"No objective reality" in quantum mechanics?

[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?

 

[gentzen]

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?

I was more alluding to the social rules (or mathematical/logical rules?) of this interpretation game when I wrote "unclear whether it is even allowed," and didn‘t really worry about myself whether measurement devices exist in space and time.

 

[Fra]

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?

As I see it, one is trying to describe the general intrinsic inference process that could be universaööy applied to even very simple observers, such as elementary particles or hypothetical primordal subsystem that took the role of "observer" sa during the first nanosecond after big bang. After all this is where a lot of the crazy stuff might have happened(crazy as in that the laws of physics aa we know them was forged). And even our best telescopes can't probe that far. The problem is that as observers loose complexity they are bound to loose memory as well. So wether the big bang was the beginning or a bounce, it is the beginning to the extent that any past memories would be lost as observers was desintegrated.

Imagine an obsever that is only a bit. It can not possibly by any stretch of imagination encode 4d spacetime relations to its environment?

/Fredrik

 

[PeterDonis]

Imagine an obsever that is only a bit.

A single qubit won't cause decoherence, so it can't be an "observer".

 

[PeterDonis]

wether the big bang was the beginning or a bounce, it is the beginning to the extent that any past memories would be lost as observers was desintegrated.

While in a model in which there was a "bounce", your statement about any previous memories being lost is probably correct, it has nothing to do with quantum measurement or decoherence. Decoherence does not require that all of the information is retrievable. In fact, a big part of what makes decoherence work is that much of the information is not retrievable: it is irretrievably lost in a huge number of untrackable degrees of freedom. That is what decoherence is.

For example, say you measure a qubit and the device that shows you the result is a pointer that swings on a dial to point at either "up" or "down". Your observation--what you will remember about the result--is of where the pointer is pointing. But that is an extremely coarse observation considering the number of quantum degrees of freedom involved. All the rest of the information gets irretrievably lost in the process of decoherence; nobody remembers it and no process can retrieve it. That is what makes the result permanent and irreversible.

 

[Fra]

A single qubit won't cause decoherence, so it can't be an "observer".

I agree, if we use the notion of observer as in QM as it stands.

But my view and point is thst this is precisely we need a more general measurement theory, which is viable also for small obsevers that does not qualify as classical.

It is comparable to wheb we had only SR, then someone says that even non-inertial observers should be allowed - but then yes a new theory was needed.

/Fredrik

 

[PeterDonis]

we need a more general measurement theory, which is viable also for small obsevers that does not qualify as classical.

Why do we need such a theory when the whole point is that small, "non-classical" objects cannot cause "measurements" in the first place, because they do not cause decoherence?

 

[PeterDonis]

It is comparable to wheb we had only SR, then someone says that even non-inertial observers should be allowed - but then yes a new theory was needed.

This is not correct. You can treat non-inertial observers just fine in SR.

What SR cannot treat is curved spacetime. The need for that was not due to any problems with treating non-inertial observers; it arose from attempts to construct a relativistic theory of gravity. The reason that was needed was that it was easily shown that Newtonian gravity as it stood was not compatible with relativity.

In the case of QM, we don't have that kind of problem. Now that we understand how decoherence works, we understand the basic physical process underlying "measurements" that have irreversible results. And that understanding tells us what kinds of things can be "measuring devices". We don't need any more general version of QM for that.

 

[Fra]

Why do we need such a theory when the whole point is that small, "non-classical" objects cannot cause "measurements" in the first place, because they do not cause decoherence?

The motivation for such quest is admittedly depending on ones ideas on howto solve the outstanding open problems such as unifying forces (including gravity) without requiring extremes of fine tuning.

Some people may think the foundations of QM or unifying the other interactions have little todo with quantum gravity but from my perspective they are likely related.

One thing that the guides me is that a i find the idea that natures causal interrelations between its parts, is dictated by constraints that are defined requiring more information that available thoe parts as very irrational. It meana then to seek to evolve the rules if inference for hypothetical microoservers to the normal laws of physics a inferred from classicä obsevers as we scale the comolexity. Decoherence ia doing it the wrong may by redeuction. I want the inside view of the same process as i think it will have more power.

/Fredrik

 

[PeterDonis]

the idea that natures causal interrelations between its parts, is dictated by constraints that are defined requiring more information that available thoe parts

Relationships between the parts are also information. That's basically what quantum entanglement is. Of course there is more information than if you just considered each part individually in isolation, and ignored its relationships with other parts, and just added all those individual pieces up. But that's just as wrong in classical physics as it is in QM.

 

[Fra]

Relationships between the parts are also information. That's basically what quantum entanglement is.

Yes, encoded by their interactions. But what are those interactions and how do they scale with energy scale? In the external view one naturally faces a problem of fine tuning.

For me part of the problem of unification of forces is to understand the details of how the interactions chosen by nature emerge as you scale up the complexity of the interacting parts. This is the "inside view" that is dual to the external high energy view. In the inside view there is instead an evolutionary problem which supposedly solves the fine tuning problem as well as the mess with renormalzation which hardly is a physical problem but a pathology of our models. With the right "constructing principles" meaning here "inference and betting rules" of the microscopic agents i see hope to make progress that i see hard otherwise.

One idea is that the "naked actions" ie the actions relative the simple observer itself must be much simpler than the "dressed action" seen when including a part of the enviromment. Trying to explain the dressed actions from the naked actions creates a fine tuning problem on the "space of naked actions" this is why this has a low explanatory value.i want to see a learning evolutionary explanatory chain, not a reductionst explanation (that needs fine tuning)

/Fredrik

 

[PeterDonis]

what are those interactions and how do they scale with energy scale?

That's what quantum field theory and the renormalization group are for.

In the external view one naturally faces a problem of fine tuning.

I don't know what you're referring to here.

renormalzation which hardly is a physical problem but a pathology of our models

This was a common view of renormalization decades ago, but I don't think it is now, nor has it been since Wilson and others developed the modern view of renormalization group theory in the late 1960s and 1970s. The common view now is that, as (IIRC) Weinberg says in one of his articles about QFT, renormalization is something you would have to do even if everything was finite, in order to properly understand what you are actually measuring when you measure something like the mass or charge of the electron.

One idea is that the "naked actions" ie the actions relative the simple observer itself must be much simpler than the "dressed action" seen when including a part of the enviromment. Trying to explain the dressed actions from the naked actions creates a fine tuning problem on the "space of naked actions" this is why this has a low explanatory value.i want to see a learning evolutionary explanatory chain, not a reductionst explanation (that needs fine tuning)

Some references for where you are getting your understanding from would be helpful here. What you are saying does not look like anything in actual QFT. In particular, your description of "naked" vs. "dressed" actions seems wrong: that distinction has nothing to do with "the simple observer itself" vs. "including a part of the environment".

 

[kurt101]

A single qubit won't cause decoherence, so it can't be an "observer".

Why do we need such a theory when the whole point is that small, "non-classical" objects cannot cause "measurements" in the first place, because they do not cause decoherence?

Can you explain this further or provide a reference to help me understand this?

This goes against my understanding. For instance, if you are able to distinguish what slit an electron went through in a double slit experiment by measuring which slit the electron went through using photons, the interference goes away. Is that not decoherence by a small "non-classical" object?

 

[PeroK]

Can you explain this further or provide a reference to help me understand this?

This goes against my understanding. For instance, if you are able to distinguish what slit an electron went through in a double slit experiment by measuring which slit the electron went through using photons, the interference goes away. Is that not decoherence by a small "non-classical" object?

That is definitely not decoherence!

 

[PeterDonis]

if you are able to distinguish what slit an electron went through in a double slit experiment by measuring which slit the electron went through using photons, the interference goes away. Is that not decoherence by a small "non-classical" object?

No. The device that measures which slit the electron went through is not "a small non-classical object". The photons are, but the photons are not all there is to the device that does the measurement. Something has to detect the photons and determine whether they indicate that an electron passed through or not. That something will be a large, classical object.

 

[gentzen]

Can you explain this further or provide a reference to help me understand this?

This goes against my understanding. For instance, if you are able to distinguish what slit an electron went through in a double slit experiment by measuring which slit the electron went through using photons, the interference goes away. Is that not decoherence by a small "non-classical" object?

The difference between environmental decoherence and more liberal interpretations of the word "decoherence" also created some puzzlement in a previous thread:

Such a phenomenon of suppression of interference is what is called decoherence.

This was somewhat surprising for me, because I expected "decoherence" to be the same as "environmental decoherence".
...
Interestingly, the first version of that SEP entry from 2003 did include "environment" as part of decoherence:

It is this phenomenon of suppression of interference through suitable interaction with the environment that we refer to by ‘suppression of interference’, and that is studied in the theory of decoherence. For completeness, we mention the overlapping but distinct concept of decoherent (or consistent) histories.

In 2012 Bacciagaluppi included sections on decoherent histories in the entry and weakened his statement to "It is this phenomenon of suppression of interference through suitable interaction with the environment that we call ‘dynamical’ or ‘environmental’ decoherence." Finally in 2020, he fully embraced that "suppression of interference" is what is meant by "decoherence" (if further qualifications are omitted).

My impression is that while the inclusion of "decoherent histories" might have been Bacciagaluppi's personal decision, there was also a real shift in the meaning of "decoherence" over time. What you can measure is the "suppression of interference" (within a well defined subsystem), so as control of decoherence became important for quantum computers and other quantum technologies, it made sense to separate the well defined "measurable" concept from the less well defined "explanatory" concept.

My personal conclusion in that previous thread was:

I guess the difference between "irreversible" and "not reversed, maybe accidental, maybe intentional, or maybe because fundamentally impossible" is less important for (most) practical purposes than was once believed. What is important for decoherence in quantum computing is whether it is actually suppressed (by error correction methods), not whether is would have been possible (or easy) to suppress it.
...
To conclude, please just continue to use the word decoherence as you did before, and rely on the context to disambiguate whether it means "environmental decoherence" or just "suppression of interference".

But you mention a similar puzzlement over "measuring which slit the electron went through". A measurement should create a permanent record. But of course, no record will be forever, it just has to be permanent for a long enough period of time. So the question arises why the state of a single qubit should not count as a measurement, in cases where it is sufficiently permanent for the purposes of the experiment. Well, I would personally rather try to avoid this, because the "sufficiently permanent" will typically be implicitly understood, and then you risk unproductive discussions (a la Frauchinger-Renner) with people who don't understand this.

 

[PeterDonis]

A measurement should create a permanent record.

No, that's too strong a criterion, since "record" implies that a human can retrieve the information. There is no such requirement for a "measurement", at least not in the sense that @kurt101 is using the term, namely "something that eliminates the interference in a double slit experiment". There is no need for anything to make a "permanent record" of which slit the electron went through in order for the interference to go away. All that is required is that something is present at the slit for the electron to interact with that causes decoherence. The decoherence does not have to lead to a human-retrievable "record" of which slit the electron went through.

the question arises why the state of a single qubit should not count as a measurement

Because, as I have already stated, a single qubit does not cause decoherence.

 

[gentzen]

No, that's too strong a criterion, since "record" implies that a human can retrieve the information. There is no such requirement for a "measurement", at least not in the sense that @kurt101 is using the term, namely "something that eliminates the interference in a double slit experiment".

Even so the main intention of my post was to help @kurt101 reconcile his understanding with yours, I didn't worry at all how he used the term "measurement" when I wrote: "A measurement should create a permanent record." My intention was to translate the obviously ambiguous term "measurement" into the seemingly less ambiguous synoymous term "permanent record". That synonymous term was supposed to be appropriate both to the everyday meaning of the word "measurement" and to how it is normally used in QM. I didn't notice the "difficulties" of reconciling the normal meaning of the word "record" to how the term "measurement" is used in QM.

On the one hand, a "record" should contain the stored information in a way that it can in principle be retrieved. And because humans are normally able to achieve things that are achievable in principle, this indeed implies that a human can retrieve the information. Now the "record" could be in a place unreachable by humans (for example, it could head away from the solar system with a sufficiently high speed such that no human born on Earth has any chance to every catch it again), but that doesn't contradict the information being "in principle" retrievable by humans.

However, there is a sense in which the information stored in the "records" in typical QM experiments like the Stern-Gerlach experiment cannot be retrieved by humans, not even in principle. The results of the spin measurements do impact where the atoms will land. But in a typical SG experiment, it will be impossible to reconstruct the exact sequence of up/down measurement results (or even a reasonable approximation to it), even in principle. For some CERN experiments, such a reconstruction of an exact sequence might exist temporarily, but even here most are discarded immediately, because the generated data volume is just too big to be stored. So the "records" are gone long before even a single run of an experiment is over, and hence using the term "record" for such a "measurement" is indeed inappropriate, even without the qualifier "permanent".

There is no need for anything to make a "permanent record" of which slit the electron went through in order for the interference to go away.

Making the interference go away is very easy. Even too easy. Calling everything which makes the interference go away a "measurement" is certainly a bad idea. I don't think that it captures the way that the word "measurement" is normally used in QM.

All that is required is that something is present at the slit for the electron to interact with that causes decoherence. The decoherence does not have to lead to a human-retrievable "record" of which slit the electron went through.

Because, as I have already stated, a single qubit does not cause decoherence.

A single qubit does not cause environmental decoherence. But it can make the interference go away. I guess I see why you think that this answers "the question ... why the state of a single qubit should not count as a measurement". But the trouble is that some people do count it as a measurement. So I tried to come up with a stronger argument for convincing kurt101 to not follow those people.

 

[PeterDonis]

A single qubit...can make the interference go away.

How?

 

[Morbert]

This might be related: A photon incident on double-slit apparatus with no detectors at the slits has a wavefunction that looks like $\frac{1}{\sqrt{2}}(|\psi_A\rangle + |\psi_B\rangle)$ after the particle has passed through the slits (labelled $A$ and $B$). If an optical crystal is placed behind the slits, such that the photon is transformed into a signal and idler photon pair, we instead get a wavefunction like $\frac{1}{\sqrt{2}}(|\psi_A\rangle_s|\phi_A\rangle_i + |\psi_B\rangle_s|\phi_B\rangle_i)$ This makes the interference pattern go away, even though no correlation with the collective degrees of freedom of some macroscopic apparatus has been established. I.e. If we compute $|\langle x|\Psi\rangle|^2$, it will be the idler photon terms, and not any detector terms, that zero the interference.

 

[Delta2]

The only objective reality imho is that there seem to exist conscious observers . Everything else (not only information and conclusions regarding QM or QFT but also regarding daily life such as that we are human beings with flesh and bones) is information in the mind of observers. But we can't be sure what exactly the observers are cause that is also (self) information in the mind of the observers.