A very basic question about Heisenberg Uncertainty

[Ken G]

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.

It's an interesting question if introductory students suffer a moment of cognitive dissonance when we tell them about forces that act at a distance, or if they are just fine with it. They probably come in ready to believe almost anything we tell them, so we would need to do a careful probe to discern whether or not that would seem fundamentally surprising to them. It would probably need to happen at the high school level, or else they'll already have been exposed to the notion. It would be an illuminating exercise, well worth reporting in a pedagogy journal.

I agree with you that framing the weirdness of entanglement is tricky, we don't want a kind of "Emperor's New Clothes" phenomenon where students need to act like they are surprised by how weird it is just to seem like they get what we are saying! Personally, I really don't care the time when the correlated observations are taken, whether they are in each other's light cones or not does not impress me because I don't see any signal that is connecting them, so who cares if a non-existent signal is superluminal or not! To me, the "weirdness" is "correlation without signal"-- I take it as a given that there is no signal there because we don't have a signal in the theory. So the weirdness is there even if the correlations are within each other's light cones.

 

[VantagePoint72]

my first proper introduction to QM was in large part via Landau and Lifshitz.

Off topic, but as long as this has strayed into pedagogy, I'm really not a fan of the L&L series. They're a fantastic reference when you already know the material, but I tend to think instructors who use them to complement lectures just have a fetish for conciseness. I recently had lunch with a professor I had way back in the undergrad days who had used an L&L book for both classes I took with him. I always wondered why, because he was a great lecturer and never even bothered to refer to them. We were discussing philosophy of physics and he said, "I never had much interest in it. I can't read the papers. I like stuff loaded up with equations, a page full of text scares me." And I thought, "Well that explains that..."

It's an interesting question if introductory students suffer a moment of cognitive dissonance when we tell them about forces that act at a distance, or if they are just fine with it. They probably come in ready to believe almost anything we tell them, so we would need to do a careful probe to discern whether or not that would seem fundamentally surprising to them. It would probably need to happen at the high school level, or else they'll already have been exposed to the notion. It would be an illuminating exercise, well worth reporting in a pedagogy journal.

You know, this is a very interesting question. A good friend of mine is a high school physics teacher. I think he's roughly in the part of the semester now where he talks about Coulomb's law (which looks non-local until you learn the rest of Maxwell's equations) and I believe next term he has the course in which Newton's theory of gravitation is introduced. I think I'll talk to him and see if we can set up a series of short surveys and get some insight into how his students react to action-at-a-distance. Probably wouldn't be rigorous enough to get published, but I'd be really interested to see the results.

 

[atyy]

Off topic, but as long as this has strayed into pedagogy, I'm really not a fan of the L&L series. They're a fantastic reference when you already know the material, but I tend to think instructors who use them to complement lectures just have a fetish for conciseness. I recently had lunch with a professor I had way back in the undergrad days who had used an L&L book for both classes I took with him. I always wondered why, because he was a great lecturer and never even bothered to refer to them. We were discussing philosophy of physics and he said, "I never had much interest in it. I can't read the papers. I like stuff loaded up with equations, a page full of text scares me." And I thought, "Well that explains that..."

Actually, I like L&L because their QM book begins with philosophy :) L&L begin with the Heisenberg cut, and mention that it is necessary and that it is strange. It is often said that the great physicists don't care about philosophy, but L&L are a counterexample. As is Weinberg's recent text. Feynman himself obviously cared about it, although he seems to not to quite have put his finger on the problem, being too close in time to Bohm and Bell. Even Dirac in a Scientific American article mentions the measurement problem, and says that it seems too hard to solve, so he will concentrate on the easier one of infinities! He also says he hopes the measurement problem will go away because maybe quantum mechanics is not the final theory.

 

[Ken G]

Probably wouldn't be rigorous enough to get published, but I'd be really interested to see the results.

I'd be interested also, so hopefully you can post the answer in this thread if you get one.

 

[DennisN]

Regarding Newton and the apparent gravitational action-at-a-distance, here's the quote/caveat from the man himself:

"It is inconceivable that inanimate brute matter should, without the mediation of something else, which is not material, operate upon, and affect other matter without mutual contact...[] That gravity should be innate, inherent and essential to matter, so that one body may act upon another at a distance through a vacuum, without the mediation of any thing else, by and through which their action and force may be conveyed from one to another, is to me so great an absurdity that I believe no man who has in philosophical matters a competent faculty of thinking can ever fall into it. Gravity must be caused by an agent acting constantly according to certain laws; but whether this agent be material or immaterial, I have left to the consideration of my readers."

(source)

 

[DennisN]

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.

You could check out e.g.
http://en.wikipedia.org/wiki/History_of_quantum_mechanics#Founding_experiments

 

[DennisN]

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.

Here may be something to think about:

We belong to a species (Homo sapiens) that has evolved to being able to physically and naturally (i.e. without fancy equipment) observe, evaluate and manipulate an environment which involves just a couple of orders of magnitude of length, let's say five (ca 10^-3 m to ca 10⁺¹ m - the exact numbers can be debated, but anyway, roughly five).

Now, take a look at this table:

http://en.wikipedia.org/wiki/Orders_of_magnitude_(length)

There are 50+ orders of magnitude of length listed there.

If we humans can't experience these 45+ other orders of magnitude of length in a natural way, would it not be weird if we expected that physical processes in the domains of these other orders of magnitude would be easy to understand for us?

 

[Ken G]

If we humans can't experience these 45+ other orders of magnitude of length in a natural way, would it not be weird if we expected that physical processes in the domains of these other orders of magnitude would be easy to understand for us?

Another interesting thing about that table is that there are about 14 orders of magnitude between currently known phenomena (like the de Broglie wavelength of protons at CERN) and the "bottom" at the Planck scale. That's about the same gap as is between that smallest well-studied scale and the wavelength of visible light, the latter being a subject of common study in Newton's day. Since we regard the bigger as being comprised of the smaller, we might then conclude that we are as far from understanding what "underpins" the quantum domain as Newton was from understanding the Higgs mechanism. So if you think quantum is weird, buckle your seatbelt. I doubt we'll cross that chasm in the next few decades, but maybe in the next few centuries...

 

[Demystifier]

We were discussing philosophy of physics and he said, "I never had much interest in it. I can't read the papers. I like stuff loaded up with equations, a page full of text scares me."

That's one of the reasons why I like dBB. It's philosophy of physics, but expressed with equations. :)

 

[Ozgen Eren]

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

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.

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

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".

No, non locality does not violate the everyday conception. Because you can observe magnets, wireless communication, the interaction of planets. Even though you don't know why, you can actually observe that some objects can affect others from distance instantly (Although its not actually instant, you can't tell the difference. But when you add Maxwell in it, it even sounds more logical to have those effects due to some moving "invisible" things.). Thus its not weird, I know, and have seen that its possible.

And no relativity is also not casual for me, as I haven't seen anything that had a time difference with me. Actually I haven't seen any proof that time itself exists. Its a pretty valid claim for the things that are too small to observe without interfering or close to speed of light, I'm not arguing with that.

All I'm saying is that, there should be a huge evidence that rules out every other claim to give up conceptions like that. That's why I'm approaching with full suspicion on quantum mechanics, not to its accuracy on experiments but to its main perception. It makes me wonder, if every subtle position jump is possible, we should be able to observe at least 1 really weird thing on macro scale. The probability for me to suddenly appear on somewhere else is really really low, I agree. 0,0...as many zeros as you would like...0001 maybe. But every second, there are almost infinitely many random events with really low probability. But if you sum them up for any second, and consider say last 100 years, at least couple random and really weird stuff should have happened.

I am sure both quantum physics and relativity and all other weird theorems works almost perfect with experimental data. All I want to be sure is that the world actually cannot be expressed in terms of what we have already seen.

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.

He did a better everyday conception than most actually. Everyone were able to see "huge rounded bulks" on the sky and everything drops to the ground, whenever they are free on air. This was an everyday conception. The apple moving towards the ground with no apparent connection is something almost everyone have seen. I mean he could have suggested that some particular objects pushed each other, but not all. I claim that you can make a theory experimentally work if you plug enough rules in it. That's what he didn't do. He used three laws and both were somehow familiar and casual(observed by any human). All I am suspecting is that, are we looking for few trivial and strong postulates and derive the rest or are we just making up a new rule in addition to what already is there.

They probably come in ready to believe almost anything we tell them, so we would need to do a careful probe to discern whether or not that would seem fundamentally surprising to them.

I mean no disrespect but in my opinion that's what anyone who didn't ever question quantum mechanics do.

 

[atyy]

I mean no disrespect but in my opinion that's what anyone who didn't ever question quantum mechanics do.

But actually, you are not questioning quantum mechanics enough! You can find the great physicists questioning quantum mechanics much more than you, Take for example, Dirac's essay http://blogs.scientificamerican.com...volution-of-the-physicists-picture-of-nature/. Among the major problems he mentions is the "role of observation" or the measurement problem. It is interesting that he says it is too difficult to solve at the moment (1963), but he hopes it will go away if quantum mechanics is falsified by data some day.

At present, quantum mechanics is not falsified by any data, and is consistent with all observations we have made except for dark matter and massive neutrinos, but those can probably be accommodated by quantum mechanics.

The first breakthrough in the measurement problem was Bohmian Mechanics. The Bell theorem is another breakthrough, showing an essential nonlocality if reality exists (with some loopholes, the major ones being superdeterminism and retrocausation).

 

[VantagePoint72]

No, non locality does not violate the everyday conception. Because you can observe magnets, wireless communication, the interaction of planets. Even though you don't know why, you can actually observe that some objects can affect others from distance instantly (Although its not actually instant, you can't tell the difference. But when you add Maxwell in it, it even sounds more logical to have those effects due to some moving "invisible" things.). Thus its not weird, I know, and have seen that its possible.

Well, you've pretty much proved my point. All the phenomena you list are things that were discovered to be local after all. This is described by Maxwell's equations for the first two, and Einstein's field equations in general relativity for the last one. The lesson was that doing something "over here" causes the effect to propagate out at or lower than the speed of light through a field. The field than acts on something "over there". Nothing instantaneous, despite what Newtonian physics once seemed to imply. You "not being able to tell the difference" between instantaneous and actually instantaneous is a great, giant chasm of difference. "Non-local" doesn't mean due to some moving invisible things, it means something over here instantaneously affecting something over there. You've demonstrated, as I suggested, that those unfamiliar with relativity tend not to actually see anything wrong with this. However, this is a lack of familiarity with physics on your part, not a failure of imagination on ours. Now, the non-locality of quantum mechanics is a bit different and does not actually violate the letter of the law of relativity (though probably the spirit of it). However, you cannot hope to understand modern perspectives on quantum mechanics without first understanding the background.

I mean no disrespect but in my opinion that's what anyone who didn't ever question quantum mechanics do.

This is tremendously arrogant. Physicists have been questioning and trying to attack quantum mechanics with all its strange implications from every angle for a century. We believe it because it has held up. If you honestly think you are the first person (or at least the first in this thread) to say, "This is very strange, can reality really be like that?" then you are being ridiculous and have completely the wrong attitude to learn something. All of us have asked that. The difference is that we then went and learned physics and realized that reality doesn't care about our preconceptions of how it ought to work.

 

[DrChinese]

I mean no disrespect but in my opinion that's what anyone who didn't ever question quantum mechanics do.

I mean no disrespect, but what you are saying is closely akin to Einstein's position in 1935. So you are in good company even if a few years behind. :-) I would strongly recommend that you read the EPR paper (Einstein is the E), in which he claims Quantum Theory must be incomplete (much as you say):

http://www.drchinese.com/David/EPR.pdf

That led to Bell's paper nearly 30 years later, which is what everyone else here is influenced by.

http://www.drchinese.com/David/Bell_Compact.pdf

You can judge it for yourself. Or if you want to read a simple explanation of Bell, I might suggest one of my web pages:

http://drchinese.com/David/Bell_Theorem_Easy_Math.htm

Of course experiments such as below support the QM side in this debate. Once you are up-to-date on this area, your questions will be more meaningful. Until then, it may as well be 1935.

http://arxiv.org/abs/quant-ph/9810080

 

[PeterDonis]

But if you sum them up for any second, and consider say last 100 years, at least couple random and really weird stuff should have happened.

Your estimate of the order of magnitude of the probability of such a macroscopic "weird" event is way, way off. Macroscopic objects contain a lot of quantum particles--on the order of $10^{25}$ to $10^{30}$ of them, and all of them would have to act "weird" in exactly the same way at exactly the same instant for a macroscopic "weird" event to happen. Even a rough upper bound on the probability of that happening in a single Planck interval would therefore be something like 2 to the power $10^{25}$. There are about $10^{50}$ Planck intervals in a year, so the probability of it happening in a year would be $10^{50}$ divided by $2^{10^{25}}$, which works out to about 1 in $2^{10^{25}}$ per year (since the $10^{50}$ in the numerator is far too small to matter). That's a lot smaller than 1 in 100 or so per year ("a lot smaller" is actually a vast understatement, even with the italics).

 

[Ken G]

To add to that, there are many everyday circuits that require "weird" things to happen-- no doubt the computers we are typing this into and using to communicate would simply not work if they were forced to never do anything "weird," like having a single particle do something that has a very small probability for any macro system to do. So we find that reality as we know it involves an important combination of the fact that large groups of things have a much more limited access to "weird behavior", for essentially probability reasons, than individual particles do.

So the issue here is not that we should believe anything we are told about quantum mechanics, and are being too easily fooled by ideas that violate our preconceptions. At issue is when to drop our preconceptions when faced with overwhelming evidence of the need to do just that.

 

[DennisN]

I mean no disrespect but in my opinion that's what anyone who didn't ever question quantum mechanics do.

Neither I mean any disrespect, but you can be very sure that quantum mechanics have been thoroughly questioned, and will continue to be questioned. As an example, it happens very frequently on this forum! Trust me, I HAVE questioned QM (but it was a long time ago, and not here on this forum). But I've also always been a big fan of the experimental side of things, so when experiments all point in the same direction, then I do not find it particularly philosophically hard to appreciate the theory. Generally, regarding "questioning", I see questioning as part of learning (and maybe the beginning of learning), but to start to get into the scheme of things, you must sooner or later become familiar with the theory and the experimental facts.

If you honestly think you are the first person (or at least the first in this thread) to say, "This is very strange, can reality really be like that?" then you are being ridiculous and have completely the wrong attitude to learn something. All of us have asked that. The difference is that we then went and learned physics and realized that reality doesn't care about our preconceptions of how it ought to work.

Nicely put. :)

Ozgen Eren, I also think Feynman formulated it rather well (and fun) IMHO:


EDIT: Here is a longer clip from the QED lecture by Feynman:

 

[ChrisVer]

No, non locality does not violate the everyday conception. Because you can observe magnets, wireless communication, the interaction of planets. Even though you don't know why, you can actually observe that some objects can affect others from distance instantly (Although its not actually instant, you can't tell the difference. But when you add Maxwell in it, it even sounds more logical to have those effects due to some moving "invisible" things.). Thus its not weird, I know, and have seen that its possible.

affect others from distance instantly... Well I can tell you for sure that the Sun you see is not the Sun "now", but the Sun 8 minutes ago, and this can be explained by the speed of light (so you CAN indeed tell the difference and explain it). This becomes even more terrible to not-take into account when you look at cosmological redshifted objects. So you are not questioning science, you are trying to defy it.

And no relativity is also not casual for me, as I haven't seen anything that had a time difference with me. Actually I haven't seen any proof that time itself exists. Its a pretty valid claim for the things that are too small to observe without interfering or close to speed of light, I'm not arguing with that.

Probably then you never heard of the experiments that have verified Relativity (from cosmic muon decays to Hafele-Keating experiment). You are still trying to defy science and not question it by making such statements.

All I'm saying is that, there should be a huge evidence that rules out every other claim to give up conceptions like that. That's why I'm approaching with full suspicion on quantum mechanics, not to its accuracy on experiments but to its main perception. It makes me wonder, if every subtle position jump is possible, we should be able to observe at least 1 really weird thing on macro scale. The probability for me to suddenly appear on somewhere else is really really low, I agree. 0,0...as many zeros as you would like...0001 maybe. But every second, there are almost infinitely many random events with really low probability. But if you sum them up for any second, and consider say last 100 years, at least couple random and really weird stuff should have happened.

Probably if you do that in a macro-scale, the time you will find will over-exceed the age of the universe. A very small probability can reach times like this. Take for example the case of particles that appear to have a lifetime greater than the age of universe- and they are particles (like protons) filling up this world. We haven't ever observed them decaying.

I am sure both quantum physics and relativity and all other weird theorems works almost perfect with experimental data. All I want to be sure is that the world actually cannot be expressed in terms of what we have already seen.

Since they do, and they haven't been disproven, a scientist has to accept them, even if it goes against his logic. Otherwise scientists wouldn't look at reality, but they would try to be logically correct [like philosophers]. For every statement, there should be an experiment to prove it wrong or right. If it is proven right, you have to accept that statement or any other that gives the same result.

He did a better everyday conception than most actually. Everyone were able to see "huge rounded bulks" on the sky and everything drops to the ground, whenever they are free on air. This was an everyday conception. The apple moving towards the ground with no apparent connection is something almost everyone have seen. I mean he could have suggested that some particular objects pushed each other, but not all. I claim that you can make a theory experimentally work if you plug enough rules in it. That's what he didn't do. He used three laws and both were somehow familiar and casual(observed by any human). All I am suspecting is that, are we looking for few trivial and strong postulates and derive the rest or are we just making up a new rule in addition to what already is there.

And it was a good idea to write down those 3 laws, but then we observed points that those laws were not predicting. GR took over that law for gravitational pulling. And I never saw the curvature of spacetime, but I saw the perihelion precession of Mercury, the deflection and the gravitational redshift of light... That's what a scientist does.

I mean no disrespect but in my opinion that's what anyone who didn't ever question quantum mechanics do.

It depends on how the question is written. In general, a question of perception is carrying certain assumptions. If one of the assumptions is wrong, then your questioning won't lead you to a fruitful result. When logic tells you A and nature gives you B, then you can't impose A because that's how you understand it. Also when you question in an insisting way, you'd better have looked deeper in what the current science has to say.