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Taamati
2008-Sep-30, 05:19 AM
Hi Guys

I was just wondering when you have quantum particles entangled, are they in fact the same particle?

Do they occupy an unseen dimension but to us it appears as two seperate but linked particles? It's hard to explain, but couldn't that account for the seemingly instant transfer of information?

I mean can't quantum particles occupy 2 places at once?

Ken G
2008-Sep-30, 02:10 PM
If you want to imagine that the same particle is in two places at once, you will have trouble dealing with the observation that both particles can be detected and destroyed in two separate detectors. With the usual "two slit" kinds of situations, some use loose words like "the particle was in two places" or "went through both slits", but you can still only ever detect one particle if you look for it. So that's quite a bit different from entanglement.

Grey
2008-Sep-30, 02:45 PM
I was just wondering when you have quantum particles entangled, are they in fact the same particle?I'm not sure this would help describe the situation. Especially since entangled particles don't even necessarily have to be the same kind of particle. Most of the cases that are easiest to describe and experiment are, but in a certain sense, any two particles that interact, ever, can be said to be entangled. In some cases, that can produce some surprising results, while in other cases it doesn't.


I mean can't quantum particles occupy 2 places at once?Yes and no. If you want to think of a particle as "spread out" like its wave function, then maybe you can say it's in two places at once. A particle can even have a significant nonzero chance to be measured at two spots that are not very close, with negligible chance to be in the area between those spots. But any time you check to be sure, the particle will always be found only in one spot or the other. You'll never see it show up in both locations if you actually do the measurement.

On the other hand, one fun thing to note is that on a Feynman diagram, a positron going forward through time is the same as an electron going backward in time. If you wanted to take those diagrams as a serious representation of what's going on, rather than a convenient method of summing the possible interactions a particle could have, you could decide that there's really only one electron in the universe, weaving backward and forward through time. I don't think anyone takes a description like that seriously, but if you're set on having a particle be in more than one place at a time, that's definitely the way to go. :)

Len Moran
2008-Sep-30, 05:06 PM
Yes and no. If you want to think of a particle as "spread out" like its wave function, then maybe you can say it's in two places at once. A particle can even have a significant nonzero chance to be measured at two spots that are not very close, with negligible chance to be in the area between those spots. But any time you check to be sure, the particle will always be found only in one spot or the other. You'll never see it show up in both locations if you actually do the measurement.



I much prefer to stick to what (as far as I understand) quantum mechanics essentially does - it predicts observations, it does not attach any familiar macroscopic reality to anything outside of the measurement. To say that "maybe" a particle exists as something spread out independently of our observations is to describe something that can never be accessed directly by us - a reality that is mind independent. The difference between that mind independent reality and the reality that ties us inexorably to the experiment is (in my opinion) fundamental to what we can and cannot expect physics to ever tell us about nature outside of our observations.

I appreciate that you are not at all suggesting the observation can show the particle in both locations, but I do feel that you seem to be attempting to describe nature in familiar terms outside of the measurement. I'm not sure to what extent you actually consider this to be possible - and I may have taken you out of context in this regard perhaps, but it seems increasingly important for me to be clear (at the quantum level) about what we mean by descriptions that do not involve observations.

Grey
2008-Sep-30, 05:27 PM
I appreciate that you are not at all suggesting the observation can show the particle in both locations, but I do feel that you seem to be attempting to describe nature in familiar terms outside of the measurement. I'm not sure to what extent you actually consider this to be possible - and I may have taken you out of context in this regard perhaps, but it seems increasingly important for me to be clear (at the quantum level) about what we mean by descriptions that do not involve observations.Mostly I was just pointing out that, while Taamati may have heard that a particle can be in two places at one time in quantum mechanics, that's only true with the very loosest of readings of quantum theory, and that it shouldn't be taken as a literal description of reality.

Taamati
2008-Sep-30, 11:21 PM
ok thanks, I wasn't aware that the particles could be different and still be entangled.

So are the particles linked through some other dimension? Like maybe through some kind of wormhole or something? Something outside of our normal perception of space-time?

Grey is right, I do only have a loose understanding of quantum theory, but it is interesting.

Ken G
2008-Oct-01, 12:45 AM
So are the particles linked through some other dimension? Like maybe through some kind of wormhole or something? Something outside of our normal perception of space-time?
The normal way that we track the connection if via something called a multiple particle wave function. There is really no need to assert the presence of anything else, the wave function performs the required service. Whether that is the "real" connection is a very different question, but no one has come up with anything that works any better, so it behooves us to simply use it and attach any picture of what it is that you choose.

Grey
2008-Oct-01, 03:51 AM
You're right, it is a fascinating subject. It's also important to remember that although quantum entanglement gives some surprising results, they're a pretty specific kind of surprising, partly because of the statistical nature of any prediction from quantum theory. In particular, the results of measuring any properties of either particle alone give perfectly normal (and as far as we can tell, random just like any other quantum measurement) results. It's only if we compare the results of measurements on both ends that we see things that make us say, "hey, wait, if our measurements of these two particles were really independent, they shouldn't be correlated as well as they are; something funny is going on here". In particular, there doesn't seem to be any indication that we could use quantum entanglement to send any kind of message.

afterburner
2008-Oct-05, 04:18 AM
Hi all, I was just wondering.

If the "synthesis" of the electron created a wave in spacetime, and if the wave moved faster than the electron...wouldn't the space time wave travel through both slits before the electron? And assuming that such a spacetime wave could actually change the course of the electron, wouldn't that account for the entanglement? And obviously the electron would only go through one hole. I hope that was clear enough.

I'm just thinking on e-paper here. :eh:

[Edit]: And depending on the lag between the electron and the wave, couldn't we theoretically test the above by changing the distance between the slits, also assuming that the wave does not dissipate too fast? Has this been done before?

Grey
2008-Oct-05, 03:52 PM
If the "synthesis" of the electron created a wave in spacetime, and if the wave moved faster than the electron...wouldn't the space time wave travel through both slits before the electron? And assuming that such a spacetime wave could actually change the course of the electron, wouldn't that account for the entanglement? And obviously the electron would only go through one hole. I hope that was clear enough.One of the possible interpretations of quantum mechanics is that the particles are all classical-type particles, but that the wave function is a representation of something real (a pilot wave), rather than a mathematical abstraction. This turns out to allow quantum mechanics to be purely deterministic like classical mechanics. Of course, to do that, the pilot wave has to have some pretty unusual features, such as being able to move faster than light. I'm not sure anyone takes that particular interpretation that seriously (indeed, I think de Broglie mostly came up with it to show that it could be done, rather than because he was convinced that it was the right way of looking at things). Bohm refined the idea in the 1950's (you can find a more detailed discussion here (http://en.wikipedia.org/wiki/Bohm_interpretation)). It's not the most commonly held interpretation, but it's not an invalid one. The upshot is that if you want to decide that quantum mechanics is best described as being made of fairly conventional particles (which definitely exist and have real properties even if they aren't being measured at the time) whose interactions are determined by superluminal pilot waves, that can fit with the actual experimental data as well as more conventional* interpretations.


[Edit]: And depending on the lag between the electron and the wave, couldn't we theoretically test the above by changing the distance between the slits, also assuming that the wave does not dissipate too fast? Has this been done before?Not really. For it to fit the data, the pilot wave has to be able to travel at superluminal speeds. Even if we changed the scenario, any pilot wave which could explain the existing data could go back and affect the electron no matter how you shuffle things around.

* I say "conventional", but of course even the most conventional interpretations of quantum theory are pretty non-conventional to anyone but a physicist. :)

Ken G
2008-Oct-05, 05:50 PM
One of the possible interpretations of quantum mechanics is that the particles are all classical-type particles, but that the wave function is a representation of something real (a pilot wave), rather than a mathematical abstraction.This is the part I just don't get in the Bohm-deBroglie approach. Why can't the "pilot wave" also be a mathematical abstraction? What criteria is it satisfying that allows us to distinguish it as real, as opposed to abstract? I have always felt that "waves tell particles where they can go", just in the wave function approach. It doesn't have to be something real, if it just instructions of some kind.
This turns out to allow quantum mechanics to be purely deterministic like classical mechanics. I'm not sure it's completely deterministic-- not from the point of view of the scientist, anyway. Again there's the question of, what does "really deterministic" mean, if scientists have no access to that determinism? That's a criticism often leveled at "divine providance" kinds of arguments.

afterburner
2008-Oct-05, 09:08 PM
This is the part I just don't get in the Bohm-deBroglie approach. Why can't the "pilot wave" also be a mathematical abstraction? What criteria is it satisfying that allows us to distinguish it as real, as opposed to abstract? I have always felt that "waves tell particles where they can go", just in the wave function approach. It doesn't have to be something real, if it just instructions of some kind.

Can't we test this?

If the pilot wave is something physical, then it should exert some kind of influence after the particle that created it disappears. But, if as soon as the particle disappears, the pilot wave also disappears, then THAT would be strange indeed.

The latter case is closer to a purely mathematical/abstract "waves tell particles where they can go" scenario, since these instructions seem to come from nowhere, and disappear in the same way, which we might call the "nature" of the particle. This is another way of saying we don't know what is going on.

The first scenario seems more classical in nature, and that is the appeal to accept that view point.

However, there is one more thing to consider about the latter scenario...It could also be the case that the appearance of a new particle restructures every part of the universe accordingly, meaning that the pilot wave does not actually travel at any speed, but is present throughout the universe during the time that the particles is. The equipment to test this is nowhere near the sensitivity it needs to be to test this scenario, unfortunately. I would imagine that we would have to have a control firing of particles in one location, and then test that norm against an intense burst/creation of particles in another, distant location, and see if we detect any changes in the behavior of the control. But the burst would have to be "louder" than the ambient creation of particles throughout the universe, and the universe is rather large, hence the difficulty...But if true, this would provide for some really nifty communication devices....

Am I understanding this right?

Ken G
2008-Oct-06, 02:10 AM
The latter case is closer to a purely mathematical/abstract "waves tell particles where they can go" scenario, since these instructions seem to come from nowhere, and disappear in the same way, which we might call the "nature" of the particle. This is another way of saying we don't know what is going on.Yeah, so far no one has thought of an experiment that can establish any separate reality to the "pilot wave". If you ask me, the entire business is an artifice invented to assuage certain philosophical prejudices we have built up from a lifetime of not being a quantum system. But it certainly does no harm to attach these philosophical preferences, it is so far not testable either way.

I would imagine that we would have to have a control firing of particles in one location, and then test that norm against an intense burst/creation of particles in another, distant location, and see if we detect any changes in the behavior of the control.That could then be used to communicate information superluminally, so would require scrapping relativity-- the other most successful physical theory of all time. It could happen, who knows, but attaching philosophical preferences to one theory at the expense of another equally powerful one seems a dubious practice in the absence of such an experimental result. So yes, by all means, do the experiment if it is possible-- just don't be surprised if it doesn't cooperate. There is a long history of people trying to "beat" relativity, and an equally long history of failure to do so-- just as there is with quantum mechanics.

Grey
2008-Oct-06, 03:29 PM
This is the part I just don't get in the Bohm-deBroglie approach. Why can't the "pilot wave" also be a mathematical abstraction? What criteria is it satisfying that allows us to distinguish it as real, as opposed to abstract? I have always felt that "waves tell particles where they can go", just in the wave function approach. It doesn't have to be something real, if it just instructions of some kind.

...

I'm not sure it's completely deterministic-- not from the point of view of the scientist, anyway. Again there's the question of, what does "really deterministic" mean, if scientists have no access to that determinism? That's a criticism often leveled at "divine providance" kinds of arguments.It doesn't have to be anything real; it can be a mathematical abstraction, but then that's no longer the model that de Broglie came up with. It's no longer the deterministic model that is the whole point of the exercise. And I really do think that's the point that de Broglie especially wanted to make. That you can create a version of quantum mechanics where all the particles have well defined properties even when you aren't measuring them, and the universe unfolds in a strictly deterministic fashion from initial conditions. You're correct that (unless we come up with some clever unexpected discoveries that would rock the foundations of modern physics), we can't determine precisely what those initial conditions are. The point is just that it's in principle possible that the universe behaves in a deterministic fashion, provided you allow for a superluminal pilot wave.

Grey
2008-Oct-06, 03:44 PM
Am I understanding this right?I think you're mostly understanding it right, but the "pilot wave" as espoused by de Broglie and Bohm doesn't have any properties other than affecting particles in a certain way that happens to match the behavior we see in quantum experiments (that's not by chance of course; since the idea was conceived to be able to do just that). A pilot wave does pretty much have to be able influence anything in the universe instantly, which could in principle violate causality. But it does so in only specific ways that carefully preserve that causality. The net result is that, just like all the other possible interpretations of quantum theory, it doesn't actually have any effect on the predictions made from the theory, or the behavior we see. Pilot wave, many worlds, sum over histories, and many others. Any of these is compatible with quantum mechanics, but none makes a distinct prediction that would allow us to choose it and eliminate the others. Hence the attitude of some physicists that you should shut up and calculate (http://scitation.aip.org/journals/doc/PHTOAD-ft/vol_57/iss_5/10_1.shtml).

Ken G
2008-Oct-06, 04:15 PM
It doesn't have to be anything real; it can be a mathematical abstraction, but then that's no longer the model that de Broglie came up with. In what way? You use the word "model"-- if I say, "god did it", does that also count as a model? To call something a model, it should be testable. What test tells me if it is abstract, or "part of deBroglie's model", if I have no access to the information that is supposed to be present? Is not the "mind of god" inaccessible information of exactly the same type?

And I really do think that's the point that de Broglie especially wanted to make. That you can create a version of quantum mechanics where all the particles have well defined properties even when you aren't measuring them, and the universe unfolds in a strictly deterministic fashion from initial conditions.I agree that was his goal, but I claim he did not succeed. He found a way that appears to satisfy that criterion, but when you look under the hood, it really doesn't. It is an entirely philosophical construction and one can say that one is doing philosophy not science, but "determinism" is also a scientifically definable concept, used in the context of making predictions. As deBroglie's approach does not make deterministic predictions, I conclude it is fundamentally not a deterministic model, in the scientific sense of the word.
The point is just that it's in principle possible that the universe behaves in a deterministic fashion, provided you allow for a superluminal pilot wave.I agree with the correctness of that statement if "deterministic" is interpreted as a philosophical, rather than scientific, claim. But I see no content in it that changes if I substitute "omnipotent deity" for "superluminal pilot wave". When the scientist has no access to the connection between the "pilot" and the "phenomena" it governs, the problem with calling it science persists either way. To me, this is the very sensible heart of the Copenhagen interpretation: add nothing to the science that cannot be distinguished by experiment from divine will, even when expressible in mathematically-inspired language. If I ask deBroglie, what controls the actual outcome, he'd say "in my model, it's the initial conditions that do that", but if I ask a religious person, they will say it is in the "mind of god that does that". Now, what scientific tests distinguishes these claims?

dhd40
2008-Oct-06, 05:35 PM
If you want to imagine that the same particle is in two places at once, you will have trouble dealing with the observation that both particles can be detected and destroyed in two separate detectors. With the usual "two slit" kinds of situations, some use loose words like "the particle was in two places" or "went through both slits", but you can still only ever detect one particle if you look for it. So that's quite a bit different from entanglement.

Maybe, itīs not really on-topic. But concerning the double slit situation: Letīs assume that we can open and close the slits in a way which guarantees that they are never open at the same time. Letīs find some way to do this in an absolutely random way (in time).
If we now perform the “standard” double-slit-experiment, we will not know through which slit the individual photons pass. And because we donīt know this, interference has to show up (at least thatīs how I understand the usual reasoning).
Really? We know that thereīs always only one open slit. But we donīt know which one is open. So we donīt know which slit the photon went through. Still, thereīs always only one open slit, ……

Iīm pretty sure that Iīm mistaken, but I donīt see why. :confused:

Ken G
2008-Oct-06, 06:09 PM
If we now perform the “standard” double-slit-experiment, we will not know through which slit the individual photons pass. And because we donīt know this, interference has to show up (at least thatīs how I understand the usual reasoning).One must be careful not to mix the concepts of "mixed states" with "superposition states"-- the latter is a fundamental statement of the information in a system and is what gives interference, the former is just an expression of your own limited knowledge and does not yield interference. What this means in your example is that you would have to open and close the slits extremely quickly, on timescales comparable to the wave period (you have essentially no knowledge of the location of the photon over those timescales, so no way to tell, even in principle, which slit the photon went through), to preserve an interference pattern. The wave function always tells you how to do this, that's its job. If you open and close the slits slowly, you won't get interference-- even if you don't know which slit is open. Interference is not just about your knowledge, that often gets muddled in popular descriptions of quantum mechanics (like in the awful cat paradox).

Grey
2008-Oct-06, 06:21 PM
In what way? You use the word "model"-- if I say, "god did it", does that also count as a model? To call something a model, it should be testable. What test tells me if it is abstract, or "part of deBroglie's model", if I have no access to the information that is supposed to be present? Is not the "mind of god" inaccessible information of exactly the same type?Sure, you could make a model of quantum theory where "god did it" is the essence of the model. But to make it a model, rather than just a statement, you'd have to quantify it. That is, de Broglie didn't just say, "well, you could say that there's a pilot wave, and it makes the particles act in a manner consistent with the predictions of quantum theory". That would be just a statement. He explained quantitatively what properties such a pilot wave would have to have, and exactly how it would have to operate in order to account for the known observations. That's a model. I wouldn't call it a distinct theory, because it makes no predictions outside of the raw mathematics of quantum mechanics for things that we'd observe, but it's a perfectly possible mechanism which could underly those predictions. And that last sentence is an accurate description of every interpretation of quantum mechanics. Even the fairly minimalist Copenhagen interpretation falls in that category (saying that it's meaningless to ask what value some potentially measurable quantity has if you don't actually measure it is making an untestable assessment of what's going on).


It is an entirely philosophical construction and one can say that one is doing philosophy not science...Hey, all of the interpretations of quantum theory are pretty much philosophy and not science. Exploring different mechanisms that could potentially underly quantum behavior might someday lead to new insight about that behavior, but at this point, it's all just interesting speculation.


But I see no content in it that changes if I substitute "omnipotent deity" for "superluminal pilot wave".Well, you can call the elements of your model whatever you want. But although the pilot wave as described by de Broglie and Bohm behaves in a manner that makes the term "pilot wave" a reasonable description (in that it acts like a wave, and serves to direct the interactions of particles), it doesn't bear much in common with what most people think of when they hear the term "omnipotent diety". Most people would expect that an omnipotent diety could do anything without restriction, and that's actually not consistent with what we see in the universe (well, unless it's an omnipotent diety that just really likes statistics and sticking firmly to a fairly bizarre set of rules for some reason).


If I ask deBroglie, what controls the actual outcome, he'd say "in my model, it's the initial conditions that do that", but if I ask a religious person, they will say it is in the "mind of god that does that". Now, what scientific tests distinguishes these claims?None. And the Copenhagen interpretation pretty much says the actual outcome is completely random, and determined at the moment of measurement. The many worlds interpretation says there isn't just one outcome, but that all potential outcomes happen in alternate universes, and we only see one of them. The "shut up and calculate" school says that we should stop asking the question of what controls the actual outcome, and just look at what the actual outcome is. None of those views can be distinguished by scientific tests, which is probably why there are so many of them. If you think there's no value at all in speculating about whether there might be some underlying mechanism to quantum mechanics if those speculations don't make any testable predictions... well, I guess that's fine actually, but I wouldn't suggest that you announce that really loudly at a conference of string theorists. :)

Ken G
2008-Oct-07, 06:29 AM
Sure, you could make a model of quantum theory where "god did it" is the essence of the model. But to make it a model, rather than just a statement, you'd have to quantify it. That is, de Broglie didn't just say, "well, you could say that there's a pilot wave, and it makes the particles act in a manner consistent with the predictions of quantum theory". But the pilot wave does none of that quantifying-- the wave function already does that. The pilot wave makes no predictions whatsoever, it merely allows the practitioner of the wave function to imagine that there is something supplemental happening that makes it deterministic. It's purely philosophical-- quantitatively philosophical, but it is not quantitative in the sense that it can be quantified by experiment.


Even the fairly minimalist Copenhagen interpretation falls in that category (saying that it's meaningless to ask what value some potentially measurable quantity has if you don't actually measure it is making an untestable assessment of what's going on).The Copenhagen interpretation (a la Bohr) does not need to make that statement-- it is an interpretation about what is meaningful and necessary, it has no need to say anything about what is meaningless.


Hey, all of the interpretations of quantum theory are pretty much philosophy and not science.That is certainly true. Which is why I feel the scientific approach is the one with the minimum extraneous elements-- greatest parsimony.


Exploring different mechanisms that could potentially underly quantum behavior might someday lead to new insight about that behavior, but at this point, it's all just interesting speculation.That is also true-- best to be versed in all possible angles. Viewed as a philosophical angle from which to look at quantum mechanics, I have no beef with deBroglie-Bohm. I merely say it is not a deterministic theory, because it gives nothing determinable.

Most people would expect that an omnipotent diety could do anything without restriction, and that's actually not consistent with what we see in the universe (well, unless it's an omnipotent diety that just really likes statistics and sticking firmly to a fairly bizarre set of rules for some reason).Not my model-- I'll say the deity makes choices that will statistically satisfy a wave function in the ensemble average. It's deterministic, in the same sense as Bohm-- no one can determine it, but it is determined.

dhd40
2008-Oct-07, 10:16 AM
One must be careful not to mix the concepts of "mixed states" with "superposition states"-- the latter is a fundamental statement of the information in a system and is what gives interference, the former is just an expression of your own limited knowledge and does not yield interference. What this means in your example is that you would have to open and close the slits extremely quickly, on timescales comparable to the wave period (you have essentially no knowledge of the location of the photon over those timescales, so no way to tell, even in principle, which slit the photon went through), to preserve an interference pattern. The wave function always tells you how to do this, that's its job. If you open and close the slits slowly, you won't get interference-- even if you don't know which slit is open. Interference is not just about your knowledge, that often gets muddled in popular descriptions of quantum mechanics (like in the awful cat paradox).

I hope I understood what you explained. So if the light intensity would be adjusted to e.g. 1 photon/second, and the (random) slit-switching frequency was 1000 switches/second (on the average), would we see interference, or not?

Ken G
2008-Oct-07, 03:10 PM
I hope I understood what you explained. So if the light intensity would be adjusted to e.g. 1 photon/second, and the (random) slit-switching frequency was 1000 switches/second (on the average), would we see interference, or not?
We would not. The 1000 switches per second is not nearly fast enough to achieve interference, unless the slits are many kilometers wide and the light you are using is deep in the radio regime. You could do the same thing in a sink with some water, it's all just wave mechanics after all. Opening and closing the slits fast enough to produce an interference pattern is something you could do with any kind of wave.

Grey
2008-Oct-07, 03:24 PM
But the pilot wave does none of that quantifying-- the wave function already does that.I disagree. The model used by de Broglie and Bohm involves a quantum potential, which (within the model) has a concrete effect on the trajectory of a particle. It's true that it gives the same predictions as a calculation using the wave function for actual observations (it has to, or it couldn't possibly be correct). But it's as much a different description as Maxwell's use of electric and magnetic fields to describe the motion of particles, rather than a direct assessment of electric forces from other charges. A treatment using an electric field likewise gives the same results. Is an electric field real, or merely a convenient fiction? Regardless of the answer to that, I'd argue that Maxwell's treatment of electromagnetism is a distinct model from a treatment that does not use an electric field for it's predictions, even though those predictions are precisely the same.

But this is a basic philosophical (again!) issue. When assessing what a theory is, some people feel that a theory is nothing other than the sum of its predictions, so that two theories that have very different formalism (but make all the same predictions) are in fact the same theory after all. Others disagree, and instead feel that the differing mechanics make them two distinct theories, in spite of the sameness of their predictions. You appear to believe the former, and that's a perfectly valid position, but there are also plenty of scientists and who believe the latter.


The Copenhagen interpretation (a la Bohr) does not need to make that statement-- it is an interpretation about what is meaningful and necessary, it has no need to say anything about what is meaningless.It doesn't need to, but it certainly does. Or at least Bohr and Heisenberg, the people most responsible for the Copenhagen interpretation, did so. For example, Heisenberg explicitly said, "Some physicists would prefer to come back to the idea of an objective real world whose smallest parts exist objectively in the same sense as stones or trees exist independently of whether we observe them. This however is impossible." and "The concept of the path of an electron between two successive measurements is meaningless." Bohr said "Everything we call real is made of things that cannot be regarded as real." and in his 1935 paper discussed at length the fact that it was meaningless to talk about any measurable quantity independently of specifying the actual experimental apparatus and the measurement process itself. Bohr and Heisenberg didn't always agree about the proper interpretation of quantum mechanics, but neither of them was shy about saying that certain questions or statements were meaningless.

I think you're thinking of the Copenhagen interpretation as being synonymous with the "shut up and calculate" school, where you avoid asking any questions at all about what the results of quantum mechanics really mean, and restrict yourself to just looking at the predictions and the experimental results themselves. But that's not actually the case. The Copenhagen interpretation is a pretty minimal interpretation of quantum theory, but it is an interpretation* nevertheless, and one that is no more supported by experiment than any of the others.


That is certainly true. Which is why I feel the scientific approach is the one with the minimum extraneous elements-- greatest parsimony.If you like. But that's "shut up and calculate", not Copenhagen.


That is also true-- best to be versed in all possible angles. Viewed as a philosophical angle from which to look at quantum mechanics, I have no beef with deBroglie-Bohm. I merely say it is not a deterministic theory, because it gives nothing determinable.I'd say that it's not even a distinct theory, just a model that could possibly underly the observed quantum behavior. If you want to say it's not deterministic, well, okay, but I think most physicists would disagree.


Not my model-- I'll say the deity makes choices that will statistically satisfy a wave function in the ensemble average. It's deterministic, in the same sense as Bohm-- no one can determine it, but it is determined.Well, you're pretty much right that it's deterministic, but you're still just making a statement there, not describing a model. If I just say "I've come up with this new idea called an electric field; it acts in such a way that Coulomb's law is obeyed by all particles", I haven't made a model of the electric field, I've just made a statement about it, and that doesn't add anything to a treatment via Coulomb's law. If instead, I write up Maxwell's laws, explain how I can determine what the value of the electric field is at every point, and how charged particles respond to the field, then I have made a model of the electric field. It still will make the same predictions as I'd get from your statement, but there's a significant difference between those two formulations.

Grey
2008-Oct-07, 03:31 PM
I hope I understood what you explained. So if the light intensity would be adjusted to e.g. 1 photon/second, and the (random) slit-switching frequency was 1000 switches/second (on the average), would we see interference, or not?Ken's right here, and suggests using very long wave radio to get interference. You could go the other way, and increase your slit-switching frequency up to a few hundred trillion times per second, comparable to the frequency of visible light, and then get interference in the visible spectrum. I have no idea whether you could come up with a switching mechanism fast enough to actually do that, but that's an engineering problem. :)

Ken G
2008-Oct-07, 04:52 PM
I disagree. The model used by de Broglie and Bohm involves a quantum potential, which (within the model) has a concrete effect on the trajectory of a particle.These are examples of pretending that a theory is providing more than it is-- more than the ability to make verifiable predictions. We have here a theory with two very clearly separable parts: one part that makes verifiable predictions, and one part that is purely extraneous that allows us to imagine a certain ontology is present, for no reason other than that we wish to do so. I say deification of quantum mechanics would be exactly like that.

But it's as much a different description as Maxwell's use of electric and magnetic fields to describe the motion of particles, rather than a direct assessment of electric forces from other charges.It is a different description, we can agree there. My deification example is also a different description. What is either adding to the science, that can be adjudicated scientifically? They are both examples of pretending (or what is termed "magical thinking").

There is nothing wrong with pretending, we do it all the time at our leisure, but there is an aura of some different kind of claim being made by deBroglie-Bohm that I just don't see the meat behind. There is what is verifiable, and there is the pretty pictures we choose to adopt in support of what we can verify, and the latter does not have "scientific" or "non-scientific" flavors, because the science ends with what the models suggest can actually be verified. When deBroglie-Bohm actually suggests something that can be verified, then we can say it is more scientific than deification-- not before.


A treatment using an electric field likewise gives the same results. Is an electric field real, or merely a convenient fiction? Neither. It certainly isn't real, because it exists in our minds (that is how it is defined, as it is its whole purpose to replace a complex array of experimental results for which it forms a kind of shorthand or mnemonic). It is certainly convenient, but "fiction" doesn't seem appropriate because of its connotations of being embellished or manipulated intentionally. I would say that the concept of electric field connects in some ways with something real (if it had no bearing on what is real it would not be useful), and in some ways with a fiction, but probably closer to the latter because it cannot possibly be what is "really there", given all the sensory and intellectual filters we had to pass the reality through to get to the concept of electric field. Still, how severe are those limitations is one of the hardest problems in the philosophy of science, I don't claim to know just "how far" from the truth the concept of electric field really is.


Regardless of the answer to that, I'd argue that Maxwell's treatment of electromagnetism is a distinct model from a treatment that does not use an electric field for it's predictions, even though those predictions are precisely the same.That's where I would not agree-- a model is its predictions, not its pedagogy. D'Alembert takes the m*a across the equals sign in F = m*a and calls it another force, and asserts that all forces always balance. This is mathematically identical to Newton's model, differing only in its philosphical picture of the kinds of tensions that "exist" in dynamical and static problems. Is it a different model? No, it is the same model, in different philosophical clothes. Is there value in seeing the model in both getups? Certainly, you never know where the next big idea will come from.


But this is a basic philosophical (again!) issue. When assessing what a theory is, some people feel that a theory is nothing other than the sum of its predictions, so that two theories that have very different formalism (but make all the same predictions) are in fact the same theory after all. Others disagree, and instead feel that the differing mechanics make them two distinct theories, in spite of the sameness of their predictions. You appear to believe the former, and that's a perfectly valid position, but there are also plenty of scientists and who believe the latter.I agree, this is philosophy. Thus one can either say "to each his own", or go a step further and ask "what is the most useful and informative definition of a model". I would argue that my approach to the definition is more useful because it makes a better distinction between what science is and what it is not, and I say the alternative approach leads to all kinds of confusion about what kinds of questions scientists can really give informed answers about. But you know my stance on that, especially in regard to quantum entanglement.


It doesn't need to, but it certainly does. Or at least Bohr and Heisenberg, the people most responsible for the Copenhagen interpretation, did so. I think you will find that is not the case. Bohr and Heisenberg had a bit of a disagreement on this point, which is glossed over in the "Copenhagen interpretation". I always favor Bohr's approach, in its mature form, where I never see Bohr making any ontological or epistemological claims that do not stem directly from the most straightforward inclusion of the facts of the situation. He basically says that quantum mechanics is what you get when you insist on coupling quantum mechanics to macro systems as your way of gaining knowledge about the former, and much of its strangeness stems directly from all the untracked information you always invoke when you do that coupling. It's just the truth of the situation, and he never adds anything to that. It is all the other interpretations that add philosophical substructure to satisfy various prejudices (which is not automatically a bad thing to do, if it ever actually produces something verifiable-- but so far, it does not).


For example, Heisenberg explicitly said, "Some physicists would prefer to come back to the idea of an objective real world whose smallest parts exist objectively in the same sense as stones or trees exist independently of whether we observe them. This however is impossible." and "The concept of the path of an electron between two successive measurements is meaningless." Yes, that was Heisenberg's take.

Bohr said "Everything we call real is made of things that cannot be regarded as real." and in his 1935 paper discussed at length the fact that it was meaningless to talk about any measurable quantity independently of specifying the actual experimental apparatus and the measurement process itself. Well that is just plain true, look at the words. How is it meaningful to talk about a measurable quantity without talking about the measuring apparatus? Bohr is simply looking at what science is really doing, nothing more. Anything else is adding substructure that does not actually appear in how the theory is verified.


Bohr and Heisenberg didn't always agree about the proper interpretation of quantum mechanics, but neither of them was shy about saying that certain questions or statements were meaningless.But note that Bohr is not saying that a trajectory is meaningless, or that an atom is meaningless, he is just being consistent with his own language-- measurement means measurement. That's all he is saying. I think all kinds of strange claims are attributed to Bohr's interpretation that are simply not justified, he was very careful to simply look at what was actually being done.



I think you're thinking of the Copenhagen interpretation as being synonymous with the "shut up and calculate" school, where you avoid asking any questions at all about what the results of quantum mechanics really mean, and restrict yourself to just looking at the predictions and the experimental results themselves. The difference between Bohr's approach and the "shut up and calculate" approach is that the calculation is only half the science, and Bohr understands that. In a world with no measuring apparatuses, what good is a calculation? Bohr is again simply being true to the fact that science is blind and deaf unless you can incorporate how you do measurements into the meaning of your calculations.

The way I would sum it up is, if scientists were themselves quantum entities, quantum mechanics would have an entirely different flavor. It would probably be a completely different theory. I also think that Bohr would have agreed with that statement, though of course his thinking took various turns at various times.


If you want to say it's not deterministic, well, okay, but I think most physicists would disagree.Most physicists have not thought too deeply about what "deterministic" means in the context of measurement theory. My whole argument is that if they were to do so, if they were to be challenged to define "deterministic" in a scientific way, they would then see that deBroglie-Bohm is not deterministic, it is a simulation of determinism, invoking "magical" and unverifiable elements to accomplish that appearance, in much the same way as things like intelligent design that science is so often critical of. If I can write an equation for magical "design forces" that explain why the laws of the universe are what they are, have I made intelligent design a science?


Well, you're pretty much right that it's deterministic, but you're still just making a statement there, not describing a model. If I just say "I've come up with this new idea called an electric field; it acts in such a way that Coulomb's law is obeyed by all particles", I haven't made a model of the electric field, I've just made a statement about it, and that doesn't add anything to a treatment via Coulomb's law.You are saying that I still need Coulomb's law to get anywhere, so saying "an electric field did it" doesn't add anything, whereas Maxwell's equations allow me to recover Coulomb's law another way. I agree, that's what I'm saying too. But for deBroglie-Bohm to avoid that exact same criticism, you have to tell me how I can get Schroedinger's equation from something that deBroglie-Bohm have introduced. You cannot-- because the parts that they added are exactly the parts that do not give us Schroedinger's equation. They keep that equation as a central part, and add nothing that informs that equation, it merely gives it a philosophical underpinning. I say the deification does exactly the same thing-- I get a philosophical underpinning that allows me to say how reality decides what happens, consistent with Schroedinger. I agree with your criticism-- I'm not saying deification and deBroglie-Bohm are equally good science, I'm saying they are equally extraneous science. And if one is not considered science, the other shouldn't be either, on the very same grounds.