In what sense is QM not understood ?

[Ken G]

Newton's theory explains why the simple theory works, but it raises a whole new set of "why?" questions. This illustrates another important idea: that the only thing that can explain why a theory works, is a better theory.

That's a key point I don't think a lot of people recognize about a physics theory, no matter how accurate or widely accepted it is: it never tells us "why" nature works the way she does, it only tells us why some previous theory worked as well as it did! To explain why we get the observations we do, we would actually need a theory that described what we are doing when we make an observation, which requires that we can model ourselves, modeling ourselves, and so on. That's why I hold it is never possible to use physics to say "why" we observe what we do, and we should not make that our goal for doing physics. But we'd still like to have theories that give a consistent and complete account that connects nature to the observed result, and that's just what quantum mechanics does not do, without invoking an interpretation that few agree on. I actually see this as a feature of QM, not a bug-- we aren't supposed to be able to map the complete connection between what nature is doing to our observation of it.

 

[Fredrik]

Yes, it is easier to describe what is happening in classical systems, but I would argue that even Kepler did that-- before there even was anything we could call classical mechanics.

Yes, I agree about this part. (More details in my reply to harrylin above).

So we cannot argue that we understand the theory of classical mechanics simply because what we are trying to predict is easier to describe pictorially-- I think when we talk about understanding a theory, what we mean is, understand why that theory provides a good description of the behavior we see, even if the behavior seems weird. The classic example is relativity-- with a few fairly reasonable sounding postulates, we obtain an explanation of very weird behavior, so we say we understand relativity. The postulates don't seem to make any unbelievable claims.

I'm not sure what you're saying here. Is it one of the following things? A) To understand the theory is to understand its mathematics and correspondence rules (the assumptions that tell us how to interpret the mathematics as predictions about results of experiments), or B) To understand the theory is to understand why its predictions are accurate.

If you meant A, then what we need to do before we can say that we understand the theory, is to prove the most relevant theorems, and convince ourselves that we have the right idea about how to perform measurements of the sort the theory makes predictions about. (I would say that we have accomplished this to a satisfactory degree already).

If you meant B, then what we need to do is to find a better theory. (If this is what you meant, then we have very different ideas about what it would mean to understand the theory. I would say that this is actually unrelated to "understanding the theory". It's an entirely different issue).

Hm, you probably meant neither. Maybe you meant C) To understand the theory is to know which things in the purely mathematical part of the theory correspond to things in the real world. This is of course the part that no one understands. So if we define "understand the theory" this way, then we don't understand it. But I don't use this definition. I'm using the one I labeled "A" above.

 

[Fredrik]

There's probably a more general way to think about the issue of "what is a measurement" which cuts deeper into the heart of the problem-- and that is, "what is the role of the physicist in the physics." This is the element that Bohr was so focused on, and many take issue with him for raising such a philosophical issue, but I think his insight is still the crux of the matter. So in these terms, "what we don't understand" about quantum mechanics is "why can't we escape the role of the observer." In all other areas of physics, we can imagine that the observer is just a kind of "fly on the wall", and we don't have to attach any importance at all to the fact that an observation is being carried out. That's exactly what we cannot do in quantum mechanics, and we just don't know why. How we resolve that uncertainty is exactly the role of the various interpretations, but none can produce an unequivocally demonstrable answer-- to put it mildly.

I have come to think about this role of the observer as an essential feature of the concept of "physics". Theories of physics are falsifiable statements about reality. To be falsifiable, a statement must have testable consequences. In other words, we must be able to use it to make predictions about results of measurements. And what is a measurement? It's an interaction between the system and its environment that puts some part of the environment into one of several states that a human observer can interpret as a result of the measurement. Such a state must last long enough for a human to observe it, and be distinguishable from states that correspond to other results. So that part of the environment, the "pointer" that indicates the result, has to behave in a way that will be perceived as classical.

A "classical" theory is a theory that only makes predictions that can be tested without significantly disturbing the system. So maybe we shouldn't be asking why QM is so weird, but instead be asking why there are classical theories that are actually pretty good.

That's a key point I don't think a lot of people recognize about a physics theory, no matter how accurate or widely accepted it is: it never tells us "why" nature works the way she does, it only tells us why some previous theory worked as well as it did! To explain why we get the observations we do, we would actually need a theory that described what we are doing when we make an observation, which requires that we can model ourselves, modeling ourselves, and so on. That's why I hold it is never possible to use physics to say "why" we observe what we do, and we should not make that our goal for doing physics. But we'd still like to have theories that give a consistent and complete account that connects nature to the observed result, and that's just what quantum mechanics does not do, without invoking an interpretation that few agree on. I actually see this as a feature of QM, not a bug-- we aren't supposed to be able to map the complete connection between what nature is doing to our observation of it.

Good post. No objections from me.

 

[krd]

I actually see this as a feature of QM, not a bug-- we aren't supposed to be able to map the complete connection between what nature is doing to our observation of it.

Who says we're not supposed to?

We have to keep asking questions - reformulating things. Maybe, sometime in the future - a few thousand years from now we'll arrive at the end.

Dream.

We're no where near the end. Like at the minute we do not have 3d prints, that can shoot beams and create whatever matter we want - like pressing a button and making a chocolate cake appear out of nothing. I know it sounds like impossible magic. But so would mobile phones have sound to the ancients. Although they did believe their priests could talk to god.

 

[nitsuj]

I actually see this as a feature of QM, not a bug-- we aren't supposed to be able to map the complete connection between what nature is doing to our observation of it.

Interpretation whether it be a "problem" (in context of predecessor theories) or a "feature" of nature at that level. A little poetic

Just really shocked that, for as much SR accounts for everything right down to defining dimensions, that QM doesn't mention them. outside of "hidden" dimensions, multi-universes and the like, QM doesn't seem (at a laymen level) to address what the measurements are exactly, less a classic (SR) description.

Which seems to be a dichotomy bridged by probability. And that doesn't seem to "flow naturally".

Perhaps not something that can be described with geometries,

 

[stevendaryl]

A "classical" theory is a theory that only makes predictions that can be tested without significantly disturbing the system. So maybe we shouldn't be asking why QM is so weird, but instead be asking why there are classical theories that are actually pretty good.

I think that the distinction you are making is important, but I don't see how it is the full story. It's completely understandable that if you try to measure the position of an electron using light, then you end up disturbing it and making the momentum uncertain. However, why should measuring the spin of one particle of an EPR pair disturb the spin of the other particle? As I said, it's not the fuzziness or uncertainty or nondeterminism of quantum mechanics that makes it so mysterious--it's the combination of uncertainty with very strong nonlocal correlations that makes it mysterious.

We can understand the uncertainty and nondeterminism in terms of the observer disturbing the system by his measurements. But then why would such measurements show such strong correlations, in the case of entangled particles?

 

[Ken G]

Hm, you probably meant neither. Maybe you meant C) To understand the theory is to know which things in the purely mathematical part of the theory correspond to things in the real world. This is of course the part that no one understands. So if we define "understand the theory" this way, then we don't understand it. But I don't use this definition. I'm using the one I labeled "A" above.

I agree that (C) is a problematic definition, but I think that is what Bohr and Feynman meant when they said that to understand quantum mechanics requires not understanding it, or more simply, no one understands it. I think what you are saying is that Bohr and Feynman are admitting to a nonworkable version of the kind of "understanding" we should be shooting for, and I think they might have agreed with that, though Feynman always expressed some discomfort around that state of affairs (though he admitted he couldn't formulate the problem so there probably wasn't any problem!).

 

[Ken G]

I have come to think about this role of the observer as an essential feature of the concept of "physics". Theories of physics are falsifiable statements about reality. To be falsifiable, a statement must have testable consequences. In other words, we must be able to use it to make predictions about results of measurements. And what is a measurement? It's an interaction between the system and its environment that puts some part of the environment into one of several states that a human observer can interpret as a result of the measurement. Such a state must last long enough for a human to observe it, and be distinguishable from states that correspond to other results. So that part of the environment, the "pointer" that indicates the result, has to behave in a way that will be perceived as classical.

A "classical" theory is a theory that only makes predictions that can be tested without significantly disturbing the system. So maybe we shouldn't be asking why QM is so weird, but instead be asking why there are classical theories that are actually pretty good.

I agree 100% with everything you just said, so lucidly.

 

[Ken G]

Who says we're not supposed to?

We have to keep asking questions - reformulating things.

I agree. And I'm suggesting just such a questioning, and reformulation: I'm suggesting that we should question whether or not we should be looking for a description of nature that has us explicitly in it, and in doubting that, we should reformulate physics in the way Bohr referred: to be a study of that which we can say about nature, and nothing more. But there's a "hitch" in that prescription, which is, we are not attempting to account for why we can say that about nature, for that is doing more than simply being what we can say about nature, that's being what we can say about what we can say about nature. The reformulation is not a capitulation to the unknown, it is a lever with which to gain purchase over the unknown-- but it comes at a price. That seems like a standard kind of "bargain" that we accept in physics all the time-- it's the kind of approach that gave us those mobile phones.

 

[harrylin]

Upon my writing that "Keppler complained that he did not understand it. Later Newton's theory of gravitation gave a first feeling of understanding of the "why" [..] due to identifying a physical cause to which those equations relate":

[..] Newton's theory is a better theory, because it makes more accurate predictions about a wider range of phenomena.
Newton's theory explains why the simple theory works, but it raises a whole new set of "why?" questions. This illustrates another important idea: that the only thing that can explain why a theory works, is a better theory.

Not necessarily a better theory is required in the sense of making more accurate predictions; in the above example, Newton's theory first of all satisfied the question of "why" Kepplers ellipses. A correct and understandable interpretation of existing equations (without making more accurate predictions) would already satisfy the "why" to the point that most of us would expect (or desire) from a theory.

[..] why should measuring the spin of one particle of an EPR pair disturb the spin of the other particle? [..] why would such measurements show such strong correlations, in the case of entangled particles?

Yes, I think that such questions hit the nail on the head.

 

[nitsuj]

However, why should measuring the spin of one particle of an EPR pair disturb the spin of the other particle? As I said, it's not the fuzziness or uncertainty or nondeterminism of quantum mechanics that makes it so mysterious--it's the combination of uncertainty with very strong nonlocal correlations that makes it mysterious.

Does a comment like above not add to that confusion?

Please correct me, I understand one particle does not "disturb" the other & that the "non-local" correlation is moot. There is no "connection" between the two, outside of their origin. What's odd is the probability is "transferred", or more reasonably; a "law" of nature regarding uncertainty seems "more proven".

Maybe I'm hung on the use of those words you chose, but the choice of words seems to imply what is often said to be the most confusing part of basic QM concepts.

 

[Doofy]

Well this has been an interesting read so far, and so many responses in such a short time. It just dawned on me what an awesome learning resource this site is. You people are all contributing to something unprecedented here, so much physics knowledge being shared all in one place on demand.

 

[stevendaryl]

Does a comment like above not add to that confusion?

I'm not suggesting that EPR-type correlations can be explained in terms of disturbance---just the opposite; I'm saying that it doesn't make sense to describe them that way. That's my point, the strangeness of quantum mechanics is not just due to nondeterminism, and it's not just due to "the observer affecting that which is being observed".

Please correct me, I understand one particle does not "disturb" the other & that the "non-local" correlation is moot. There is no "connection" between the two, outside of their origin.

That's the idea behind local hidden variables: Because the two particles share a common origin, they subsequently share state information, and so what happens to the two particles later on is naturally correlated. That's perfectly understandable. But it's wrong. You can't (at least not in a way that makes intuitive sense to most people) explain EPR-type that way: the correlations predicted by quantum mechanics are just different from what would be predicted based on the two particles sharing a common origin. That's what Bell's theorem shows.

What's odd is the probability is "transferred", or more reasonably; a "law" of nature regarding uncertainty seems "more proven".

I'm not sure I understand what you mean by that.

 

[robertjford80]

I would say that it is not just because it is nondeterministic that people say they don't understand quantum mechanics. It's the combination of nondeterminism together with extremely strong correlations that is hard to understand.

In an EPR experiment, you produce a twin pair of spin-1/2 particles. Alice measures the spin of one particle along some axis, and gets +1/2 or -1/2. Bob measures the spin of the other particle along a different axis, and gets +1/2 or -1/2.

The fact that Alice's result is nondeterministic is not hard to understand. But the fact that, in the case where Alice and Bob choose the same axis, they always get opposite result, is hard to understand. If Alice knew what axis Bob was going to choose, and Alice did her measurement a second before Bob, then she would know exactly what result Bob would get. So in that situation, from her point of view, Bob's result isn't nondeterministic--it's completely predictable.

It's the combination of perfect nondeterminism and perfect correlations that is hard to understand about quantum mechanics.

correct

 

[Delta Kilo]

To me, it is the sheer size of the Hilbert space that blows the mind. Spin directions of N billiard balls can be fully described by 2N numbers (2 for each ball). Spin of N electrons requires 2^N numbers! For instance, simulating a handful of qubits would severily strain resources of a modern PC. 100 qubits is out of the question. The numbers quickly grow beyond anything physics has ever dealt with. The only hope seems to be some sort of overriding cosmic holographic principle, as in Total Perspective Vortex:

To explain - since every piece of matter in the Universe is in some way affected by every other piece of matter in the Universe, it is in theory possible to extrapolate the whole of creation - every sun, every planet, their orbits, their composition and their economic and social history from, say, one small piece of fairy cake.

 

[nitsuj]

"What's odd is the probability is "transferred", or more reasonably; a "law" of nature regarding uncertainty seems "more proven"."

I'm not sure I understand what you mean by that.

Entanglement for me, shows that "probability" in this context is "embedded" in nature, is isotropic through out spacetime. i.e. not a result of us guessing a probability.

 

[Ken G]

I'm not suggesting that EPR-type correlations can be explained in terms of disturbance---just the opposite; I'm saying that it doesn't make sense to describe them that way. That's my point, the strangeness of quantum mechanics is not just due to nondeterminism, and it's not just due to "the observer affecting that which is being observed".

The strangeness of entanglement does seem rather different from the strangeness of the measurement problem, but I think there's a way to look at it where they are actually a similar issue. We merely need to think of the identity of a particle as something that is established by observation. Then, indistinguishability gets "collapsed" in a way that is similarly strange as whatever other measurement is being done.

You can't (at least not in a way that makes intuitive sense to most people) explain EPR-type that way: the correlations predicted by quantum mechanics are just different from what would be predicted based on the two particles sharing a common origin. That's what Bell's theorem shows.

Actually, it's fine to attribute the correlations to the common origin, indeed that is far superior (in my view) to any concept of "instantaneous effects" between the particles. What Bell's theorem says is that the ramifications of the common origin cannot be expressed in terms of a "locally real" picture, where each particle "carries with it" all the information you need to correlate the measurements.

 

[stevendaryl]

The strangeness of entanglement does seem rather different from the strangeness of the measurement problem, but I think there's a way to look at it where they are actually a similar issue. We merely need to think of the identity of a particle as something that is established by observation. Then, indistinguishability gets "collapsed" in a way that is similarly strange as whatever other measurement is being done.

There are two different, but related, ideas about the effect of measurement on the system. The first is the notion that the act of measurement affects the thing being measured. That is potentially true classically, as well as quantum-mechanically. Shining a light on an object to see it better can end up changing the object's properties. I don't think that's a hard-to-understand concept. It makes interpreting experiments more complicated, but conceptually I don't think it's too weird.

With the "disturbance" model of the uncertainty principle, we can consistently believe that an electron has both a position and momentum at every moment, but that attempts to measure one ends up changing the other in an uncontrollable way. But, as I said, that disturbance model of uncertainty does not explain why distant measurements (in the case of EPR-type experiments) should be correlated.

The second notion of measurement affecting the system is the idea that a system doesn't have a property until that property is measured. That's very different, and much weirder.

 

[Fiziqs]

To me, the problem with understanding QM doesn't lie in QM itself, but instead lies in the fact that most physicists are human, and humans are for the most part, idiots. Ergo, so-called physicists, (those who speak as if they understand QM, but in actuality don't have a clue) seem to view it as some vastly complex and mysterious phenomena, beyond the comprehension of most mere mortals. Relegated to the realm of mathematics, and probabilities. Understandable only to those few with sufficient education and insight to grasp such things, to which I say bull***t. I believe that QM, and nature in its essence, will prove to be inspiringly simple and elegant at its core. QM, which if truly understood, would appear so simple, that even a child could understand it, is not the mystery that physicists make it out to be. Someday children will marvel at our ignorance of QM, the same way that children nowadays view those who used to think that the Earth was the center of the universe. How could men have been so deluded and ignorant. Of course back then I'm sure that the discovery of objects moving backwards in the heavens, and celestial bodies orbiting other bodies seemed pretty mysterious too. Most things are when you don't understand them.

That said, I too am ignorant, I do not know what the answer is, but I know that it is simple. In my ignorance I attempt to envision a world in which such seeming mysteries, as entanglement and wave particle dualities, can be explained without the need for mysteriously vague concepts like "probabilities".

Imagine a one dimensional string. Waves can move along this string in some form. Now let's coil the string to form a sheet. Likewise let's allow waves to move across the sheet. Now let's take our sheet and roll it up into a three dimensional string, and again let's allow waves to move along this string. Now what would a wave moving along our three dimensional string look like to someone on the rolled up two dimensional sheet? What would a wave moving across our two dimensional sheet look like to someone on our three dimensional string? Let's not stop there, let's take multiple strings, and multiple sheets. What happens then?

There are a myriad ways to fold, roll, and entangle the ten, eleven, or whatever number of dimensions there ultimately are, to produce effects which may seem incomprehensible when viewed from only one or two dimensions, but when viewed as a whole, are not nearly as mysterious, even if they are difficult to model. M-theory with its membranes is only the beginning of what promises to be a glorious journey of discovery. But one which will someday seem as simplistic as our sun centered solar system model seems to us today.

So if you cannot explain the world of QM in layman's terms, it's because you don't really understand it. Forget the frigging math. Envision first, model second. Think outside the box. Be visionary, not dogmatic. It's not what you know that matters... it's what you know, that isn't so.

Nature, is at its core, elegant, simple, and beautiful. If it seems complicated and mysterious, it is only because you fail to understand it.

 

[stevendaryl]

To me, the problem with understanding QM doesn't lie in QM itself, but instead lies in the fact that most physicists are human, and humans are for the most part, idiots. Ergo, so-called physicists, (those who speak as if they understand QM, but in actuality don't have a clue) seem to view it as some vastly complex and mysterious phenomena, beyond the comprehension of most mere mortals

...

Nature, is at its core, elegant, simple, and beautiful. If it seems complicated and mysterious, it is only because you fail to understand it.

So you're saying that people find quantum mechanics mysterious because they don't understand it. Well, yeah.

 

[Fiziqs]

So you're saying that people find quantum mechanics mysterious because they don't understand it. Well, yeah.

That's exactly what I'm saying. As opposed to the idea that QM is inherently mysterious, and no amount of "understanding" will change that. This thread, if I understand its premise, is that there is among lay persons the false notion that physicists don't understand QM.

I don't believe this notion is misguided. I believe that indeed physicists don't understand QM. The mysteriousness of QM is not due to the fundamental properties of nature, but is instead a fundamental property of ignorance.

 

[stevendaryl]

That's exactly what I'm saying. As opposed to the idea that QM is inherently mysterious, and no amount of "understanding" will change that.

"Mysterious" just means the same thing as "not understood", doesn't it?

 

[Dead Boss]

Nature, is at its core, elegant, simple, and beautiful. If it seems complicated and mysterious, it is only because you fail to understand it.

I agree that nature is simple, elegant and beautiful, but I disagree that QM is not. From my brief exposure to QM, QFT and higher mathematics I think that the most elegant, beautiful and simple ideas cannot be explained in lay terms.

 

[harrylin]

[..] This thread, if I understand its premise, is that there is among lay persons the false notion that physicists don't understand QM [..].

Surely you're mistaken! This thread is about the fact that physicists admit that they don't (or not really) understand QM. It refers to for example the following sayings by Richard Feynman:

"I think that I can safely say that no one understands Quantum Mechanics. "
-The Character of Physical Law

"You see my physics students don't understand it. ... That is because I don't understand it. Nobody does."
-QED

 

[Maui]

Is it just the fact that QM deals with probabilities of measuring final states rather than the 1 input --> 1 output style of classical mechanics that makes people say it's "not understood" ?

This is a fundamental problem, on the one hand you have fundamental 'particles' that have no internal structure, yet they show deterministic properties when measured as an ensemble(classical behavior). When in combination with other fundamental 'particles'(which also lack internal structure) they can form beautiful, meaningful compounds that act in completely novel ways. This behavior seems unreal and out of this world. How could this be? What/who guides this process towards classicality? One could dive into holism and argue that reality is greater than the sum of its parts or remain agnostic and choose to adopt a more modest goal.

Is "not understood" just another way of saying "not familiar in terms of everyday human experience" ?

It's worse. It's impossible to say what qm is describing and if it's describing an outside world at all. It's also impossible to unambiguously state what the world is made out of. One can at best list all the 'particles' from the Standard Model and their interactions but they are just observed behavior(represented by their quantum numbers). People have trouble relating to the outside world as being made out of observed behavior.

 

[yoda jedi]

I'm not suggesting that EPR-type correlations can be explained in terms of disturbance---just the opposite; I'm saying that it doesn't make sense to describe them that way. That's my point, the strangeness of quantum mechanics is not just due to nondeterminism, and it's not just due to "the observer affecting that which is being observed".
That's the idea behind local hidden variables:
Because the two particles share a common origin, they subsequently share state information, and so what happens to the two particles later on is naturally correlated.
That's perfectly understandable. But it's wrong. You can't (at least not in a way that makes intuitive sense to most people) explain EPR-type that way: the correlations predicted by quantum mechanics are just different from what would be predicted based on the two particles sharing a common origin. That's what Bell's theorem shows.

i wish to ask steven, if
quoting you:
"and so what happens to the two particles later on is naturally correlated"
ok, for a pair of electrons (of course, sharing a common origin) if you observes one a counterclockwise, then the other is clockwise.
then, in a second state to the same electrons, if you change one of the electron spin, say, the clockwise to counterclokwise, the other change to clockwise ? there is an experiment showing that ?

i ask only for a pair of electrons.

.

 

[nitsuj]

That's exactly what I'm saying. As opposed to the idea that QM is inherently mysterious, and no amount of "understanding" will change that. This thread, if I understand its premise, is that there is among lay persons the false notion that physicists don't understand QM.

I don't believe this notion is misguided. I believe that indeed physicists don't understand QM. The mysteriousness of QM is not due to the fundamental properties of nature, but is instead a fundamental property of ignorance.

Can you clarify the distinction you're mentioning? It seems you are also suggesting "physicists" are mysterious in their approach to "understanding" QM.

To stevendaryl's point "Mysterious" just means the same thing as "not understood", doesn't it?

 

[Whovian]

ok, for a pair of electrons (of course, sharing a common origin) if you observes one a counterclockwise, then the other is clockwise.
the, in a second state to the same electrons, if you change one of the electron spin, say, the clockwise to counterclokwise, the other change to clockwise ? there is an experiment showing that ?.

Simple. There's no way to change the first one's spin without changing the angular momentum of the thing that changes the electron's spin, by Conservation of Angular Momentum.

 

[yoda jedi]

Simple. There's no way to change the first one's spin without changing the angular momentum of the thing that changes the electron's spin, by Conservation of Angular Momentum.

which thing ? that angular momentum is irrelevant to the state of the other electron spin.

 

[yoda jedi]

i know of experiments that control the spin of a single electron, but not for one of a pair (common origin).
but I don't see the impossibility of making the experiment.