People gonna keep thinking quantum observers are people.

That’s a very astute distinction.

Hi all,



iI my opinion, as others have said, this experiment, by itself, does not disprove the existence of an 'objective reality' anymore than QM does. After all, Wigner's friend is a well-known feature of QM, so it is not really prove anything new.
For instance, an interpretation like the de Broglie-Bohm theory can explain that experiment.



I agree that QBism, Copenaghen interpretation (CI), RQM in their own ways reject 'realism'. But how about MWI. In MWI, the only 'truly real thing' is the universal wave-function (UW). The UW never collapses in MWI. It rejects counterfactual definiteness. But the UW is still objective.
Also, the 'objective collapse' theories like the Ghirardi-Rimini-Weber model or Penrose's interpretation claim that the wave-function is real IMO. But during measurements, the wave-function localizes to a definite position. AFAIK, these theories all predict that this 'localization' occurs at some length scale (even if they differ in the precise way that occurs) spontaneously - another name for these models is 'spontaneous collapse' theories. Hence, I would not say that 'counterfactual definiteness'='objective reality', strictly speaking.

There are different 'flavors' of CI. For instance, see these papers by Michel Bitbol: http://www.bourbaphy.fr/bitbol.pdf and https://core.ac.uk/download/pdf/148348264.pdf. In brief, in my own understanding, Bitbol thinks that the wave-function is not a 'description' of reality, but it is a tool of the experimenter that enables her to make probabilistic predictions. In other words, QM, according to Bitbol is perspectival: it makes predictions of what the experimenter herself will observe. This does not mean that the observer creates reality but, rather, that the measurement is made by a peculiar perspective, namely that of the observer's. According to Bitbol this was also the position of Bohr himself. Note, however, that this interpretation does not take an ontological position on the 'objective reality'. In some sense, CI, in this 'flavor', is a statement on the limitations of science. Science cannot give us knowledge of 'reality as it is', but in its relation to the observations (in the first paper Bitbol compares Bohr's views on QM with Kant's philosophy).
This is rather similar to Rovelli's RQM. The difference is that according Rovelli each physical object has its own 'perspective'.

It isn't a live and dead cat, a blatant contradiction which cannot arise. Bob observes the cat and knows if it is dead or alive. Alice measures the cat still in superposition. That's very different than Alice measuring a dead cat and Bob a live one. — noAxioms

Thanks! Helpful clarification of my hamfisted attempt at an allegory.

Takes a theory to beat a theory — Andrew M

What if the theory that needs to be ‘beaten’ is not a theory at all but an untestable metaphysical postulate? Then it might be better to simply ignore it, or proceed as if it says nothing.

Now they’ve performed the first experiment that proves it
— Wayfarer

is this not a an objective realisation? — Mr Phil O'Sophy

The point is that two observers see different things which cannot be reconciled, but that neither of which can be shown to be incorrect.

Wigner’s friend is a well-known feature of QM — boundless

However the point of the article is the claim that what was previously only a thought experiment has now been experimentally realised.

QM, according to Bitbol is perspectival: it makes predictions of what the experimenter herself will observe. This does not mean that the observer creates reality but, rather, that the measurement is made by a peculiar perspective, namely that of the observer's. According to Bitbol this was also the position of Bohr himself. Note, however, that this interpretation does not take an ontological position on the 'objective reality'. In some sense, CI, in this 'flavor', is a statement on the limitations of science. Science cannot give us knowledge of 'reality as it is', but in its relation to the observations (in the first paper Bitbol compares Bohr's views on QM with Kant's philosophy). — boundless

I have been reading those papers from Bitbol, and this is the interpretation that makes the most sense to me also. A point that Bitbol makes is that there is an ineliminably subjective aspect to knowledge, generally - measurements are always made from a point of view or perspective. But scientific philosophy doesn’t want to acknowledge that, it wants to believe that it’s seeing reality as it is in itself, as Boundless notes. This the conceit that is being exposed by these conundrums.

So, Schroedinger's cat is either alive or dead and never both. — Dfpolis

I really do understand that. I understand that Schrodinger spun that yarn with a certain sense of sarcasm, in order to try and illustrate the bizarre implications of QM.

“What did you do to the cat, Erwin? It looks half dead!’ ~ Ms Schrodinger.

People gonna keep thinking quantum observers are people. — fdrake

That is at the very least a disputed issue.

If you reject "objective reality", is there any interpretation other than Many Worlds which is acceptable? — Metaphysician Undercover

I think the Copenhagen interpretation is not compatible with the usual folk notion of 'objective reality'. It denies that there is any fact of the matter about where a particle is between observations.

Presumably quantum phenomena were happening long before there were people.

I haven't read the paper yet, but I did a search for references to it on physicsforums and found nothing, including in more than one recent discussion on the Wigner's Friend thought experiment. So I suspect the experiment doesn't say what the MIT article claims it says. To have a situation where two observers can obtain contradictory measurements, rather than just measurements with differing levels of detail, would be too epoch-making to ignore.

The reports of the experiment are very new, so that could explain the lack of commentary. But it also means the article has not been peer-reviewed. I suggest people wait for that before they start trying to draw metaphysical conclusions from it. Further, the article is so new that nobody has really had time to analyse it fully yet.

Luboš Motl writes that the experiment is not described in enough detail to enable full peer analysis. He also challenges some of the assumptions, and hypothesises that if they were replaced with more standard assumptions, the claimed anomaly would disappear.

From the article:

But Proietti and co’s result suggests that objective reality does not exist. In other words, the experiment suggests that one or more of the assumptions—the idea that there is a reality we can agree on, the idea that we have freedom of choice, or the idea of locality—must be wrong.

So, according to the article, the notion of objective reality has not been unequivocally undermined, as your headline asserts. It might be the notion of freedom of choice or the idea of locality which have been undermined; the article only claims that it must be that one of the three is wrong.

However the point of the article is the claim that what was previously only a thought experiment has now been experimentally realised. — Wayfarer

Right! And this, if confirmed, would a wonderful thing :wink:

However, since all interpretations of QM give (almost) the same predictions, we cannot use that experiment to falsify or verify one in particular.

I have been reading those papers from Bitbol, and this is the interpretation that makes the most sense to me also. A point that Bitbol makes is that there is an ineliminably subjective aspect to knowledge, generally - measurements are always made from a point of view or perspective. But scientific philosophy doesn’t want to acknowledge that, it wants to believe that it’s seeing reality as it is in itself, as Boundless notes. This the conceit that is being exposed by these conundrums. — Wayfarer

Correct. The point of Bitbol is that in the case of Wigner's friend experiment, Wigner's friend sees a definite experimental outcome, whereas according to Wigner there is a superposition (which includes his friend, too).
But the point is that you cannot really make this comparison until Wigner asks to his friend to tell the precise outcome he has seen. The two perspective are different and they are both 'right'.

Each observer has her own perspective. When they communicate the two perspectives are not separate anymore. At this point, we can make a reasonable comparison.

Rovelli's view is similar, but according to him we can define a perspective with all physical systems. In Rovelli's view, communication is replaced by a physical interaction.

In fact, in both cases before the communication (in Bitbol's version of CI) or the physical interaction (in Rovelli's RQM), a comparison is impossible.

But Proietti and co’s result suggests that objective reality does not exist. In other words, the experiment suggests that one or more of the assumptions—the idea that there is a reality we can agree on, the idea that we have freedom of choice, or the idea of locality—must be wrong.

So, according to the article, the notion of objective reality has not been unequivocally undermined, as your headline asserts. It might be the notion of freedom of choice or the idea of locality which have been undermined; the article only claims that it must be that one of the three is wrong. — Janus

Given that, it appears to me that the article says nothing new.

Experiments have already 'confirmed' Bell's Theorem, which says that one of QM, locality and counterfactual definiteness (similar to freedom of choice) must be wrong. Since we haven 't given up on QM, that means CFD or locality must be wrong. And if we accept that, then the Proietti result gives us no reason to accept that objective reality doesn't exist.

Caveat - it's a couple of years since I read Bell's paper, so I may be misremembering.

If that's right, then belief in 'objective reality' goes back to being a metaphysical question - which is where I feel it belongs.

Presumably quantum phenomena were happening long before there were people. — fdrake

This is the issue at the heart of the 'observer problem'. This very issue was why Einstein asked (dismissively) 'Doesn't the moon exist when we're not looking at it?' See Does the Universe Exist if we're Not Looking?

This is the issue at the heart of the 'observer problem'. — Wayfarer

Except 'observation' has been occurring since before humans existed, all a 'measurement' is (AFAIK) is a mapping from a superposition of eigenstates of a quantum operator to a single eigenstate of a quantum operator. In that framework, an observable is just a (Hermitian) linear operator on quantum states. None of these objects are people, and none of them have to occur due to an experimental measurement.

Reality happens outside of the lab too.

Except 'observation' has been occurring since before humans existed — fdrake

But you're not seeing why there is a controversy about this issue. You're simply adopting, or assuming, the perspective of scientific realism, without showing any indication that you understand what exactly about the discoveries of 20th c physics threw this into question.

You will find many titles in the science sections of bookstores with variations on the theme of 'what is real?' Why do you think that might be?

But you're not seeing why there is a controversy about this issue. You're simply adopting, or assuming, the perspective of scientific realism, without showing any indication that you understand what exactly about the discoveries of 20th c physics threw this into question. — Wayfarer

Tell me why people are required for the natural formation of salt (which requires quantum mechanical effects due to the ionic bond). Specifically tell me why a person needs to have something to do with the electron orbiting around the sodium atom in its outer orbital to make it donate that electron to the chlorine atom, filling its outer orbital. Where are the people involved? Why are people needed for the formation of ionic bonds? What role do we play in the formation of ionic bonds in compounds that existed long before the first human?

Tell me why people are required for the natural formation of salt (which requires quantum mechanical effects due to the ionic bond). — fdrake

Do you know why, and in what circumstances, Albert Einstein asked the question 'Does the moon continue to exist if we're not looking at it?'

//discussed in this thread.//

'Does the moon continue to exist if we're not looking at it?' — Wayfarer

Yes, it was an analogy to express suspicion about the physical intuition underlying particles existing in superposition. Einstein saw quantum mechanics as more of a mathematical trick than a physical theory, and was notoriously hostile to it in public. It's actually intended to be a reductio as absurdum to the idea that reality depends upon an observer; achieved by equating the notion of an observer with that of a human. Einstein thought that nature worked deterministically everywhere.

Bohr; who had a more relational view and a higher opinion of the physicality of the wavefunction; eventually was vindicated, and he famously said:

Isolated material particles are abstractions, their properties being definable and observable only through their interaction with other systems.

Note, not just the experimental apparatus humans make, but systems; like the composite system of a sodium atom and a nearby chlorine atom, jointly constraining and driving the location of the outer orbital electron of the sodium atom to join the outer orbital of the chlorine atom, thereby making a compound with properties neither atom had by itself.

Nature is suffused with interaction top to bottom, labs and their measurements are a relatively novel mode of interaction for it.

That's not the way I see it. To me, the underlying philosophical issue is the presumption of mind-independence which underlies scientific realism, the assumption of which is what is behind this proposition:

Presumably quantum phenomena were happening long before there were people. — fdrake

Whereas, that is what is called into question - that's why it is controversial and why it provoked so many impassioned arguments. (Werner Heisenberg recalls being reduced to tears on occasions by the vehemence of the debates surrounding the famous 1927 Solvay conference were many of these issues came to a head.)

What Einstein couldn't accept was any place given to the role of the observer in the determination of the outcome. He demanded that any truly scientific account must be strictly objective, with no reference to the observer - hence, 'mind-independent'. This is what he thought was the criterion for what is scientifically acceptable, and was why he asked the question.

Later, the EPR paradox was intended by him to be the final nail in the coffin for Bohr's interpretation - but as is now known, when it was finally experimentally tested long after Einstein's death it again undermined realism.

People gonna keep thinking quantum observers are people. — fdrake

Instruments take measurements, but only humans are observers, which is why Niels Bohr would say things like 'nothing exists until it is measured'.

I agree that QBism, Copenaghen interpretation (CI), RQM in their own ways reject 'realism'. But how about MWI. In MWI, the only 'truly real thing' is the universal wave-function (UW). The UW never collapses in MWI. It rejects counterfactual definiteness. But the UW is still objective. — boundless

Agree. MWI says there is an objective reality, but it is entirely in superposition, and measurement just entangles the measurer with the measured thing. It does not collapse any wave function. Hence there is no defined state of anything (like dead cat), and hence no counterfactual (or even factual) definiteness.

Instruments take measurements, but only humans are observers, which is why Niels Bohr would say things like 'nothing exists until it is measured'. — Wayfarer

In quantum mechanics the departure from this ideal (of nature as an inert 'objective' substrate - me) 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. — Bohr, Remarks after the Solvay Conference

I think the section you have underlined actually mitigates against your argument, don’t you?

↪fdrake I think the section you have underlined actually mitigates against your argument, don’t you? — Wayfarer

Oh no, I think it makes the point than an observer need not be human quite nicely. The next bolded bit, which leads on from it, extends the logic to every physical process. IE - a physical process can 'observe' another one, and a human doesn't have to mediate between them.

This ties back into my demand for you to describe what necessary role humans play in the formation of the ionic bond in sodium chloride - the only right answer is none at all.

So you think physical processes have subjective features? Do you subscribe to panpsychism?

I don't think that subjective and objective are particularly useful terms. At what point does objective light reflecting from an object become a subjective interpretation of that object's properties? The whole dichotomy has way too much baggage to be useful. It paints a picture of a person standing apart from the world and passively contemplating the 'raw data' of their senses, whereas reality builds emergent structures out of itself within circumscribed developmental environments. The felt roughness of sandpaper does nothing to destroy its grain; we are of the world as much as we are in it.

Indeed - I think that is very much the kind of understanding that has emerged from science in the last several decades. But I think it sits oddly alongside what you said previously.

Notice that Bohr says that ‘the objective world of nineteenth century science’ has become untenable. I think he’s correct in saying that, but isn’t it very much the substance of his disagreements with Einstein?

"This ties back into my demand for you to describe what necessary role humans play in the formation of the ionic bond in sodium chloride - the only right answer is none at all."

On the other hand, from a metaphysical perspective one could argue as philosopher and perceptual researcher Evan Thompson does:

"The problem cannot be ‘How do we go from mind-independent nature to subjectivity and
consciousness?’ because natural objects and properties are not intrinsically identifiable ; they are identifiable only in relation to the ‘conceptual imputations’ of intersubjective experience."

"If we had a complete, canonical, objective, physicalist account of the natural world, including all
the physical facts of the brain and the organism, would it conceptually or logically entail the subjective facts of consciousness? If this account would not entail these facts, then consciousness must be an additional, non-natural property of the world. One problem with this whole way of setting up the issue, however, is that it presupposes we can make sense of the very notion of a single, canonical, physicalist description of the world, which is highly doubtful, and that in arriving (or at any rate
approaching) such a description, we are attaining a viewpoint that does not in any way
presuppose our own cognition and lived experience."

:up:

natural objects and properties are not intrinsically identifiable — Joshs

On account of 'absence of own-being'.

Where in Thomson's writing is that passage from?

It denies that there is any fact of the matter about where a particle is between observations. — andrewk

That seems to be fundamental, there cannot be any fact of the matter about where a particle is between observations. To me that says that the particle, as a particle, is a product of the observation process. Maybe we shouldn't think of the particle as a particle, if it doesn't exist as we would think that a particle exists. Actually, I've heard that from physicists, that they just call it a particle, but it isn't really a particle in any normal sense of the word. That's my in depth understanding of "particle" physics.

What if the theory that needs to be ‘beaten’ is not a theory at all but an untestable metaphysical postulate? Then it might be better to simply ignore it, or proceed as if it says nothing. — Wayfarer

The main postulate of MWI is that the universe is represented by a unitarily evolving quantum state interpreted realistically. It provides empirical predictions that have been thoroughly and successfully tested (so far).

What I think you mean is that one of the theory's predictions can not be tested (that there are many worlds), at least at this point. But that prediction can only be eliminated by changing either the physical theory (as with Bohmian Mechanics, Objective collapse) or by interpreting the quantum state in some other way (as with Copenhagen, RQM, etc.) Neither of those changes are trivial and they have various consequences that need to be considered on their own merits.

No interpretation is a cop out, but MWI cannot have those observers in different world branches since they communicate. Alice knows the polarity and tells Bob that she does. Bob knows that the particle is still in superposition and tells Alice so. That cannot happen if the two are in different branches. — noAxioms

In the experiment, Alice can communicate to Bob that she has measured a definite polarity (without the polarity itself being revealed) while the lab she is in remains isolated (and Bob does not communicate back, which would presumably constitute a measurement entangling him with Alice). So there are actually three MWI branches here. One where Alice measures a horizontal polarization, one where she measures a vertical polarization, and one that is the superposition of those two branches where Bob detects interference (and knows that Alice has made a measurement).