A very basic question about Heisenberg Uncertainty

[DrChinese]

... Its unexplained statistical nature is the thing that's bugging me, forcing us to say "thats just the way it is".

That is the central issue with most folks upon becoming familiar with QM. But that would be equally true in a classical universe: that's just the way it is. In classical theory, why does c (speed of light) take on that specific value? That's just the way it is. Why do particles take random values? That's just the way it is.

So no difference really except on the aesthetic side.

 

[Ozgen Eren]

There is a completely deterministic version of QM: it's called de Broglie-Bohm theory. The price you pay for determinism is non-locality: the wave function that deterministically "guides" a particular particle updates itself instantaneously across the entire universe. If non-locality seems less weird to you than non-determinism, then here you are. (The way de Broglie-Bohm theory accounts for the observed statistical uncertainty in quantum experiments is by saying that we simply can't make accurate enough measurements--if we could, we would see that every particle always has a definite position and momentum, it's just that the laws that govern how these evolve has a non-local term in it due to the wave function, so a photon or electron can "swerve" at any moment because something happened on the other side of the universe, and to us, locally, that looks like a "random" swerve that we can't predict.)

If you would like a deterministic theory that is also local, however, that's not possible: John Bell showed that, and experiments have shown him to be correct. So if non-locality is as weird to you as non-determinism, then there is no possible theory that is less weird than QM with its ordinary non-deterministic interpretation.

There is one known deterministic explanation that has not been falsified by any experiment - superdeterminism, which I mentioned above as one of the topics that you could look at.

Although weirdness is in the eye of the beholder, most people find that superdeterminism is a lot weirder than quantum non-determinism. (Also although it has not been falsified by any experiment, it makes no prediction that QM does not also make, so it has no tangible advantage over QM).

Actually I don't find non-locality weird, since it doesn't break causation. I haven't look into this in just yet but Broglie-Bohm and Bell looks like they will answer most of my questions. Thanks again for all the suggestions for further reading I will definitely make them worth.

And either way, since both theories (ordinary non-deterministic QM and the de Broglie-Bohm deterministic nonlocal variant) are still QM, there is no possible theory that we know of that is less weird than QM. So once again, do you know of one? If so, please give references. If not, why do you think there must somehow be one?

Are you trying to imply I can't question QM if I don't have any better suggestion or did you just misread my last answer. I think there must be a better explanation because all we have is a finite sequence of experimental data. And mathematically you can find infinite number of functions that satisfies any finite sequence. Unless you can't prove QM is the simplest answer you can't blame me for doubting its not the simplest. And just in case if you are still wondering, no I do not have any better answer. But I am humble enough to think that does not mean there isn't any.

That is the central issue with most folks upon becoming familiar with QM. But that would be equally true in a classical universe: that's just the way it is. In classical theory, why does c (speed of light) take on that specific value? That's just the way it is. Why do particles take random values? That's just the way it is.

So no difference really except on the aesthetic side.

I actually meant about the non-deterministic side of it., but I didn't know about the Broglie-Bohm.

 

[phinds]

Ozgen Eren, it appears that you will not be satisfied unless there is a valid deterministic theory, whether it is known to us now or not, that explains what QM explains. I suggest you study the single slit experiment and see if you can think of any way that there could EVER be a deterministic theory that would explain how you can get those results.

 

[Ahmad Kishki]

Sir, you must give up your notion of "particle" and "waves", because at the quantum level there is a dual nature of matter, commonly referred to as wave-particle duality. Firsty, we identify a localised "thing" as a particle and an unlocalised "thing" is a wave. Quantum mechanics (through many experiments specially the double slit interference of electrons) tells us that at the micro level the distinction between a wave and a particle becomes hazy. This is called a quantum particle. To understand Heisenberg's uncertainty, we must understand what is this "quantum particle".

The quantum particle is simply a wave packet. This wave packet can be spread all over space to give you an ordinary wave or concentrated at a point to give you a pulse (or the dirac delta distribution). This wave packet is formed by the superposition of many waves of different wavelengths (mathematically speaking). (The waves that undergo superposition is the wave function of the system)

Now, how does this relate to Heisenberg's uncertainty? Suppose you measure the position of a particle with great accuracy, this would mean that the quantum particle becomes a pulse at the position you found the quantum oarticle at. Here, you know the position of this pulse, but you don't know which wavelength was responsible for the particle in the first place since a pulse of infinite height is really the constructive interference of many many mnay waves at a point and destructive interference at all other points. So the uncertainty in wavelength is huge. Let's remember the debroglie wavelength which states that wavelength is inversely proportional to momentum, so in other words the uncertainty in momentum is great.

Suppose you measure the wavelength, you would get a single wave which is completely unlocalised in space, and since you cannot really say a wave is here or there, but rather a wave is everywhere, then you have a huge uncertainty in position, but not momentum since as i said momentum is just inversely proportional to wavelength.

I must pause here and say that this is not related to any aspect of the measuring process, and is simply how matter behaves at the micro level.

Heisenberg's uncertainty simply gives the lower limit of these uncertainties in measurement.

I really don't want to sidetrack this thread with something that will further confuse the OP, but I must point out to you that the concept of "wave particle duality" was dumped some 80+ years ago and is only still around due to some misguided belief that it makes things easier on beginning students. There is no wave particle duality because quantum objects are not waves and they are not particles. Those are classical concepts. Quantum objects are only that ... quantum objects. If you measure particle behavior you will see some particle-like characteristics and if you measure wave behavior you will see some wave characteristics, but that does not make quantum objects particles or waves and does not (as it was thought to do 80 years ago) mean there is a wave particle duality.

I am not sure if your definition of the wave particle duality differs from mine, in any case i said what you said exactly about quantum objects. Regardless, i never came acrosd "wave particle duality" was dumped, so please link me up.
I checked wikipedia quickly only to find:
"Wave–particle duality is the concept that every elementary particle or quantic entity exhibits the properties of not only particles, but also waves. It addresses the inability of the classical concepts "particle" or "wave" to fully describe the behavior of quantum-scale objects. As Einstein wrote: "It seems as though we must use sometimes the one theory and sometimes the other, while at times we may use either. We are faced with a new kind of difficulty. We have two contradictory pictures of reality; separately neither of them fully explains the phenomena of light, but together they do".[1]

 

[PeterDonis]

Are you trying to imply I can't question QM if I don't have any better suggestion

No, but you are not just "questioning QM"; you are questioning it specifically on the grounds that it is "weirder" than other "possible theories". So it's perfectly reasonable for me to ask, what are these other "possible theories" and why do you think there must be some that are not as weird as QM? If there aren't any--if it's impossible for any theory that's not at least as weird as QM to explain the experimental facts--then that's the answer to your question.

I think there must be a better explanation because all we have is a finite sequence of experimental data.

But you are not just claiming that there must be multiple possible theories (an infinite number, in fact) that can explain any finite sequence of experimental data. I agree with that. You are making a stronger claim: that of all those possible theories, there must be some that are not as weird as QM. That is the claim I am questioning. My counterclaim is that the weirdness comes from experiments--experiments have shown us that the world just is that weird, so any theory that can explain how the world works must also be that weird.

 

[PeterDonis]

I don't find non-locality weird, since it doesn't break causation.

No, it just breaks relativity--at least, nobody has ever found a nonlocal theory that is fully compatible with relativity. The de Broglie-Bohm theory is non-relativistic, and AFAIK nobody has been able to construct a consistent relativistic version of it.

 

[PeterDonis]

I suggest you study the single slit experiment and see if you can think of any way that there could EVER be a deterministic theory that would explain how you can get those results.

The de Broglie-Bohm theory can explain it (it duplicates all the experimental predictions of standard QM), and it is deterministic. It's just also nonlocal and non-relativistic.

 

[Ozgen Eren]

But you are not just claiming that there must be multiple possible theories (an infinite number, in fact) that can explain any finite sequence of experimental data. I agree with that. You are making a stronger claim: that of all those possible theories, there must be some that are not as weird as QM. That is the claim I am questioning. My counterclaim is that the weirdness comes from experiments--experiments have shown us that the world just is that weird, so any theory that can explain how the world works must also be that weird.

Yes I agree that the experiment results force weirder theorems, but every explanation we have breaks a major concept. So I just assume among those infinitely many possible explanations, there should be one that doesn't violate position, momentum and similar concepts however complex their equations are. I can't prove it without showing one and I have never heard of Bells theorem before so I'm not sure if he actually disproves what I am saying now, I will check it out more in detail soon.

 

[PeterDonis]

I just assume among those infinitely many possible explanations, there should be one that doesn't violate position, momentum and similar concepts however complex their equations are.

Perhaps one issue is that you are changing your definition of what is "weird" vs. "not weird". First it was non-determinism vs. determinism; now it appears to be "doesn't violate position, momentum, and similar concepts". (Note that the de Broglie-Bohm theory satisfies this latter criterion: all particles have well-defined positions and momenta at all times. But, as I have said, the price you pay is non-locality and violation of relativity.) Of course "weird" is really a subjective term and you can use it however you want; however, that makes it of limited value for discussion unless we can agree on a fixed definition of what it means.

 

[atyy]

(Note that the de Broglie-Bohm theory satisfies this latter criterion: all particles have well-defined positions and momenta at all times. But, as I have said, the price you pay is non-locality and violation of relativity.)

Just a note. Earlier in the thread I asserted that it is impossible (except in special cases) for quantum particles to have well-defined positions and momenta at all times. PeterDonis's statement and mine are not in contradiction, because we are using different definitions of position and momentum. I was referring to the non-commuting canonically conjugate positions and momenta.

 

[PeterDonis]

PeterDonis's statement and mine are not in contradiction, because we are using different definitions of position and momentum. I was referring to the non-commuting canonically conjugate positions and momenta.

Yes, I should have included that the "position" and "momentum" of the particles in de Broglie-Bohm theory, the ones that have well-defined values at all times, are different from the canonically conjugate quantum variables with those names, which are the ones the Uncertainty Principle applies to. Those variables are still there in de Broglie-Bohm theory, but they involve the wave function as well as the particles themselves (so the "particles" themselves are not quite the same as the "quantum particles" that appear in ordinary QM--the latter are more like combinations of the de Broglie-Bohm particles and the de Broglie-Bohm wave function).

 

[bahamagreen]

Ozgen Eren,

There are others here that may be able to explain it better, but what you are confronting in QM is a somewhat different kind of idea about the relationship between a measurement and the attribute measured. The thing is, the measured attribute "does not exist" independently of the measurement... how can that be?

The way I think of it, which is extremely hand wavy, is something like this:

A wave can be decomposed into a set of fundamental sine waves of various frequencies, phases, and amplitudes. Those sine waves combined will reconstruct the original wave. This is Fourier... and the first conclusion might be that any wave is comprised of a set of these sine waves... sine waves are fundamental.

But, the choice of using sine waves to do this is not required; one could also use cosine waves, or triangle waves, or impulse waves, or square waves, or any kind of wave (like the complicated wave of a musical instrument) as the choice of "seed wave" with which to perform the decomposition of the original wave.
Now, think of the "measurement process" as choosing a "seed wave" to use for decomposing into component waves and think of the set of component waves as the "measured attribute".

If you choose the seed wave to be the sine wave or some other simple wave, that is like preparing a measuring apparatus to measure something like position, or momentum, or some other simple physical attribute. The choice of seed wave (how you design the measurement) determines the nature of the attribute that you measure.

If you chose a complicated seed wave for your decomposition, what you are doing is arraigning a more complicated apparatus and condition, which may be really impossible to construct, but the resulting measurement would correspond to some attribute, which may be bizarrely weird and really impossible to figure out what it corresponds to in physics... but you will get the measure of an attribute.

So, every possible seed wave used to decompose the original into components will result in measurement of some attribute... and there are an infinite number of seed wave forms to use, and an infinite number of resulting attributes, virtually none but a few resulting from the simple seed waves will look like anything we think of as a normal attribute, like position, or momentum.

The designs and implementation of real measuring conditions are pretty much limited to the ones that use the more simple seed waves for deconstruction and those measures result in the basic attributes with which we recognize, and some that are a little weird. If we could design a measurement using a seed wave of great complexity, the attribute measured would be incomprehensible and unrecognizable, but would stand just as "real" as a simple measure that results in a position or momentum.

The quantum object does not have intrinsic attributes apart from measurement in the same way that an original wave does not have a position or momentum attribute until the choice of seed wave used for deconstruction is selected and employed. The original wave does not have any particular set of decomposition components prior to choosing with wave form you want to use to do so... it has no intrinsic attributes before you design and implement a measurement.

It is the familiarity of the simple ones that makes one think they might be there all the time.

 

[DrChinese]

So I just assume among those infinitely many possible explanations, there should be one that doesn't violate position, momentum and similar concepts however complex their equations are. I can't prove it without showing one and I have never heard of Bells theorem before so I'm not sure if he actually disproves what I am saying now, I will check it out more in detail soon.

Heisenberg showed that knowledge of the value of one quantum property (say X) leads non-commuting properties (say Y or Z) to have undefined values in the limit where X is certain. If you attempt to provide values that are consistent with experiment, you will need your theory to be non-local.

So as several have mentioned, you get to make trade-offs between what important concept you must reject: locality, causality, observer independent reality, etc.

To give you an idea of how difficult the experimental regimen is currently: there are experiments in which particles are entangled AFTER they are observed (and no longer even exist). They can be entangled outside each others' light cone as well in the same experiment. So you will find a lot of hoops to jump through in your quest for a viable theory* that is not "weird". *Other than QM, which of course predicts this behavior. If people did not take QM seriously, such behavior could never be even considered.

 

[Ozgen Eren]

Perhaps one issue is that you are changing your definition of what is "weird" vs. "not weird". First it was non-determinism vs. determinism; now it appears to be "doesn't violate position, momentum, and similar concepts". (Note that the de Broglie-Bohm theory satisfies this latter criterion: all particles have well-defined positions and momenta at all times. But, as I have said, the price you pay is non-locality and violation of relativity.) Of course "weird" is really a subjective term and you can use it however you want; however, that makes it of limited value for discussion unless we can agree on a fixed definition of what it means.

I thought it was trivial. When our everyday conception is violated, I call it weird. If you can't apply it to your car for example. That includes the cases where matter don't have defined position, speed or energy. (you can't talk about a car that doesn't have a specified position, speed or energy. you can just say you don't know it in our daily life, but if exists, you know its somewhere and you probably don't assume it may teleport a second later) Or I call it weird when an event don't have a cause. (you can't say the car just moved randomly, I guess you would look for a reason, like a driver right?) Thats what I mean by weird. I understand how quantum dynamics approximate classical physics on large scale. But I just find it really hard to believe every possible explanation for our experimental result set have to be weird.

On wikipedia it summarized Bells theorem by "No physical theory of local hidden variables can ever reproduce all of the predictions of quantum mechanics.". So in the manner I just described, I guess he proves a "not weird explanation" do not exist.

 

[Ozgen Eren]

To give you an idea of how difficult the experimental regimen is currently: there are experiments in which particles are entangled AFTER they are observed (and no longer even exist). They can be entangled outside each others' light cone as well in the same experiment. So you will find a lot of hoops to jump through in your quest for a viable theory* that is not "weird".

Did someone grouped all "weird" experimental results that led us to quantum physics, or where can I find them? I know about death spiral of electron, blackbody radiation, photoelectric effect and double slit experiment. But it appears there are a lot more of it.

 

[Ozgen Eren]

Ozgen Eren,

There are others here that may be able to explain it better, but what you are confronting in QM is a somewhat different kind of idea about the relationship between a measurement and the attribute measured. The thing is, the measured attribute "does not exist" independently of the measurement... how can that be?

The way I think of it, which is extremely hand wavy, is something like this:

A wave can be decomposed into a set of fundamental sine waves of various frequencies, phases, and amplitudes. Those sine waves combined will reconstruct the original wave. This is Fourier... and the first conclusion might be that any wave is comprised of a set of these sine waves... sine waves are fundamental.

But, the choice of using sine waves to do this is not required; one could also use cosine waves, or triangle waves, or impulse waves, or square waves, or any kind of wave (like the complicated wave of a musical instrument) as the choice of "seed wave" with which to perform the decomposition of the original wave.
Now, think of the "measurement process" as choosing a "seed wave" to use for decomposing into component waves and think of the set of component waves as the "measured attribute".

If you choose the seed wave to be the sine wave or some other simple wave, that is like preparing a measuring apparatus to measure something like position, or momentum, or some other simple physical attribute. The choice of seed wave (how you design the measurement) determines the nature of the attribute that you measure.

If you chose a complicated seed wave for your decomposition, what you are doing is arraigning a more complicated apparatus and condition, which may be really impossible to construct, but the resulting measurement would correspond to some attribute, which may be bizarrely weird and really impossible to figure out what it corresponds to in physics... but you will get the measure of an attribute.

So, every possible seed wave used to decompose the original into components will result in measurement of some attribute... and there are an infinite number of seed wave forms to use, and an infinite number of resulting attributes, virtually none but a few resulting from the simple seed waves will look like anything we think of as a normal attribute, like position, or momentum.

The designs and implementation of real measuring conditions are pretty much limited to the ones that use the more simple seed waves for deconstruction and those measures result in the basic attributes with which we recognize, and some that are a little weird. If we could design a measurement using a seed wave of great complexity, the attribute measured would be incomprehensible and unrecognizable, but would stand just as "real" as a simple measure that results in a position or momentum.

The quantum object does not have intrinsic attributes apart from measurement in the same way that an original wave does not have a position or momentum attribute until the choice of seed wave used for deconstruction is selected and employed. The original wave does not have any particular set of decomposition components prior to choosing with wave form you want to use to do so... it has no intrinsic attributes before you design and implement a measurement.

It is the familiarity of the simple ones that makes one think they might be there all the time.

This analogy was actually pretty useful thank you. So I guess we also think of uncertainty as taking some Fourier constants of one seed wave, and some Fourier constants of another, given that limited number of constants will construct the original wave. Thus we can never end up knowing both properties at the same time, we just vary their intervals depending on the choice of the constants. And I also guess that this just came out of the modeling of experiments.

 

[PeterDonis]

When our everyday conception is violated, I call it weird.

But you said nonlocality didn't bother you, and nonlocality certainly violates our "everyday conception".

That includes the cases where matter don't have defined position, speed or energy.

Or I call it weird when an event don't have a cause.

The de Broglie-Bohm theory is not "weird" by these criteria. I notice you didn't include nonlocality or violation of relativity in this list, though.

On wikipedia it summarized Bells theorem by "No physical theory of local hidden variables can ever reproduce all of the predictions of quantum mechanics.". So in the manner I just described, I guess he proves a "not weird explanation" do not exist.

So nonlocality and violation of relativity are "weird" by your definition?

 

[VantagePoint72]

But you said nonlocality didn't bother you, and nonlocality certainly violates our "everyday conception".

Actually, you'd be surprised how many somewhat physics literate non-physicists (read: people who took high school physics and still remember some of it) are entirely unphased about the notion of non-locality. Until, that is, you remember the Newtonian physics is very non-local, being chock full of instantaneous action-at-a-distance forces. It's a funny thing, but people actually have to be taught first that non-locality is a weird thing in light of special relativity, before appreciating that quantum mechanics is weird for (kind of) having it. For people who haven't been taught to think relativistically, locality is a foreign concept that violates "everyday conception".

 

[atyy]

Actually, you'd be surprised how many somewhat physics literate non-physicists (read: people who took high school physics and still remember some of it) are entirely unphased about the notion of non-locality. Until, that is, you remember the Newtonian physics is very non-local, being chock full of instantaneous action-at-a-distance forces. It's a funny thing, but people actually have to be taught first that non-locality is a weird thing in light of special relativity, before appreciating that quantum mechanics is weird for (kind of) having it. For people who haven't been taught to think relativistically, locality is a foreign concept that violates "everyday conception".

The worst of these are the lattice theorists, thinking they can provide a non-perturbative definition of the standard model

 

[PeterDonis]

Until, that is, you remember the Newtonian physics is very non-local, being chock full of instantaneous action-at-a-distance forces.

Hm, good point. Apparently I have done a very good job of retraining my intuitions to be relativistic. ;)

 

[Ken G]

Say we have 1,2 and 3 as experimental result. Then we say most probable function that satisfies it is x=y (like classical physics until it failed). Then we get 1,2,3,7 disproving x=y (7 is like the experiments that disproved classical physics). We can still write infinite functions (theories in this case) to satisfy the experimental data we have. All those infinitely many possible explanations will have different metaphysical assumptions. And I just think quantum physics is like drawing a really weird but valid curve to explain the sequence, instead of trying to fit a smooth polynomial (a theory with our usual perception is a smooth polynomial in this case).

Yes, I think that's a good way to frame the question. So then the question becomes, what are we trying to do-- fit the data with the best polynomial we can, and just tolerate it if we can't do it very exactly (that's more or less what Ptolemy and Copernicus did, when they were stuck with using only circles), or do we generalize our thinking until we can fit it even better (that's more or less what Kepler did when he used ellipses instead of circles). It would have been quite a shock to Ptolemy to imagine ellipses in space, indeed it might have shook him to his core (the ancient Greeks firmly believed in the perfection of the cosmos, and the perfection of circles). Yet today, we teach about ellipses, without batting an eye! Consider this wonderful quote by Feynman:
"We always have had … a great deal of difficulty in understanding the world view that quantum mechanics represents. At least I do, because I'm an old enough man that I haven't got to the point that this stuff is obvious to me. Okay, I still get nervous with it. And therefore, some of the younger students … you know how it always is, every new idea, it takes a generation or two until it becomes obvious that there's no real problem. It has not yet become obvious to me that there's no real problem. I cannot define the real problem, therefore I suspect there's no real problem, but I'm not sure there's no real problem."
In that quote, he seems to agree with you that it would be nice to "fit a polynomial" in your words, but by the same token, he suspects this is just a prejudice of his, that will likely vanish in future generations-- much as the desire for circles in space vanished when ellipses came along.

 

[atyy]

Consider this wonderful quote by Feynman:
"We always have had … a great deal of difficulty in understanding the world view that quantum mechanics represents. At least I do, because I'm an old enough man that I haven't got to the point that this stuff is obvious to me. Okay, I still get nervous with it. And therefore, some of the younger students … you know how it always is, every new idea, it takes a generation or two until it becomes obvious that there's no real problem. It has not yet become obvious to me that there's no real problem. I cannot define the real problem, therefore I suspect there's no real problem, but I'm not sure there's no real problem."

I always consider this to represent Feynman's misunderstanding of quantum mechanics. In his lectures, he says the only weird thing in QM is the double slit. That of course has a local hidden variable model, so yes, there is no real problem.

 

[VantagePoint72]

In his lectures, he says the only weird thing in QM is the double slit.

He actually did acknowledge this omission some years later and said something to the effect of, "Interference and entanglement together cover all the weirdness of quantum mechanics."

 

[Ken G]

It's a funny thing, but people actually have to be taught first that non-locality is a weird thing in light of special relativity, before appreciating that quantum mechanics is weird for (kind of) having it. For people who haven't been taught to think relativistically, locality is a foreign concept that violates "everyday conception".

I think you have a valid insight here, but I'd say the weirdness of non-locality goes much deeper, and predates relativity. I believe I recall Newton himself saying that no rational person could believe that action at a distance was really true, he just didn't have anything better at the time. So I think he would have been happy to hear about relativity, even though it unseated his theory from the top of the pyramid. To me, the fundamental source of the expectation of locality comes from our most basic experiences, that we only sense reality at a given place and time. We can infer things about elsewhere, but we are just telling a story-- all we actually perceive is entirely local. So if that's the way we ourselves move through our reality, that's also the kind of way for reality to work that would not seem weird to us.

 

[atyy]

He actually did acknowledge this omission some years later and said something to the effect of, "Interference and entanglement together cover all the weirdness of quantum mechanics."

Ah that's interesting. Do you know where I can look that up?

Incidentally, the Quantum Bayesians have made at least a partial defence of the remarks in Feynman's lectures (they acknowledge that nowadays, most Foundations people will think it's the Bell inequalities that epitomize quantum weirdness). It's in the introduction of http://arxiv.org/abs/0906.2187.

 

[VantagePoint72]

I believe I recall Newton himself saying that no rational person could believe that action at a distance was really true, he just didn't have anything better at the time.

True, but I think we can all agree that whatever implications the average student draws from Newton's theories, Newton himself did not have "everyday conceptions" about most things.

 

[Ken G]

I always consider this to represent Feynman's misunderstanding of quantum mechanics. In his lectures, he says the only weird thing in QM is the double slit. That of course has a local hidden variable model, so yes, there is no real problem.

I doubt Feynman would count deBroglie-Bohm as a valid solution to that problem. For example, in his lectures he made the point that you could create a "unified theory of everything" by simply taking every equation of physics, express it as a quantity that must vanish, square that quantity, and add it to all the other such quantities. Then the "theory of everything" would simply say that the sum of all those squared quantities must equal 0, and you have a single equation that includes all the rest. But he pointed out this could not count as a true unification, because it does not have the conceptual status as a unification of disparate phenomenon, it is just a kind of mathematical trick. One can view deBroglie-Bohm in a similar way-- not a conceptual unification, but rather a mathematical trick that allows the known unknowns in the final state to be descendants of the unknown unknowns in the initial state. I don't mean to derail the discussion by turning it into a critique of deBroglie-Bohm as I think that interpretation has interesting insights, I just mean to say that I feel Feynman's quote still holds-- if we don't include solutions that might be regarded simply as mathematical tricks, we still have the fundamental strangeness of the two-slit experiment applied to quanta. The strangeness is we have initial states that as far as we can tell have been prepared identically, which perform differently. It's the usual magic of the Born rule, and brushing that magic back to the initial state is not necessarily a resolution of the quandary.

 

[Ken G]

True, but I think we can all agree that whatever implications the average student draws from Newton's theories, Newton himself did not have "everyday conceptions" about most things.

You mean he was a mystic? That's true, but I still think it is reasonable that he rejected action at a distance because it just doesn't seem like how reality works. I can call to someone across the room, but the sound does not just appear in their head like ESP-- there is a pressure variation in the air in their ear. Since we don't routinely manipulate gravity or electromagnetic forces at a distance, all the influences we have on our environment in our daily lives are like this when we analyze them-- they are all local. So I think it does come as a surprise when we teach students, well before relativity, that mass and charges exert forces on each other at a distance. One way to check this is to ask a student, when they first start out in physics and long before relativity, if they think they can see something in the distance because they are remote sensors, or if they think there is something coming from that distant object that is arriving at their eyes. It would be interesting to check this, I've never tried.

ETA: put differently, this relates to two rather different meanings of "local." Post-relativity, local can mean that signals and influences must propagate slower than the speed of light. But pre-relativity, we still have a concept of local, which is that signals and influences have to propagate at some finite speed, there must be something there that moves from point A to point B to "carry the influence." The speed would have no universal limit, but every signal would have a speed-- that's what rules out action at a distance. And whatever the speed was, some type of entanglement experiment could violate it-- all relativity does is tell us what speed we need to worry about.

 

[VantagePoint72]

Ah that's interesting. Do you know where I can look that up?

Hrmph, one can never escape as easily without a source as one likes. Honestly, it's second hand information—something a colleague told me recently when discussing Feynman and quantum computing—and I hadn't bothered to check it. A bit of cursory research turns up nothing, so perhaps I'm just spreading hearsay by a Feynman admirer.

You mean he was a mystic? That's true, but I still think it is reasonable that he rejected action at a distance because it just doesn't seem like how reality works. I can call to someone across the room, but the sound does not just appear in their head like ESP-- there is a pressure variation in the air in their ear. Since we don't routinely manipulate gravity or electromagnetic forces at a distance, all the influences we have on our environment in our daily lives are like this when we analyze them-- they are all local.

I mean, I don't disagree with anything you say here, obviously. I think locality is intuitive. As for Newton, all I meant was to say that as a man with his hand uncommonly secure on the pulse of nature, it's not surprising to me that he had intuitive misgivings for the reasons you discuss. I was just suggesting that those without Newton's gifts (which is quite possibly everyone) can probably be forgiven for not jumping to the same intuitive discomfort when they study his theories.

So I think it does come as a surprise when we teach students, well before relativity, that mass and charges exert forces on each other at a distance. One way to check this is to ask a student, when they first start out in physics and long before relativity, if they think they can see something in the distance because they are remote sensors, or if they think there is something coming from that distant object that is arriving at their eyes. It would be interesting to check this, I've never tried.

Well, perhaps your experience with students differs from mine. All I can say is that most of the time when I discuss things like Bell's theorem with people with a high school physics background (which, granted, doesn't happen that often) the "quantum mechanics is non-local!" punchline rarely seems to land.

 

[atyy]

Hrmph, one can never escape as easily without a source as one likes. Honestly, it's second hand information—something a colleague told me recently when discussing Feynman and quantum computing—and I hadn't bothered to check it. A bit of cursory research turns up nothing, so perhaps I'm just spreading hearsay by a Feynman admirer.

I too am a Feynman admirer, and learned a lot from his lectures. The beautiful statements about frogs and composers still resonate with me. Unfortunately, I'm even worse than those lattice theorists, being a biologist, so I simply don't find nonlocality or contextuality weird :) The thing I find weird is the measurement problem, but that's probably because although I read Feynman's Volume III on my own, my first proper introduction to QM was in large part via Landau and Lifshitz. Of course if the lattice theorists actually get a lattice standard model, the measurement problem will probably be solved.