Living Opponents of the Copenhagen Interpretation

[bhobba]

It was one of Bohr's insights that emerged during his analysis of EPR. There is an agreement to be found between relativity and QM rather than a disagreement. The concept of information becomes clearer.

I would agree with that.

Thanks
Bill

 

[carllooper]

You say "what matters is if a theory is 'physically right or wrong' not whether it is 'philosophically so'." By a theory being "physically right or wrong" here, you could mean two things: (1) you could mean "physically" in the sense of having to do with the real physical matter of the world, and thus be saying something like "a physically true theory correctly describes what is actually out there in the world," or (2) which is practically the opposite of that, you could mean "physically" in the sense of "as in physics"--which, these days, often explicitly repudiates the idea that theories are describing a "world out there," but rather insists on the above point about just being about accurate experimental predictions.

I don't why they would be opposed.

Physics (2) is, by definition, the study of the physical world (1).

I don't know what kind of physics would repudiate the concept of a "world out there". Physics certainly poses some challenges to classical philosophy, but it's not within physics scope to solve that for philosophy. Philosophy has to solve that.

Experiment plays an important role in physics. Without an experiment, it can be difficult to decide if something has some physical meaning or remains science fiction. Physics is very particular about that - far more than science fiction.

Prediction is only required if one needs to test out any potential conflicts or otherwise between a theory and it's experimental side. If the theory didn't include any predictions then there's no way to map the theory to the experiment. But many theorisations don't need to involve prediction. They can be working within a context in which the physical or experimental aspect has already been tested, and are elaborating the theoretical side, ie. in a mathematically consistent way that wouldn't violate the theory already tested.

C

 

[atyy]

But whereas scientific advancement has often occurred by trying so solve paradox, it seems to me that part of the philosophy behind QM is very much the opposite. Rather than trying to solve the paradox, there is a sense in which they are deliberately embracing it. Rather than saying, "there is something that is going on here that we don't understand and we need to study it further," they are saying, "we understand exactly what is going on here, and it is that the particles cease to exist"--at just the point where they reach the limit of their ability to understand them! Thus it is with the alleged "completeness" of quantum theory. It amounted to transforming their inability to understand into a "scientific conclusion" (that was really a philosophical error, in my view) that (a) is taught to all students of science nowadays, and (b) actually puts a stop to the process of scientific inquiry.

But here you are simply wrong. Quantum mechanics is widely acknowledged to have a problem called the "measurement problem". It is the most important problem in the foundations of quantum mechanics. Von Neumann's proof of the alleged "completeness" (to use your term) of quantum mechanics is widely known to be wrong, particularly after Bell's 1966 explanation of the error.

 

[jbmolineux]

I STRONLY disagree with that:
https://www.google.com.au/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&cad=rja&uact=8&ved=0CCAQFjAA&url=http://www.phys.washington.edu/users/vladi/phys216/Weinberg_Against_philosophy.doc&ei=fVJoVIXUNeL2mQWH_oCwCA&usg=AFQjCNHg_elaIirwh-1Q7Al_kVaI8Fz8YA&sig2=h2vnb14frw18jhYKClXrLw

But this forum is not the place to discuss it. The philosophy forums is the place.
It's not. Wienbergs view, which is the same as mine, is pretty common.

My post concerned the statement - 'One can imagine a lot of classically trained physicists walking around the conference in a huff saying things like "what is all this rubbish". And Bohr is trying to win them over to QM.'

I think by that time no one wasn't won over to QM. What Bohr was on about is his particular Copenhagen view - in this case complementarity. But shut up and calculate would soon predominate, if it hadn't already.
Does Euclidean geometry describe the actual world? And if it does precisely how does QM differ from it.

Thanks
Bill

Carl, I couldn't agree more with Weinberg's article. He literally demonstrates exactly what I am arguing--how bad philosophy has repeatedly led physics astray.

But the remedy for bad philosophy is not no philosophy. As Weinberg himself says, everyone has underlying philosophical assumptions. It's getting them right that matters, not simply not having them. The philosophy that says "physics should have no philosophy" is just one more bad philosophy. Thus "shut up and calculate" has allowed "the particle that cannot be measured does not exist" to dominate. And since you stop searching for what you don't believe exists, it has literally put a stop to scientific inquiry. Or so it seems to me.

The remedy for bad philosophy is good philosophy, and Weinberg seems to demonstrate a good deal of it in rejecting much of the bad philosophy that has come to dominate the academy. Since the bad philosophy has (according to Weinberg's article) done so much damage to physics--isn't that all the more reason for the importance of the conversation not being won by those who are in error? Doesn't that show precisely that good philosophy is important? As I think CS Lewis says, "good philosophy needs to exist, if for no other reason than that bad philosophy must be answered."

As to your question about Euclidean geometry, I'm not sure I would say that it "describes the world." I believe mathematical theories are different than scientific theories in that they aren't about the physical world. That's why you don't test them with measurements or experiments. But theories in physics ARE about the world, which is why you do test them with measurements and experiments.

Where I believe QM goes wrong (and apparently where Wienberg also believes it goes wrong) is in the claim that what cannot be measured cannot be the content of a theory.

 

[carllooper]

To me it appears that what has happened is that physicists started with choice #2 above (that physics does not describe a "world out there" but just makes predictions that either can, or can't be experimentally verified) and were led by that conclusion to disbelief in the out-there reality of the very particles they are supposed to be studying. You can clearly trace the lineage in the latter "scientific conclusion" to the former philosophical error. You can see how this would affect physical research. If you don't believe that the thing you are studying really exists (at least in any way you can make sense of) that is OBVIOUSLY going to affect the paths that you choose to study it!

No this is not the case at all. Physics does describe the world out there, but it's just not in a way that classical philosophy might describe it. For example Kant assumes a world in which time and space are separate. And this might be something one might like to use in physics. But if you are trying to use it with Relativity Theory it won't work, because Relativity employs the concept of time and space as not separate. It's not for any philosophical reason. And it's not due to the infiltration of any wrong philosophy. It's that an aspect of the world out there makes sense in terms of Relativity Theory. Or to put it another way: the world out there doesn't make nonsense of it.

Regarding particles.

The reality or otherwise of particles depends on what you mean by reality. A particle itself is defined by the mathematics. In many philosophies mathematics and reality are the same thing. So in those philosophies you could say the particle is real. In other philosophies its the particle detection which is real and the particle which isn't. But who's to say which philosophy is correct? And does it matter? As long as one understands what is being meant, in a particular context, by terms such as "particle", or "reality", or "non-existent" or "actual" etc. that's what matters.

C

 

[jbmolineux]

But here you are simply wrong. Quantum mechanics is widely acknowledged to have a problem called the "measurement problem". It is the most important problem in the foundations of quantum mechanics. Von Neumann's proof of the alleged "completeness" (to use your term) of quantum mechanics is widely known to be wrong, particularly after Bell's 1966 explanation of the error.

Yes, I am aware that QM has a measurement problem. But historically the measurement problem combined with the positivist idea that "only the measurable is meaningful" to the actual belief that the unmeasurable is nonexistent, which a stop to inquiry, as the Weinberg article above spells out. I have heard that later the repudiation of "completeness" by Bell came to be accepted, but obviously decades of research-that-could've-been were lost by what were ultimately philosophical mistakes.

Honestly, I am unable, even, to sort out where the conversation went from there. Perhaps you guys can help me to understand. Here is how I understand what happened (and please tell me where I am wrong here).

1. Logical positivists, in their zeal to "get metaphysics out of philosophy" put forth the empirical criterion of meaning, which says that the only meaningful propositions were those that could be empirically verified.

1. Once this was accepted, as soon as one considers some fact which "exists" (in the ordinary sense) but cannot be perceived for any reason--cannot be spoken about meaningfully at all. This was problematic and counter-intuitive, and led to many absurdities. But it was the clear implication of the empirical criterion of meaning.

1. Heisenberg brought to bear the empiricist criterion of meaning onto the position / velocity of particles at the quantum level, where the mere act of observing them by shinning light on them would affect them (the measurement problem). He pointed out that under the empirical criterion, such particles actually do not have a velocity or a position.
2. This was embraced by some (Copenhagen), and flatly rejected by others, including Einstein, who believed that this result contradicted the fundamental scientific intuition.
3. Eventually, the positivists won over most of the physics community. I believe Von Neuman's later-to-be-repudiated "completeness" proof was part of it. On this basis, the idea became orthodox in physics that particles do not "exist in the classical sense" but exist in a in a state of quantum uncertainty, and literally do not have both a position and velocity.
4. In the next few decades logical positivism would die out in philosophy (as it came to be accepted that is was directly self-defeating), and apparently Von Neuman's completeness was eventually repudiated by Bell.
5. As far as I understand it, Bell's theorem was tested experimentally a number of times in a way that is supposed to have vindicated the positivist QM interpretation.

This last step, unfortunately, is where I get lost. I haven't really been able to understand the Bell Test experiments, which is why I want to learn physics. But it seems to me that as soon as positivism is repudiated in philosophy and "completeness" in physics, that there would be a metaphysical revolution in physics to get things back on track. As the Weinberg article pointed out, bad philosophy had been leading physics astray for decades. If you could get it back on track, it seems that the astray-leading should stop. But it seems to me that physics is still so-heavily influenced by bad philosophy that it still hasn't really got back on track. (And, in my view, "shut up and calculate" amounts to "there is no objective physical world to be measured," or certainly allows it to flourish….) Or maybe I just don't understand the Bell Test experiments, and how they somehow vindicate some philosophical ideas that I have been considering errors.

Answers to a few questions might help me here:

• Why is it impossible to know both the position and the velocity of a particle by the rebound of a photon hitting it?

• On Entanglement experiments - why can't the cause be explained by the same properties in each at the source? (I know I am out of my league with this question, and I believe the answer might be Bell's theorem itself...is that true?)
• Why couldn't there be an entanglement-type experiment where both particles were sent out "in the same way" so that you learn the information from one to know about the other? In other words, can you do something at the beginning to ensure that the position and velocity are the same, and then measure the position of one and the velocity of the other?

 

[bhobba]

I believe mathematical theories are different than scientific theories in that they aren't about the physical world. That's why you don't test them with measurements or experiments. But theories in physics ARE about the world, which is why you do test them with measurements and experiments.

That's a misconception.

Both Euclidean Geometry and QM are mathematical models and make predictions that can be tested. Pure mathematics is something different again .

Where I believe QM goes wrong (and apparently where Wienberg also believes it goes wrong) is in the claim that what cannot be measured cannot be the content of a theory.

QM doesn't say that. Its simply a theory whose primitive is observations. It silent on the issue of what's going on aside from that - but we have conjectures (called interpretations) that have their own take.

Thanks
Bill

 

[Fredrik]

Quantum mechanics is widely acknowledged to have a problem called the "measurement problem".

The only reason for this is that what most people think of as QM is a great theory of physics plus the unnecessary and unscientific assumption that a pure state "provides a complete and exhaustive description of an individual system". Quantum mechanics as I would define it, doesn't have a measurement problem. There are things that this theory is unable to tell us, but that's not a problem. It's either something that only a better theory can answer, or something that's forever beyond the reach of science. If it's the latter, that's certainly a problem, but it's a problem with the real world, not a problem with the theory.

This is how Wikipedia describes the measurement problem:

If observers and their measuring apparatus are themselves described by a deterministic wave function, why can we not predict precise results for measurements, but only probabilities?​

This sounds like a very good question, until you realize that the claim that matter is described by pure states is either a tautology (if you define "describes" in a way that makes this idea true) or an unnecessary and unscientific assumption added on top of a perfectly fine theory (if you leave "describes" undefined). It's the same assumption that Ballentine worded "a pure state provides a complete and exhaustive description of an individual system".

To a person who hasn't made this assumption, the question above looks very naive. Consider my theory of a six-sided die defined in post #86. No reasonable reasonable person would ask "If dice are described by this theory, why can we not predict precise results for measurements, but only probabilities?" The reason why people ask such silly questions about QM is that the standard presentation of the theory makes it very tempting to literally identify pure states with systems. The temptation is so strong that a lot of people are simply unable to see that when they do, they have left science behind and made an unnecessary assumption

 

[bhobba]

Physics does describe the world out there, but it's just not in a way that classical philosophy might describe it.

Modern physical theories are mathematical models, exactly the same as the archetypical mathematical model - Euclidean Geometry.

Since antiquity it has been recognised as THE pristine intellectual achievement:
http://poetry.about.com/od/poems/l/blmillayeuclid.htm
Euclid alone has looked on Beauty bare.
Let all who prate of Beauty hold their peace,
And lay them prone upon the Earth and cease
To ponder on themselves, the while they stare
At nothing, intricately drawn nowhere
In shapes of shifting lineage; let geese
Gabble and hiss, but heroes seek release
From dusty bondage into luminous air.
O blinding hour, O holy, terrible day,
When first the shaft into his vision shone
Of light anatomized! Euclid alone
Has looked on Beauty bare. Fortunate they
Who, though once only and then but far away,
Have heard her massive sandal set on stone.

All modern physics does is carry on that exemplary tradition.

Thanks
Bill

 

[jbmolineux]

No this is not the case at all. Physics does describe the world out there, but it's just not in a way that classical philosophy might describe it. For example Kant assumes a world in which time and space are separate. And this might be something one might like to use in physics. But if you are trying to use it with Relativity Theory it won't work, because Relativity employs the concept of time and space as not separate. It's not for any philosophical reason. And it's not due to the infiltration of any wrong philosophy. It's that an aspect of the world out there makes sense in terms of Relativity Theory. Or to put it another way: the world out there doesn't make nonsense of it.

Regarding particles.

The reality or otherwise of particles depends on what you mean by reality. A particle itself is defined by the mathematics. In many philosophies mathematics and reality are the same thing. So in those philosophies you could say the particle is real. In other philosophies its the particle detection which is real and the particle which isn't. But who's to say which philosophy is correct? And does it matter? As long as one understands what is being meant, in a particular context, by terms such as "particle", or "reality", or "non-existent" or "actual" etc. that's what matters.

C

Carl, I certainly don't believe that any philosophical error on Einstein's part caused any problems in physics! Kant's idea that time and space are mind-dependent seems to me to be a philosophical precursor to relativity.

Yes, different people have different ideas of what constitutes "reality," "non-existent," "particle," etc.--as you point out. But then you go on to say that "as long as one understands what is being meant"--that’s what matters. But since there is no agreement about what is meant by those terms, how can it be understood what is being meant?

Further, the question of whether something even exists if it can't be measured by the technology of a certain time is certainly relevant! If scientists are taught something doesn't exist, it puts a stop to the inquiry that drives scientific progress!

 

[bhobba]

Yes, I am aware that QM has a measurement problem. But historically the measurement problem combined with the positivist idea that "only the measurable is meaningful" to the actual belief that the unmeasurable is nonexistent, which a stop to inquiry, as the Weinberg article above spells out

You are getting a bit confused between a theory that is silent about things other than observations, and philosophical guff that says that's all there is.

Thanks
Bill

 

[Fredrik]

That's a misconception.

Again the statement you're disagreeing with is something that I would say is definitely true. There is no piece of mathematics that says anything about the real world all on its own. Hilbert space theory doesn't say anything about the result of experiments. To turn it into a theory of physics, we have to add correspondence rules that tell us how to interpret the mathematics as predictions about possible results of experiments. The same goes for every piece of mathematics and the corresponding application to the real world. In the theory I defined in post #86, everything was pure mathematics that said nothing about the real world until I made that final assumption, the correspondence rule.

In some cases, it's so obvious what the correspondence rules are supposed to be that we may not even realize that we are using correspondence rules. Euclidean geometry is probably the best example. It's a piece of pure mathematics that was intended to be applied to the real world. We have known since we were kids how mathematical things in Euclidean geometry correspond to real-world things. Those correspondences are what turn this piece of pure mathematics into a theory of applied mathematics.

In general, I would say that the difference between pure mathematics and applied mathematics is correspondence rules. Applied mathematics is a subject that includes applied geometry, applied probability theory and physics.

 

[bhobba]

Kant's idea that time and space are mind-dependent seems to me to be a philosophical precursor to relativity.

Its the other way around.

Kant's carry on about the a priori nature of Euclidean geometry negatively affected the equally great mathematician Gauss from publishing his discoveries about non Euclidean geometry. This was required for Riemann to develop Riemannian geometry, the extension of that to pseudo Riemannian geometry being the basis of General Relativity.

Thanks
Bill

 

[jbmolineux]

Weinberg is dismissing the idea that philosophers can tell physicists which theories are plausible and which ones are not. I think everyone in physics agrees with him. But the idea that you can't find the right theory just by thinking about it is a philosophical principle that it's important to get right. So it would be more accurate to say that Weinberg is supporting the quoted sentence than to say that he's disagreeing with it.Now this is something I strongly disagree with. bhobba's example of Euclidean geometry shows that a theory can make good predictions even if's only an approximate description of the world. Here's an example of a theory that makes good predictions without even approximately describing the world:

Define $\Omega=\{1,2,3,4,5,6\}$. Let $\Sigma$ be the set of all subsets of $\Omega$. For each $E\in\Sigma$, let $|E|$ denote the cardinality of $E$, i.e. the number of distinct elements of $E$. Define $P:\Sigma\to[0,1]$ by
$$P(E)=\frac{|E|}{|\Omega|}$$ for all $E\in\Sigma$. Now let's turn this piece of mathematics into a theory about the real world by specifying that for each $E\in\Sigma$, $P(E)$ is the fraction of times we'll get a result in the set $E$ if we repeatedly throw a standard six-sided die a large number of times.

This theory isn't even approximate description of the world. In particular, it says nothing about what's actually happening to the die between state preparation (the throw) and measurement (the moment when it has landed with some side up).

Weinberg is dismissing the idea that philosophers can tell physicists which theories are plausible and which ones are not. I think everyone in physics agrees with him. But the idea that you can't find the right theory just by thinking about it is a philosophical principle that it's important to get right. So it would be more accurate to say that Weinberg is supporting the quoted sentence than to say that he's disagreeing with it.Now this is something I strongly disagree with. bhobba's example of Euclidean geometry shows that a theory can make good predictions even if's only an approximate description of the world. Here's an example of a theory that makes good predictions without even approximately describing the world:

Define $\Omega=\{1,2,3,4,5,6\}$. Let $\Sigma$ be the set of all subsets of $\Omega$. For each $E\in\Sigma$, let $|E|$ denote the cardinality of $E$, i.e. the number of distinct elements of $E$. Define $P:\Sigma\to[0,1]$ by
$$P(E)=\frac{|E|}{|\Omega|}$$ for all $E\in\Sigma$. Now let's turn this piece of mathematics into a theory about the real world by specifying that for each $E\in\Sigma$, $P(E)$ is the fraction of times we'll get a result in the set $E$ if we repeatedly throw a standard six-sided die a large number of times.

This theory isn't even approximate description of the world. In particular, it says nothing about what's actually happening to the die between state preparation (the throw) and measurement (the moment when it has landed with some side up).

Fredrik, I believe that theory IS describing the world. It's describing the fact that a six-side die is a cube which--if it is spun into the air and allowed to hit a hard surface from an undetermined height, at an undetermined angle and rate of rotation--are each equally-likely to end up facing upwards when it stops bouncing.

Obviously if you take out the "undetermined height, angle, and rate of rotation" aspects, the theory would not hold. I think Carl Popper describes it as a "propensity" that exists in the die which, by the very nature of its shape, leads to these results when it is repeatedly dropped in the way described above.

Or does anyone have another explanation of why the theory actually holds? Does it really have nothing to do with the shape of the die? If it doesn't have to do with the shape of the die (as described above), why does it work with a normal six sided die, and not a weighted die?--or an 8-side die?

Heck, I've never even tested that theory in a laboratory and I know it holds just by knowing the shape of the die! Has anyone here tested it in a laboratory? Since physics is just about the results of experiments about what can be repeatedly testable, wouldn't we have to test it in a lab to even be able to say that it holds?--and yet, having neither tested it in a laboratory nor bothered to read about what-we-already-know-would-happen if someone DID test it, we all know that the theory is true!

 

[bhobba]

Again the statement you're disagreeing with is something that I would say is definitely true.

I usually agree with your views.

However here I must dissent.

Euclidean Geometry as usually presented is a mathematical model whose statements can be tested eg do the angles of a triangle add up to 180%.

Hilbert's treatment however is another matter:
http://en.wikipedia.org/wiki/Hilbert's_axioms

Hilbert space theory doesn't say anything about the result of experiments.

No it doesn't. Its the mapping of resolutions of the identity to the primitive of observations that does that. That's what QM does - and that's what makes it a mathematical model.

The usual presentation of Euclidean geometry speaks of points of no size and lines of no width that are idealisations of stuff out there.

Thanks
Bill

 

[jbmolineux]

You are getting a bit confused between a theory that is silent about things other than observations, and philosophical guff that says that's all there is.

Thanks
Bill

Bill, if scientific theories were actually "silent about things other than observations" then they wouldn't be able to report anything other than those observations themselves. It's precisely their ability to say MORE THAN the observations themselves that makes them valuable! I certainly agree that its "philosophical guff" which says "that's all there is" but I think its a mistake to say that reporting-only-the-content of observation is all that the theory is saying!

 

[atyy]

The only reason for this is that what most people think of as QM is a great theory of physics plus the unnecessary and unscientific assumption that a pure state "provides a complete and exhaustive description of an individual system". Quantum mechanics as I would define it, doesn't have a measurement problem. There are things that this theory is unable to tell us, but that's not a problem.

I basically agree with you for the definition of "complete" you are using here: that a more complete theory than quantum mechanics cannot exist. In other words, you are saying that quantum mechanics is incomplete, like our other theories including Newtonian gravity, classical special relativity, classical general relativity, non-relativistic quantum mechanics, and the standard model of particle physics. However, where I slightly disagree is that I think a full solution to the measurement problem should also provide explicit examples of what the more complete theory is.

There are two classes of theory. The first class is potentially complete as a model of reality. This class requires experiments to tell us that the theory is incomplete. As an example, I would say give the classical Maxwell's equations without point charges. It is conceivably a theory of some reality independent of observers. However, experiments tell us that the classical Maxwell's equations are incomplete - there is also gravity, and there is quantum mechanics.

The second class of theory makes successful predictions, but we can know that they are incomplete even before they are falsified. As an example, I would give the standard model of particle physics, or quantum Einstein gravity. Both theories match almost all observations (except the neutrino mass and dark matter), yet we know that they are incomplete, because they fail to be predictive or even consistent above a certain energy. It is for this reason that there are scientific research programmes asking what the completion of quantum gravity is. Similarly, Bohmian mechanics, Many-Worlds, constraints on hidden variables by Bell are scientific research programmes asking what the possible completions of quantum mechanics are.

As an aside, I do realize you are using Ballentine's words to describe what he is opposing. Let's switch from quantum mechanics to Ballentine interpretation. "A pure state is a complete and exhaustive description of an individual system" is correct quantum mechanics, since it just means complete within the theory, which is a choice of system/apparatus, operators, commutation relations, Hilbert space. Similarly, when I assert that "momentum and position are the complete and exhaustive description of an individual system" in single particle Newtonian mechanics, I am not asserting that Newtonian mechanics is the final theory.

Edit: I think I might disagree with you in a major way - at a quick reading, Wikipedia's description of the measurement problem is incorrect.

 

[atyy]

Or maybe I just don't understand the Bell Test experiments, and how they somehow vindicate some philosophical ideas that I have been considering errors.

I read the rest of your post too, but I'll need to think a bit before a more detailed reply. But here I just want to say that it isn't right to claim that the Bell Test experiments vindicate "nonrealism" (whatever that means). Rather the Bell Test experiments say that if we believe in reality, then it is nonlocal in a certain sense. In particular, the predictions of quantum mechanics cannot be generated by any local deterministic theory. I should also say the experiments at present do have loopholes, so we are not forced to accept that reality is nonlocal. Also, even if the experiments were perfect, there are (at least) two loopholes that would allow reality to be local. The first loophole is superdeterminism, and the second loophole is retrocausation.

 

[carllooper]

Carl, I certainly don't believe that any philosophical error on Einstein's part caused any problems in physics! Kant's idea that time and space are mind-dependent seems to me to be a philosophical precursor to relativity.

Yes, different people have different ideas of what constitutes "reality," "non-existent," "particle," etc.--as you point out. But then you go on to say that "as long as one understands what is being meant"--that’s what matters. But since there is no agreement about what is meant by those terms, how can it be understood what is being meant?

Further, the question of whether something even exists if it can't be measured by the technology of a certain time is certainly relevant! If scientists are taught something doesn't exist, it puts a stop to the inquiry that drives scientific progress!

I didn't mention anything at all about mind-dependence in Kant.

Regarding understanding. Let's suppose you are reading a paper in which someone claims the wave function is real. What you really want to know by that is what they mean by that - what do they mean by "real". It seems such a simple thing but it's not. There's a history there. And you need to work through that and the context in which the paper is presented, to get an idea of what they mean by that word "real". Once you understand what they are on about it almost doesn't matter whether they had used the word "real" or said something like the "wave function is a chicken".

C

 

[jbmolineux]

I read the rest of your post too, but I'll need to think a bit before a more detailed reply. But here I just want to say that it isn't right to claim that the Bell Test experiments vindicate "nonrealism" (whatever that means). Rather the Bell Test experiments say that if we believe in reality, then it is nonlocal in a certain sense. In particular, the predictions of quantum mechanics cannot be generated by any local deterministic theory. I should also say the experiments at present do have loopholes, so we are not forced to accept that reality is nonlocal. Also, even if the experiments were perfect, there are (at least) two loopholes that would allow reality to be local. The first loophole is superdeterminism, and the second loophole is retrocausation.

Yes, that was my understanding as well--although I've heard it both claimed that the Bell Test experiments vindicated "non-realism" and that it was simply LOCAL realism that they violated. But that leads me to another question--why is non-local realism so hard to fathom? What's so spooky about "action at a distance?" Didn't that die with gravity and electromagnetism? Don't those both clearly involve action-at-a-distance? (Or is it that they are not instantaneous but travel at the speed of light?)

 

[jbmolineux]

I didn't mention anything at all about mind-dependence in Kant.

Regarding understanding. Let's suppose you are reading a paper in which someone claims the wave function is real. What you really want to know by that is what they mean by that - what do they mean by "real". It seems such a simple thing but it's not. There's a history there. And you need to work through that and the context in which the paper is presented, to get an idea of what they mean by that word "real". Once you understand what they are on about it almost doesn't matter whether they had used the word "real" or said something like the "wave function is a chicken".

C

Yeah, but the "history" there is a mess!

 

[bhobba]

Bill, if scientific theories were actually "silent about things other than observations" then they wouldn't be able to report anything other than those observations themselves

Scientific theories in general aren't silent about such things - only some like QM - probability theory is another example.

But again what you say is a misconception - QM contains the full machinery of Hilbert space for example, and that contains all sorts of things. Its only specific things that are mapped to stuff out there. And indeed it is those other things that often lead to powerful consequences such as Gleason's Theorem and the existence of Born's rule.

Thanks
Bill

 

[bhobba]

why is non-local realism so hard to fathom? What's so spooky about "action at a distance?" Didn't that die with gravity and electromagnetism? Don't those both clearly involve action-at-a-distance? (Or is it that they are not instantaneous but travel at the speed of light?)

Non local realism is a-priori just as valid as its converse.

A choice, without direct experimental support, reveals more about the prejudices of the person concerned than 'reality' - just like Einstein's belief QM was incomplete - and Bohr's belief it was. Neither are right - or wrong.

Personally I think QM violates both locality and realism of naive realism - but that's just me and simply tells you about my world view - it means diddly squat.

That said it doesn't remove the responsibility of actually having an opinion.

Thanks
Bill

 

[carllooper]

Yeah, but the "history" there is a mess!

So? Do you think the history of philosophy is any tidier? Or the history of any other discipline? There's no magic wand to be found and waved and everything suddenly becomes tidy.

 

[bhobba]

So? Do you think the history of philosophy is any tidier? Or the history of any other discipline? There's no magic wand to be found and waved and everything suddenly becomes tidy.

Especially philosophy. Do you know of a single issue (other than something trite) that philosophers agree on? Mathematicians and physicists agree on all sorts of things. And in general they make progress - not philosophers:
http://www.ralphmag.org/EQ/gauss-kant.html

Rather amusing - but still illustrating an important point.

Thanks
Bill

 

[Fredrik]

Fredrik, I believe that theory IS describing the world. It's describing the fact that a six-side die is a cube which--if it is spun into the air and allowed to hit a hard surface from an undetermined height, at an undetermined angle and rate of rotation--are each equally-likely to end up facing upwards when it stops bouncing.

Obviously if you take out the "undetermined height, angle, and rate of rotation" aspects, the theory would not hold. I think Carl Popper describes it as a "propensity" that exists in the die which, by the very nature of its shape, leads to these results when it is repeatedly dropped in the way described above.

Or does anyone have another explanation of why the theory actually holds? Does it really have nothing to do with the shape of the die? If it doesn't have to do with the shape of the die (as described above), why does it work with a normal six sided die, and not a weighted die?--or an 8-side die?

To say that a theory describes (an aspect of) the real world should mean that the purely mathematical part of the theory describes a fictional universe that bears a strong resemblance to (that aspect of) the real world. In this case, the purely mathematical part of the theory doesn't contain concepts such as "shape", "die", "velocity", "angular momentum", etc., so I would say that it definitely doesn't describe those aspects of the real world.

You asked for an explanation of why the theory holds. The only thing that can explain why a theory holds is a better theory, and there are undoubtedly many theories that are better than this one in the sense that they can reproduce all the predictions of my theory, and make a few new ones. One such theory is classical mechanics, but I'm sure that if we tried, we could come up with something a lot worse than that, but still better than my theory, that includes concepts like "shape". The fact that it's intuitively obvious to you that such a theory must exist doesn't mean that my simple theory describes what's going on in terms of things like "shape".

Heck, I've never even tested that theory in a laboratory and I know it holds just by knowing the shape of the die! Has anyone here tested it in a laboratory? Since physics is just about the results of experiments about what can be repeatedly testable, wouldn't we have to test it in a lab to even be able to say that it holds?--and yet, having neither tested it in a laboratory nor bothered to read about what-we-already-know-would-happen if someone DID test it, we all know that the theory is true!

We've all rolled enough dice to be fairly certain what would happen, but yes, the shape of a standard die is what makes it seem obvious to us that the theory must be very accurate. We will think about the theory in terms of "shape" and other familiar concepts from better theories, but the theory itself doesn't contain them.

 

[DennisN]

Further, the question of whether something even exists if it can't be measured by the technology of a certain time is certainly relevant! If scientists are taught something doesn't exist, it puts a stop to the inquiry that drives scientific progress!

Hi, jbmolineux! I just want to mention this short text by Anton Zeilinger:, which was a response to a general question "2014 : What Scientific Idea Is Ready For Retirement?"

The idea to be abandoned is the idea that there is no reality in the quantum world.
[...]
But, whether it has a well-defined position or not, the buckyball very well exists. It is real in the double-slit experiment, even when it is impossible to assign its position a well-defined value.

 

[carllooper]

Yes, that was my understanding as well--although I've heard it both claimed that the Bell Test experiments vindicated "non-realism" and that it was simply LOCAL realism that they violated. But that leads me to another question--why is non-local realism so hard to fathom? What's so spooky about "action at a distance?" Didn't that die with gravity and electromagnetism? Don't those both clearly involve action-at-a-distance? (Or is it that they are not instantaneous but travel at the speed of light?)

The spookiness of action-at-a-distance has it's origins in the fact that action-at-a-distance was once considered, by certain ruling powers, to be supernatural. In and of itself this wouldn't be a problem. After all it's just a word. But anyone found studying the supernatural could be punished. And that's where the word acquires meaning and a certain influence on a culture. Scientists, however, were able to side-step the ban on studying the supernatural by calling what they studied "natural' instead. Perhaps gravity might been formalised earlier had not this taboo around action-at-a-distance been so heavily suppressed when it was.

Or maybe there's some other reasons for the reservations.

In any case it's sometime easier just to retain the language in which a concept has been historically framed. So we could, for example, call non-local realism: 'spooky', but without in any way requiring that non-local realists be sentenced to hang at dawn. In the same way we can retain the word "planet" without in any way suggesting that the so called continue to be considered as wandering gods.

C

 

[atyy]

Yes, I am aware that QM has a measurement problem. But historically the measurement problem combined with the positivist idea that "only the measurable is meaningful" to the actual belief that the unmeasurable is nonexistent, which a stop to inquiry, as the Weinberg article above spells out. I have heard that later the repudiation of "completeness" by Bell came to be accepted, but obviously decades of research-that-could've-been were lost by what were ultimately philosophical mistakes.

To be fair to the early quantum mechanicians, they did realize the problem and discussed it extensively. That is why we still have language like the "Heisenberg cut" and "von Neumann chain". Von Neumann's model of measurement is still one way of presenting the measurement problem that the reversible time evolution of the wave function alone does not produce definite experimental outcomes, contrary to experience. Von Neumann did make an error, but he could hardly have made the wrong claim if he had not been looking for a solution. Also, we know that not everyone accepted von Neumann's erroneous proof before Bell. Bohm, for example, produced a hidden variables theory that reproduces non-relativistic quantum mechanics. Also, Messiah's famous textbook does state that hidden variables that Einstein advocated was not excluded, and that he would adopt Copenhagen just because it was consistent with all experiments up to then.

The most common modern flavour of the Copenhagen interpretation does not deny a common-sense reality, but in fact explicitly states its existence, together with the idea that the wave function is a tool to describe that reality. I will follow the spirit of modern Copenhagen-style interpretations, such as Leifer and Spekkens, who say "the picture we have in mind is of the quantum state for a region representing beliefs about the physical state of the region, even though we do not yet have a model to propose for the underlying physical states."

• Why is it impossible to know both the position and the velocity of a particle by the rebound of a photon hitting it?

It is impossible to know both the position and momentum of a quantum particle, because it does not and cannot have both position and momentum simultaneously. Quantum position and and quantum momentum refer to the results of different experimental procedures, and the procedures are such that you cannot perform both of them in the same place at the same time. We consider these procedures to measure "position" and "momentum", because the quantum "properties" reduce to the classical properties in the classical limit of quantum mechanics.

• On Entanglement experiments - why can't the cause be explained by the same properties in each at the source? (I know I am out of my league with this question, and I believe the answer might be Bell's theorem itself...is that true?)
• Why couldn't there be an entanglement-type experiment where both particles were sent out "in the same way" so that you learn the information from one to know about the other? In other words, can you do something at the beginning to ensure that the position and velocity are the same, and then measure the position of one and the velocity of the other?

That is Bell's great achievement to show that if quantum mechanics is correct, the underlying reality cannot be explained by local properties, for some reasonable definition of "local".

Yes, that was my understanding as well--although I've heard it both claimed that the Bell Test experiments vindicated "non-realism" and that it was simply LOCAL realism that they violated. But that leads me to another question--why is non-local realism so hard to fathom? What's so spooky about "action at a distance?" Didn't that die with gravity and electromagnetism? Don't those both clearly involve action-at-a-distance? (Or is it that they are not instantaneous but travel at the speed of light?)

It is considered "spooky" because Einstein liked to use colourful language. I'm not sure what Einstein had in mind, but it seems to be that spooky action of a distance is in conflict with a classical conception of special relativity, in which the action at a distance is instantaneous and faster than light.

At present, we know that quantum spooky action at a distance is not compatible with any local deterministic theory, but it does not allow faster than light communication, and is compatible with special relativity.

Are there models for the underlying reality compatible with quantum mechanics and relativity? That is still a matter of research. My personal view is that one promising area is to assume that relativity is not exact, for example in lattice models of quantum field theory. However, a problem with that approach is that at present we do not have a lattice model of chiral fermions interacting with nonabelian gauge bosons.

Two very good reviews of the measurement problem are (each is perhaps slightly biased in different ways, but I believe they have striven to be technically accurate):

http://arxiv.org/abs/quant-ph/0209123
Do we really understand quantum mechanics?
Franck Laloe

http://arxiv.org/abs/0712.0149
The Quantum Measurement Problem: State of Play
David Wallace

I should also mention the Transactional Interpretation, which I don't know enough about to know if it is technically right, but is interesting because it tries to use the retrocausation loophole in the Bell Tests.

 

[jbmolineux]

Especially philosophy. Do you know of a single issue (other than something trite) that philosophers agree on? Mathematicians and physicists agree on all sorts of things. And in general they make progress - not philosophers:

I agree completely. The mess that exists currently in philosophy is significantly worse than that of the sciences. Moreover, as the Weinberg article suggested, it is actually RESPONSIBLE for much of the mess in the sciences--which is essentially my major thesis. There are a handful of things about which most philosophers today generally agree, however, and among them is that the philosophy that underlay the development of QM is self-refuting. But my minor thesis is that it still continues to wreak havoc in the sciences today through the idea that "the measurement is all that is being measured" (or "shut up and calculate" or however you want to say it), which is just an extension of that self-refuting philosophical error that has now been discredited in the field of philosophy, but continues to dominate the philosophy of quantum physicists.

Thus, as Atyy says, "It is impossible to know both the position and momentum of a quantum particle, because it does not and cannot have both position and momentum simultaneously." But originally, for the founders of QM, that was just an extension of the empirical criteria of meaning to the theoretical limit of the measurements of that day (and, as I understand it, was directly related to the technological limits of spectroscopes of that time).

But it has become part of the orthodoxy of QM, and physicists today believe themselves to be simply eschewing philosophical bias ("shutting up and calculating") in accepting that. But it seems to me that they are not only inheriting a bias, but they are swallowing whole a philosophical error that is now discredited for being self-refuting, which both historically and logically is the direct and only precursor to the idea that the particles literally "do not have both a position and a velocity."

By the way, I am not suggesting that physics needs to go the field of academic philosophy in order to learn. They need less, not more, of the filth that has been coming out of that decadent institution for the last century or more. By "good philosophy" I just mean careful, accurate, wise thinking about the principles that underlie science, and on questions such as the ones that we're discussing, and those that have become deeply intertwined in physics and particularly QM.