Miss-Interpreting Quantum Collapse.

[Skwinty]

If someone understands the theory, then I don't see how it could be said that they don't understand the, er ... well, the field, I guess? The metaphysical interpretation isn't clear, and never can be: it's metaphysics, after all. I think understanding of the rules and how to make predictions can suffice for understanding, but I can understand being more stringent and perhaps requiring knowledge of why these rules (rather than other rules) are used and why different interpretations exist; and I can understand some people wishing to be able to form a picture in their heads. But not having a consensus on a metaphysical interpretation is a silly reason to say no-one understands a theory.

Feynmann never made this statement in reference to classical physics or relativity. These laws of physics have been probed to the limit, whereas quantum mechanics lies in the realm of sub atomics beyond the planck scale. Once we are able to probe beyond the planck scale, then a more comprehensive understanding will be achieved.

If there was only one interpretation of quantum mechanics now, then our understanding would be a lot better. Also, if quantum mechanics was fully understood now, why is the unification of general relativity and quantum mechanics proving to be a tough nut to crack?

 

[schrodingasdawg]

Feynman never made this statement in reference to classical physics or relativity.

Why does this matter?

These laws of physics have been probed to the limit, whereas quantum mechanics lies in the realm of sub atomics beyond the planck scale. Once we are able to probe beyond the planck scale, then a more comprehensive understanding will be achieved.

I think what you mean is that our present physical theories are suspected of breaking down at the Planck scale. I also don't see how knowing when a theory breaks down has anything to do with how well-understood the theory is in its own right. Are you suggesting that classical mechanics was not understood before it was observed to break down at atomic scales and quantum mechanics was developed?

If there was only one interpretation of quantum mechanics now, then our understanding would be a lot better.

I don't see how this follows.

Also, if quantum mechanics was fully understood now, why is the unification of general relativity and quantum mechanics proving to be a tough nut to crack?

Because gravity isn't renormalizable.

http://en.wikipedia.org/wiki/Quantum_gravity#Nonrenormalizability_of_gravity

 

[Skwinty]

Why does this matter?

I think his statement was meant as a joke; if it wasn't, it's just silly. It's no more meaningful to say "no-one understands quantum mechanics" than it is to say "no-one understands classical mechanics," "no-one understands general relativity," or similar statements about any field of physics---or the other sciences, or anything else in life.

I don't know. You raised the issue.

Are you suggesting that classical mechanics was not understood before it was observed to break down at atomic scales and quantum mechanics was developed?

The probing and measurements have confirmed the limitations of the theory. Just as Newtons theories were surpassed by relativity, so will Einsteins theories be surpassed by some thing else.

It is clear to me that classical mechanics and relativity is very well understood. Quantum mechanics is understood as far as the original equations and calculations are concerned.

IMO, the fact that these theories were shown to have limitations through progress in technology, indicates to me that they were not fully understood in their day. They were the best available, but their limitations still not fully comprehended.

Will we ever have a complete and full comprehension of the universe and all it's laws of physics? I hope so, but the undeniable fact is, every time we think we have understood something, then along comes another enigma which casts doubt on on our comprehension.

 

[Skwinty]

Because gravity isn't renormalizable.

String theory seems to get around this problem quite nicely

 

[Fredrik]

Well, it's not really clear how classical mechanics is to be interpreted. The interpretation that the configuration of a system at any time is caused by its configuration at a prior time is the most common (probably because it's the "common sense" interpretation), but there is a more obscure teleological interpretation of classical mechanics associated with the Lagrangian formulation---i.e., the system takes the path that it does between preparation and measurement because it is "trying" (not to imply intent) to achieve its final state with the most extreme action.

I'm not talking about why physical systems evolve in time the way they do. I'm talking about much simpler things than that. Consider non-relativistic classical mechanics of a point particle. It doesn't matter if you think of the Newtonian or the Lagrangian version. No matter what the force/Lagrangian is, the solution is (in both cases) a function [latex]x:I\rightarrow\mathbb R^3[/latex], where I is some interval.

What I'm saying is that in this theory, there's no controversy about interpreting x(t) (for any value of t) as the particle's position at time t. (The moon is there even if no one is looking at it). That's the obvious interpretation I'm talking about. In QM there's no consensus about what a state vector represents. Even an assumption as seemingly harmless as "it's a mathematical representation of the system's properties at the given time" takes us deep into many-worlds territory, unless we postulate something more or less magical (exact wave function collapse, special properties of consciousness, etc.) to avoid that.

If someone understands the theory, then I don't see how it could be said that they don't understand the, er ... well, the field, I guess?

I mostly agree, but note that you could become an expert at using the theory to calculate probabilities of possibilites without understanding those things I mentioned above. Understanding the interpretational issues is a completely different thing than understanding how to use the theory. So I guess it's a matter of taste if we should include the interpretational issues when we say things like "I understand the theory".

 

[schrodingasdawg]

I don't know. You raised the issue.

No, I didn't. I stated the statement was silly and no more meaningful than saying the same thing about classical mechanics or GR. I didn't say Feynman did say the same thing about those theories. However, you put special emphasis on the fact that he didn't say these things. I asked you why it would matter whether he did or not, i.e. why did you emphasize this fact?

IMO, the fact that these theories were shown to have limitations through progress in technology, indicates to me that they were not fully understood in their day.

I don't see how this follows. Classical mechanics was fairly well-understood in its own right (though it is much better understood today). Knowing the limits of applicability of a theory or model is important, but doesn't increase or decrease understanding of the theory in the areas where it is applicable.

 

[schrodingasdawg]

I'm not talking about why physical systems evolve in time the way they do. I'm talking about much simpler things than that. Consider non-relativistic classical mechanics of a point particle. It doesn't matter if you think of the Newtonian or the Lagrangian version. No matter what the force/Lagrangian is, the solution is (in both cases) a function [latex]x:I\rightarrow\mathbb R^3[/latex], where I is some interval.

What I'm saying is that in this theory, there's no controversy about interpreting x(t) (for any value of t) as the particle's position at time t. (The moon is there even if no one is looking at it). That's the obvious interpretation I'm talking about. In QM there's no consensus about what a state vector represents. Even an assumption as seemingly harmless as "it's a mathematical representation of the system's properties at the given time" takes us deep into many-worlds territory, unless we postulate something more or less magical (exact wave function collapse, special properties of consciousness, etc.) to avoid that.

Ah, this is true. My apologies for misunderstanding what you meant.

I still think it's silly to say "no-one understands quantum mechanics" because of this, but I think we don't disagree here. The knowledge that the interpretation of the state vector is not clear and that no experiment can distinguish the interpretations I think actually qualifies as understanding.

 

[Skwinty]

The knowledge that the interpretation of the state vector is not clear and that no experiment can distinguish the interpretations I think actually qualifies as understanding.

My take on this is:

Quantum mechanics has an awkward formulation that is quantitatively very successfull but confuses or hides the true nature of reality. The whats and why's of quantum mechanics still need to be revealed.

The basic concepts required to understand the macroscopic world have no meaning when we look into the microscopic world. This means we must modify our language and reasoning when trying to understand the universe on atomic and sub atomic scales.

Hence the question: Can we understand quantum mechanics without the ability to probe beyond the planck scale?

 

[BillyJoe]

I agree. I can still recognize and sorta agree with the point I was making (that she doesn't say something that is specific enough to rule out The Correct Interpretation of HUP), but it seems more likely that her words are in reference to The Common Misunderstanding of HUP.

Yes it is a probability thing with me as well, but, whatever the case, I think her explanation collapses for someone with the Common Misunderstanding, which sort of makes her article a waste of time and space.

I'm not sure this reflects well on me, though. It suggests that over the last three years, I've changed from someone who is looking for agreement to someone who is willing to be cynical.

It probably comes from having to correct the same errors over and over and over again.

 

[BillyJoe]

I don't think there's anything wrong with the quote in the OP...She's simplifying it, but isn't that the appropriate thing to do in this type of article?

Not if it misses the target audience.
Not if it fails to correct the misunderstanding of those who misunderstand HUP.
Not if it actually reinforces that misunderstanding.
I think it does all of these things.

and I don't see anything in there that proves that she has misunderstood the uncertainty principle.

I have said as much, but I think the probability is that she has, but I'm not going to argue with you.

Interactions will cause the apparence of collapse. For example, the double slit experiment has been performed with C70 molecules, with different values of the density of the air that the molecules must pass through. The result is that the lower the air pressure, the more the interference pattern looks like "each particle took both paths", and the higher the air pressure, the more it looks like "each particle took only one path".

The problem is that you are not explaining the HUP.

 

[Fredrik]

Not if it misses the target audience.
Not if it fails to correct the misunderstanding of those who misunderstand HUP.
Not if it actually reinforces that misunderstanding.
I think it does all of these things.
...
The problem is that you are not explaining the HUP.

The quote in the OP isn't even about the HUP. It's about wavefunction collapse. Why would her claims about wavefunction collapse only be acceptable if it corrects people's misunderstandings about the HUP, and why is it a problem that my post doesn't explain the HUP?

I have written an explanation of the HUP by the way. You can read it here.

 

[Dancing David]

My take on this is:

Quantum mechanics has an awkward formulation that is quantitatively very successfull but confuses or hides the true nature of reality. The whats and why's of quantum mechanics still need to be revealed.

Sorry Skwinty, this is true of all science and thepries. They are all approximate models of reality. None of them actual explain the underlying mechanisms.

The basic concepts required to understand the macroscopic world have no meaning when we look into the microscopic world. This means we must modify our language and reasoning when trying to understand the universe on atomic and sub atomic scales.

Hence the question: Can we understand quantum mechanics without the ability to probe beyond the planck scale?

And again the same sentence is applicable to all science. Scientific theories do not reveal "the true nature of reality". They model the possible.

 

[Fredrik]

They are all approximate models of reality.
...
Scientific theories do not reveal "the true nature of reality". They model the possible.

QM might not even do that. It predicts probabilities of possible results of experiments with amazing accuracy, but to be considered a "model" of reality, even an approximate model, I think it should at least make a statement about what the system is doing between state preparation and measurement. (I would say that this is the difference between a "theory" and a "model", but everyone might not agree with my definitions. A theory makes predictions, and a model is a theory that also describes what's going on). And there's no obvious way to interpret QM as making statements of that sort. See #99 and #105. (The simplest and most straightforward interpretation, other than the idea that QM doesn't describe reality at all, is the many-worlds interpretation).

 

[Skwinty]

Sorry Skwinty, this is true of all science and thepries. They are all approximate models of reality. None of them actual explain the underlying mechanisms.


And again the same sentence is applicable to all science. Scientific theories do not reveal "the true nature of reality". They model the possible.

Hi Dancing David,
No need to apologise,
The point is IMO, Richard Feynmann never said nobody understands the theory of quantum mechanics, he said nobody understands quantum mechanics.

I think it is safe to say that we understand the theories of classical mechanics as well as the underlying mechanisms.

The same can be said for relativity.

However, this cannot be said for quantum mechanics.
Now, I am not a physicist, so ultimately you are not disagreeing with me, rather you are disagreeing with Richard Feynmann, Brian Greene and many other respected physicists.

 

[schrodingasdawg]

QM might not even do that. It predicts probabilities of possible results of experiments with amazing accuracy, but to be considered a "model" of reality, even an approximate model, I think it should at least make a statement about what the system is doing between state preparation and measurement. (I would say that this is the difference between a "theory" and a "model", but everyone might not agree with my definitions. A theory makes predictions, and a model is a theory that also describes what's going on). And there's no obvious way to interpret QM as making statements of that sort. See #99 and #105. (The simplest and most straightforward interpretation, other than the idea that QM doesn't describe reality at all, is the many-worlds interpretation).

This is nonsense. Why should a "model of reality" have to provide a classical trajectory? QM describes just fine how the wave-function evolves between preparation and measurement, and there's no reason "reality" can't be "modeled" using wave-functions instead of classical trajectories.

 

[Robin]

The quote is utter rubbish, as you have discerningly detected.

Feynman, in "Six Easy Pieces", in describing the double slit experiment says "the photons disturb the electrons", he says this several times.

Fenyman's lectures were originally addressed at first year physics students.

Smythe's remarks are prefaced by the caveat that they are "crudely simplistic" but they are more or less how Feynman puts it.

Smythe does not, as BillyJoe suggests, say that the photons cause how the collapse occurs. If you are going to call her quote utter rubbish then you need to call Feynman's "Six Easy Pieces" rubbish also.

As you say later we should not confuse the mathematics with the reality, but neither should you confuse any other sort of description with reality. The mathematics is the most accurate description. Putting it in words always results in a less accurate description.

Feynman again, after he has outlined the mathematical approach:

One might still like to ask: "How does it work? What is the machinery behind the law?" No one has found any machinery behind the law. No one can "explain" any more than we have just "explained". No one will give you a deeper representation of the situation. We have no idea about a more basic mechanism from which these results can be deduced.

Richard Feynman "Six Easy Pieces"​

Where Smythe gets is wrong is in saying that scientists have "great difficulty" recording and measuring particles because it implies that there might be a way in which the double slit experiment might be set up to tell which slit the particle passes through without losing the interference pattern.

But of course this is not just "difficult", it is impossible to have a method of detecting which slit the electron passes through without losing the interference pattern.

 

[Robin]

This is nonsense. Why should a "model of reality" have to provide a classical trajectory? QM describes just fine how the wave-function evolves between preparation and measurement, and there's no reason "reality" can't be "modeled" using wave-functions instead of classical trajectories.

This is how Heisenberg put it:

It has been stated in the beginning that the Copenhagen interpretation of quantum theory starts with a paradox. It starts from the fact that we describe our experiments in the terms of classical physics and at the same time from the knowledge that these concepts do not fit nature accurately. The tension between these two starting points is the root of the statistical character of quantum theory. Therefore, it has sometimes been suggested that one should depart from the classical concepts altogether and that a radical change in the concepts used for describing the experiments might possibly lead back to a non-statistical, completely objective description of nature.

This suggestion, however, rests upon a misunderstanding. The concepts of classical physics are just a refinement of the concepts of daily life and are an essential part of the language which forms the basis of all natural science. Our actual situation in science is such that we do use the classical concepts for the description of the experiments, and it was the problem of quantum theory to find theoretical interpretation of the experiments on this basis. There is no use in discussing what could be done if we were other beings than we are. At this point we have to realize, as von Weizsäcker has put it, that 'Nature is earlier than man, but man is earlier than natural science.' The first part of the sentence justifies classical physics, with its ideal of complete objectivity. The second part tells us why we cannot escape the paradox of quantum theory, namely, the necessity of using the classical concepts.

Werner Heisenberg, "Science and Philosophy" 1958​

Presumably when he is saying "it has sometimes been suggested that one should depart from the classical concepts altogether" he is referring to the many-worlds interpretation which came out in 1957, although I have no evidence that he saw this.

But I agree that it is valid to say that the wave function is modelling reality.

 

[Robin]

I think it is safe to say that we understand the theories of classical mechanics as well as the underlying mechanisms.

The same can be said for relativity.

I don't think that is safe to say. We certainly don't understand the underlying mechanisms of relativity.

And quantum mechanics is the best understanding we have of the underlying mechanism of classical mechanics.

Also, Richard Feynman suggests that perhaps there is nothing to understand, that this is really how nature works.

But even if we had some more basic mechanism that explains why quantum mechanics behaves the way it does, we would only end up with some other unexplained mechanism.

 

[Fredrik]

This is nonsense. Why should a "model of reality" have to provide a classical trajectory?

I didn't say anything about a classical trajectory. (I would interpret that as a curve in a classical phase space, not a Hilbert space). And I said pretty much the same thing that you agreed with earlier, so I'm a bit surprised that you're the one saying this.

QM describes just fine how the wave-function evolves between preparation and measurement,

Yes, of course. QM wouldn't work otherwise.

and there's no reason "reality" can't be "modeled" using wave-functions

Right, but it's certainly not obvious that we should think of the evolution of the wavefunction as a description of what happens to the system, because (as you agreed earlier), it's far from clear what the wavefunction is a mathematical representation of. All we know for sure is that it makes sense to think of it as a representation of the statistical properties of an ensemble of identically prepared systems, but that's just a fancy way of saying that it might not correspond to anything at all in the real world. (The members of that ensemble do not all exist at the same time).

It's tempting to think of it as a representation of the properties of the system, and if we give in to that temptation, it's very natural to think of the time evolution of the wavefunction as a description of what happens between state preparation and measurement. But it wouldn't be a description of just one world, as we're already in many-worlds territory. I'm not saying that the MWI must be wrong, but it certainly isn't the only option.

 

[BillyJoe]

Just having another look:

Ingrid Hansen Smythe:

Does the mind of the observer truly shape reality as claimed? After all it is true that, at the quantum level, a scientist has great difficulty recording and measuring particles and their interactions without changing the results of the investigation. Is this because the scientist’s mind is influencing the experiment? Is it because the scientist perceived the experiment and, as a result of perceiving, changed the results? No. The answer is far more mundane. To put it in crudely simplistic terms, as soon as the scientist switches on the light to see what’s going on, other particles, like photons, get in the way. It is the photons that are responsible for messing up the results, not the thoughts of the experimenter.

What does she mean by "messing up the results"? Do physicists regularly "mess up their results"? By "switching on the light"?? Surely they have learnt not to do that by now!!!

Does she mean that in order for an experimenter's thoughts to shape reality (assuming for the moment that that is actually what happens), he would necessarily have to turn on the lights - so as to justify her claim that "it is the photons messing up the results, not the Observer's mind"

This is looking less and less intelligible to me.
(...if only for a different reason )

BJ