I "No objective reality" in quantum mechanics?

[dendros]

TL;DR Summary

After having read some headlines, I'm curious if what they say (there is no objective reality, i.e no reality in the absence of an observer) could be true or not.

As per title and the TL;DR, I'm curious if there could be some truth in these statements of the headlines I had read recently or are they just sensationalist fluff.

Personally, I find these statements very hard to believe. In fact, impossible to believe. But I'm not a QM expert, not even an amateur so I'm not sure at all on how things work in this field so that is why this thread was created.

Are there some knowledgeable members in this forum that can shed some light on this?

 

[malawi_glenn]

It would help ton if you could be more specific than

After having read some headlines

 

[A. Neumaier]

Summary: After having read some headlines, I'm curious if what they say (there is no objective reality, i.e no reality in the absence of an observer) could be true or not.

Since there are observers, the question is moot. Therefore different observers have different opinions on the question.

But at least everything objectively observed has an objective reality. This includes everything faithfully reported in the scientific literature.

 

[martinbn]

Summary: After having read some headlines, I'm curious if what they say (there is no objective reality, i.e no reality in the absence of an observer) could be true or not.

As per title and the TL;DR, I'm curious if there could be some truth in these statements of the headlines I had read recently or are they just sensationalist fluff.

Personally, I find these statements very hard to believe. In fact, impossible to believe. But I'm not a QM expert, not even an amateur so I'm not sure at all on how things work in this field so that is why this thread was created.

Are there some knowledgeable members in this forum that can shed some light on this?

You need to give more context, otherwise we can only guess. You also have to be careful with the meaning of words ('Cause you know sometimes words have two meanings). In quantum mechanics especially when it comes to interpretations "reality" means the existence of values of observables. And the lack of reality refers to the non existence of values without measurement. For example it is meaningless to say that an electron has position if the position is not being measured. On the other hand the reality, i.e. the existence, of the electron is not denied.

 

[Fra]

(there is no objective reality, i.e no reality in the absence of an observer) could be true or not

This has two conceptual parts that i think ita best to separate

1. Reality (ie what's inside the "back box")

2. Objectivity (ie does different observers agree with each other on what is in the "blac box")

The essence of QM is that your rational inferences about reality are driven by the data from measurements. Without measurements/observers there is no food for inference. That doesn't mean there is nothing, it just means there can be no rational inference an no one to ask these qustions.

Wether different observers must come to an timely agreement or wether nauture would allow disagreeing observers is a deeper question. But the conventional way is to demand different observers to infer the same laws and constants as a design constraint in building theories otherwise the theory is inconsistent. But this may not be true if you bring observers into the physics rather than beeing a gauge choice. Ie consider the observer communication to follow the laws of physics.

/Fredrik

 

[dendros]

Sorry for the delayed response, I was busy.

An example of what caused me to open this thread: https://www.popularmechanics.com/science/a40460495/objective-reality-may-not-exist/

But I don't really understand what they're talking about except that it seems like there was an experiment that led a team of scientists from Brazil to arrive at the conclusion that "objective reality might not exist", as per title.
Could that be true or the article distorted what scientists were saying?

 

[Lord Jestocost]

On the issue of "objective reality" (see https://en.wikiversity.org/wiki/Does_objective_reality_exist?)

Physics cannot answer such a questions because it is beyond its scope. The philosopher David J. Chalmers puts it in a nutshell in “Ontological Anti-Realism”:

“The basic question of ontology is ‘What exists?’. The basic question of metaontology is: are there objective answers to the basic question of ontology? Here ontological realists say yes, and ontological anti-realists say no.” [bold by LJ]

 

[dendros]

@Lord Jestocost, I'm not talking about philosophy but science. In that article I linked to, is said that there was an experiment done by scientists from Brazil and its conclusion was, apparently, that there is no objective reality. At least, that is what the title of this article says.
But I don't understand why the article says that because it seems to describe Heisenberg uncertainly principle, complementarity, etc and these are, from what I understand, nothing new in respect to QM.

Perhaps there is something that eludes me so I'm asking here because I want to learn more on this subject.

 

[martinbn]

@Lord Jestocost, I'm not talking about philosophy but science. In that article I linked to, is said that there was an experiment done by scientists from Brazil and its conclusion was, apparently, that there is no objective reality. At least, that is what the title of this article says.
But I don't understand why the article says that because it seems to describe Heisenberg uncertainly principle, complementarity, etc and these are, from what I understand, nothing new in respect to QM.

Perhaps there is something that eludes me so I'm asking here because I want to learn more on this subject.

May be one needs to look at the actual paper and the science news article.

 

[dendros]

Well, this seems to be the paper: https://www.nature.com/articles/s42...2570&CJEVENT=057c36e4014111ed802502350a180511

Any thoughts?

 

[PeroK]

Well, this seems to be the paper: https://www.nature.com/articles/s42...2570&CJEVENT=057c36e4014111ed802502350a180511

Any thoughts?

I started to read it and got the feeling it was quite mad. I then looked at the other publications by the authors and they appear to be medical specialists, rather than physicists.

 

[malawi_glenn]

I then looked at the other publications by the authors and they appear to be medical specialists, rather than physicists.

Some of the authors work here: https://ictqt.ug.edu.pl/pages/people/
I do think this parkinsons research paper is just a bad search engine thingy

 

[dendros]

@PeroK, interesting. In what way is it "quite mad", if I may ask? I had a similar feeling but it was based on those headlines that I read ("no objective reality", etc).

 

[PeroK]

@PeroK, interesting. In what way is it "quite mad", if I may ask? I had a similar feeling but it was based on those headlines that I read ("no objective reality", etc).

Any modern paper that cites Bohr's complinentarity as its basis seems dubious to me. That's of no more than historical interest now, surely?

Then, the description of the double-slit experiment reads like a pop-science account.

Reading further, I don't know what to make of it.

 

[PeterDonis]

An example of what caused me to open this thread: https://www.popularmechanics.com/science/a40460495/objective-reality-may-not-exist/

Pop science articles are not a good way to learn actual science. That's true for any area of science, but it's particularly true for QM. You need to be looking at textbooks and peer-reviewed papers.

the description of the double-slit experiment reads like a pop-science account.

Which is to be expected, since the source is a pop science magazine, not a peer-reviewed journal.

 

[Jarvis323]

Which is to be expected, since the source is a pop science magazine, not a peer-reviewed journal.

He was talking about the description from the paper.

https://www.nature.com/articles/s42...2570&CJEVENT=057c36e4014111ed802502350a180511

 

[Algr]

In that article I linked to, is said that there was an experiment done by scientists from Brazil and its conclusion was, apparently, that there is no objective reality. At least, that is what the title of this article says.

Lots of us aren't very happy with objective reality, but it is easy in any field of discovery to start seeing things you want, rather then what is really there. This is doubly true with pop-sci coverage of physics. Lots of people thought Einstein had disproven objective reality when he showed that two events could be simultaneous for some people, but not for others. But really it is just looking at the same event from a different angle.

'Cause you know sometimes words have two meanings

oooooohh, it really makes you wonder...

 

[Jarvis323]

Summary: After having read some headlines, I'm curious if what they say (there is no objective reality, i.e no reality in the absence of an observer) could be true or not.

As per title and the TL;DR, I'm curious if there could be some truth in these statements of the headlines I had read recently or are they just sensationalist fluff.

Personally, I find these statements very hard to believe. In fact, impossible to believe. But I'm not a QM expert, not even an amateur so I'm not sure at all on how things work in this field so that is why this thread was created.

Are there some knowledgeable members in this forum that can shed some light on this?

Well, this seems to be the paper: https://www.nature.com/articles/s42...2570&CJEVENT=057c36e4014111ed802502350a180511

Any thoughts?

If you notice, the paper that the article is based on doesn't mention the word objective even once, and even in the pop-sci article, the authors aren't quoted using the word objective.

The pop-sci author is sort of expanding the article by making a connection between the results of the paper and the wider issue about objectivity in QM, but to me it seems a bit of a stretch.

Other papers about the Wigner's friend throught experiment have been published which I think are more relevant to the issue about objectivity.

Experimental test of local observer independence​

https://www.science.org/doi/10.1126/sciadv.aaw9832

A no-go theorem for the persistent reality of Wigner’s friend’s perception​

https://www.nature.com/articles/s42005-021-00589-1

A strong no-go theorem on the Wigner’s friend paradox​

https://www.nature.com/articles/s41567-020-0990-x

To me, the issue is still quite muddled because it seems dependent on careful definitions, as well as your interpretation of QM (note that I am not an expert though).

As an example, in Rovelli's relational interpretation, which claims QM is about facts and not states, he divides the facts into "stable" ones and "relative" ones.

Stable Facts, Relative Facts​

https://link.springer.com/article/10.1007/s10701-021-00429-w

 

[malawi_glenn]

Lots of us aren't very happy with objective reality, but it is easy in any field of discovery to start seeing things you want, rather then what is really there

Indeed.

If you think about it, without objective reality, how can we make objective scientific claims?

 

[Jarvis323]

Lots of us aren't very happy with objective reality, but it is easy in any field of discovery to start seeing things you want, rather then what is really there.

Reminds me of Bertrand Russel's message to the future.

 

[Lord Jestocost]

Well, this seems to be the paper: https://www.nature.com/articles/s42...2570&CJEVENT=057c36e4014111ed802502350a180511

Any thoughts?

To my mind, the authors seem merely to show what was always clear for some founders of quantum mechanics: Separate images that we form from different experimental viewpoints (the experiential reality) do not combine into one comprehensive whole that allows us to speak of an underlying “objective reality” in a classical sense. We have no single image (call it objective) that would corresponds to a particle as “particle by itself” and a wave as “wave by itself”. As Carl Friedrich von Weizsäcker remarks in “The Structure of Physics” (the book is a newly arranged and revised English version of "Aufbau der Physik" by Carl Friedrich von Weizsäcker):

“In the first months of 1927 there was a technical disagreement between Bohr and Heisenberg about the conjectured correct interpretation of quantum mechanics which even led to serious personal irritations. While they were separated for a few weeks, Bohr going to Norway for a skiing trip while Heisenberg remained back in Copenhagen, each found his own solution: Heisenberg the uncertainty of position and momentum, Bohr the complementarity of wave and particle. At Bohr’s return they eventually agreed to the formulation of complementarity being the cause of the uncertainty. That is the fourth possible solution to the problem of duality. Matter and light ‘by themselves’ are neither particles nor waves. Yet if we wish to visualize them we must use both pictures. And the validity of one picture imposes limitations on the validity of the other. This is the main point of the Copenhagen interpretation.” [bold by LJ]

 

[dendros]

Then I don't understand why the article was given that title ("there might be no objective reality") if the paper does not seem to talk about this but about Bohr's complementarity principle.
Reading the paper and from its language, I got the impression that the authors really want Bohr's principle to be true.
But I'm not knowledgeable on this, especially regarding math. What's your opinion on its mathematical formalism and the experiment described in it?

 

[Fra]

If you think about it, without objective reality, how can we make objective scientific claims?

By the process of evolving and negotiating an agreement between observers.

By the same process(the only one at hand) we can not know if this is just a stable illusion we agree on in the interacting group of observers or a global universal eternal "fact".

/Fredrik

 

[malawi_glenn]

By the process of evolving and negotiating an agreement between observers.

This sounds like an objective claim to me, there is no objective reality in which this claim is always true

 

[dendros]

I feel I should explain why I don't agree with statements such as "there is no objective reality": because these are contradicting logic.

If reality depends on observers then how did observers appear in first place? For example, how did the BB happen if there was no one to observe it since it was, by definition, the beginning of everything including observers?

And so on.

 

[Fra]

This sounds like an objective claim to me, there is no objective reality in which this claim is always true

One does not need to assume equilibrium. If one does one can define the regular observer equivalence and thus objectivity. In the general case the situation is off-equilibrium and the observers are evolving. There is indeed no guarantee that equilibrium will be attained in the timescale of interactions - but we can still do science and observera can do rational inference. I think the learning process is more important than some ultimate final state of enlightment.

/Fredrik

 

[PeroK]

I feel I should explain why I don't agree with statements such as "there is no objective reality": because these are contradicting logic.

You perhaps take this too literally. The issue is mapping classical concepts down to elementary interactions.

If reality depends on observers then how did observers appear in first place? For example, how did the BB happen if there was no one to observe it since it was, by definition, the beginning of everything including observers?

The BB wasn't the beginning of everything. It's only as far back as we can successfully project and understand.

Moreover, QT is not necessarily the last word on Cosmology.

 

[Fra]

how did the BB happen if there was no one to observe it since it was, by definition, the beginning of everything including observers?

Conceptually i thing of BB as tracing evolution back to when the simplest possible observers started to be distinguished and interact. Their early "communication" I see as the TOE unification end as its extreme. This is another perspective as compare to the TOE energy scale when forcea unify from the perspective of current models. But a dual view might be the low complexity limit of observer complexity. So we face a self organisation challenge vs fine tuning challange. Pick your choice.

/Fredril

 

[Jarvis323]

I feel I should explain why I don't agree with statements such as "there is no objective reality": because these are contradicting logic.

If reality depends on observers then how did observers appear in first place? For example, how did the BB happen if there was no one to observe it since it was, by definition, the beginning of everything including observers?

And so on.

An observer isn't necessarily a person, it can just be a small system of subatomic particles. For the purported experimental proof of observer dependence, they modeled Wigner and his friend with 6 photons. Then they say the below in their discussion, which I am not sure is agreeable to everyone.

One might further be tempted to deny our photonic memories the status of “observer.” This, however, would require a convincing revision of our minimal definition of what qualifies as an observer, which typically comes at the cost of introducing new physics that is not described by standard quantum theory. Wigner, for example, argued that the disagreement with his hypothetical friend could not arise due to a supposed impossibility for conscious observers to be in a superposition state (2). However, the lack of objectivity revealed by a Bell-Wigner test does not arise in anyone’s consciousness, but between the recorded facts. Because quantum theory does not distinguish between information recorded in a microscopic system (such as our photonic memory) and in a macroscopic system, the conclusions are the same for both: The measurement records are in conflict regardless of the size or complexity of the observer that records them. Implementing the experiment with more complex observers would not necessarily lead to new insights into the specific issue of observer independence in quantum theory. It would, however, serve to show that quantum mechanics still holds at larger scales, ruling out alternative (collapse) models (20). However, this is not the point of a Bell-Wigner test—less demanding experiments could show that.

It should also be noted that there are assumptions involved.

If one holds fast to the assumptions of locality and free choice, this result implies that quantum theory should be interpreted in an observer-dependent way.

https://www.science.org/doi/10.1126/sciadv.aaw9832

 

[dendros]

Can a field be an "observer" since photons are excitations of the electromagnetic field?

If yes, it would mean that basically any physical entity can be an "observer" and saying that there is no objective reality (i.e no observers -> no reality) is just a more convoluted way to say that there is no reality without physical entities, which is pretty obvious.

Am I wrong in my interpretation?

 

[PeroK]

Can a field be an "observer" since photons are excitations of the electromagnetic field?

If yes, it would mean that basically any physical entity can be an "observer" and saying that there is no objective reality (i.e no observers -> no reality) is just a more convoluted way to say that there is no reality without physical entities, which is pretty obvious.

Am I wrong in my interpretation?

Let me try an analogy. Let's assume that your post is "real". We can pin it down to data somewhere on a disk and thereby to magnetic bits. But, if we try to dig deeper, we may find that we cannot ultimately identify elementary particles that represent your post.

All we know is that the IT system reliabily renders your post on a screen. But, we cannot dig down to a bedrock objective reality in the way that classical physicists would have imagined.

 

[DrChinese]

...saying that there is no objective reality (i.e no observers -> no reality) is just a more convoluted way to say that there is no reality without physical entities, which is pretty obvious.

Am I wrong in my interpretation?

There is a different meaning here of the phrase "objective reality", as used in the world of QM. It's jargon, and should be read as such. Yes, the argument goes all the way to Einstein and Bohr. But since Bell, the arguments have changed - as has some of the lingo. Different people tend to use different jargon.

Essentially, it means that a particle has a definite position, a definite momentum, and definite spin components independent of an observer. Generally, most physicists would say: we live in an observer dependent world; the nature of an observer's choice of measurement basis shapes reality, and all particles (at least for some particle properties) are in superpositions in a variety of bases at all times.

It doesn't mean that each observer's reality is different and subjective. Observers can agree objectively (i.e. when they perform identical measurements).
----------------------
I too will supply an analogy:

Is the moon there when no one observes it?
a. Yes, the moon is there. The particles are "there", i.e. the total number of particles essentially remains the same (ignoring other effects).
b. No, of course "there" must consider the Uncertainty Principle. All of the particles comprising the moon are never (in the quantum sense) "there" in specific spots.

 

[PeterDonis]

No, of course "there" must consider the Uncertainty Principle. All of the particles comprising the moon are never (in the quantum sense) "there" in specific spots.

Note that this sense of "there" would imply that the moon is not "there" even when it is observed, since we never observe all of its individual particles. We observe something like its center of mass position and apparent size, each with finite error bars. That does not pin down a particular position, or even a particular wave function of any form, for any individual particle of the moon. (These sorts of considerations are why I favor your other meaning of "there", the a. one, according to which obviously the moon is there whether we observe it or not, since its particles, or more precisely its quantum degrees of freedom, are there, however much uncertainty we might have about the exact state of each degree of freedom.)

 

[dendros]

Thanks to all for trying to explain this weirdness.

I guess I'm more of a classical mindset and I find subjectivity hard to swallow.

But if I understand somehow what you're saying then there might be no absolute reality, just like there is no absolute space and time as Newton believed. Did I understood somewhat correctly?

 

[Fra]

It doesn't mean that each observer's reality is different and subjective. Observers can agree objectively (i.e. when they perform identical measurements).

The question is if two observers can perform identical measurements? That presumes a view where its just about condtitional probabilities. This is fine the observers are different physicists that use their common classical reality to get relative frequencies of events.

But it gets problematic if one considers more general observers that has to rely on lossy communication due to massive differences i information capacity etc? This can be thought of as differences in other prior information(background info) not encoded in the prior probability. One could also think that two obaervera with same prior probability and same prior info would already be indistinguishable. Ie. They are the same observer already.

Here the physical process of comparing views is tricky and may even be irreversible. This is supposed to relate to rhe BB issue. We would need to ask what i means for a BB superlight "observer" to be able to construct even the same measurement as an Earth based macroscopic device?

/Fredrik

 

[PeroK]

But if I understand somehow what you're saying then there might be no absolute reality, just like there is no absolute space and time as Newton believed. Did I understood somewhat correctly?

Perhaps the fundamental law of nature is the Heisenberg Uncertainty Principle (HUP). And the more general UP. This puts a limitation on the knowledge you can have about results of measurements. If elementary particles were classical, you could prepare an electron in a state where it had a definite position and a definite momentum. And that would equate to an objective reality. But, the HUP does not allow an objective reality in that sense. You can know the state of an electron, but that implies that its position and momentum are not elements of an objective reality.

That is the QM view of nature. Ironically, the classical view would not allow the complexity of chemistry and there could be no life in a classical objectively real universe.

So, whatever you think of QM "weirdness", we owe everything to it, in terms of representing a universe where complex life can exist.

 

[dendros]

A question: are observers in the QM the equivalents of frames of reference from classical physics?
Just curious.

 

[PeroK]

A question: are observers in the QM the equivalents of frames of reference from classical physics?
Just curious.

No. Standard QM adopts Newtonian space, time and reference frames. Relativistic QM, such as full QFT, adopts Minkowski spacetime and the reference frames of SR.

 

[PeroK]

PS to give you an example. In classical EM we have Rutherford scattering, which assumes a classical trajectory with classical uncertainty - finite width of an electron beam. The interaction is modeled using Coulomb's law, with classical KE and PE.

In QED, we have Moller scattering with the interaction modeled by Feynman diagrams, where the probability amplitude for the scattering angles are computed by a particular integration over all virtual photon exchanges.

In the former, there is an assumed objectively real path for each electron. In the latter there is no objectively real path, but only a probability of detection depending on angle.

At low energies the predictions are approximately the same. But, at high energies Coulomb's law and Rutherford scattering break down and the QED predictions turn out to be correct.

 

[dendros]

No. Standard QM adopts Newtonian space, time and reference frames. Relativistic QM, such as full QFT, adopts Minkowski spacetime and the reference frames of SR.

I'm sorry for my badly put question.

What I wanted to ask is: is an observer (in QM) and a frame of reference (in general physics, be it Newtonian or Relativistic) the same thing?

 

[PeroK]

I'm sorry for my badly put question.

What I wanted to ask is: is an observer (in QM) and a frame of reference (in general physics, be it Newtonian or Relativistic) the same thing?

No. An observer is more likely an interaction in QM. With perhaps the observer being a macroscopic measuring device.

 

[dendros]

Thanks.

In fact, I find the term "observer" to be a rather nebulous concept (is it a conscious entity, a particle, a device, is it microscopic or macroscopic, etc) but this could be due to my lack of knowledge, while it's probably much clearer to those that are knowledgeable in QM.
That'a why I asked the above question.

Is there a precise definition of an observer in QM?

 

[PeroK]

Is there a precise definition of an observer in QM?

Probably not. You could start with the Wikipedia page and an Internet search.

I don't think the definition of observer is critical to the fundamental point that there is no underlying objective reality in QM. Bohmian Mechanics notwithstanding!

 

[PeterDonis]

Is there a precise definition of an observer in QM?

Not really. However, we do understand a lot more about what makes an object an "observer" than we did when QM was originally developed. The key advance was decoherence theory, which was started in the 1970s and early 1980s and which has continued to advance. Decoherence theory makes it clear that, generally speaking, what makes something an "observer" (i.e., something that causes a "measurement" to be made, so that mathematically speaking, we use a collapsed wave function corresponding to the observed measurement result to predict future measurement results--without necessarily making any commitment regarding interpretation, i.e., "what is really going on") is that it causes decoherence. Heuristically, when a system we want to "measure" (say a qubit) interacts with a system that causes decoherence, decoherence spreads the information about the interaction among a huge number of untrackable degrees of freedom. That makes the interaction (at least for all practical purposes) irreversible and makes a record of the measurement result that cannot be undone. That basically matches the original highly hand-waving definition of "measurement" that was used by the original developers of QM.

 

[dendros]

@PeterDonis, thanks for clarification, I feel that I understand a bit better on that subject.

But I feel the need to repeat a previous question I asked: if reality requires observers to exist then how did observers themselves appear, i.e how did they came into existence? By being themselves observed by other observers? If so, how did the latter appear?

This sounds very confusing to me but I guess QM is not intuitive at all. Ultimately, it seems to boil down to the chosen interpretation of QM and that's all for now.

 

[PeterDonis]

if reality requires observers to exist

On the viewpoint I described, this is backwards. "Observers"--systems that can cause decoherence--are "built" out of "reality" just like the things that they "observe". "Observation" is just an interaction between quantum degrees of freedom. There's nothing special about it or about its relationship to "reality"; it's in the same position with regard to "reality" as any other interaction.

 

[Fra]

if reality requires observers to exist

then how did observers themselves appear, i.e how did they came into existence? By being themselves observed by other observers? If so, how did the latter appear?

From a third person seeing perspective, the observers are ordinary matter. And observers beeing observed by other observers are nothing else than a subsystem interacting with its physical environment. Ie all matter in the universes constantly "observes" each other. So your question is the same as what is the origin of matter and how did the first particles form? Nothing poppes into existence but its an evolution and nonone yet fully understands this.

So observers and communication vs matter and interactions? Is it all a game of words?

No, I think the two views offer different mathematical ways to abstract and understand nature. The convential matter/interaction view is a sort of external or third person description. That has som pros and som cons. The observer/inference view is an intrinsic view which also jas pros and cons. One can also see it aa thinking tools for the theory builder.

To unify the views is like finding the intrinsic set of interacting views that from certain third person perspective looka just like the regular laws of physics. If we can do that, we will probably acquire a deeper understanding of the nature of causality and why we have "laws of physics" that we have.

/Fredrik

 

[PeroK]

@PeterDonis, thanks for clarification, I feel that I understand a bit better on that subject.

But I feel the need to repeat a previous question I asked: if reality requires observers to exist then how did observers themselves appear, i.e how did they came into existence? By being themselves observed by other observers? If so, how did the latter appear?

You still seem to be equating "existence" with "definite measurement". A measurement doesn't bring something into existence. (Although much that is written on QM seems to imply that.)

To some extent QFT is clearer in this respect. The various fields exist across spacetime (EM field, electron field, quark fields etc.). The fields interact. Observers are sub-systems that interact with each other or can be designed to interact with a microscopic system.

Quantum Theory generally, therefore, has to explain the generally classical behaviour of the macroscopic observers or measurement devices. And the behaviour of the microscopic system being observed/measured. Neither of these is trivial conceptually. There is no easy way to comprehend this.

I don't believe, however, that it is incumbent on QM to say where the universe came from originally. That might be possible. Note that classical physics certainty has no answer to how the universe originated. In that respect QM is a huge step forward although IMO not the last word on the origin of universe itself.

 

[gentzen]

To some extent QFT is clearer in this respect. The various fields exist across spacetime (EM field, electron field, quark fields etc.). The fields interact. Observers are sub-systems that interact with each other or can be designed to interact with a microscopic system.

Physical existence means existence in space and time, and in this respect QFT is indeed clearer. But "normally" this is not clearly expressed, and I have no idea to which extent you, or vanhees71, or anybody else claiming QFT to be "clearer" has anything along those lines in mind. (I have some feeling what AN has in mind for QFT, and that is the reason why I take it serious and try to make sense of it.)

Numbers like 3 or i exists outside of space and time, but this is a different sort of existence, and not what is normally meant when existence is discussed. The sort of existence of a wavefunction in configuration space is "unclearer" than that. Maybe it means an existence purely in time, but without space.

Quantum Theory generally, therefore, has to explain the generally classical behaviour of the macroscopic observers or measurement devices. And the behaviour of the microscopic system being observed/measured. Neither of these is trivial conceptually. There is no easy way to comprehend this.

Especially when it is unclear whether it is even allowed to assume the concrete existence in space and time of those macroscopic measurement devices. Bohr and many others where pretty clear that you do have to assume their existence, and that it would be misguided to try to derive that existence from the mathematics of QM.

I don't believe, however, that it is incumbent on QM to say where the universe came from originally. That might be possible. Note that classical physics certainty has no answer to how the universe originated. In that respect QM is a huge step forward although IMO not the last word on the origin of universe itself.

I like those wise words.

 

[PeterDonis]

Especially when it is unclear whether it is even allowed to assume the concrete existence in space and time of those macroscopic measurement devices.

Since we are such "macroscopic measurement devices", it doesn't seem like we need to "assume" the existence of at least one class of such devices. If we don't exist, who is having this whole conversation in the first place?