I Wheeler's delayed choice doesn't change the past

[Lord Jestocost]

That's why I call Bohr's and Heisenberg's approach "mystic". They want to find something "behind the phenomena", an explanation of the world so to say.

Niels Bohr (Source: https://en.wikiquote.org/wiki/Niels_Bohr):

“I consider those developments in physics during the last decades which have shown how problematical such concepts as "objective" and "subjective" are, a great liberation of thought. The whole thing started with the theory of relativity. In the past, the statement that two events are simultaneous was considered an objective assertion, one that could be communicated quite simply and that was open to verification by any observer. Today we know that 'simultaneity' contains a subjective element, inasmuch as two events that appear simultaneous to an observer at rest are not necessarily simultaneous to an observer in motion. However, the relativistic description is also objective inasmuch as every observer can deduce by calculation what the other observer will perceive or has perceived. For all that, we have come a long way from the classical ideal of objective descriptions.

In quantum mechanics the departure from this ideal has been even more radical. We can still use the objectifying language of classical physics to make statements about observable facts. For instance, we can say that a photographic plate has been blackened, or that cloud droplets have formed. But we can say nothing about the atoms themselves. And what predictions we base on such findings depend on the way we pose our experimental question, and here the observer has freedom of choice. Naturally, it still makes no difference whether the observer is a man, an animal, or a piece of apparatus, but it is no longer possible to make predictions without reference to the observer or the means of observation. To that extent, every physical process may be said to have objective and subjective features. The objective world of nineteenth-century science was, as we know today, an ideal, limiting case, but not the whole reality. Admittedly, even in our future encounters with reality we shall have to distinguish between the objective and the subjective side, to make a division between the two. But the location of the separation may depend on the way things are looked at; to a certain extent it can be chosen at will.”

Is there any “mystic” in this reasoning? Wheeler has put Bohr's view in a nutshell: "No elementary phenomenon is a phenomenon until it is a registered (observed) phenomenon.” Bohr never wanted to find something behind the phenomena.

My point is that this is semantics. We know what the Math says...

Niels Bohr (Source: https://en.wikiquote.org/wiki/Niels_Bohr):

“No, no, you are not thinking, you are just being logical.” (In response to those who made purely formal or mathematical arguments)

 

[vanhees71]

Well, what's the statement by Bohr? I always wonder what he really wants to say in writing a lot of complicated text. Are the subjective elements in "modern physics" or not?

My personal opinion is very clear: There are none! Physics is an empirical science and deals with objective reproducible facts and theoretical reasoning about such facts.

 

[zonde]

Delayed choice is particularly tough for dynamical explanation in the mechanical universe (“ant’s-eye view”) for the reason that zonde describes here.

I don't get it, why do you consider delayed choice experiment as tough for dynamical explanation. If you are satisfied with 50% visibility you even do not need any non-locality and can explain it with shared hidden variables.
Ok, if you want more that 50% visibility (as can be observed in experiments) you need non-locality or something IMO more drastic.

When physics contradicts your view of reality you have three choices: live with contradiction, reject the physics, or amend your view of reality.

Hmm, there is no physics that contradicts my apparently dynamical view of reality.
.

 

[Demystifier]

They want to find something "behind the phenomena", an explanation of the world so to say. That's not the purpose of the natural sciences however but belongs to the "complementary" other part of human experience, namely emotions and particularly religion.

I wouldn't say that finding something "behind the phenomena" has much to do with emotions or religion. If finding the "behind the phenomena" should not be classified as a part of natural sciences, then it's quite obvious that it should be classified as philosophy.

 

[vanhees71]

You don't even need any hidden variables. Standard QED does an excellent job. After thinking for a long time about these issues, I don't understand anymore, why one must invent interpretations of standard QT that create problems instead of simply using the one provided by Born's probability interpretation and taking it seriously. QT, interpreted in this way, is not more mysterious than any classical theory of physics and it's very successful.

As any hitherto discovered theory of physics it's incomplete in not providing a consistent description of quantum gravity nor does it give a clear hint at what observable consequences of a quantum theory of gravitation to expect. That's the real issue, not some quibble of some philosophers who don't want to accept that the natural sciences force us to learn how nature behaves and that this is not always according to our always preliminary and incomplete worldviews. Due to this anti-science attitude of philosophers/theologians people like Giordano Bruno were burnt!

 

[Demystifier]

After thinking for a long time about these issues, I don't understand anymore, why one must invent interpretations of standard QT that create problems instead of simply using the one provided by Born's probability interpretation and taking it seriously. QT, interpreted in this way, is not more mysterious than any classical theory of physics and it's very successful.

Well, one must invent non-minimal interpretations precisely because one wants to see (as you nicely expressed it) behind the phenomena. In classical physics the view behind the phenomena is almost automatic, but in quantum physics it isn't.

Now if you want to ask why does one want to see behind the phenomena, nobody expressed it better than Einstein:
"I want to know how God created this world. I'm not interested in this or that phenomenon, in the spectrum of this or that element. I want to know His thoughts, the rest are details."
And to avoid miss-conclusions, it has to be said that for Einstein "God" is a philosophical, not a religious concept. What he calls "God" is not very different from the contemporary concept of the "Theory Of Everything".

 

[zonde]

You don't even need any hidden variables. Standard QED does an excellent job. After thinking for a long time about these issues, I don't understand anymore, why one must invent interpretations of standard QT that create problems instead of simply using the one provided by Born's probability interpretation and taking it seriously. QT, interpreted in this way, is not more mysterious than any classical theory of physics and it's very successful.

I can understand your viewpoint and it seems rater sensible, but I have my reasons to look for interpretation. I am interested what hides behind statistical nature of QT.

As any hitherto discovered theory of physics it's incomplete in not providing a consistent description of quantum gravity nor does it give a clear hint at what observable consequences of a quantum theory of gravitation to expect.

Have you any idea for a topic to discuss along these lines?

 

[vanhees71]

Sure, Einstein was a kind of pantheist, mostly inspired by Spinoza, but that's well beyond the realm of objective science.

Also, why are you saying "In classical physics the view behind the phenomena is almost automatic"? $\dot{\vec{p}}=\vec{F}$ is as abstract an description as Schrödinger's equation, and both are simply justified by describing the phenomena (within their realm of validity only of course). In which sense let's me Newton's equation of motion let view behind the phenomena almost automatically, while the Schrödinger equation doesn't? I don't think that any scientific theory can tell us "how God created this world". It's not even a question you can sensibly pose within the natural sciences! It's another level of human experience, and it's clearly a individual subjective one, which is precisely the realm the natural sciences do not consider.

 

[vanhees71]

I can understand your viewpoint and it seems rater sensible, but I have my reasons to look for interpretation. I am interested what hides behind statistical nature of QT.

What makes you think there should hide anything behind the statistical nature of QT. Why shouldn't nature (or rather our observations of phenomena) be inherently probabilistic?

Have you any idea for a topic to discuss along these lines?

[/QUOTE]
Well, I've no clue. I think without any clue of a quantum theory of gravity nor any empirical hint at quantum effects concerning gravity, it's wild speculation anyway!

 

[Demystifier]

Also, why are you saying "In classical physics the view behind the phenomena is almost automatic"? $\dot{\vec{p}}=\vec{F}$ is as abstract an description as Schrödinger's equation, and both are simply justified by describing the phenomena (within their realm of validity only of course). In which sense let's me Newton's equation of motion let view behind the phenomena almost automatically, while the Schrödinger equation doesn't?

In classical physics, we know that it is not really about force F or momentum p, but about particle trajectories x(t). F and p are just auxiliary quantities that help to get the thing we are really interested about, that is x(t). The two crucial properties of x(t) are
1) It is a quantity that we directly observe, e.g. as a trajectory of a planet.
2) It is objective in the sense that, according to the theory, it does not depend on whether we observe it or not.

If classical physics is ultimately about x(t), then what is quantum physics ultimately about? If it is about the actual values of observables as functions of time, then the problem is that we don't have an explicit formula for that. If it is about probabilities of observables at given time, then the problem is that probabilities are not objective, in the sense that the theory does not say what is a probability of an observable when it is not measured.

 

[Lord Jestocost]

After thinking for a long time about these issues, I don't understand anymore, why one must invent interpretations of standard QT that create problems instead of simply using the one provided by Born's probability interpretation and taking it seriously.

... and so finally ending up in the "ensemble interpretation" due to the implicit assumption that quantum randomness stems not from utter lawlessness but from hidden causes.

 

[Fra]

Niels Bohr (Source: https://en.wikiquote.org/wiki/Niels_Bohr):

“I consider those developments in physics during the last decades which have shown how problematical such concepts as "objective" and "subjective" are, a great liberation of thought. The whole thing started with the theory of relativity. In the past, the statement that two events are simultaneous was considered an objective assertion, one that could be communicated quite simply and that was open to verification by any observer. Today we know that 'simultaneity' contains a subjective element, inasmuch as two events that appear simultaneous to an observer at rest are not necessarily simultaneous to an observer in motion. However, the relativistic description is also objective inasmuch as every observer can deduce by calculation what the other observer will perceive or has perceived. For all that, we have come a long way from the classical ideal of objective descriptions.

This analogy between relativity and QM is great.

I think the REASON why Bohr insists for the NEED for a classical measurement device, in order to define the experiements, is simply that of objectivity. Even though it is subjective in the sense of conditional upon the actual choice and settings of the device, it complies to objectivity(*) in the sense that in the classical world different observers can easily communicate without distorting each other.

So I think that Bohr is right that quantum theory as it stands requires a classical backdrop, for attaching all the things, like probability concepts etc.

(*) I see complications there, but they elaborations that i think we could not expect anything in 1935 or so to be aware of. But when looking at unficiations and gravity things do get more complex. And the question of "objectivity" actually takes on a whole new level, far beyond Bohr and Einsteins ideas. This is to question the objectivity in inferred laws of physics, and what if there exists no classical measurement device, say at unification energies at big bang? then what happens to these ideas? But that belongs to the BTSM anyway so i will not do more than hint. But I think that in even in that light, the insights of Bohr in the early days was extremely sound and clear. Even though i also agree that some of the actual papers are sometimes hard to parse.

/Fredrik

 

[zonde]

What makes you think there should hide anything behind the statistical nature of QT. Why shouldn't nature (or rather our observations of phenomena) be inherently probabilistic?

There are clicks in detectors from which experimentalist calculates statistics. There is at least that much behind the statistical nature of QT (this part is actually not very hidden). And actually you don't have to go any further to run into problems with "inherently probabilistic clicks in detectors"

Well, I've no clue. I think without any clue of a quantum theory of gravity nor any empirical hint at quantum effects concerning gravity, it's wild speculation anyway!

I think there is enough empirical hints (of course I might be wrong) to think about quantum gravity.
Say "charge" of gravitation field is mass. But formation of bond state in QM releases some mass and reduces gravity "charge" of component particles.
.

 

[microsansfil]

If classical physics is ultimately about x(t), then what is quantum physics ultimately about? If it is about the actual values of observables as functions of time, then the problem is that we don't have an explicit formula for that. If it is about probabilities of observables at given time, then the problem is that probabilities are not objective, in the sense that the theory does not say what is a probability of an observable when it is not measured.

What do we expect from a theory of the physical science ? give us rules/axioms to build predictives models or tell us something about the physical world ? In other words, are we frustrated to constat that quantum mechanics has given up the ambition of providing explanations (causal assignment) to stick to the predictive function only ( probabilistic inference ) ?

Best regards
Patrick

 

[OCR]

Even though i also agree that some of the actual papers are sometimes hard to parse.

If you think the writings of Bohr are, sometimes, a bit hard to parse... try parsing this:

In which sense let's me Newton's equation of motion let view behind the phenomena almost automatically, while the Schrödinger equation doesn't?

Please, can some of you slow down your thinking to match your key strokes, and make some attempt to proofread... just a little ?

Thank you, and carry on.

 

[vanhees71]

Hm, I just asked a question. What's wrong with that?

 

[vanhees71]

... and so finally ending up in the "ensemble interpretation" due to the implicit assumption that quantum randomness stems not from utter lawlessness but from hidden causes.

No, it's much simpler: There are no hidden causes, but nature behaves just fundamentally in a random way with the laws for the probabilities for measurement results given by quantum theory.

 

[vanhees71]

In classical physics, we know that it is not really about force F or momentum p, but about particle trajectories x(t). F and p are just auxiliary quantities that help to get the thing we are really interested about, that is x(t). The two crucial properties of x(t) are
1) It is a quantity that we directly observe, e.g. as a trajectory of a planet.
2) It is objective in the sense that, according to the theory, it does not depend on whether we observe it or not.

If classical physics is ultimately about x(t), then what is quantum physics ultimately about? If it is about the actual values of observables as functions of time, then the problem is that we don't have an explicit formula for that. If it is about probabilities of observables at given time, then the problem is that probabilities are not objective, in the sense that the theory does not say what is a probability of an observable when it is not measured.

Classical mechanics is ultimately about a trajectory in phase space, given by the dynamical evolution (since the state of the system in classical mechanics is represented by a point in phase space).

Quantum mechanics is ultimately about the evolution of the probabilities (or probability distributions) given by the statistical operator and the eigenvectors of observables in any picture of time evolution.

 

[Fra]

If you think the writings of Bohr were, sometimes, a bit hard to parse... try parsing this:

"Please, can some of you slow down your thinking to match your key strokes, and make some attempt to proof read... just a little ?"

In isolation that looks scrambled grammatically yes, but if you read it "in context" a human parser has little problem to understand it. But i figure posts here are not as polished as as wordings in a formal paper.

I also think I see Demystifiers point which caused the question to be asked: With some exceptions, in classical mechanics the it is easier to create an intuitive picture of what "really happens" as one can often make causal pictures in 4D of "mechanical mechanisms". This is surely much harder in quantum mechanics, as mechanistic intuition obviously fails.

/Fredrik

 

[Fra]

If it is about probabilities of observables at given time, then the problem is that probabilities are not objective, in the sense that the theory does not say what is a probability of an observable when it is not measured.

As I see it, question or "choosing observables" imples a change in the internal structure of the observer (or measurement device, or information processing agent if you prefer).

Thus technically, asking totally different questions correspond to different observers, so i do not see a problem with this. This is why its "subjective". But its not subjective in a mystical way imo?

/Fredrik

 

[microsansfil]

Classical mechanics is ultimately about a trajectory in phase space, given by the dynamical evolution (since the state of the system in classical mechanics is represented by a point in phase space).

Quantum mechanics is ultimately about the evolution of the probabilities (or probability distributions) given by the statistical operator and the eigenvectors of observables in any picture of time evolution.

In the context of Statistical mechanics it seem that Classical and Quantum formulations are highly analogous : "Classical and quantum dynamics of density matrices" http://www.scs.illinois.edu/mgweb/Course_Notes/chem544/notes/Ch9.pdf

best regards
Patrick

 

[Fra]

In the context of Statistical mechanics it seem that Classical and Quantum formulations are highly analogous : "Classical and quantum dynamics of density matrices" http://www.scs.illinois.edu/mgweb/Course_Notes/chem544/notes/Ch9.pdf

In particular they both fit well into what Smolin calles the Newtonian schema. Where you indeed have an underlying determinstic evolution in a timeless statespace, by means of timeless laws.

Its more on the causal part that thet differ. One simple way I like to think of the difference is to draw an analogy to economy, and how a palyer can determine market value of things. Here we can consider market value as the "observables". One traditional way is to value the substance value, corresponding to in some realist sense the "actual" hard physical values. The other way is to consider a pure expectation picture, where the actual values are no more and no less than the expected value of all other players in the market. And these expectations (wether rational or not) determine the actions fo the players on the market. The former is like classical mechanics and the latter is more like quantum mechanics, where the causality are based on expectations, rather than actualities.

If you analyze the substance value picture, and require that the substance values must be experimentally determinend, you really end up in a situation such as QM. Because not even traditional hard values have objective values. For example gold, and diamonds? The value of gold is also in principle subject to speculation and expectations.

I suspect its this that is strange to some with QM.

/Fredrik

 

[vanhees71]

As I see it, question or "choosing observables" imples a change in the internal structure of the observer (or measurement device, or information processing agent if you prefer).
Thus technically, asking totally different questions correspond to different observers, so i do not see a problem with this. This is why its "subjective". But its not subjective in a mystical way imo?
/Fredrik

It's not subjective. Why should it be subjective? If I measure observable $A$, I get something different than measureing observable $B$, supposed $B$ is not a unique function of $A$. There's nothing subjective in this, nor is it very specific to QT.

The main difference between classical and quantum physics is not the measurement of observables but the meaning of the state, i.e., what can be "prepared". In classical physics you can in principle determine the state of the system such that all possible observables have a determined value, while in QT this is only possible for sets of compatible observables, and the meaning of the state, even if it's one implying maximal possible knowledge about the system, i.e., the preparation in a pure state, is probabilistic. That's the true content of the Heisenberg-Robertson uncertainty principle. A great deal of confusion concerning the meaning of the quantum state is thus due to Heisenberg, who got one of his most famous discoveries (the uncertainty relation) wrong. He was immediately corrected by Bohr, but unfortunately the wrong first interpretation by Heisenberg stuck, and you get misleading statements about the measurability or non-measurability of observables still today. Indeed, you can measure any observable as precisely as you like (modulo practical technical problems with precise measurements) for any state the measured system is prepared in. What you can't do is to prepare precisely two incompatible observables, but still you can measure either of them with arbitrary precision, in whatever state the system is prepared in. Neither preparation nor measurement is subjective if the used procedures/protocols fulfill the constraints of reprodicibility making them to scientific empirical facts rather than subjective imaginations.

 

[Fra]

It's not subjective. Why should it be subjective? If I measure observable $A$, I get something different than measureing observable $B$, supposed $B$ is not a unique function of $A$. There's nothing subjective in this, nor is it very specific to QT.

I think you react on the word "subjective", with subjective i simply mean that the expectations and the probabilitis are conditional to (ie subjective) to the measurement device (including its choice of observables).

So I agree that in the first level of analysis, there is no actual subjectivity as in "ambigousness".

I use the word subjective synonymous to "conditional to observer" which also means conditional to the choice of observables. But wether there "choice" of observables is "free" or not, is a different discussion. It also depends on if we are talking about the freedom of experimenter to tweak the detectors, or freedom of a nucleus to "choose" its observables. In the former case, there is a FAPP freedom, but in the later case i think the nucleus aligns its "choice of observables" in order to stabilize itself in its environment and get maximal predictive power. However in the latter case Bohrs idea of the requirement for a CLASSICAL measurement device also breaks down. So for this reason it tried to keep the discussion at the current QM level, in order to stay on topic.

/Fredrik

 

[vanhees71]

As I see it, question or "choosing observables" imples a change in the internal structure of the observer (or measurement device, or information processing agent if you prefer).

Thus technically, asking totally different questions correspond to different observers, so i do not see a problem with this. This is why its "subjective". But its not subjective in a mystical way imo?

/Fredrik

Well, of course, to measure the position of a particle you need a different device than to meausure its momentum. This is not specific to QT but also the case within classical physics. Indeed there's no problem with this, and it's in no way mystical at all.

I think you react on the word "subjective", with subjective i simply mean that the expectations and the probabilitis are conditional to (ie subjective) to the measurement device (including its choice of observables).

So I agree that in the first level of analysis, there is no actual subjectivity as in "ambigousness".

I use the word subjective synonymous to "conditional to observer" which also means conditional to the choice of observables. But wether there "choice" of observables is "free" or not, is a different discussion. It also depends on if we are talking about the freedom of experimenter to tweak the detectors, or freedom of a nucleus to "choose" its observables. In the former case, there is a FAPP freedom, but in the later case i think the nucleus aligns its "choice of observables" in order to stabilize itself in its environment and get maximal predictive power. However in the latter case Bohrs idea of the requirement for a CLASSICAL measurement device also breaks down. So for this reason it tried to keep the discussion at the current QM level, in order to stay on topic.

/Fredrik

But this is an abuse of the word "subjective". All you describe are objective properties of objective observations in nature. Of course the probabilities depend on which quantity is measured. That's not even surprising, let alone mystical or subjective. For me it doesn't make sense to say "a nucleus chooses its observables". A nucleus just is a welldefined entity of nature. What I observe at it (e.g., it's position or momentum) is my free choice, and QT helps me to tell the probabilities for the outcome of the corresponding measurement, provided I've given the state of the nucleus (which is a formal mathematical description of (an equivalence class of) a specific prepartion procedure.

I've also never understood Bohr's "classical measurement device". According to QT everything is quantum, including macroscopic systems making up measurement devices. The classical behavior of the relevant macroscopic observables (which are coarse-grained by averaging many microscopic degrees of freedom over microscopically large, macroscopically small space-time regions) is emergent.

 

[Demystifier]

Classical mechanics is ultimately about a trajectory in phase space,

Perhaps in the Hamilton formulation, but not in the Newton or Lagrange formulation. In the latter two formulations, what matters is the configuration space, not the phase space.

 

[Demystifier]

Classical mechanics is ultimately about a trajectory in phase space, given by the dynamical evolution (since the state of the system in classical mechanics is represented by a point in phase space).

Quantum mechanics is ultimately about the evolution of the probabilities (or probability distributions) given by the statistical operator and the eigenvectors of observables in any picture of time evolution.

Can classical mechanics be formulated without an explicit reference to measurement?
Can quantum mechanics be formulated without an explicit reference to measurement?
If the first answer is "yes" and the second "no", don't you feel that it is a problem?

 

[vanhees71]

True, but that doesn't make any principal difference to the distinction between classical and quantum physics. If you derive classical approximations from quantum theory it's clear that the Hamilton formulation is the only save starting point. E.g., in QFT you always have to start with the "Hamiltonian path integral" to be sure to get the correct "Lagrangian path integral". A naive application of the Lagrangian form already leads to wrong results in quite simple cases as, e.g., for the thermal-field theory treatment free charged Klein-Gordon field in the grand-canonical approach at finite chemical potential!

 

[vanhees71]

Can classical mechanics be formulated without an explicit reference to measurement?
Can quantum mechanics be formulated without an explicit reference to measurement?
If the first answer is "yes" and the second "no", don't you feel that it is a problem?

There is of course no such problem since no physics can be formulated without reference to measurements/observations. Physics is about measurements and observations!

 

[Demystifier]

There is of course no such problem since no physics can be formulated without reference to measurements/observations. Physics is about measurements and observations!

http://www.informationphilosopher.com/solutions/scientists/bell/Against_Measurement.pdf

Theoretical physics is distilled from experiments, there are no doubts about it. However, ones the distillation process is over, one may want to formulate the theory without an explicit reference to measurement. For instance, Landau and Lifshitz have written a great book on classical mechanics without mentioning measurements. On the other hand, it seems that something similar cannot be done for quantum theory (in the standard form).