In what sense is QM not understood ?

[nitsuj]

Hmmm... I'd say James S Saint takes it, but it was a close one

 

[kith]

If you want to explain all of reality with physics, you merely need classical physics polished up by an actual philosopher familiar with rationality and logic.

Please elaborate.

I don't see a fundamental difference between classical and quantum mechanics with respect to ontology. Both are physical theories which can be used to predict certain aspects of the behaviour of nature correctly and both don't tell us anything about how reality really is. If I want to explain reality, I need to employ an interpretation. This is certainly more straightforward in classical mechanics, but that's not a fundamental difference.

 

[nitsuj]

Yea he/she should have left out physics in the comment

"If you want to explain all of reality [STRIKE]with physics[/STRIKE]..."


And in the context I see "philosophically logical reality" different from the realities of QM.

A fantastic task for someone...James are you up for it?

 

[Fredrik]

I don't see a fundamental difference between classical and quantum mechanics with respect to ontology. Both are physical theories which can be used to predict certain aspects of the behaviour of nature correctly and both don't tell us anything about how reality really is.

So when you drop an apple and watch it fall to the ground (as Newton's theory says that it will), the apple might actually be doing something completely different that doesn't involve falling at all? That's what it would mean for classical mechanics to not tell us anything about how reality really is. It clearly does, at least approximately. It can be thought of as an approximate description of our world, or as an exact description of a fictional world that resembles our own.

QM on the other hand is very different. A preparation procedure can be represented by a wavefunction that's non-zero over a large region, and we still have no idea if the particle is actually spread out all over that region, or if it's entirely contained in some small volume inside it.

 

[Dead Boss]

QM on the other hand is very different. A preparation procedure can be represented by a wavefunction that's non-zero over a large region, and we still have no idea if the particle is actually spread out all over that region, or if it's entirely contained in some small volume inside it.

Well maybe it's neither. Maybe the particle does not have such clear cut existence as we'd like to think.

 

[nitsuj]

Yea he/she should have left out physics in the comment

"If you want to explain all of reality [STRIKE]with physics[/STRIKE]..."


And in the context I see "philosophically logical reality" different from the realities of QM.

A fantastic task for someone...James are you up for it?

ah ha ha, there was actually a reply.

Man these moderators are fast.

 

[Whovian]

Well maybe it's neither. Maybe the particle does not have such clear cut existence as we'd like to think.

It doesn't matter at all, as all would just be different interpretations of quantum mechanics, and wouldn't yield any testable discrepancies. (While my view is it's some odd entity that's just acting like itself in all experiments, again, this yields no testable predictions(?))

 

[stevendaryl]

It doesn't matter at all, as all would just be different interpretations of quantum mechanics, and wouldn't yield any testable discrepancies. (While my view is it's some odd entity that's just acting like itself in all experiments, again, this yields no testable predictions(?))

That could very well be the case. I think that there is a sense in which we clearly do not understand quantum mechanics--as I said, we don't really understand what a "measurement" is, at a fundamental level. However, this lack of understanding doesn't seem to translate into a practical question that could be answered by an experiment. I would not say that there are no consequences to choosing one interpretation or another--there definitely are consequences. For example, the fuzzy notion that "consciousness collapses the wave function" really has consequences, since it predicts that there can be no macroscopic superpositions of humans, but there can be macroscopic superpositions of human-built devices. But since we have no feasible way of detecting macroscopic superpositions, it would seem that this fuzzy notion can't be practically tested.

I think we're in a similar boat when it comes to quantum theories of gravity. My gut feeling is that it is unlikely that observations will give us any guidance as to whether we are on the right track, since the predictions made by quantum gravity tend to involve extreme conditions of black holes or the early universe which are not replicable.

So I think that we may be in the unfortunate situation of knowing that we don't
understand things, but having no idea how to improve our understanding through experiment and observation.

 

[bhobba]

You are demonstrating how physicists (presuming you to be one) really are not qualified to debate logic or philosophical issues. And by the way, I suspect that you are not aware of the tight association between a philosopher and logic

Are you aware of the tight association of logic and mathematics and that physics is written in the language of mathematics?

QM accepts that positive attracts negative and visa versa as fundamental

QM accepts nothing of the sort. Insofar as I can make sense of such a statement I presume it refers to EM. It explains EM as the result of local gauge invariance which explains why positive and negative varieties attract:
http://quantummechanics.ucsd.edu/ph130a/130_notes/node296.html

However you need to understand a bit of math - you know - formalised logic.

But then, that gets back to the topic of this thread

Yes indeed. IMHO the essence of QM is the principle of superposition. The sense it is 'not understood' is the weirdness of this idea that things like a particle that in everyday experience has a specific property like following a specific path can partly follow many paths at the same time or be in two positions at the same time or similar weird superposition's that defy our everyday experience. It is understood however in the sense it can be used to make testable predictions that so far have not found falsification.

Thanks
Bill

 

[nucl34rgg]

This is something that I've seen repeated many times, but I'm wondering how accurate it is. I mean, we've got this mathematical framework where we deal with vector spaces, eigenstates, superpositions, mixed states etc. that works to a high degree of accuracy.

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

What I wonder about is how the founders of QM figured out that the mathematics we use in QM (operators, bras, kets etc.) was the right thing to use. They didn't just pull it out of thin air, they must have reasoned their way to at least some of it, eg. Schrodinger didn't just get out a pen and write down $H\Psi = i\hbar \frac{d}{dt}\Psi$ out of nowhere. Why isn't that considered "understanding" it?

QM is perfectly well understood by most physicists, and has been for a little over 70 years. There are axiomatic derivations of QFT and QM. The real reason QM has a bad rap in popular culture is that people often say, "Nobody understands QM." to illustrate the idea that the quantum world, at first glance, seems very different qualitatively than the world we live in at the scales that we perceive things. However, if you really think about it, QM actually makes perfect sense. People often think QM is the cutting edge of physics. It was in the 1930's -- not so today.

I also don't get why people are so anti-QM.

 

[harrylin]

QM is perfectly well understood by most physicists, and has been for a little over 70 years.

Not so according to a lot of smart physicists who knew/know perfectly well how to apply QM. But perhaps you're smarter than them?

[..] if you really think about it, QM actually makes perfect sense. [..] I also don't get why people are so anti-QM.

This has nothing to do with "anti-QM", for example Feynman was very much pro-QM. And I think that most of us can't wait to hear you explain how QM makes perfect sense concerning the issues that were brought up here - for sure, I'd like to hear how entanglement makes perfect sense to you.

 

[nucl34rgg]

Not so according to a lot of smart physicists who knew/know perfectly well how to apply QM. But perhaps you're smarter than them?

This has nothing to do with "anti-QM", for example Feynman was very much pro-QM. And I think that most of us can't wait to hear you explain how QM makes perfect sense concerning the issues that were brought up here - for sure, I'd like to hear how entanglement makes perfect sense to you.

Knowing the axiomatic framework upon which QM is based, what the limitations are of QM, along with how to apply QM is equivalent to understanding QM. By this meaning of "understanding," physicists understand QM in the same way that they understand any other scientific theory. Is this "understanding" counterintuitive at times? Yes, of course, but if you think carefully about it, you will find that it makes sense and most other ways actually don't or are needlessly contrived.

Also, I never meant to imply I was "smarter." I am pretty dumb. I simply meant to say that most physicists understand QM. Go ask any HEP professor if they understand QM and define understanding as I did, for example, and I'd venture to guess they will admit they understand it.

Entanglement is perfectly understood in the context of statistical correlation.
http://en.wikipedia.org/wiki/Correlation_and_dependence

Watch this video. It will clear up a lot of things.
http://media.physics.harvard.edu/video/?id=SidneyColeman_QMIYF

Feynman clearly understood QM (probably better than most other people alive at the time), and when he said his famous quote, he meant to illustrate the idea that the quantum world, at first glance, seems very different qualitatively than the world we live in at the scales that we perceive things.

Often times, these types of statements about not understanding are really veiled "Why?" questions or recursive "But what is that, really?" questions. Which, eventually you will run into a brick wall. Let me give you a simple example. Define formally the concept of "set." Another hilarious one is, "What is mass, really?", and of course my favorite: "But why is it like that?" If you use the usual idea of "understanding" you can quickly convince yourself that one doesn't actually really understand anything except perhaps first order logic by merely asking the types of questions shown above.

In short, what does it mean to "understand" something? If you can mathematically describe it and apply it, I would say it's well understood.
It's unfortunate that a few quotes in passing by a physicist decades ago that were meant as pedagogical encouragement are now taken as the gospel current opinion on the topic. The theory of QM was fully worked out, with its limitations understood (yes it has limitations...that is why QFT, string theory etc are researched) long ago. The implications of QM are not "understood" in the conventional sense because there are different implications depending upon certain assumptions taken. QM is not, nor has it ever claimed to be the end all complete perfect description of reality (do we even know what it means to have such a description?).

Contrary to popular belief, the goal of science is not to find an exact description of reality and what is "true" about our universe. If it were, it would be logically fallacious to attempt to do so empirically. The goal of science is to use models to make predictions that fit empirically observed phenomena. It's a work in progress and it always will be. Empirical science is not, nor can it ever be "ontologically complete." This is perfectly fine. QM works and people understand it.

 

[nitsuj]

That was a good read nucl34rgg.

In particular this part.

If you use the usual idea of "understanding" you can quickly convince yourself that one doesn't actually really understand anything except perhaps first order logic by merely asking the types of questions shown above."

 

[harrylin]

[..] Entanglement is perfectly understood in the context of statistical correlation.
http://en.wikipedia.org/wiki/Correlation_and_dependence

Watch this video. It will clear up a lot of things.
http://media.physics.harvard.edu/video/?id=SidneyColeman_QMIYF

I've joined physicsforums because I would like to understand QM in the sense meant in the topic header. Thus I've been discussing Bell's Theorem and related issues with the experts here; and obviously none of them thinks that watching a video or reading a Wikipedia article (which they may have written) will make one understand it, in that sense (funny lecture though, makes me think of Woody Allen). Which brings me to the next point:

Feynman clearly understood QM (probably better than most other people alive at the time), and when he said his famous quote, he meant to illustrate the idea that the quantum world, at first glance, seems very different qualitatively than the world we live in at the scales that we perceive things.

OK then, like him I now also claim that no one understands QM. And you may quote me on that, pretending that I only meant to illustrate the idea that the quantum world, at first glance, seems very different qualitatively than the world we live in at the scales that we perceive things. However, I did not say that nor suggest that, and neither did Feynman. What I mean is very different from that, as a matter of fact it is closer to what you suggest next:

Often times, these types of statements about not understanding are really veiled "Why?" questions or recursive "But what is that, really?" questions. [..]

Well, that is generally what "why" questions and the word "understand" mean - as also already discussed in this thread and numerous other threads.

[..] If you can mathematically describe it and apply it, I would say it's well understood. [..]

Then you may not be able to understand why Feynman and many other experts agree that QM is not understood. As you realized, he knew perfectly well how to mathematically describe and apply QM - he even excelled in it. We all know that that is not the sense in which QM is said to be "not understood". And in what sense it is meant, has been elaborated already by others in this thread.

[..] Contrary to popular belief, the goal of science is not to find an exact description of reality and what is "true" about our universe. [..]

That would be unachievable. However:

The goal of science is to use models to make predictions that fit empirically observed phenomena .[..]

The problem that we are discussing here, is that we even lack a plausible model of how and why QM works. To quote Feynman also on that one:

"The more you see how strangely Nature behaves, the harder it is to make a model that explains how even the simplest phenomena actually work. So theoretical physics has given up on that."

It is in that sense that QM is "not understood" - as many people have tried to explain now (see for example posts #2, 16, 74, 78, 79).

PS: I think that in what way QM is "not understood" has been sufficiently explained by now.

 

[kith]

So when you drop an apple and watch it fall to the ground (as Newton's theory says that it will), the apple might actually be doing something completely different that doesn't involve falling at all?

Yes. There needn't even be an apple, like in a computer simulation.

Questions of ontology can't be examined with the methods of the natural sciences. Which makes them kind of boring for my taste, but I think it is important not to forget that physics doesn't tell us how the world really is, but describes a model world which behaves similar to our own.

Now the differences between QM and classical mechanics are empirical, not ontological. At least, "classical" ontologies for QM are possible (dBB, MWI). I think Demystifier has even written a paper about a probabilistic interpretation of classical mechanics (or linked to it).

 

[Fredrik]

Yes. There needn't even be an apple, like in a computer simulation.

I'm not so fond of this view. We can say things like "I can't even be sure that you exist", and we'd be right, if we mean "sure" in the strictest possible sense. But science treats experimental results as facts. So you can't reject the idea that classical mechanics describes reality (approximately) without also rejecting science in its entirety. And you don't need to reject science to reject the idea that QM describes reality. That's a crucial difference.

So I don't think ontologies of classical theories are nearly as problematic as ontologies of quantum theories. There is however the problem that a single classical theory may admit more than one ontology. (Unfortunately I don't have a good example that I fully understand myself. One example that comes to mind is the version of GR that's mentioned in "Black holes and time warps: Einstein's outrageous legacy", in which spacetime is flat, and measuring devices are deformed by the properties of matter).

I think it is important not to forget that physics doesn't tell us how the world really is, but describes a model world which behaves similar to our own.

Yes, I often say this myself. Sometimes people yell at me when I do.

 

[Ken G]

Questions of ontology can't be examined with the methods of the natural sciences. Which makes them kind of boring for my taste, but I think it is important not to forget that physics doesn't tell us how the world really is, but describes a model world which behaves similar to our own.

Yes, I'd say physics has a very subtle and interesting relationship with ontology (which is metaphysics). Philosophy gave birth to physics to answer questions like "what is", but physics kind of took a different turn, along the lines of the Feynman quote that it has given up on the "what is" question and focused instead on the "how can we understand it or at least predict it" question. That's when physics and metaphysics parted company, citing irreconcilable differences.

However, they still chat on the phone. Physics uses ontology as a kind of crutch, a way to picture what is happening to help motivate the mathematics-- without actually requiring that we believe it is really happening. To make that point crystal clear, just recall the last time you used Newton's "force of gravity" to solve a problem-- you probably thought quite ontologically about that force, yet knew there is probably no such thing in "real life."

The problem is when we try to import the physics theories back into the philosophy, to answer what is "really happening." That's the job of metaphysics, but it is highly subjective, like a lot of philosophy. Some feel the goal of philosophy is not to answer the questions definitively, but rather, to explore the range of possible answers. That is certainly what metaphyics is in quantum mechancial interpretations, but it was generally done by physicists, rather than philosophers, because they were the ones who understood the physics. But it's still a metaphysical conversation-- and that's something that physicists sometimes have a little bit of a hard time accepting.

 

[Ken G]

I'm not so fond of this view. We can say things like "I can't even be sure that you exist", and we'd be right, if we mean "sure" in the strictest possible sense.

I think it's important not to mix ontology, which is questioning what is, with epistemology, which is questioning how we know things. Those are pretty much orthogonal issues, so we may assume we have agreed on our epistemology when we attack ontology. So it's not really relevant if we can know it or not, let's assume we have adopted basic scientific epistemology.

But science treats experimental results as facts.

That's the epistemology, that's fine-- we can all agree there.

So you can't reject the idea that classical mechanics describes reality (approximately) without also rejecting science in its entirety.

We can treat the experimental results as facts, without saying what really happened. For example, take Zeno's paradoxes. We may watch an arrow fly, and assert that it followed a continuous path, but we don't ever actually observe that-- whether we are using our eyes, or a movie camera, or a bubble chamber, it makes no difference-- we only ever get a discrete series of events, with no knowledge what happens in between except a picture (pretense?) of continuity. Indeed, Zeno found it quite paradoxical that an arrow could have a location, and a velocity, at the same time-- in eerie parallel with the quantum mechanical correspondence principle. So if Zeno could doubt the ontology of classical mechanics even before quantum mechanics, I don't see any reason we can't do it, after quantum mechanics!

So I don't think ontologies of classical theories are nearly as problematic as ontologies of quantum theories.

I would agree that we can say there are degrees of problems with the ontology-- we just shouldn't overlook the ontological headaches already present in classical mechanics.

There is however the problem that a single classical theory may admit more than one ontology.

Yes, that's another key issue-- when we have nonuniqueness, it is a clear sign that we are having trouble saying what actually is happening. Are there really forces, or is there really a Hamiltonian, or is there really action?

 

[bhobba]

Contrary to popular belief, the goal of science is not to find an exact description of reality and what is "true" about our universe. If it were, it would be logically fallacious to attempt to do so empirically. The goal of science is to use models to make predictions that fit empirically observed phenomena. It's a work in progress and it always will be. Empirical science is not, nor can it ever be "ontologically complete." This is perfectly fine. QM works and people understand it.

I must admit I can't quite follow that one. IMHO the goal of science is to find truth - and empirical checking to see if its true is what science is all about. To me the real essence of science is the willingness to always doubt - to say - yes we are after the truth but it must always be checked and rechecked.

Thanks
Bill

 

[Fredrik]

Yes, that's another key issue-- when we have nonuniqueness, it is a clear sign that we are having trouble saying what actually is happening. Are there really forces, or is there really a Hamiltonian, or is there really action?

I don't consider that a problem at all. As I said in post #22:

The way I see it, non-relativistic classical theories are all defined in a framework defined by Galilean spacetime. The Newtonian, Lagrangian and Hamiltonian approaches are just three different ways to consistently add matter to an empty spacetime. A specific theory in that framework is defined by its equations of motion. One way to find a new theory in this framework is to simply guess an equation of motion. (Actually, that is the Newtonian approach). The other approaches are just ways to eliminate the worst guesses. So I don't find it surprising that these approaches don't tell us anything about what's actually happening. They're not even part of the theories; they are just tools that help us eliminate the worst candidates for new theories.​

As I said in my previous post, I don't have any great examples of multiple ontologies for a classical theory. The example I gave there is the best I can think of, but I don't actually understand it.

SR might be another example. "Lorentz (a)ether theory" has been mentioned in this forum a bunch of times, and it has been claimed that it makes the same predictions as SR. I haven't seen a definition of that theory, so I can't really tell if this is true, but I wouldn't be shocked if there's a theory that singles out one inertial frame as "special", giving us "actual simultaneity" and "apparent simultaneity", and still makes the same predictions as SR.

However, if it turns out that the only meaningful way to define such a theory is to take SR and add the assumption that one frame is special, then I would just reject it, because I use a definition of "theory" such that a) specific theories are defined by their assumptions, and b) if one of the assumptions can be removed without changing any predictions, it's not a theory. For example, I don't consider "QED+there's a non-interacting blue unicorn" a theory.

So what I would mean by a non-standard ontology in this case is really a second theory (without any removable assumptions) that makes the same predictions but describes things in different terms, e.g. by claiming that there's a preferred frame, and a phenomenon that makes it undetectable. (The theory would of course have to explain how this phenomenon makes it impossible to determine which frame is special).

We can treat the experimental results as facts, without saying what really happened. For example, take Zeno's paradoxes. We may watch an arrow fly, and assert that it followed a continuous path, but we don't ever actually observe that-- whether we are using our eyes, or a movie camera, or a bubble chamber, it makes no difference-- we only ever get a discrete series of events, with no knowledge what happens in between except a picture (pretense?) of continuity.

Right, but since no experiment has disproved the hypothesis that no matter what part of the arrow's flight we choose to photograph, the result is always in agreement with the theory, I think we have the best possible reason that we could ever hope for to say that classical theories are approximate descriptions of reality.

I would even go so far as to say that this is an excellent reason to say that classical theories are approximate descriptions of reality even if we knew for sure that arrows do other things when they're not being watched. To claim otherwise seems to me to be like saying that a circle drawn on a flat surface using a pen attached to a string that's also attached to a needle at the point we want to be the center, isn't approximately circular, because it's not exactly a circle.

 

[stevendaryl]

Knowing the axiomatic framework upon which QM is based, what the limitations are of QM, along with how to apply QM is equivalent to understanding QM.

We don't know the limitations of QM. And, as I said, we don't know how to apply QM except in a "rule of thumb" way. The ambiguous part, as I said, is knowing when a measurement has been made. What's a measurement? The formalism doesn't tell you, but it does tell you that after a measurement, the system will be in a eigenstate of the operator corresponding to the observable being measured. So the rules depend crucially on knowing what a measurement is, and they don't tell us. I would say that this is very different from classical physics. For classical physics, it's also true that whether something is a measurement or not is fuzzy. But the laws of physics don't depend in any way on that distinction.

There is an inherent ambiguity in the Rules of Quantum Mechanics. Suppose we prepare a system in some state, and then later we let it interact with a detector, and then even later, we perform some other measurement on it. There are two different ways to analyze it: (1) We can consider the detector to be performing a measurement of some observable. In this case, the wave function collapses to an eigenstate after interacting with the detector, and we use this eigenstate to compute the probabilities for the final measurement. (2) We can consider the detector to be a quantum system in its own right, evolving according to Schrodinger's equation. In this case, the detector doesn't perform a measurement, and there is no collapse of the wave function to an eigenstate.

In principle, these two different ways of analyzing the situation could lead to different results, because if we treat the detector as a quantum system, there is the possibility for interference effects. In practice, interference effects involving macroscopic objects are unobservable.

 

[bhobba]

There is an inherent ambiguity in the Rules of Quantum Mechanics. Suppose we prepare a system in some state, and then later we let it interact with a detector, and then even later, we perform some other measurement on it. There are two different ways to analyze it: (1) We can consider the detector to be performing a measurement of some observable. In this case, the wave function collapses to an eigenstate after interacting with the detector, and we use this eigenstate to compute the probabilities for the final measurement. (2) We can consider the detector to be a quantum system in its own right, evolving according to Schrodinger's equation. In this case, the detector doesn't perform a measurement, and there is no collapse of the wave function to an eigenstate.

Decoherence resolves this - long before the detector registers a result the environment has decohered the system (detector and what is being measured) so it is in some actual (not a superposition) state.

Thanks
Bill

 

[Ken G]

IMHO the goal of science is to find truth - and empirical checking to see if its true is what science is all about. To me the real essence of science is the willingness to always doubt - to say - yes we are after the truth but it must always be checked and rechecked.

But it seems to me there is a fundamental contradiction there, which we have to manage somehow. I completely agree with your second sentiment (and I like Feynman's characterization of science as "distrusting experts", and love his description of it as a way to avoid fooling ourselves), but I think it challenges the first. How can we be looking for truth, if we are embracing doubt at all stages? It suggests that truth is not a destination, but a journey. I'm fine with that, as long as we recognize that we are, in effect, redefining the standard meaning of truth, to get it to fit science, rather than trying to fashion science, to get it to fit some impossible standard of truth.

 

[nucl34rgg]

I've joined physicsforums because I would like to understand QM in the sense meant in the topic header. Thus I've been discussing Bell's Theorem and related issues with the experts here; and obviously none of them thinks that watching a video or reading a Wikipedia article (which they may have written) will make one understand it, in that sense (funny lecture though, makes me think of Woody Allen). Which brings me to the next point:

OK then, like him I now also claim that no one understands QM. And you may quote me on that, pretending that I only meant to illustrate the idea that the quantum world, at first glance, seems very different qualitatively than the world we live in at the scales that we perceive things. However, I did not say that nor suggest that, and neither did Feynman. What I mean is very different from that, as a matter of fact it is closer to what you suggest next:

Well, that is generally what "why" questions and the word "understand" mean - as also already discussed in this thread and numerous other threads.

Then you may not be able to understand why Feynman and many other experts agree that QM is not understood. As you realized, he knew perfectly well how to mathematically describe and apply QM - he even excelled in it. We all know that that is not the sense in which QM is said to be "not understood". And in what sense it is meant, has been elaborated already by others in this thread.

That would be unachievable. However:

The problem that we are discussing here, is that we even lack a plausible model of how and why QM works. To quote Feynman also on that one:

"The more you see how strangely Nature behaves, the harder it is to make a model that explains how even the simplest phenomena actually work. So theoretical physics has given up on that."

It is in that sense that QM is "not understood" - as many people have tried to explain now (see for example posts #2, 16, 74, 78, 79).

PS: I think that in what way QM is "not understood" has been sufficiently explained by now.

But, don't you see how "why" questions make one run in circles, and ultimately are inherently unanswerable? The question of "why" is fundamentally unanswerable through empirical means. Science never ever tells us why. It only gives us a model of "how" that is convenient for prediction or calculation. We have no way of knowing if the model is ever REALLY true, and empirical theories that describe reality are only claimed to be "true" in that they are consistent with observed data. You cannot prove truth by sampling a few events and concluding for the general case. Thus, science doesn't "prove" something to be true. It is simply the best approach we have for approximating the "truth."

Every scientist knows this, but this is often misrepresented to the general public.

Even in mathematics, which takes a rational approach to answer mathematical questions, asking "why" beyond a certain point is unproductive because the answer eventually is just: "It follows from the definition" or rather "That's the way it is." The problem is that modern human language is capable of describing poorly defined absurdities. We are capable of asking "why" without even being precise about what we mean by "why". If we were more precise about what we meant by "why", the confusion would disappear.

Also, I wasn't trying to be rude or dismissive by suggesting a video. You really might learn something from the late Professor Sidney Coleman. That video is widely regarded as a great clarification of concepts from QM. I strongly recommend it.

Anyway, I am leaving this thread because I'm not particularly fond of Woody Allen! :) Also, my claim was "QM is understood" as a theory of physics. You countered with "The more you see how strangely Nature behaves, the harder it is to make a model that explains how even the simplest phenomena actually work. So theoretical physics has given up on that." which describes that physics is an attempt to explain a truth of nature, which I am saying is NOT the goal of science and has nothing to do with QM. Science cannot answer how the universe "actually works" or more generally it cannot tell us the underlying reality or truth (if there is even such a thing) precisely because it is fundamentally empirical. It can only suggest models that are consistent with observed phenomena to a given probability within a given measurement tolerance. I am afraid that the "understanding" you are seeking is in fact, not a physical theory, but rather a philosophical metaphysical theory that is ontological, which I am afraid everyone is ill equipped to provide because such a theory doesn't exist and cannot ever be formulated.

 

[stevendaryl]

Decoherence resolves this - long before the detector registers a result the environment has decohered the system (detector and what is being measured) so it is in some actual (not a superposition) state.
Thanks
Bill

Does decoherence really resolve it? It seems to me that the superposition just spreads to larger and larger subsystems. First, there is a particle in a superposition of states. Then it interacts with the detector, putting the detector into a superposition of states. Then the detector interacts with the environment, putting the environment into a superposition of states. I don't see that there is a point where anything becomes "actual".

 

[harrylin]

[..] Also, my claim was "QM is understood" as a theory of physics. You countered with "The more you see how strangely Nature behaves, the harder it is to make a model that explains how even the simplest phenomena actually work. So theoretical physics has given up on that." which is an attempt to explain a truth of nature, [...] It can only suggest models that are consistent with observed phenomena to a given probability within a given measurement tolerance. [..]

Ehm no, nobody did an attempt to explain a truth of nature, and the issue that Feynman brought up was that no satisfying model for QM exists, for example his proposed model could not explain partial reflection* from a glass plate in a satisfying way - but I won't go again into such details as my earlier mistake was probably that I replied too much, so that my primary comment may have gone unnoticed. I'll stress it now: you made a claim which you next started to defend about how QM is understood, and what the goal of science supposedly is, and so on. Those are not the topic of this thread, and we all know in what way QM is understood. So thanks for elaborating on that, but it's really besides the point. And I'm also leaving this thread now- not because of videos or Woody Allen but because I think that already sufficient to-the-point answers have been given about the sense in which QM is said to be not understood.

*in his book on QED, he admits this somewhere around p.20. I was so disappointed that I did not read on for a year or so

 

[bhobba]

Does decoherence really resolve it? It seems to me that the superposition just spreads to larger and larger subsystems. First, there is a particle in a superposition of states. Then it interacts with the detector, putting the detector into a superposition of states. Then the detector interacts with the environment, putting the environment into a superposition of states. I don't see that there is a point where anything becomes "actual".

Decoherence does resolve it. It decoheres in a very very short time so it does not spread.

The correct description is first there is a particle in a superposition of states, it interacts with the environment and in a very short time dechoreres into a state that is not in a superposition (it is in what is called a mixed state - but one where it is in some definite state 100% for sure - but the exact state is described by bog standard probability theory - quantum weirdness is no longer present) by leaking phase to the environment very very quickly, usually well before it even reaches the detector. In a few situations like the double slit experiment that leakage occurs at the detector - but that is not the norm. In some very very special circumstances like superconductivity no leakage occurs at all and then things are really weird - but not the measurement type problem weird.

Thanks
Bill

 

[Fiziqs]

Anyway, I am leaving this thread because I'm not particularly fond of Woody Allen! :) Also, my claim was "QM is understood" as a theory of physics. You countered with "The more you see how strangely Nature behaves, the harder it is to make a model that explains how even the simplest phenomena actually work. So theoretical physics has given up on that." which describes that physics is an attempt to explain a truth of nature, which I am saying is NOT the goal of science and has nothing to do with QM. Science cannot answer how the universe "actually works" or more generally it cannot tell us the underlying reality or truth (if there is even such a thing) precisely because it is fundamentally empirical. It can only suggest models that are consistent with observed phenomena to a given probability within a given measurement tolerance. I am afraid that the "understanding" you are seeking is in fact, not a physical theory, but rather a philosophical metaphysical theory that is ontological, which I am afraid everyone is ill equipped to provide because such a theory doesn't exist and cannot ever be formulated.

Let me see if I can give an analogy to demonstrate exactly why some people argue that physicists do not understand QM.

There is an ancient device known as the Antikythera mechanism. It was a coral encrusted device salvaged from an ancient shipwreck. After many years, and careful study, reseachers were able to reconstruct the device. It turned out to be a device for keeping track of the motion of the sun and moon, and perhaps some of the planets. It was quite intricate, and able to model the motion of the sun and moon quite closely considering the time in which it was made.

There are even more modern examples of such devices that are remarkable in the accuracy with which they can model the motions of celestial bodies. Accounting for even small anomalies in the motions of the moons and planets. These are also quite remarkable devices for their time.

But there is just one problem with these devices. For although they can model the motions of the sun, planets, and moons with remarkable detail, they say absolutely nothing about "why" these objects move the way they do. They are nice models, but the makers of these models had no understanding of the forces involved.

QM is very much the modern equivalent, it can accurately predict the behavior of matter, but today's mathematical models are much like those ancient brass ones, they tell us absolutely nothing about "why" these particles behave the way they do. In the same way that those ancient model builders failed to "understand" the very thing that they were modeling, today's physicists fail to "understand" the forces at work within their own model. They can model it quite precisely, but being able to model it, doesn't equate to understanding it.

Those ancient brass models, and today's modern mathematical ones, both fall short in one regard, they make no attempt to explain "why".

 

[stevendaryl]

Decoherence does resolve it. It decoheres in a very very short time so it does not spread.

I don't understand that. I thought that the only difference between the environment (meaning, I think, the electromagnetic field, plus possibly other fields) and any other system is just the huge number of states. But I don't see how that changes things in principle. In particular, I don't see how the environment can select a single state out of a superposition. It can become correlated--is that what you mean? If originally you have

|ψ_environment>(|ψ_{device, A}> + |ψ_{device, B}>)

(that is, the device is in a superposition of two states, A and B) then it will rapidly decohere to get:

|ψ_{environment+system, A}> + |ψ_{environment+system, B}>

But it's still a superposition. Of course, the environment is actually described by quantum field theory, rather than quantum mechanics, but I think the same principles hold.

The correct description is first there is a particle in a superposition of states, it interacts with the environment and in a very short time dechoreres into a state that is not in a superposition (it is in what is called a mixed state - but one where it is in some definite state 100% for sure - but the exact state is described by bog standard probability theory - quantum weirdness is no longer present) by leaking phase to the environment very very quickly, usually well before it even reaches the detector.

I really don't see how that is true, unless you are treating the environment as a mixed to start with. Pure states evolve into pure states---however, as was explained by Everett in his thesis on the Universal Wave Function, a pure state with many degrees of freedom will look exactly like a mixed state, if you average over the degrees of freedom that you don't care about. But that's exactly what I was talking about--that for practical purposes, interference becomes impossible, and the lack of interference effects destroys the "nonclassical" character of quantum probabilities. It's still a superposition, though. There is no "collapse" to a single value in the physics.

In a few situations like the double slit experiment that leakage occurs at the detector - but that is not the norm. In some very very special circumstances like superconductivity no leakage occurs at all and then things are really weird - but not the measurement type problem weird.

Thanks
Bill

I'm not really sure if we are talking about two different ways of looking at the same thing, or whether you're actually claiming something contrary what I believe to be the case. Certainly for many-particle quantum mechanics, the system cannot evolve from a pure state to a mixed state. Once you go to quantum field theory, I'm not sure, any longer, but my feeling is that it's still impossible. But, as I said, for practical purposes, you can treat a system as being in a mixed state if it is correlated with an environment having many degrees of freedom.

 

[bhobba]

Those ancient brass models, and today's modern mathematical ones, both fall short in one regard, they make no attempt to explain "why".

But they do - simply not in the terms you judge as telling why. Why do objects fall - space time is curved - why is space time curved - it is dynamical - why is it dynamical - because nature has no prior geometry - why does nature have no prior geometry - no one knows - its just the way nature is - it's very intuitive when you understand it and incredibly beautiful and elegant mathematically, but no one right now really knows why. Some deeper theory may tell us why eventually - but it too will contain something where we say - its just the way nature is. Such is what any explanation contains. Its only those that don't understand this that say there is no attempt to explain why.

IMHO such a view is a variant of this its only math so it can't be reality type argument. They ignore once it is mapped to stuff external to us out there in reality land, such as for example the points and lines of Euclidean Geometry are, it is a description of reality and transcends mere math. 10 year olds leaning Geometry for the first time usually grasp it immediately but for some reason a few adults that frequent forums like this have trouble with it.

Thanks
Bill

 

[stevendaryl]

Here's the way I understand the transition from pure state to an apparent mixed state, according to Everett: Suppose the complete wave function, system and environment, are described by the pure state |ψ> that is a product state:

|ψ> = $\sum$ c_i,a |$\phi$_i> |$\chi$_a>

where |$\phi$_i> is a basis for the system you care about, and
|$\chi$_a> is a basis for everything else (the environment, for example).

Now, suppose we want to compute the expected value of some operator $O$ that only involves the system, not the environment. In that case,

<$\chi$_b|<$\phi$_j| $O$ |$\phi$_i> |$\chi$_a>
= δ_a,b <$\phi$_j| $O$ |$\phi$_i>

So <$O$>
= <ψ|$O$|ψ>
= $\sum$ c^*_j,a c_i,a <$\phi$_j| $O$ |$\phi$_i>

For operator $O$, this result is the same as if the system were in the mixed state described by the density matrix ρ given by:

ρ = $\sum$ c^*_j,a c_i,a |$\phi$_i> <$\phi$_j|

So, as far as operators that only depend on the system, the system acts as if it were in a mixed state.

 

[bhobba]

I don't understand that.

Hmmm. Your math below is not correct. The +'s you have should be tensor products and you need to do something called tracing over the environment - this is what causes dechorence when you work through the math.

It is not in a superposition of states after decoherence - it is in a mixed state - which is different. This is illustrated by the supposed Schroedinger Cat paradox. Without dechorence the cat is in a superposition of being alive and dead until you observe it - that is the mystery and supposed paradox. With decohrence it is not in a superposition of states - it is either alive or dead - not in a weird mixture of alive and dead. We don't know if it is alive or dead but with 100% certainty it is one or the other - not a weird quantum combination of being alive and dead at the same time - the environment has decohered that possibility out by a leaking of phase. This is the essence of quantum weirdness - this ability to be partly in two mutually exclusive possibilities such as alive and dead at the same time. Decoherence removes such weirdness at the classical level to be exactly the same as classical probability theory. When you flip a coin you know it is heads or tails - one or the other - not a combination of the two at the same time. Observing a system with decohence taken into account is like lifting your hand on a flipped coin - you see if its heads or tails - but you know its one or the other and is not in any sense a mystery or problem.

Check out:
ftp://orthodox-hub.ru/ftp2/books/_%D4%E8%E7%E8%EA%E0_%CC%E0%F2%E5%EC%E0%F2%E8%EA%E0/RevModPhys/RevModPhys%201984-2008/root/data/RevModPhys%201984-2008/pdf/RMP/v064/RMP_v064_p0339.pdf

If that article is a bit advanced then I suggest Griffiths - Consistent Quantum Theory:
https://www.amazon.com/dp/0521539293/?tag=pfamazon01-20

That text explains it at about the most elementary level possible - but even then the math is not trivial - but still understandable with effort.

Thanks
Bill

 

[stevendaryl]

QM is very much the modern equivalent, it can accurately predict the behavior of matter, but today's mathematical models are much like those ancient brass ones, they tell us absolutely nothing about "why" these particles behave the way they do.

I am disputing the claim (which some people are making) that what is weird about quantum mechanics is that it doesn't say why particles behave the way they do. I don't care about whether it says why, because at some point, "why" questions have to end with: they do it because that's the way they work.

That isn't what's weird about quantum mechanics. The fact that there is no "why" for how particles behave applies just as well to classical mechanics. What's weird about quantum is, as I have said, the fact that it doesn't say how particles behave. It says how measurements behave, and it doesn't really say what a measurement is, or which interactions count as measurements. We have rules of thumb for answering the question of what interactions count as measurements, but that's all we really have.

 

[stevendaryl]

Hmmm. Your math below is not correct. The +'s you have should be tensor products and you need to do something called tracing over the environment - this is what causes dechorence when you work through the math.

Check out:
ftp://orthodox-hub.ru/ftp2/books/_%D4%E8%E7%E8%EA%E0_%CC%E0%F2%E5%EC%E0%F2%E8%EA%E0/RevModPhys/RevModPhys%201984-2008/root/data/RevModPhys%201984-2008/pdf/RMP/v064/RMP_v064_p0339.pdf

If that article is a bit advanced then I suggest Griffiths - Consistent Quantum Theory:
https://www.amazon.com/dp/0521539293/?tag=pfamazon01-20

That text explains it at about the most elementary level possible - but even then the math is not trivial - but still understandable with effort.

Thanks
Bill

I think I understand it pretty well. I've read all those papers (long ago). And I don't think my math was mistaken. The + does NOT mean tensor product. It means a linear combination of states.

Math symbols are pretty tedious to type, so I was just using |A> |B> to mean the tensor product of |A> and |B>. I think that's pretty common, at least I've seen it many times. |A> (|B> + |C>) means the tensor product of |A> with the state |B> + |C>, which in turn is a superposition of |B> and |C>. So we have an equation:

|A> (|B> + |C>) = |A>|B> + |A>|C>

To give an example, if you have two electrons, and you ignore all degrees of freedom except the spin degrees of freedom, then a general pure state can be written as

α |up>|up> + β |up> |down> + γ |down> |up> + δ |down> |down>

where |up> |down> is the state in which the first electron has spin up and the second has spin-down, etc.

 

[bhobba]

I think I understand it pretty well. I've read all those papers (long ago). And I don't think my math was mistaken. The + does NOT mean tensor product. It means a linear combination of states.

Math symbols are pretty tedious to type, so I was just using |A> |B> to mean the tensor product of |A> and |B>. I think that's pretty common, at least I've seen it many times. |A> (|B> + |C>) means the tensor product of |A> with the state |B> + |C>, which in turn is a superposition of |B> and |C>. So we have an equation:

|A> (|B> + |C>) = |A>|B> + |A>|C>

To give an example, if you have two electrons, and you ignore all degrees of freedom except the spin degrees of freedom, then a general pure state can be written as

α |up>|up> + β |up> |down> + γ |down> |up> + δ |down> |down>

where |up> |down> is the state in which the first electron has spin up and the second has spin-down, etc.

Your claim the final outcome after decoherence is not correct. It is not:
|ψenvironment+system, A> + |ψenvironment+system, B>

There is now no plus here - it is not now in a superposition of states as indicated by a plus - it is in a mixed state which is described by a density matrix with no off diagonal elements - meaning there is no superposition of states. The physical interpretation of a mixed state is a number of pure states presented to an experimenter randomly:
http://en.wikipedia.org/wiki/Quantum_state#Mixed_states

The system after dechorence is in a specific pure state, with each pure state being classically well defined like the cat alive or dead except the exact state is random like flipping a coin.

If you have read all those texts and understood them then I am at a loss to understand your concern.

Thanks
Bill