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

[vanhees71]

The statement you quoted here is true and doesn't entail any interpretation. What you choose to believe ontologically about it can and does differ between different interpretations of QM.

Of course, it's of high interest to understand all aspects of QT, including also the philosophical ones. It took me years to come to the conclusion that the "mystic approach" is simply due to an old-fashioned philosophical balast of the founding fathers, mostly Bohr and Heisenberg. Just taking the Born rule seriously for me resolves all problems, particularly the EPR problem (although the EPR paper is not reflecting Einstein's philosophical quibbles with QT very well; he's written a much clearer paper (Dialectica article in German):

http://onlinelibrary.wiley.com/doi/10.1111/j.1746-8361.1948.tb00704.x/abstract

I wonder, why this is still not accepted by many physicists and why modern textbooks still teach the confusing collapse hypothesis of some Copenhagen-interpretation flavors (admittedly that's not Bohr's fault, who, AFAIK, didn't include collapse in his version of the Copenhagen interpretation).

 

[Lord Jestocost]

It took me years to come to the conclusion that the "mystic approach" is simply due to an old-fashioned philosophical balast of the founding fathers, mostly Bohr and Heisenberg.

What the heck is that "mystic approach"? "Mystic" related to what? Related to 19th century physics? Related to some not pronounced implicit assumptions about "reality" which might merely based on your personal psychological predispositions? In case you don’t grasp Bohr’s and Heisenberg’s thinking, that’s your problem, but don’t coin the term “mystic” as a “killer argument”. Science should be based on arguments.

Listen, what Wheeler writes about Bohr:

“You tell me what isn’t the plan of physics, our friend rejoins. If you understand quantum mechanics so well, why don’t you tell we what is the plan of physics?
No one knows, we reply. We have clues, clues most of all in the writings of Bohr [23-25], but no answer. That he did not propose an answer, not philosophize, not go an inch beyond the soundest fullest statement of the inescapable lessons of quantum mechanics, was his way to build a clean pier for some later day’s bridge to the future.”

J. A. Wheeler in „Quantum Theory and Measurement“ (edited by John Archibald Wheeler and Wojciech Hubert Zurek), Princeton, New Jersey 1983, page 201

 

[bhobba]

What the heck is that "mystic approach"? "Mystic" related to what?

Related to someone like Dirac and even Einstein. Einstein and Bohr were good friends but in QM their approaches were different. Einstein would accuse Bohr of being 'mystical' - it was two different approaches. Actually Bohr wasn't too bad - he was more agnostic to it than a believer - others like Pauli, yes the great and rational Pauli, were worse. Worse still was the great Von-Neumann and Wigner that advocated the so called Princeton school based on the, it turns out to be wrong, but for reasons not known at the time, Von-Neumann's argument consciousness caused collapse:

See the attached document

As the above argues, correctly IMHO, even today we are still feeling the effects of those early years and it has been detrimental to understanding QM, especially at the beginner and even intermediate level. Fortunately books like Ballentine banish it, not that I agree 100% with Ballentine on every issue, but it is way more rational than others.

Thanks
Bill

 

[bhobba]

why don’t you tell we what is the plan of physics?

I am surprised his old student and friend, Feynman, didn't see the fallacy of that - first you have to show physics has a plan.

Thanks
Bill

 

[.Scott]

I believe Hawking is exaggerating in telling us (page 83):
«...the Universe doesn't have just a single history, but every possible history, each with its own probability; and our observations of the current state affect its past and determine the different histories of the Universe, just as the observations of the particles in the double slit experiment affect the particles' past.»
Do you agree?

I may come closer to agreeing with this than most. If you follow the Multi World Interpretation (MWI) - in the sense that the universe is continuously spawning slightly different copies of itself that do not further interact, then I have no counter argument for you. Otherwise, consider this:

The first problem with "changing the past" is the notion of choosing an observation - acting independently of "predestination". Let's say that we have two copies of the universe, one where you (Alice) observe something at noon, the other where you observe something different at noon. Would it be safe to say that these universes were the same up until noon? That's certainly what we had in mind when we set up the experiment (notwithstanding that this experiment cannot actually be set up).

But when we test this in the only ways we can, we sometimes find a violation of the Bell inequality. Alice makes measurements "randomly", and when her choices and results are compared to those of Bobs, it appears that Alice mucked with the past. Of course, the results are symmetric, Alice can accuse Bob of mucking with the past as well.

But since we apparently only end up with a single copy of the past, whose to say that it has changed? And we have an equal problem with the future. Just as we can't duplicate the universe, we can't really inject information into it from "outside" either.

So if we allow ourselves to claim that either the present or the future are not specific, why should we treat the past any differently?

To a large extent, this is a matter of semantics. We can all agree on what restrictions QM places on experimental results. But when we go from the Math to the English language, we discover that English has ambiguities that are well tolerated in common affairs - but work poorly in describing the Physics.

Let me describe the Bell Inequality by moving "cause" into the future. Remember that the only purpose of this is to change the English semantics describing the QM results. I am not claiming that this is a "better" description - only a consistent one:

Both Alice and Bob make their measurement choices of the entangled particle independently. Since Alice did this based on a telescope facing the Northern sky and Bob did it based on the Southern sky, we presume that no information has yet been exchanged between them. But we will allow for an indefinite present, so Alice may not have measured the result, and if she did, it may be + or -. Similarly, Bob's may be any of the three as well. When they compare results, certain Alice/Bob combinations are inconsistent with QM and will not persist. They can be made to violate Bell's inequality without any FTL issue.

 

[DrChinese]

But since we apparently only end up with a single copy of the past, whose to say that it has changed?

I would argue that there is plenty of "evidence" there is more than one past. If light only goes in one path from source to detector, why is it possible to increase its intensity by blocking some paths (those providing destructive interference)? After all, this effect occurs even when photons are sent one at a time. Ditto with a suitable double slit setup. On the other hand, these particular setups are usually used as an example of the path integral approach. Which of course implies that the photon took all possible paths. (I placed "evidence" in quotes above because this should not be considered an absolute proof.)

My real point is that there is much we don't know about time. I'm not even sure if there is 1 past, 1 present, and/or 1 future - or many. And I'm not sure if time only flows in 1 direction, 2 directions (time symmetric), or in fact no direction (RBW)!

Mine is a minority position in this thread. However, these questions are far from settled scientifically.

 

[.Scott]

I would argue that there is plenty of "evidence" there is more than one past. If light only goes in one path from source to detector, why is it possible to increase its intensity by blocking some paths (those providing destructive interference)? After all, this effect occurs even when photons are sent one at a time. Ditto with a suitable double slit setup. On the other hand, these particular setups are usually used as an example of the path integral approach. Which of course implies that the photon took all possible paths. (I placed "evidence" in quotes above because this should not be considered an absolute proof.)

My real point is that there is much we don't know about time. I'm not even sure if there is 1 past, 1 present, and/or 1 future - or many. And I'm not sure if time only flows in 1 direction, 2 directions (time symmetric), or in fact no direction (RBW)!

Mine is a minority position in this thread. However, these questions are far from settled scientifically.

My point is that this is semantics. We know what the Math says - it's just a matter of how it is described in common English terms.
And I would not say that there is a single right way to describe it. If you decide you want only a single history, you can find other ways to make the Math work.

 

[zonde]

My point is that this is semantics. We know what the Math says - it's just a matter of how it is described in common English terms.
And I would not say that there is a single right way to describe it.

It's not enough to know what math says. In science we want to test it experimentally. And there are certain assumptions that have to be satisfied for us to test it experimentally. One is that cause and effect has to produce factual records for us to analyze them at a later time. Another is that we assume experimenter has some ability to determine experimental conditions FAPP independently from the relationship he is testing.
So if we propose different pasts that produce the same factual records there is no way in principle how we could ever falsify existence of multiple pasts.
And if we propose retrocausal relationships where effect comes before cause we have paradoxical situation where experimenter can not determine the conditions of cause based on records about observed effect.

So even so there no single right way how to describe the math, there certainly are unscientific ways how to describe the math. So I can't agree it's just semantics.

 

[vanhees71]

I am surprised his old student and friend, Feynman, didn't see the fallacy of that - first you have to show physics has a plan.

Thanks
Bill

Sure, physics has a plan, as nearly any endeavor of men. The plan is to investigate and describe as good as we can the reproducible phenomena in a quantitative way, no more no less. 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. 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. Of course, both realms of human experience are equally important, but it's as important to keep them strictly separated.

 

[Fra]

(1)

That's fine for the status quo, the book argues that to advance physics from the status quo (e.g., obtain quantum gravity and explain dark energy and dark matter data)

Do you by this mean that the rational behind these "interpretational discussions" - as contrasting to fruitless discussions that aim to make no difference to current theory - is a process of trying to improve the understanding of the theory in a bigger context IN ORDER to figure out what is the most plausible way to generalize or modidy the theory to allow solving the open questions? Then I fully agree.

Thus, if one participates in these discussions holding the viewe that the status quo is all we need, and there may be open questions but to which the foundational issue has no relevance, then it is a rational conclusion that the discussions are somewhat meaningless. A researcher thinking like this, thinks of the unlike thinkers as perpahs irrational philosophers that only talk.

But if one has the opinion (all researchers have their own opinions) that the open questions, are very likely to related to some of the foundational issues then it is rather irrational to ignore these things. A researcher thinking like this, may view the unlike thinkers as ostriches, not wanting to look at the real problems simply because they are too hard. And it simply feels better to reject these questions.

(2) About the mysticism of QM. I rather always found it to be a mystery WHY so much people keep confusing the idea of observer or context dependent information, with the completely off topic discussion about human brain and consciousness. As far as I know Bohr never comitted this mistake, but others did, and those who did was most probably also the people that kept misinterpreting Bohr, making his probably insightful views dirty.

Observer dependence, subjective probabilities or context dependent inferences has imo NOTHING mystical over it? It is rather and extremely rational information processing picture? This has absolutely nothing to do with humans or consciossness.

In this respect Bohr was probably more clear than others that in instead of insisting that the experiment requires and "observer" which can by some be misinterpreted to mean a conscious human, he said it requires a CLASSICAL measurement device, as that is the "conditional reference" required to formulate questions and experiments. But the meaning is similar if you only think about it. the reason for instead talking about observers, is in the extension where you want to generalize these ideas to thinkings that was not existing at Bohrs time.

/Fredrik

 

[bhobba]

Sure, physics has a plan, as nearly any endeavor of men. The plan is to investigate and describe as good as we can the reproducible phenomena in a quantitative way, no more no less. 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. 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. Of course, both realms of human experience are equally important, but it's as important to keep them strictly separated.

I agree with you, but the question is does it need a plan at all. Its science - that's enough IMHO - no more needs to be said. Is science a plan? Yes - but is it any different than the plan of any science - conjecture, experiment, conjecture, experiment - and so on.

Thanks
Bill

 

[.Scott]

It's not enough to know what math says. In science we want to test it experimentally. And there are certain assumptions that have to be satisfied for us to test it experimentally. One is that cause and effect has to produce factual records for us to analyze them at a later time. Another is that we assume experimenter has some ability to determine experimental conditions FAPP independently from the relationship he is testing.
So if we propose different pasts that produce the same factual records there is no way in principle how we could ever falsify existence of multiple pasts.
And if we propose retrocausal relationships where effect comes before cause we have paradoxical situation where experimenter can not determine the conditions of cause based on records about observed effect.

So even so there no single right way how to describe the math, there certainly are unscientific ways how to describe the math. So I can't agree it's just semantics.

All that is required by the scientific method is that hypothesis be testable and that experiments be reproducible. Causation is a human characterization of events. We have an ingrain notion that we are able to influence the future. This presumption is completely independent of whether we live in a deterministic universe or not.
When we model a system, we often tend to put it into cause/effect terms - with the cause and effect events having a time-like relation and the cause preceding the effect. And QM can often be cast into those familiar terms. But QM doesn't intrinsically work that way. QM applies limits to the entire experiment - and across the entire duration of the experiment.

At the end of the experiment, we have results, we have analysis of the results, and we have a story about those results. For example, we have results that show a violation of the Bell inequality. On analysis we see that the results are consistent with our Mathematical model (QM). Then we make up a story about how our experiment worked - and that usually includes the construction of a list of events in time-like sequence which each causing the next.

I agree that the cause/effect way of looking at things is very useful - and I do not oppose it. But we need to recognize that cause and effect are in the man, not in the nature. QM is the science, cause/effect is part of our technology.

 

[zonde]

All that is required by the scientific method is that hypothesis be testable and that experiments be reproducible. Causation is a human characterization of events. We have an ingrain notion that we are able to influence the future. This presumption is completely independent of whether we live in a deterministic universe or not.

Yes, everything is right. But I'm not sure why you switched from causation to determinism in the last sentence. They are different things. Maybe you perceive them as equivalent?

When we model a system, we often tend to put it into cause/effect terms - with the cause and effect events having a time-like relation and the cause preceding the effect. And QM can often be cast into those familiar terms. But QM doesn't intrinsically work that way. QM applies limits to the entire experiment - and across the entire duration of the experiment.

Well, if you mean that QM makes statistical predictions then yes, it complicates things a bit. But other than that cause and effect still applies to QM. And I have no idea what do you mean by "QM doesn't intrinsically work that way". And surely we can split experiment with large ensemble into two sub experiments with half as big ensembles and still describe them with QM.

At the end of the experiment, we have results, we have analysis of the results, and we have a story about those results. For example, we have results that show a violation of the Bell inequality. On analysis we see that the results are consistent with our Mathematical model (QM). Then we make up a story about how our experiment worked - and that usually includes the construction of a list of events in time-like sequence which each causing the next.

The story part goes beyond minimal QM.

I agree that the cause/effect way of looking at things is very useful - and I do not oppose it. But we need to recognize that cause and effect are in the man, not in the nature. QM is the science, cause/effect is part of our technology.

I agree that cause and effect are in the man. But all of the science is in the man just as well.
And I would not say that cause/effect is part of our technology unless by technology you mean all the things we have got by birth and by our upbringing.

 

[RUTA]

(1)
Do you by this mean that the rational behind these "interpretational discussions" - as contrasting to fruitless discussions that aim to make no difference to current theory - is a process of trying to improve the understanding of the theory in a bigger context IN ORDER to figure out what is the most plausible way to generalize or modidy the theory to allow solving the open questions? Then I fully agree.

Thus, if one participates in these discussions holding the viewe that the status quo is all we need, and there may be open questions but to which the foundational issue has no relevance, then it is a rational conclusion that the discussions are somewhat meaningless. A researcher thinking like this, thinks of the unlike thinkers as perpahs irrational philosophers that only talk.

But if one has the opinion (all researchers have their own opinions) that the open questions, are very likely to related to some of the foundational issues then it is rather irrational to ignore these things. A researcher thinking like this, may view the unlike thinkers as ostriches, not wanting to look at the real problems simply because they are too hard. And it simply feels better to reject these questions.
/Fredrik

Yes, that sums up my attitude and many in foundations of physics, e.g., Smolin. But, in defense of my colleagues who are not motivated to seek new physics in this fashion, I certainly can’t say they’re wrong to ignore these concerns. As you say, “all researchers have their own opinions.”

 

[.Scott]

I agree that cause and effect are in the man. But all of the science is in the man just as well.
And I would not say that cause/effect is part of our technology unless by technology you mean all the things we have got by birth and by our upbringing.

By technology I meant everything we invent to change our environment. Once we believe we have a working model of the universe (Science), we often use that model to improve our situation. When we invent new technology, we are tapping into our intrinsic cause/effect method of thinking - one that presumes we are able to change the future.

 

[.Scott]

Yes, everything is right. But I'm not sure why you switched from causation to determinism in the last sentence. They are different things. Maybe you perceive them as equivalent?

They are related. If you presume determinism, then seeing the past as a function of the future is no great challenge. Without determinism, you are presuming that decisions are being made from "outside" the universe - and the tendency is to view this new information as affecting only the future.

 

[vanhees71]

By technology I meant everything we invent to change our environment. Once we believe we have a working model of the universe (Science), we often use that model to improve our situation. When we invent new technology, we are tapping into our intrinsic cause/effect method of thinking - one that presumes we are able to change the future.

Well, and I think engineering, i.e., the functioning of all our technology is a strong hint that evidence-based science, which hinges in its very foundations on the assumption that causality holds true, really "works" (even in the literal sense). I dare to doubt that any philosophical speculations have brought anything like this for the development of mankind (to the better or worse, one must of course admit) as science (physics, chemistry, biology, which are the input for the more applied sciences like engineering, chemical industry, and medicine and agriculture, respectively).

 

[.Scott]

Well, and I think engineering, i.e., the functioning of all our technology is a strong hint that evidence-based science, which hinges in its very foundations on the assumption that causality holds true, really "works" (even in the literal sense). I dare to doubt that any philosophical speculations have brought anything like this for the development of mankind (to the better or worse, one must of course admit) as science (physics, chemistry, biology, which are the input for the more applied sciences like engineering, chemical industry, and medicine and agriculture, respectively).

Yes. It certainly works. As I said earlier, I do not oppose it at all. I only note that "works" (as you are using it here) is an anthropocentric assessment.

 

[RUTA]

It's not enough to know what math says. In science we want to test it experimentally. And there are certain assumptions that have to be satisfied for us to test it experimentally. One is that cause and effect has to produce factual records for us to analyze them at a later time. Another is that we assume experimenter has some ability to determine experimental conditions FAPP independently from the relationship he is testing.
So if we propose different pasts that produce the same factual records there is no way in principle how we could ever falsify existence of multiple pasts.
And if we propose retrocausal relationships where effect comes before cause we have paradoxical situation where experimenter can not determine the conditions of cause based on records about observed effect.

So even so there no single right way how to describe the math, there certainly are unscientific ways how to describe the math. So I can't agree it's just semantics.

zonde sums up nicely what bothers some people about delayed choice experiments. Again, it’s not the physics per se, the physics works beautifully (theory and experiment agree nicely), it’s the implications for one’s view of reality. When physics contradicts your view of reality you have three choices: live with contradiction, reject the physics, or amend your view of reality. We’re seeing all three of these responses on this thread. In our book, we advocate the third option, i.e., moving from the “ant’s-eye view” to the “God’s-eye view” of reality. Delayed choice is particularly tough for dynamical explanation in the mechanical universe (“ant’s-eye view”) for the reason that zonde describes here. That’s why Zeilinger’s delayed choice experiment (cited earlier) is the first experiment we present and discuss in our chapter on QM. As I stated earlier, no amount of formal analysis dispells the challenge presented by zonde here — the ramafications of my choice occurred before my choice was made. How can that be? QM just doesn’t care about “freely made choices,” so live with it, reject QM, or change your view of reality.

 

[vanhees71]

Well, if I have some prejudices about "reality" (I usually do not use this word anymore in physics discussions because the philosophers spoiled any definite meaning of that word), then empirical objective reproducible facts help to get rid of these prejudices. The predicted possibility of delayed choice (by Wheeler using quite simple foundations of QT which can be easily explained to students in the QM 1 lecture) is confirmed by many high-precision experiments in various forms (mostly with photons but also with other systems). So instead of hinging on classical prejudices we should accept the empirical facts related to the predictions of QT (but not more!). As a physicist you cannot really simply reject QM (or rather QT since one has to include relativistic QFT here). For that it's too successful a description of nature. So, as I said above, rather change your view of "reality" (reality for me is simply any reproducible objective observational fact).

 

[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).