The "no medium needed" view has worked pedagogically very well for years.

Ken G
2010-Dec-22, 02:57 PM
But Wilczek also realizes that we do not expect there to be any frames where the laws of physics work differently. You can bet that the physics of photon propagation inside a superconductor is different in the frame of the superconductor-- you don't have to look at some external radiation to get that, you can just watch the motion of a single photon within that superconductor. When a photon has a rest frame because it is moving at less than c (as also happens in glass, for example), you can enter a frame where the photon is effectively motionless (or more correctly, the wave that tells the photon where to go is a stationary wave). This is only possible, without violating relativity, because you see the medium, and all its charges, whizzing by at a known speed, and you can account for their effects on the photon wave function. None of our current theories attribute any direct influence between the CMB and the "Higgs condensate", there isn't even any evidence that the rest frame of the condensate, if there is one (which presumably there is not), would even be the same as the rest frame of the CMB (which is clearly observable). So I cannot see it as anything but a descriptive analogy, until there is an actual theory that suggests waves propagate in a medium that has a physically meaningful rest frame. Such a theory may one day exist-- but not this day.

Wilczek may be straddling a line between where he is simply using a pedagogically useful analogy to help motivate one way to picture how particles acquire mass, and a personal opinion about how a future theory that describes that will work. In the mean time, it simply isn't known if the CMB rest frame has any physical significance beyond a convenient frame for analyzing spatial hyperslices of a cosmological model on the largest scales-- let us not repeat the error of those who always spoke of Maxwell's equations as being designed for an aether rest frame before any experiment actually indicated that they were!

Arcane
2010-Dec-23, 08:21 PM
Thank you guys for the great insight on the subject. I am not sure why I never heard that a Proton can go thru both slits... I have only ever heard that about light. It still doesn't make complete sense because I thought it was special to light since light has no mass. But I am reading and learning. Thanks again.

Strange
2010-Dec-23, 10:05 PM
"Goes through both slits" is one way of thinking about it. Other interpretations are available. We don't (and probably can't) know what really happens. [Cue another long discussion on what "really happens" means :)]

The two-slit experiment has been done with "particles" as big as a fullerene molecule (C60) which is about 800 times bigger than a proton

forrest noble
2010-Dec-25, 07:18 PM
Thank you guys for the great insight on the subject. I am not sure why I never heard that a Proton can go thru both slits... I have only ever heard that about light. It still doesn't make complete sense because I thought it was special to light since light has no mass. But I am reading and learning. Thanks again.

Remember going through both slits might somehow seem possible for a proton but how about a fullerene molecule (C60) which is about 800 times bigger than a proton (explained by Strange above)? The double-slit experiment has been conducted with this molecule and a wide variety of light frequencies, atomic particles, atoms, and other molecules, always with the same result. Using De Broglie's interpretation there would be no mystery at all to any of it and only one possible outcome, interpretation, or explanation (the particle goes though one slit, its waves go through both slits) no matter what the size of matter being used. I think such large molecules as fullerene would seem to be very difficult for the normal QM interpretation or justify their standard explanation or stretch credibility by proposing that this molecule somehow splits or otherwise goes through both slits at the same time. Fullerene might be the largest possible molecule that could be used for this experiment. Anything heavier may not show any measurable influence concerning the influence of the related waves.

Shaula
2010-Dec-25, 11:21 PM
I think such large molecules as fullerene would seem to be very difficult for the normal QM interpretation or justify their standard explanation or stretch credibility by proposing that this molecule somehow splits or otherwise goes through both slits at the same time. Fullerene might be the largest possible molecule that could be used for this experiment. Anything heavier may not show any measurable influence concerning the influence of the related waves.
Why do you think that? You seem to have a slightly odd version of standard QM here. The fullerene molecule doesn't split to form a wave, it doesn't act like a particle going through one of the slits while some mysterious thing goes through both. The fullerene molecule is best described as a wave in this experiment. The system's waveform interferes with itself giving rise to the above effects. Normal QM has absolutely no problem describing a fullerene molecule (or any object) producing an interference pattern due to its interaction with two slits. It is only when you try to force explanations like the one above onto it that things seem wrong. Given a carefully enough isolated system there is no reason at all that larger molecules (a virus has been proposed) cannot display quantum behaviour.

Ken G
2010-Dec-26, 04:35 AM
Personally, the language I prefer is careful to avoid taking any stance that cannot be justified by experiment, and is not required by the theory we are using to understand said experiment. Thus, we should not say the particle goes through both slits, nor should we say the particle goes through one slit or the other but the wave goes through both. Certainly the wave goes through both, but both the experiments and the theory are agnostic about what the particle is doing "along the way." So I feel we should make no claim at all about the particle's path-- an experiment that is not set up to determine that path does not allow us to say anything about the path.

Now, one might counter that if we do equip the experiment with the ability to detect the path, we always get only one or the other, so why should it not always be one or the other if the experiment does not have that capability? But the fact that the interference pattern at the screen is different if we equip the experiment with the ability to detect the path tells us that this is a fundamentally different setup. I think the lesson of quantum mechanics is that there is no such thing as what "actually happens" when we have not the capability to say what happens-- because what actually happens, and our capability to say what happens (in principle, not worrying about any information that we are simply not privy to), are just two different ways of saying exactly the same thing.

Shaula
2010-Dec-26, 09:13 AM
The misconception seemed to be that the particle's path was some sort of hidden variable that exists deterministically and is somehow masked by quantum fuzziness or our ability to measure things. Local hidden variables contradict experimental evidence AIUI. QM describes a wavefunction, not a particle. The concept 'particle' is something we apply to model some aspects of a wavefunction. Just like we can use 'wave' for others. But fundamentally neither are adequate descriptions and we need to go back to what the maths is telling us. Things come as wavefunctions, not particles or waves.

forrest noble
2010-Dec-27, 09:40 PM
Why do you think that? You seem to have a slightly odd version of standard QM here. The fullerene molecule doesn't split to form a wave, it doesn't act like a particle going through one of the slits while some mysterious thing goes through both. The fullerene molecule is best described as a wave in this experiment. The system's waveform interferes with itself giving rise to the above effects. Normal QM has absolutely no problem describing a fullerene molecule (or any object) producing an interference pattern due to its interaction with two slits. It is only when you try to force explanations like the one above onto it that things seem wrong. Given a carefully enough isolated system there is no reason at all that larger molecules (a virus has been proposed) cannot display quantum behaviour.

I think Stange's example of Fullerene, otherwise called Bucky balls, is a good example of very serious problems with QM theory explanations in that the exact geometry of buggy balls are well known. For a Fullerene molecule to turn into a wave function just because they "know" they are going to pass through a slit surely pushes credibility of such an explanation to the absolute limit. The same thing concerning explanations for every other particle. Once it (the particle) "knows" that it's going through a slit it becomes a wave that can take up many different paths at the same time. For photons or atomic particles one might wish to consider such a possibility, but believing the Bucky balls could become a wave is a clearly fantastic example, I think, concerning some of the proposals and seemingly unrealistic explanations of QM.

Shaula
2010-Dec-27, 10:28 PM
No, Forrest, that is your rather novel interpretation of QM. That was the point I was trying to make. QM does not claim the particle turns into a wave or that the wave turns into a particle. The fullerene molecule is ALWAYS defined by its wavefunction. It doesn't turn into one, it doesn't magically predict what you are about to do to it so that is can morph into something. The fullerene system is described by a wavefunction. It is not a problem with QM in the slightest. It is a problem with your conceptualisation of it. Just because we know the geometry of a proton's constituents doesn't mean we have issues describing it as a wave. Buckyballs are described by wavefunctions. Everything is. It is just that as systems get more complex it is harder to maintain a coherent state in which we see wave aspects of it.

Len Moran
2010-Dec-27, 10:59 PM
I think Stange's example of Fullerene, otherwise called Bucky balls, is a good example of very serious problems with QM theory explanations in that the exact geometry of buggy balls are well known. For a Fullerene molecule to turn into a wave function just because they "know" they are going to pass through a slit surely pushes credibility of such an explanation to the absolute limit. The same thing concerning explanations for every other particle. Once it (the particle) "knows" that it's going through a slit it becomes a wave that can take up many different paths at the same time. For photons or atomic particles one might wish to consider such a possibility, but believing the Bucky balls could become a wave is a clearly fantastic example, I think, concerning some of the proposals and seemingly unrealistic explanations of QM.

I appreciate that Shaula has essentially addressed this, but I would like to add the following.

I think that, quite contrary to what forest noble says, these experiments with massive molecules do show the “quantum wave nature” of objects that would normally fall into the “matter category” and show directly how the continuous action of decoherence gradually takes away the "quantum-ness" of these objects and transforms them into the familiar classical objects of our experience.

The experiment using massive molecules such as C70 and fullerene by the Zeilinger group does not use two physical slits – the wavelengths would not allow the construction of the required narrow and narrowly spaced slits. The more massive and faster the particle, the shorter the wavelength – this is the only restriction on using the traditional two slit setup for large particles. Instead of using two slits to obtain interference they use three identical free standing gold gratings with about a thousand slits in each, making use of the Talbot-Lau effect, the purpose being to demonstrate the wave nature of the molecules as they pass through the gratings. The Talbot-Lau effect is a true interference phenomenon and is applicable to any wave passing through it – light and dark bands will be observed. In the case of matter particles (molecules) a density pattern equal to the grating patterns at multiples of the Talbot length are observed and thus demonstrating the wave nature of the matter particles. It can also be demonstrated that the interference effects are not between adjacent molecules even though the experiments were not single shot - the interference is essentially the equivalent of one molecule passing through the system at a time.

In the Zeilenger experiment many molecules are passed through three gratings. The first grating induces coherence in the beam, the centre grating acts as the actual diffraction grating and the final grating is placed at a distance equal to multiples of the “Talbot length” behind the diffraction grating. By moving this final grating along the x direction (moving it toward and away from the diffraction grating), the number of molecules registered behind the final grating will fluctuate between a minimum and maximum value if (an only if) the molecules exhibit wave characteristics.

By pumping pressurised gas into the apparatus, the interference effects are seen to gradually disappear with an increase of pressure, which is precisely the action of decoherence.

The above is my summary of these experiments as contained in the book “Decoherence and the Quantum to Classical Transistion” by Maximilian Schlosshauer.

forrest noble
2010-Dec-28, 12:41 AM
'I don't like it, and I'm sorry I ever had anything to do with it.'
Erwin Schrodinger talking about Quantum wave function interpretation above.

"Science is the belief in the ignorance of experts." Richard Feynman.

No physicist that I ever heard of in modern history ever questioned the wave character of matter. It would seem that maybe the primary reason that De Broglie waves are not considered to be physical is because it would seemingly require the ZPF to also be physical and have aether like characteristics.

Len Moran, thanks for the info. We might wish to forget that the wave function interpretation is totally illogical concerning the macro-world but must remember that some of the greatest minds that have every lived also considered the wave function interpretation of matter concerning the micro-world to also be ridiculous, as seen above/ below by some of their quotes.

http://en.wikiquote.org/wiki/Richard_Feynman
http://en.wikiquote.org/wiki/Quantum_mechanics

Shaula
2010-Dec-28, 02:17 AM
Forrest, that is not even an argument. "Clever people found it distasteful". So? Evidence backs it. That is all there is to it. Physics is about evidence and measurements. They support the theory no matter how unpalatable that may be to you and other people. By all means use that distaste to push the boundaries of understanding by looking for new answers but this is Q&A And here the mainstream rules. And the mainstream interpretation of QM is not what you have stated and doesn't have the problems you've claimed. We've derailed this topic enough so I'll stop there.

forrest noble
2010-Dec-28, 05:37 AM
Forrest, that is not even an argument. "Clever people found it distasteful". So? Evidence backs it. That is all there is to it. Physics is about evidence and measurements. They support the theory no matter how unpalatable that may be to you and other people. By all means use that distaste to push the boundaries of understanding by looking for new answers but this is Q&A And here the mainstream rules. And the mainstream interpretation of QM is not what you have stated and doesn't have the problems you've claimed. We've derailed this topic enough so I'll stop there.


Forrest, that is not even an argument. "Clever people found it distasteful". You are correct, this is not a logical argument. As you have stated the most mainstream of answers are preferred in this Q &A section.


So? Evidence backs it. That is all there is to it. This I think is arguable, but that it is the presently preferred mainstream answer is not arguable.


By all means use that distaste to push the boundaries of understanding by looking for new answers All related answers are extremely simple according to De Broglie

but this is Q&A And here the (prevailing) mainstream rules. added: (prevailing) I totally agree.


And the mainstream interpretation of QM is not what you have stated and doesn't have the problems you've claimed. De Broglie's interpretation today is not the preferred mainstream model, but whether the mainstream model has problems concerning "the wave function interpretation" in QM depends upon who in the mainstream you are talking to.


We've derailed this topic enough so I'll stop there. I also agree that the possibilities and conjecture concerning related details extends beyond the thread's scope as you have rightfully pointed out, but more-so my "postings" than yours :)

regards

Ken G
2010-Dec-28, 06:56 AM
Once it (the particle) "knows" that it's going through a slit it becomes a wave that can take up many different paths at the same time. For photons or atomic particles one might wish to consider such a possibility, but believing the Bucky balls could become a wave is a clearly fantastic example, I think, concerning some of the proposals and seemingly unrealistic explanations of QM.It's not a problem with QM, more a problem with choosing appropriate language to describe QM. I would not say a particle "becomes" a wave function, the wave function has a very different purpose than the particle concept. The wave function tells us where the particle may be found, but it does not tell us the mass of the particle, or its charge, or even that it is a particle. All that we bring to the table alongside the wave function. What the particle "is" or "becomes", on the other hand, is not something that QM takes a position on, we can safely classify that as the pedagogy we use to help us find appropriate language, and it's tricky. Interestingly, since we would scientifically tend to associate the predictions of QM with a stronger form of realism than we would its pedagogies, this means that all the verbiage about what we imagine the particle "is" or "is doing" ends up farther from what we can think of as "real" than does the strange mathematics of QM that leads to the testable predictions.

Len Moran
2010-Dec-28, 11:59 AM
Len Moran, thanks for the info. We might wish to forget that the wave function interpretation is totally illogical concerning the macro-world but must remember that some of the greatest minds that have every lived also considered the wave function interpretation of matter concerning the micro-world to also be ridiculous, as seen above/ below by some of their quotes.


I’m afraid I don’t understand the extremity of that view – the wavefunction is not totally illogical concerning the macroworld, the very basis of the Zeilinger group’s experiments was to confirm the mathematics that gave rise to decoherence theory - it is decoherence theory that mathematically accounts for the macroworld, and it does so using the quantum formalism - what is totally illogical about that? In the experiments I described, the coherence is gradually removed by the increasing density of gas molecules combining with the C70 molecules - the transition from the quantum to the classical is actually being observed in real time as the interference fades. In the traditional two slit experiment, if you pump gas between the slits and the diaphragm, the same thing happens, the interference fades - confirming the predictions of decoherence theory.

But it is essentially all down to observations – that’s all we ever get, to talk about actual waves or particles as if they have an intrinsic existence outside of our conception of them is to take realism too far – the wave function is surely not an interpretation, it is a mathematical tool that allows us to predict observations.

I don’t consider that sub atomic particles really do have “physical” waves or have “physical” particle like properties, or that interference is a “physical” thing, like ripples on a pond - I don't consider that the C70 molecule is a "physical" wave - it is the observations that give us these conceptions, but that doesn't mean that such properties exist in that form outside of observations - science tells us no such thing. The C70 molecule may be approaching the macroscopic, and maybe the "matter" part of those objects seem to approach familiar physical objects like stones, but it is decoherence that gives us that transition - a stone is an object because of decoherence, a C70 molecule is in between, it may seem more tangible, less likely to be thought of in terms of an electron and its predictive wave function, but its similarity shows up through the experiments I have described. A rock passing through such an experiment is never going to show interference, but it is decoherence that predicts that to be the case, and it is the experiments of the Zeilinger group that confirm those predictions on particles that lie between the microscopic and the macroscopic such as the C70 molecule.

The wavefunction predicts what we observe, it doesn’t explain what we observe – nothing in physics “explains” (in the realist sense) what we observe, to talk in such realist terms about a particle not being able to go through two slits at the same time, or going through one with the waves going through both is to assign an unjustified reality to such concepts outside of our involvement. It’s fine to assign pictures to things, and it helps physics enormously to do such things, but when those pictures start to become the basis for “explaining” (in the realist sense), then that’s the time to say “stop”.

Shaula
2010-Dec-28, 12:57 PM
The wavefunction predicts what we observe, it doesn’t explain what we observe – nothing in physics “explains” (in the realist sense) what we observe, to talk in such realist terms about a particle not being able to go through two slits at the same time, or going through one with the waves going through both is to assign an unjustified reality to such concepts outside of our involvement. It’s fine to assign pictures to things, and it helps physics enormously to do such things, but when those pictures start to become the basis for “explaining” (in the realist sense), then that’s the time to say “stop”.
Please, not another "what is really real in the real reality we really live in" debate.... Physics is about the measurements. Can we just stick to that? I am agreeing with what you are saying here Len but would like to try to avoid yet another epic fifty post philosophical discourse on the nature of perception and the application of analytical models to the underlying reality of the universe. I worry that they put people off asking questions on here because someone always trots out a tired version of Plato's cave.

OT: QM says that any piece of matter can act like a wave or a particle as they are both observational manifestations of the underlying entity that we use to describe an object (the wavefunction). As objects get larger they tend to be harder to make act like a wave but the potential is still there. The mainstream view is that wave and particle are just labels we apply to help us model something very complex.

Len Moran
2010-Dec-28, 03:02 PM
Please, not another "what is really real in the real reality we really live in" debate.... Physics is about the measurements. Can we just stick to that? I am agreeing with what you are saying here Len but would like to try to avoid yet another epic fifty post philosophical discourse on the nature of perception and the application of analytical models to the underlying reality of the universe. I worry that they put people off asking questions on here because someone always trots out a tired version of Plato's cave.


Well ok, but I actually never tire of trying to place physics in a perspective where I can at least start to try and distinguish between measurement, prediction, models, ontology and philosophy – that task seems important to me within physics and I don’t see it as a tired version of Plato’s cave - I always see those questions (in part) as being very relevant to the way questions are framed and answered.

Anyway, I take your point if not entirely agreeing with it.

Shaula
2010-Dec-28, 03:34 PM
Anyway, I take your point if not entirely agreeing with it.
Don't get me wrong, I think that these discussions can be important and should be had. But there is a time and a place and I do not believe that questions threads are that time or place. Last thing you want your doctor so say when you ask him "What is wrong with me?" is "That all depends how you define wrong. And where you place the concept of self in a post-holistic interpretation of wellness..." I think these questions need their own threads where all interested parties can contribute. To turn so many of the casual questions on here into a debate about the nature of reality and ontology of physics is non-productive IMO.

FWIW I have a passing interest in the philosophy of physics but I find the logic chopping that it seems to inspire somewhat tedious. People go around and around the same few points scoring logical or linguistic points until the whole conversation threatens to vanish up its own paradox. Reminds me far too much of university days for my liking.

forrest noble
2010-Dec-28, 07:05 PM
Ken G,

As always your explanation is certainly mainstream and I think more logical than most QM explanations, thanks.

Len Moran,

You are quite well spoken. As you can tell I side with De Broglie's explanation as being totally logical but realize the yours is the present mainstream QM view. The problem, I believe with the standard explanation is that a particle and a wave cannot co-exist. To explain this, one interpretation is that the particle moves as a probability wave and during this time it does not exist as a particle. In the case of Fullerene C70 we can "send off" the molecule at varying speeds. If its direction is in accord with its wave function and is somewhat redirected regardless of its speed, we might even say that all matter including ourselves move as a wave function and our exact position could never be known because even ourselves have a wave function and accordingly could not co-exist with our wave function. To be more extreme we might say that since everything has some motion, that only wave functions exist but matter does not.

I realize your views are more moderate than others but it seems clear to me that the QM explanation is extremely difficult to logically defend, if at all, concerning the idea that wave functions cannot co-exist with matter.

Even though I think this discussion is still related to the OP, I think that I may be provoking discussion less related to the OP so I will put a new question on the related topic in this section so as not to be the cause of confusion concerning the OP or others reading these postings.

astromark
2010-Dec-28, 08:25 PM
I fail to see any conflict here... Mater does exist. It has motion.
It has participable mass and behaves as a wave energy front...
Testable seen and confirmed. The answer is BOTH. Who dares to suggest other than this... ?

Shaula
2010-Dec-28, 10:42 PM
The problem, I believe with the standard explanation is that a particle and a wave cannot co-exist. To explain this, one interpretation is that the particle moves as a probability wave and during this time it does not exist as a particle.

..and our exact position could never be known because even ourselves have a wave function and accordingly could not co-exist with our wave function
Once again - strawman or you don't grasp what current theories actually say is happening. Where do you get these interpretations from? Why do you think that what we call particle and wave are more fundamental than what we actually seem to observe? You sound like someone looking at an orange wall and complaining that it cannot be orange because you only understand red and yellow and they cannot coexist.

The logical problems with the current model come from trying to use words to describe the maths. Do the maths and the predictions fall out. That is what a model is. Please, please stop trying to imply that because you don't like what you hear that means that there is a fundamental flaw in things. Keep that to the ATM or to speculative thread. This was a question about wave particle duality - not yet another soap box for you to promote the idea that the current mainstream is a teetering house of cards.

astromark
2010-Dec-29, 03:22 AM
--- I am sorry,. But it is a holiday week and I am in good company and spirit...:clap:
--- " Your, teetering house of cards." Is very secure as I use gluon's to stick it... :razz:
That photons behave as do protons and electrons is arguable... what does behave really mean ?
To exhibit a manor of motion as a particle while also being a wave like energy form.. Yes, both. :whistle:
How could we not know where we are...? I can see and measure many objects near to here... I can establish ' Relativity.'

Ken G
2010-Dec-29, 03:44 PM
Please, not another "what is really real in the real reality we really live in" debate.... Physics is about the measurements. Can we just stick to that? But that simply isn't true, physics is not just about the measurements. Pick up any physics book, and I expect it will take you no more than a few randomly chosen pages to find language that is not just about measurements. Why do we need to pretend otherwise? (For example, you'll quickly find statements like "matter is made of atoms" or "the force of gravity is mg". Not only are these expressed as statements about reality, rather than descriptions of some theory that makes correct predictions, the mental infrastructure needed to give these statements their correct interpretation (the latter one) would be extremely unwieldy, and that's why no one ever does it. So if we live in a world where we regard physics as our means of understanding our reality, what's so wrong about wanting to know what it is that we are actually doing, what our words actually mean? I view these kinds of discussions as a kind of antidote to the intellectual laziness that we get away with most of the time but occasionally bites us.


I am agreeing with what you are saying here Len but would like to try to avoid yet another epic fifty post philosophical discourse on the nature of perception and the application of analytical models to the underlying reality of the universe. I worry that they put people off asking questions on here because someone always trots out a tired version of Plato's cave.Then why do they keep asking such philosophical questions if they are put off by answers that actually address what they asked? Let's have a contest-- we look at Q&A, and you score a point for every question like "what theory best explains measurement X", and I score a point for every question like "what existed before the Big Bang" or "why is there life"? I'll bet the final tally would be pretty close-- suggesting that in fact we have a pretty equal balance between what basic facts people are confused about, versus what are the larger interpretive ramifications of those facts. Indeed, this very thread was started by questions about what protons and electrons are "like", in terms of their nature, it was not a question framed in terms of just the outcomes of the measurements.

The mainstream view is that wave and particle are just labels we apply to help us model something very complex.Right, and the philosophical thinking gets more to the "something very complex" piece. I think of standard scientific explanations as a kind of set of directions for getting from point A to point B, but philosophical discourse is more around the landscape being traversed. It's all a question of whether or not you like looking out the window-- and how sometimes it can actually be easier to follow the directions if you try to recognize the landmarks out there. So I would say the problem with language like "a particle and a wave cannot co-exist" is the ever-present confusion between what a model does, and what is actually happening. We can say that a model can invoke wave and particle elements, and we can also be troubled that reality itself must not be so schizophrenic, and we can use that, not as a complaint about the models, but as an entry to understanding what the model is, and is not-- as well as a peek into what might be happening in the reality that the models are not addressing, and at what point we should regard that as motivation for new models. I agree that the motivation for new models should be when the predictions don't work, not when we get a sense of discomfort, but the sense of discomfort is still telling us something-- perhaps that we need to think about the models differently.

Shaula
2010-Dec-29, 05:26 PM
Ken, the original question was about how protons and electron behaved. It was about how they behaved in the double slit experiment. That was answered about two thousand words ago and since then what it has boiled down to is "but that can't be real because it doesn't make sense when you word it like this" - and now it is getting sidetracked into the nature of reality (the debate I was trying to avoid). Does a proton behave like a photon when you fire it at two slits is not a philosophical question and not one in which a discourse on the nature of shared reality or the logic of QM is required.

I'm sorry but I have to disagree with you 100% on what physics is about. It is about measurements and predictions. You cannot measure reality, you cannot measure truth. These are outside physics. Physics has to be repeatable, measurable and so on. Concepts that depend on a person's mind have no place in basic physics. There may be some larger wrapper for physics that encompasses that as well but it is not physics. The very best physics can say is that something behaves as if it works like this model. It cannot tell you what it is really.

And statements like "a wave and a particle cannot coexist" do not imply problems or issues with QM. They are linguistic paradoxes at best. They come from us using faulty terminology and faulty idealised models to try to describe what we are seeing. Just because we don't have a word for wave-particle doesn't mean that the concept is flawed.

I'm going to try to leave it there. Can we save the what is physics about stuff for another thread rather than see it creep into ones in which it is not really at home?

Ken G
2010-Dec-30, 07:34 AM
Ken, the original question was about how protons and electron behaved. It was about how they behaved in the double slit experiment.That's demonstrably not so. The OP did not ask if protons would make a double-slit diffraction pattern, it asked if they would "go through both slits", and if they had particle/wave properties. Those are extremely different issues than the measurable diffraction pattern. Maybe the OPer did not appreciate those differences, and meant how you interpreted it, but even then their confusion makes my point. They can benefit from being led to see the important differences between questions about properties and actions within some model versus model-independent outcomes of experiments. The very fact that you are reading a question about properties and actions within some model, and interpreting it as a question about experimental outcomes, clearly demonstrates the existence of the problem I'm referring to here-- those two things are uniformly confused on this forum (I'm almost blue in the keyboard saying this), and if we accomplish anything in answering this type of question, let's accomplish education around those differences.



I'm sorry but I have to disagree with you 100% on what physics is about. It is about measurements and predictions.That is also demonstrably incorrect. Yes, I've many times the "shut up and calculate" school of thought on physics, only here's the rub: the very same people who claim to follow that school of thought never actually do. Never. It's so easy to find examples where they go well beyond that kind of thinking, that at some point we just have to be honest with ourselves and acknowledge that the goals of physics are more than just predicting experimental outcomes. In my view, and this is borne out by the questions we see right here on Q&A, the actual goals of physics as they play out in practice is roughly an equal balance between getting the right predictions, and getting a sense of understanding or appreciation for how reality is built (to our best ability to understand, of course). Even the ever-so-common phrase "laws of physics" is quite clearly a reference to this second mode of operation of physics-- models that explain experiments are the "models of physics", not the "laws of physics." If the "shut up and calculate" school would agree to avoid forever the phrase "laws of physics" in favor of "models of physics", then they would at least be true to their own self-described convictions.


You cannot measure reality, you cannot measure truth. These are outside physics. Physics has to be repeatable, measurable and so on.To me, you are just (correctly) identifying what truth means in physics. And that's exactly how the language of physics plays out, invariably. How far do you have to get into cosmology before you see the phrase "the universe is expanding" or "the universe has a finite age", never "the simplest model that best agrees with the current observations is that a model that has the attributes of expanding for a finite age"? If your stance was correct, those two approaches would not be shorthand and longhand for the same thing, they would be two fundamentally different ways of thinking, the former being not physics and the latter being physics. Technically, only the latter is correct, but we just don't physics in the technically correct way, and we should stop pretending that we do-- or if we insist that physics is its technically correct form, we should use the correct language every time. Physics is the language used by physicists, we can't claim we're doing something different from how we explain it every time.


Concepts that depend on a person's mind have no place in basic physics.Not so. A central structural principle in physics is Occam's Razor, which recognizes that since our goal is to understand, in our minds, we should always seek the simplest explanation. That places our minds at the center of physics, not off in some unimportant or ignorable offshoot. And so it has always been, throughout the history of physics.


There may be some larger wrapper for physics that encompasses that as well but it is not physics.We like to imagine that such a clear demarkation exists, but in practice it simply does not.

The very best physics can say is that something behaves as if it works like this model. It cannot tell you what it is really.Then we must forever reject language like "matter is made of atoms", or "the universe is expanding." By your stance, physics simply does not assert those things, so physicists should not either. We cannot claim it is all right to use different language than what we are actually doing because it is some kind of shorthand-- physics is by definition the way it is described by physicists.


And statements like "a wave and a particle cannot coexist" do not imply problems or issues with QM.What they imply is a fundamental confusion between what is a statement about a model and what is a statement about reality. The ubiquitous presence of that confusion is very much what I am talking about, and at least trying to call to our attention.


Can we save the what is physics about stuff for another thread rather than see it creep into ones in which it is not really at home?No, because one cannot even answer the question "does a proton go through both slits" without educating about the difference between a model and what is actually happening. The model does not assert that the proton goes through both slits, yet it does assert that we will see a two-slit diffraction pattern. The OP is not correctly answered without that remark, and the supporting statements necessary to make it understood.