Suppose, instead, that there is a measurement at the slits
— Andrew M

Does this measurement physically affect a photon on its way to the far screen? — jgill

Generally, yes. One exception is where a detector is placed at only one slit. The detector will not interact with the photons going through the other slit. See also interaction-free measurements.

It's also possible to use polarizers to remove and/or restore the interference pattern:

For single photons, the double-slit interference pattern can be made to disappear by using a marker.
...
The which-path marker consists of two, mutually perpendicular, polarizing filters.
...
When either the vertical or the horizontal filter covers both slits, the double-slit interference pattern is preserved, albeit at a reduced intensity compared to no filter. When the vertical filter covers one slit and the horizontal filter covers the other, the double-slit pattern disappears completely. Two superimposed single-slit patterns are all that remain. This new arrangement changes the setup into a which-path experiment in the sense that it is now (in principle) possible to know which slit the photon passed through; this destroys the quantum interference.

Introducing a third polarizing filter, the quantum eraser, between the marker and the detector thwarts the which-path experiment if it is oriented 45° with respect to the marker filters. Every photon reaching the detector is now polarized in the direction of the third polarizer and it is no longer possible to know which slit each photon passes through; as a result, the interference phenomenon is restored. — Young's double-slit experiment with single photons and quantum eraser - Rueckner, Peidle

The wave function in quantum mechanics evolves deterministically according to the Schrödinger equation as a linear superposition of different states. However, actual measurements always find the physical system in a definite state. Any future evolution of the wave function is based on the state the system was discovered to be in when the measurement was made, meaning that the measurement "did something" to the system that is not obviously a consequence of Schrödinger evolution
— ”Wiki: Measurement problem

...

There are an infinite number of solutions depending upon that constant (measurement) - a superposition. Then the measurement takes place and a "collapse" occurs giving a particular solution. Did the measurement "do something" to the system, or does one simply experiment to find the appropriate value of the constant? Where is the magic? — jgill

To apply your example, consider the double-slit experiment. When there is no measurement at either slit, the originally emitted particles build up an interference pattern on the back screen. That pattern can be described by a linear superposition of each particle going through the left slit plus each particle going through the right slit, i.e.:

$|\psi\rangle = \frac{1}{\sqrt{2}}(|left\rangle + |right\rangle)$

Suppose, instead, that there is a measurement at the slits. The originally emitted (and measured) particles no longer build up an interference pattern. The particles measured at each slit subsequently end up on the back screen as described by the $|left\rangle$ and the $|right\rangle$ terms independently, and not as described by $|\psi\rangle$.

That outcome is not obviously a consequence of (continuous) Schrödinger evolution. So it seems that the measurements "did something" to the system.

Compare to an ocean wave that might be described as a linear superposition of simpler waves. In that case, observing or measuring the wave doesn't affect the system.

Lots of interesting comments there. I'll focus on the Born rule for the moment.

https://mateusaraujo.info/2021/03/12/why-i-am-unhappy-about-all-derivations-of-the-born-rule-including-mine/

I don't have these same concerns, but I think it is important than many proponents of MWI do list similar concerns about other theories in quantum foundations. — Count Timothy von Icarus

Yes, fair point.

MWI aside, the puzzle is whether the observed probabilities can be derived without positing the Born rule. The basic approach (borrowing from Carroll and Sebens, Zurek, and others) is to use two alternative rules.

1. The indifference rule: If the amplitudes for the terms in a quantum state are equal, assign the same probability to each term such that they sum to 1.
2. The reduction rule: If the amplitudes are unequal, reduce the state to terms with equal amplitudes. Then apply the indifference rule.

Now those rules may be thought to be equivalent to positing the Born rule. However they aren't mysterious in the way that the Born rule seems to be.

For example, consider a qubit in the state:

$|\psi\rangle = \sqrt{\frac{1}{3}}|0\rangle + \sqrt{\frac{2}{3}}|1\rangle$

Since the amplitudes are unequal, the reduction rule is applied. The reduced state is:

$|\psi_r\rangle = \sqrt{\frac{1}{3}}|0\rangle + \sqrt{\frac{1}{2}}(\sqrt{\frac{2}{3}}|1_a\rangle + \sqrt{\frac{2}{3}}|1_b\rangle) = \sqrt{\frac{1}{3}}|0\rangle + \sqrt{\frac{1}{3}}|1_a\rangle + \sqrt{\frac{1}{3}}|1_b\rangle$

Since the amplitudes are now equal, the indifference rule is applied. There are three states, so the probability for each state is 1/3 which gives 1/3 for the $|0\rangle$ state and a total of 2/3 for the reduced $|1\rangle$ states, which matches the Born rule when applied to $|\psi\rangle$.

That's an operational derivation that bypasses issues about uncertainty, what probabilities "really are", and any reference to MWI.

If the physics in question is reversible, why do we posit a splitting universe instead of a merging one, aside from the fact that having it split in both directions (forwards and backwards in time) is incoherent?

Perhaps whenever we make a measurement we merge universes, such that we progress by such merges to one of many potential end points, final conditions, of the universe, assuming ad hoc that it has an end? This might work, but it blows up the rational-agent based derivations of the Born Rule. Rational agent models are not reversible, we don't say, "given what I observe now, what must have happened in the future, what endpoint must I be most likely to be converging on?" — Count Timothy von Icarus

As it happens, David Deutsch describes both splitting and merging in his version of the Wigner's friend thought experiment (for which reversibility is relevant). See an earlier thread here.

The principle is really just an extrapolation from what is observed on the microscopic scale. Cast in quantum computing terms, qubits and quantum logic gates are reversible. Applying a Hadamard gate to a computational basis state splits it into an equal superposition state, and applying a Hadamard gate again merges the superposition back to the original state. So the question is whether that process scales up to the everyday, macroscopic level. And, if so, whether or how that should affect our reasoning.

I only skimmed this thread, but has the Born Rule problem really not come up?

To bring up the example before, it is like someone's spouse is either in spot A with probability 30% or spot B with probability 70%, except, get this, she is also in BOTH spots with probability 100%. Explaining that satisfactorily is going to be a doozy, and I don't think Dutch Book arguments really solve the problem. — Count Timothy von Icarus

See the brief discussion earlier in the thread (with a link to Sean Carroll's solution).

In the end, a few premises are needed though, and I explicitly list that one (that humans are not special, and reality doesn’t supervene on my experience). I even attempt a logical demonstration of it, but I don’t think it constitutes a proof. — noAxioms

Per "reality doesn’t supervene on my experience", it seems to me that that is how me ordinarily use language. It works well, while other uses are problematic.

It is me that nobody seems to get that? I’m not saying that the alternative (that noumena supervenes on human phenomena or human language) is necessarily wrong, but that such a stance utterly destroys any hope of acquisition of knowledge — noAxioms

I'm not saying anything like that. It seems to me obvious that without ordinary, everyday human experience we wouldn't be talking about trees falling in the forest, or quantum theory, or anything else. I'm not therefore saying that trees somehow depend on human experience. Obviously they don't.

I think you meant the John Bell quote.
Thx, fixed that. One 4-letter B-word is the same as another, no? — noAxioms

In this case ... no. ;-)

I want to know more on the subject of what Aristotle meant by this. The eternal unchanging unmoved mover. — invicta

As in illustration of an unmoved mover, consider how a cat is attracted to a saucer of milk. The milk does not move but it does move the cat.

Hopefully someone with links will come along…I don’t know where to start with him. — invicta

An excellent resource is "Ancient Philosophy: Aristotle and His Successors" by Susan Sauvé Meyer.
https://www.coursera.org/learn/aristotle

In particular, see these links (videos and transcripts):
https://www.coursera.org/lecture/aristotle/the-first-mover-of-the-cosmos-RO3zk
https://www.coursera.org/lecture/aristotle/the-unmoved-mover-qpDGr

My point was that this abductive construction isn’t in any way something unique to quantum theory. That’s not what make it different, and it certainly doesn’t indicate that physical processes require the presence of humans. Sure, the human knowledge of physics requires humans, but that knowledge isn’t necessary for trees to fall in the forest when nobody is around. — noAxioms

I agree. Trees have a separate existence and evolution to us. Nonetheless the idea of trees falling only has meaning in the context of human experience. We can point to a tree and to something falling and say that this is what is meant. That relational connection grounds our language.

Please see the Bohr quote in Andrew M’s post a few back — noAxioms

I think you meant the John Bell quote.

Nonetheless the observer - or, even better, agent or person - closes the loop in the sense that it is human experience that grounds quantum theory and the quantities that can be measured.
— Andrew M

That also seems true of say Newtonian theory. — noAxioms

Yes. Though, of course, we've found through observation that Newtonian theory is incorrect. Also, the "action-at-a-distance" aspect was suspect to Newton from the outset.

Is there some way in which human agency or observation makes a difference in (grounds) QT in a way that it doesn’t in NT? That would be a pretty incredible claim, that physics (and not just human theories/knowledge of physics) is different in the presence of humans than it is in a universe absent them. — noAxioms

No, my point is that quantum theory is constructed abductively from what we observe. As far as we know, it applies universally.

The two TVs represent the two measurements. The reality is the soccer match. Obviously, the images on the two TVs have to correlate as they represent two views of a single reality. I think the point he is making is that there's a deeper reality than the physical world and therefore it's no surprise if two measurements correspond. — Art48

Yes, that's the intention. However it's not surprising that the TV images correlate. Whereas quantum entanglement is surprising.

Isn't it the case that we know they do not have predefined values (unless we accept the pilot wave, Bohmian Mechanics interpretation)? — Art48

Yes, that's correct.

Suppose we have two spinning coins, separated by light years. Suppose if Alice causes her coin to stop spinning (analogous to doing a measurement) and it lands heads, that Bob causes his coin to stop spinning and it lands tails. Suppose Bob's coin always lands on the reverse side as Alice's coin. This is my metaphor for quantum entanglement as I understand it. Comments? — Art48

That specific example can be explained by a hidden variable theory (i.e., with unknown predefined values). When the two (quantum) coins were prepared together, one was configured to land heads, the other tails. Alice and Bob didn't know which way the coin they received would land until they measured it. But when they did, the hidden variables were revealed.

However to see why hidden variables are ultimately unsatisfactory, consider the following classical game (called the CHSH game):

Alice and Bob are in separate rooms with no way to communicate with each other. A referee in each room randomly flips a coin which the players see. Each player responds by saying "red" or "blue".

For Alice and Bob to win the game, their responses have to be the same if and only if at least one of the coins show heads. They can agree on a strategy beforehand.

The game is repeated many times. What classical strategy maximizes their probability for winning and what is that probability? Have a go at answering it, then check below.



Now let's try a specific quantum strategy. For each game, Alice and Bob share a single entangled particle pair. To match the responses above, I'll call the two possible states "red" and "blue" (in this case, Alice and Bob would both report the same color if they measured their particles at the same angle).

Alice and Bob agree beforehand to each measure their particle at an angle that depends on the coin flips. On heads, Alice's measurement angle will be 0°, on tails 45°. On heads, Bob's measurement angle will be 22.5°, on tails -22.5°. They each measure their particle and report the color result.

In three of the cases, at least one of the coins will be heads and the difference in Alice and Bob's measurement angles will be 22.5°. QM predicts that they will measure the same result cos²(22.5°) ≈ 85% of the time. In the double-tails case, the difference in Alice and Bob's measurement angles will be 67.5°. QM predicts that they will measure a different result sin²(67.5°) ≈ 85% of the time. Thus Alice and Bob win the game on average 85% of the time.

So the quantum strategy is better than the best possible classical strategy, violating the Bell inequality. That's Bell's Theorem (one of many variants). So at least one of the classical assumptions is incorrect (hidden variables, locality, or statistical independence).

Even looking at the measurement dials has no impact on the collapse or not. Of course, the descriptions you quote give a special role to an observer (writing down the reading of the dial say), but 1, it doesn’t take a human (or actual ‘observation’) to do that, and 2, it being written down isn’t what causes collapse. If the dial says |here>, then the wave function is collapsed whether or not anything (or person) reads that dial since the dial is not inside Walmart. — noAxioms

Yes, decoherence doesn't depend on whether anyone looks or not.

Nonetheless the observer - or, even better, agent or person - closes the loop in the sense that it is human experience that grounds quantum theory and the quantities that can be measured.

A further point is that you won't ever find a contradiction when comparing what has been observed. It's the assumptions about what hasn't been observed that can lead to trouble as Bell's Theorem shows. Sometimes it's not even clear what should count as a measurement. For example, consider this passage from Sidney Coleman's lecture:

Neville Mott worried way back in 1929 about cloud chambers. He said: “Look, an atom releases an ionizing particle at the center of a cloud chamber in an s-wave. And it makes a straight line track.

Why should it make a straight line track? If I think about an s-wave, it is spherically symmetric. Why do they not get some spherically symmetric random distribution of sprinkles? Why should the track be a straight line? — Quantum Mechanics in Your Face - Sidney Coleman

One possible answer is that the cloud chamber is making a rapid series of measurements that constrain the particle to a straight line. Mott's answer (now the generally accepted answer) is that the cloud chamber is also a quantum system which evolves together with the particle and the potential paths not on the straight line cancel out. So nothing clearly qualifies as a measurement here.

Having imbibed a bit too much at the local pub he enters a state of superposition, thoroughly confused, an unknowing victim of a partial differential equation. — jgill

You might enjoy this excerpt from Coleman's lecture:

Now I will give an argument due to David Albert[21] with respect to Zurek’s question. Zurek asked: “Why do I always have the perception that I have observed a definite outcome?” To answer this question, no cheating: we can’t assume Zurek is some vitalistic spirit loaded with élan vital unobeying the laws of quantum mechanics. We have to say the observer—well I don’t want to make it Zurek, that would be using him without his permission, I’ll make it me, Sidney—has some Hilbert space of states, and some condition in Sidney’s consciousness corresponds to the perception that he has observed a definite outcome, so there is some projection operator on it, the definiteness operator. If you want, we could give it an operational definition: the state where the definiteness operator is +1 is one where a hypothetical polite interrogator asks Sidney: “Have you observed a definite outcome?”, and he says: “Yes”. In the orthogonal states he would say: “No, gee, I was looking someplace else when that sign flashed” or “I forgot” or “Don’t bother me, man, I’m stoned out of my mind” or, you know, any of those things. — Quantum Mechanics in Your Face - Sidney Coleman

In Coleman's account, these are all potential paths that can interfere.

The entire quantum subject would be better served if "observer" were eliminated everywhere and replaced by "measurement". — jgill

I think it's useful since we are observers. At any rate, since our understanding of quantum mechanics is incomplete, there's going to be differences of opinion on the best way to talk about it.

This is true even for experts in the field. For example, here's an argument I quoted earlier 'for observation'. And, in the other direction, here's John Bell's argument 'against measurement':

Here are some words which, however legitimate and necessary in application, have no place in a formulation with any pretension to physical precision: system, apparatus, environment, microscopic, macroscopic, reversible, irreversible, observable, information, measurement.

The concepts 'system', 'apparatus', 'environment', immediately imply an artificial division of the world, and an intention to neglect, or take only schematic account of, the interaction across the split. The notions of 'microscopic' and 'macroscopic' defy precise definition. So also do the notions of 'reversible' and 'irreversible'. Einstein said that it is theory which decides what is 'observable'. I think he was right - 'observation' is a complicated and theory-laden business. Then that notion should not appear in the formulation of fundamental theory. Information? Whose information? Information about what?

On this list of bad words from good books, the worst of all is 'measurement'. It must have a section to itself.

Against 'measurement'
...
The first charge against 'measurement', in the fundamental axioms of quantum mechanics, is that it anchors there the shifty split of the world into 'system' and 'apparatus'. A second charge is that the word comes loaded with meaning from everyday life, meaning which is entirely inappropriate in the quantum context. When it is said that something is 'measured' it is difficult not to think of the result as referring to some pre-existing property of the object in question.
...
In other contexts, physicists have been able to take words from everyday language and use them as technical terms with no great harm done. Take for example, the 'strangeness', 'charm', and 'beauty' of elementary particle physics.

No one is taken in by this 'baby talk', as Bruno Touschek called it. Would that it were so with 'measurement'. But in fact the word has had such a damaging effect on the discussion, that I think it should now be banned altogether in quantum mechanics.

The role of experiment
Even in a low-brow practical account, I think it would be good to replace the word 'measurement', in the formulation, by the word 'experiment'. For the latter word is altogether less misleading. However, the idea that quantum mechanics, our most fundamental physical theory, is exclusively even about the results of experiments would remain disappointing.

In the beginning natural philosophers tried to understand the world around them. Trying to do that they hit upon the great idea of contriving artificially simple situations in which the number of factors involved is reduced to a minimum. Divide and conquer. Experimental science was born. But experiment is a tool. The aim remains: to understand the world. To restrict quantum mechanics to be exclusively about piddling laboratory operations is to betray the great enterprise. A serious formulation will not exclude the big world outside the laboratory. — Against ‘measurement’ - John Bell, 1990

Then the casual physics dilettante could think, Yes, if I aim my flashlight at my keys on the table I don't disturb the keys, but if the keys were quantum size I might disturb them in the act of "observing" them. Just a comment. — jgill

If you do lose your quantum car keys, you can just call your local quantum mechanic.

The soccer game metaphor is presented from 5:00 to about 16:37. Imagine a soccer game as reality, as the thing in itself. The physical world is likened to seeing the game on TV, on two different TV channels using two different TV monitors. Each TV channel uses its own camera so the images on the two TV monitors correspond but are not identical. — Art48

Just to comment on the quantum mechanics:

The card shuffling idea is incorrect. I don't know where they get that from. It's necessary to keep track of each pair of entangled particles. It's only the particle pairs that exhibit the correlation when measured.

Also that the measurements perfectly correlate is not the puzzle. Since the entangled particles were prepared together, they could easily have had predefined values. No big deal.

The segment with the pink and green cubes also doesn't get at why anyone should be puzzled.

Throw in the notion that one's looking causes immaterial non-local influences, and I would expect the average person to be more confused after watching that film than before.

Also, the metaphor of the soccer match and TV's also doesn't end up addressing entanglement. There's really a soccer match. And the TV's (also real!) receive a local signal. No mystery there.

Regarding non-locality, most physicists reject it because it doesn't play nicely with the theory of relativity.

Per QM, the system could either be weakly measured (giving some information without destroying the superposition) or strongly measured resulting in rapid decoherence. Alternatively, the system could be transformed such that the probabilities change (including to certainty).
— Andrew M

Can you elaborate these three, Andrew? What would be the act of a weak measurement? And, how could the probabilities involved with a specific system be changed to certainty without some form of measurement? To me, such a change would require a cause, and the cause would be a matter of "fixing" the system, like cheating if you're a gambler. But if "fixing" was possible then there would be no real mystery unless only the cheaters had figured it out. — Metaphysician Undercover

Note that Wigner's friend is a thought experiment that is not practical (to say the least) with humans, but may one day be done using human-level AI's on a quantum computer.

With that said, suppose the friend's superposition state is:

|psi> = 0.6|here> + 0.8|there>

This means that there's a 36% chance of Wigner measuring the friend to be here as opposed to there (square the numbers to get the percentages).

To visualize the math, consider an XY axis where the |here> and |there> states are arrows (unit vectors) on the positive X and Y axes respectively and |psi> is an arrow from the origin to coordinates (0.6, 0.8).

If Wigner wants to test whether the friend really is in that superposition state, he can measure in the { |psi>, |-psi> } basis (where |-psi> is orthogonal to |psi>) and confirm that the result is |psi> (i.e., he sees +1 on his measurement dial rather than -1). Wigner can do this test multiple times and will always get the same result. This interaction is a weak measurement in the sense that information is extracted while leaving the friend in the initial superposition state.

Wigner can then measure her state in the { |here>, |there> } basis. This collapses |psi> to either |here> or |there>.

Alternatively, Wigner can rotate the friend's state onto one of the definite states, say, |here>. Wigner can then measure in the { |here>, |there> } basis which will confirm that she is here.

Allowing that causes of change which come from the inside are very real, and distinct from causes of change which come from the outside, forces the conclusion that systems theory does not provide an adequate representation. — Metaphysician Undercover

There's no problem with the friend acting inside the lab (e.g., moving here or there). However the subsequent measurement and/or rotation operations would be performed by Wigner from outside the lab.

Until Walmart opened its otherwise impervious doors. Contrary to most of the posts you’re getting, it has nothing to do with anybody actually looking at anything, with good or bad eyes. — noAxioms

Except maybe the measurement dials!

That’s just a different ordering, but any ordering can have a counting number assigned to each item in order. It’s still ordered. — noAxioms

:up:

A Heisenberg cut is a form of relational expression, that a system on one side of the cut is in some state (as represented by the wave function) relative to the system on the other side of the cut. Yes, the placement of the cut is arbitrary. The cut was first introduced as an epistemic cut (what one system knows about the other) but became a metaphysical one once the interpretation moved away from its epistemic roots. — noAxioms

:up:

The proper analogy would be that jgill observed interference effects until he and his wife met up and she pointed out that she had been standing there all the time.
— Andrew M

So we say that jgill has extremely bad eyes, and all he sees until he's about three metres from his wife is a strange interference pattern? Suppose he's 50 metres away. How would he interpret the interference pattern as probabilities for the actual location of his wife? Consider that if this is a true analogy, the closer that he gets, he ought to be able to observe changes to the interference pattern which would increase his certainty. — Metaphysician Undercover

It's the Wigner's friend thought experiment where the system in question (in this case Walmart rather than a laboratory) is isolated from the rest of the environment.

Per QM, the system could either be weakly measured (giving some information without destroying the superposition) or strongly measured resulting in rapid decoherence. Alternatively, the system could be transformed such that the probabilities change (including to certainty).

Wrong, but interesting. :smile: — jgill

Interesting, yes. See Wigner's friend.

"(any enumeration, not just an ordered one)"

How can any enumeration not be ordered? — noAxioms

I meant, "not just in order of smallest number to largest number."

If collapse isn’t physical and isn’t epistemological, then what is it? — noAxioms

Pragmatic. At some point it's necessary to ground a quantum mechanical description in a definite observation or measurement. So a Heisenberg cut is employed. As Heisenberg put it:

"The dividing line between the system to be observed and the measuring apparatus is immediately defined by the nature of the problem but it obviously signifies no discontinuity of the physical process. For this reason there must, within limits, exist complete freedom in choosing the position of the dividing line." — Heisenberg cut - Wikipedia

Neo-Copenhagen interpretations such as RQM, QBism and Consistent histories also employ this in their respective ways. All of these interpretations conform to the predictions of standard QM, including for Wigner's friend-style experiments. Whereas objective collapse theories are, in principle, experimentally differentiable from standard QM.

Very clever experiments but I did notice

"As to how the day-to-day reality of objects that we observe, such as furniture and fruit, emerges from such a different and exotic quantum world, that remains a mystery."
— Macro-Weirdness: Quantum Microphone Puts Naked-Eye Object in 2 Places at Once — Wayfarer

OK. But as physicist Sidney Coleman says (via Peter Woit):

The problem is not the interpretation of quantum mechanics. That’s getting things just backwards. The problem is the interpretation of classical mechanics. — Quantum Mechanics in Your Face - Sidney Coleman (transcript and slides)

Which is to say, we view quantum mechanics through the lens of our classical concepts and intuitions. Coleman notes that, "The thing you want to do is not to interpret the new theory in terms of the old, but the old theory in terms of the new."

Now people say the reduction of the wave packet occurs because it looks like the reduction of the wave packet occurs, and that is indeed true. What I’m asking you in the second main part of this lecture is to consider seriously what it would look like if it were the other way around—if all that ever happened was causal evolution according to quantum mechanics. What I have tried to convince you is that what it looks like is ordinary everyday life. — Quantum Mechanics in Your Face - Sidney Coleman (transcript and slides)

To this, Peter Woit comments:

While some might take this and claim Coleman as an Everettian, note that there’s zero mention anywhere of many-worlds. Likely he found that an empty idea that explains nothing, so not worth mentioning. — Peter Woit

In the lecture, Coleman says, "The position I am going to advocate is associated with Hugh Everett in a classic paper" (referring to Everett's paper, '"Relative State" Formulation of Quantum Mechanics').

Then, from the Q&A:

Question: You said in passing that you were a follower of Everett.

Coleman: Yeah, but that's a tricky thing to say. That's like saying you're a Christian. I mean, Everett wrote this one truly wonderful paper and then everyone got on their horse and rode off in all directions. The position I'm advocating is a position that, at least in my case, was certainly largely inspired by Everett's paper. Whether it's really Everett's position or not I would prefer not to discuss. — Quantum Mechanics in Your Face - Sidney Coleman (video)

Which all leads to:

It's not a valid analogy, though. The strangeness of the observer problem in physics is that the act of observation itself is instrumental in determination of the outcome. The proper analogy would be that, prior to you seeing your wife, she didn't exist in any specific location at all, she's not simply in an unknown location. — Wayfarer

The proper analogy would be that @jgill observed interference effects until he and his wife met up and she pointed out that she had been standing there all the time.

Show me a macroscopic entity existing in superposition. — Wayfarer

A piezoelectric "tuning fork".

A piezoelectric "tuning fork" has been constructed, which can be placed into a superposition of vibrating and non-vibrating states. The resonator comprises about 10 trillion atoms. — Quantum superposition - Wikipedia

Researchers have demonstrated a device that can pick up single quanta of mechanical vibration similar to those that shake molecules during chemical reactions, and have shown that the device itself, which is the width of a hair, acts as if it exists in two places at once—a "quantum weirdness" feat that so far had only been observed at the scale of molecules.

"This is a milestone," says Wojciech Zurek, a theorist at the Los Alamos National Laboratory in New Mexico. "It confirms what many of us believe, but some continue to resist—that our universe is 'quantum to the core'." — Macro-Weirdness: 'Quantum Microphone' Puts Naked-Eye Object in 2 Places at Once

As I thought had been established, the interference pattern in the double-slit experiments is independent of time and space (shown by its rate independence), thus indicating an extraspatiotemporal cause. — Wayfarer

No, I suggested that idea was a deux ex machina.

Everything follows quantum principles, whether acorns, water waves or particles.

This new ontological picture requires that we expand our concept of ‘what is real’ to include an extraspatiotemporal domain of quantum possibility,” write Ruth Kastner, Stuart Kauffman and Michael Epperson.

Considering potential things to be real is not exactly a new idea, as it was a central aspect of the philosophy of Aristotle, 24 centuries ago. An acorn has the potential to become a tree; a tree has the potential to become a wooden table. — Quantum mysteries dissolve if possibilities are realities - Tom Siegfried

Yes, an acorn has the potential to become a tree, as Aristotle would have correctly noted. That simply means that an acorn has the capability or possibility to develop into a tree under certain conditions.

That shouldn't suggest an "extraspatiotemporal" limbo world where tree potentialities exist and evolve until they are actualized as trees in our "spatiotemporal" world. Instead the world we inhabit just is where an acorn develops into a tree.

Similarly, under certain conditions, a series of particles in a dual-slit experiment will form an interference pattern. That is a true statement about the world that we can observe for ourselves.

The positing of an "extraspatiotemporal" domain as against a "spatiotemporal" domain is a variation of The Ghost in the Machine metaphor.

We don't live in a classical world supplemented by an extraclassical domain of existents, possibilities and processes. We live in a quantum world.

Yes I think I see what you're getting at.

So whether they're discharged one electron at a time, or at a faster (or is that 'higher'?) rate, then you still get the same pattern.

Yes, the pattern will be the same in both cases.

The fact that the effect can't be replicated by a physical (water) wave is, I think, due to the interference pattern not actually being 'waves' as such, but something for which the interference patterns of waves is just an analogy.

I think it's deeper than just an analogy. The wave dynamics are central to the math.

The argument that started this was about whether this means that time (being 'rate') is not a factor; which also that means that space (i.e. proximity of particles) is not a factor (as proximity is an aspect of space-time.) So, what is causing the interference pattern is outside, or not a function of, space-time.

Deux ex machina?

Cantor's proof (by contradiction) shows that the set of real numbers is uncountable and thus can't be enumerated. Since the set of real numbers can't be enumerated, the diagonalized number therefore can't be computed.
— Andrew M

But that number (from Cantor’s proof) is generated from a countable list of rationals, not an uncountable list of reals. So it can be computed. It doesn’t require the ordering of the reals. That was my point,. — noAxioms

Cantor's proof assumes an enumeration of the set of real numbers (any enumeration, not just an ordered one), and then proves that there can't be one. So it's therefore not possible to compute the diagonal for the set of real numbers. The specific diagonal that is computed as part of the construction is necessarily for a different set of numbers (say, the countable list of rationals).

If it were the diagonal for the set of real numbers, then it would appear somewhere on the list, say, the i^th index. But then to calculate the number at that index to the i^th digit would result in an infinite loop.

Copenhagen-style interpretations also generally deny a physical collapse. So, in that sense, Copenhagen and Everett/MWI agree (and disagree with physical collapse theories such as GRW).
— Andrew M

I am not really clear on what a formal statement of metaphysical Copenhagen interpretation would say. I’m more familiar of its roots as an epistemological interpretation where collapse (of what is known) very much does occur, but it is just a change in what is known about a system, not a physical change. They’ve since created a not-particularly well defined metaphysical interpretation under the same name, and if it doesn’t suggest physical collapse, I’d accept that statement. — noAxioms

:up:

Some quotes to that effect here.

Just refresh my memory about what Ryle said was the correct view of the matter, if this is the incorrect view? — Wayfarer

In the Introduction, Ryle says:

This book offers what may with reservations be described as a theory of the mind. But it does not give new information about minds. We possess already a wealth of information about minds, information which is neither derived from, nor upset by, the arguments of philosophers. The philosophical arguments which constitute this book are intended not to increase what we know about minds, but to rectify the logical geography of the knowledge which we already possess. — The Concept of Mind - Gilbert Ryle

While Ryle's anti-Cartesian goal is obvious, his theory cuts across both dualist and behaviorist theories. For example, to act intelligently doesn't necessitate internal processes (per dualism), but doesn't preclude them either (per behaviorism). Activities such as thinking and imagining, while private in a conventional sense are not private in the radical Cartesian sense. We can give voice to our thoughts, identify the motives and intentions of others, and so on.

So for Ryle, the dogma that we're ghosts in machines is in error. But so is the symbiotic dogma that we're machines. That is, to reflexively reallocate mind terms to the brain (or eliminate them altogether) is just to veer from Scylla to Charybdis. As Ryle notes in a later essay:

There have always existed in the breasts of philosophers, including our own breasts, two conflicting tempers. I nickname them the "Reductionist" and the "Duplicationist" tempers, or the "Deflationary" and the "Inflationary" tempers. The slogan of the first temper is "Nothing But ..."; that of the other "Something Else as Well ..." — Thinking and Saying - Gilbert Ryle

Ryle's positive goal is to direct our attention to the contexts that our language and activities arise in. As he says:

Descartes left as one of his main philosophical legacies a myth which continues to distort the continental geography of the subject.

A myth is, of course, not a fairy story. It is the presentation of facts belonging to one category in the idioms appropriate to another. To explode a myth is accordingly not to deny the facts but to re-allocate them. And this is what I am trying to do.

To determine the logical geography of concepts is to reveal the logic of the propositions in which they are wielded, that is to say, to show with what other propositions they are consistent and inconsistent, what propositions follow from them and from what propositions they follow. The logical type or category to which a concept belongs is the set of ways in which it is logically legitimate to operate with it. The key arguments employed in this book are therefore intended to show why certain sorts of operations with the concepts of mental powers and processes are breaches of logical rules. — The Concept of Mind - Gilbert Ryle

There are the ducks, and there is the row. When you have seen the ducks, you have seen the row. But there are not four things. Yet the row is no ghost. — unenlightened

Nice example. There can be three ducks in a row, in a pond, in danger. All recognizable, yet categorically different, ways for things to be.

That is, the university is something we can see by virtue of being creatures with minds.
— Andrew M

I think Ryle, (and certainly I,) would prefer to say that the university is something that we do together; if the building is lost, and the library burns, we can meet under a tree for a tutorial on whatever we can remember of the course. — unenlightened

Ryle says "the University has been seen". But yes, if the university didn't have buildings, then one would say something different, as you have done.

Ryle points out that the foreigner's puzzle arose from his inability to understand how to use the concept of 'the University'
— Andrew M

But Ryle is creating a straw man because no one thinks like that. — Andrew4Handel

Ryle's purpose there was to illustrate what is meant by the phrase "category mistake". Not to argue that people make that mistake with respect to universities.

In the UK we have The Open University where you study from home.

I think most people understand that a University is more than just a collection of buildings and that it is not just one building but a learning institution with a wide reach. — Andrew4Handel

Of course.

It is not synonymous with the problem of squaring mental states with brain states and physicality with non physicality. — Andrew4Handel

Well, that's the issue at hand. As you note, we are unlikely to be confused about universities. But, as Ryle says:

There is a doctrine about the nature and place of minds which is so prevalent among theorists and even among laymen that it deserves to be described as the official theory. — The Concept of Mind - Gilbert Ryle

After giving an outline, he goes on to say:

Such in outline is the official theory. I shall often speak of it, with deliberate abusiveness, as ‘the dogma of the Ghost in the Machine’. I hope to prove that it is entirely false, and false not in detail but in principle. It is not merely an assemblage of particular mistakes. It is one big mistake and a mistake of a special kind. It is, namely, a category-mistake. — The Concept of Mind - Gilbert Ryle

It is likely that many on the forum have read Ryle's work, so to what extent does his critique throw important light on the mind and body connection? — Jack Cummins

Ryle's critique reframes the mind/body debate. As you may know, Ryle was part of the ordinary language philosophy movement which included other luminaries such as Wittgenstein and Austin. As Ryle points out in his introduction:

This book offers what may with reservations be described as a theory of the mind. But it does not give new information about minds. We possess already a wealth of information about minds, information which is neither derived from, nor upset by, the arguments of philosophers. The philosophical arguments which constitute this book are intended not to increase what we know about minds, but to rectify the logical geography of the knowledge which we already possess. — The Concept of Mind - Gilbert Ryle

To give a sense of what this might entail, here is one of Ryle's examples of a category mistake:

A foreigner visiting Oxford or Cambridge for the first time is shown a number of colleges, libraries, playing fields, museums, scientific departments and administrative offices. He then asks ‘But where is the University? I have seen where the members of the Colleges live, where the Registrar works, where the scientists experiment and the rest. But I have not yet seen the University in which reside and work the members of your University.’ It has then to be explained to him that the University is not another collateral institution, some ulterior counterpart to the colleges, laboratories and offices which he has seen. The University is just the way in which all that he has already seen is organized. When they are seen and when their co-ordination is understood, the University has been seen. His mistake lay in his innocent assumption that it was correct to speak of Christ Church, the Bodleian Library, the Ashmolean Museum and the University, to speak, that is, as if ‘the University’ stood for an extra member of the class of which these other units are members. He was mistakenly allocating the University to the same category as that to which the other institutions belong. — The Concept of Mind - Gilbert Ryle

Ryle points out that the foreigner's puzzle arose from his inability to understand how to use the concept of 'the University'. Note that there are at least two different ways the foreigner may have gone wrong here and, together, they demonstrate the hold that the 'Ghost in the Machine' metaphor can have on us. If the foreigner can't see the university, then perhaps it is something unseen - the ghost. That doesn't seem right, so perhaps it is reducible to the buildings he does see - the machine. So the foreigner may exorcise the ghost, leaving just the machine. But when that turns out to be unsatisfactory, the ghost soon returns to haunt the machine. And so the puzzle remains unresolved.

The solution is to recognize that these are false alternatives. Instead the university is the way in which all that the foreigner has already seen is organized. That is, the university is something we can see by virtue of being creatures with minds.

What do you think this means, to assume numbers which cannot be counted nor computed? — Metaphysician Undercover

Cantor's diagonal argument assumes that the set of real numbers are countable and then shows that that assumption leads to a contradiction.

Since axioms are produced by mathematicians who practise pure mathematics, and those people who apply mathematics have a choice as to which axioms are used, it would appear like we ought not use axioms like these, which necessitate that aspects of reality will be unintelligible to us. Instead, we ought to look for axioms which would render all of reality as intelligible. — Metaphysician Undercover

You may find that with computable numbers (which are countable):

The computable numbers include the specific real numbers which appear in practice, including all real algebraic numbers, as well as e, π, and many other transcendental numbers. Though the computable reals exhaust those reals we can calculate or approximate, the assumption that all reals are computable leads to substantially different conclusions about the real numbers. The question naturally arises of whether it is possible to dispose of the full set of reals and use computable numbers for all of mathematics. This idea is appealing from a constructivist point of view, and has been pursued by what Errett Bishop and Fred Richman call the Russian school of constructive mathematics. — Computable numbers - Use in place of the reals - Wikipedia

Also from physicist Max Tegmark:

I was seduced by infinity at an early age. Georg Cantor’s diagonality proof that some infinities are bigger than others mesmerized me, and his infinite hierarchy of infinities blew my mind. The assumption that something truly infinite exists in nature underlies every physics course I’ve ever taught at MIT — and, indeed, all of modern physics. But it’s an untested assumption, which begs the question: Is it actually true?

...

Yet real numbers, with their infinitely many decimals, have infested almost every nook and cranny of physics, from the strengths of electromagnetic fields to the wave functions of quantum mechanics. We describe even a single bit of quantum information (qubit) using two real numbers involving infinitely many decimals.

Not only do we lack evidence for the infinite but we don’t need the infinite to do physics. Our best computer simulations, accurately describing everything from the formation of galaxies to tomorrow’s weather to the masses of elementary particles, use only finite computer resources by treating everything as finite. So if we can do without infinity to figure out what happens next, surely nature can, too — in a way that’s more deep and elegant than the hacks we use for our computer simulations.

Our challenge as physicists is to discover this elegant way and the infinity-free equations describing it—the true laws of physics. To start this search in earnest, we need to question infinity. I’m betting that we also need to let go of it. — Infinity Is a Beautiful Concept – And It’s Ruining Physics - Max Tegmark

I pondered over this for several days trying to understand the arguments. I still hold to what I said. The section you mention nicely shows that the x generated from the list of computable numbers is not itself a computable number, but I was speaking of the x generated from Cantor’s original proof of some real not being a rational number. That x is computable, but not rational, and thus cannot be used as evidence that there are some real numbers not computable.
The page you linked does show other ways to demonstrate exactly this, but the diagonalization method is not one of them. — noAxioms

Cantor's proof (by contradiction) shows that the set of real numbers is uncountable and thus can't be enumerated. Since the set of real numbers can't be enumerated, the diagonalized number therefore can't be computed. A similar point is made by Carl Mummert (a professor of computing and mathematics) on Mathematics Stack Exchange.

Collapse seems to be a choice of classical description of a quantum state, in other words, an interpretation-dependent thing. In interpretations with ‘jumping’, yes, it happens all the time, everywhere. In interpretations without it (such as Everett’s relative state formulation, pre DeWitt’s MWI), it’s just a classical effect, not anything physical that happens. — noAxioms

Yes. The interpretation provides an account for how a measurement operationally returns a definite state when the formalism describes an indefinite state.

Copenhagen-style interpretations also generally deny a physical collapse. So, in that sense, Copenhagen and Everett/MWI agree (and disagree with physical collapse theories such as GRW).

"Presumably [the AI in the box] wouldn't. But an AI (unlike a human) could be run on a quantum computer as part of a carefully controlled experiment, thus testing physical collapse theories that differ from standard quantum theory."

I have serious doubts about that. It is a suggestion that there is an empirical difference between the interpretations, and yet I see not explicit prediction from any pair of interpretations that differ. — noAxioms

The empirical difference is between physical collapse theories such as GRW, and non-physical collapse interpretations (such as MWI and Copenhagen). From Wikipedia:

The fundamental idea is that the unitary evolution of the wave function describing the state of a quantum system is approximate. It works well for microscopic systems, but progressively loses its validity when the mass / complexity of the system increases.
...
Such deviations can potentially be detected in dedicated experiments, and efforts are increasing worldwide towards testing them. — Objective-collapse theory - Wikipedia

This Article is intriguing. At first I thought they had found a way to reverse time in the quantum world, but rather they rejuvenated a photon, taking it back to a previous state.

The mathematics involved is probably linear (much is in the quantum world), since most non-linear systems are not reversible. — jgill

Yes, that's right. Here's the paper for anyone else interested.

"Let’s begin with a thought-experiment: Imagine that all life has vanished from the universe, but everything else is undisturbed. Matter is scattered about in space in the same way as it is now, there is sunlight, there are stars, planets and galaxies—but all of it is unseen. There is no human or animal eye to cast a glance at objects, hence nothing is discerned, recognized or even noticed."
— Charles Pinter, Mind and the Cosmic Order — Wayfarer

So far, so good.

"Objects in the unobserved universe have no shape, color or individual appearance, because shape and appearance are created by minds. Nor do they have features, because features correspond to categories of animal sensation. This is the way the early universe was before the emergence of life—and the way the present universe is outside the view of any observer."
— Charles Pinter, Mind and the Cosmic Order — Wayfarer

Pinter's asserted view of "the way the present universe is outside the view of any observer" is a performative contradiction. That's the problem with the so-called view from nowhere in a nutshell.

My model is Aristotle's hylomorphism. We interact with nature in our capacity as natural creatures. That's the relational perspective. So the moon is round, orbits the Earth and pre-existed life on Earth from a human point-of-view.

"The computable numbers are countable since they be put in a one-to-one correspondence with the natural numbers."
— Andrew M
Not to disagree, but an assertion like that requires a demonstration that they’re countable. — noAxioms

Here's one such demonstration, concluding with:

The computable numbers are an infinite set. We have provided an injective function g that maps every computable number to a single natural number: a Godel number. Any set with such a function is countable, and therefore computable numbers are countable. — Alan Turing and the Countability of Computable Numbers - Adam A. Smith

"However the real numbers are not countable per Cantor's diagonalization proof. Thus there are some real numbers that are not computable."
— Andrew M
Interestingly, the real number generated by Cantor's diagonalization proof is a computable number, so I’m not sure if this counts as evidence that there are some real numbers not computable. Once again, not disagreeing with the conclusion, only with how it was reached. — noAxioms

It isn't a computable number (though it is a real number) - see the section entitled "A counter proof?" at the above link.

OK, they managed to test something whose outcome (the CHSH inequality violation) was already predicted by quantum theory. It’s a new test, but not one that changed the theory or any of its interpretations in any way. — noAxioms

Yes, it would be big news if standard quantum theory were ever falsified.

Thanks for the larger context Bell statement. I agree with it fully. What is ‘jumping’ in that quote? “Do we not have jumping then all the time?”. — noAxioms

He's referring to the collapse of the wave function (i.e., a discontinuous change in the otherwise continuous time evolution of the Schrodinger equation).

Meanwhile, I still don’t see what the AI in the box will do. Bell’s statement is pretty clear that a real human in there wouldn’t serve any special role or purpose, so why would an AI be any different? — noAxioms

Presumably it wouldn't. But an AI (unlike a human) could be run on a quantum computer as part of a carefully controlled experiment, thus testing physical collapse theories that differ from standard quantum theory.

That sounds mostly reasonable, but the branching part based on something making observations still bothers me a bit. What is the branching mechanism? Perhaps I should have started with that question instead. — Marchesk

Per MWI, branching is the process of a system becoming entangled with the environment (of which the observer or measuring apparatus is a part) such that interference between the different parts of the wave function no longer occurs, i.e., decoherence. That process is, for all practical purposes, irreversible. Whereas the entanglement between two microscopic systems is reversible.

To give a classical analogy, suppose you knocked over an empty glass that you quickly stood up again. The action was reversible. But suppose the glass falls on the floor and shatters into a thousand pieces. For all practical purposes, that's an irreversible action.

Scott Aaronson has a good lecture on this:

To see an interference pattern, you'd have to perform a joint measurement on the two qubits together. But what if the second qubit was a stray photon, which happened to pass through your experiment on its way to the Andromeda galaxy? Indeed, when you consider all the junk that might be entangling itself with your delicate experiment -- air molecules, cosmic rays, geothermal radiation ... well, whatever, I'm not an experimentalist -- it's as if the entire rest of the universe is constantly trying to "measure" your quantum state, and thereby force it to become classical! Sure, even if your quantum state does collapse (i.e. become entangled with the rest of the world), in principle you can still get the state back -- by gathering together all the particles in the universe that your state has become entangled with, and then reversing everything that's happened since the moment of collapse. That would be sort of like Pamela Anderson trying to regain her privacy, by tracking down every computer on Earth that might contain photos of her! — Decoherence and Hidden Variables - Scott Aaronson

I don’t agree that Nagel’s diagnosis is erroneous. I think he pinpoints something real and insidious. — Wayfarer

Yes he does. I'm further saying that the view from nowhere should be rejected in its entirety, not supplemented by a further error (the Cartesian subject).

And Bell's Theorem did nothing to validate Einstein's realist objections to 'spooky action at a distance'. — Wayfarer

Yes, Bell showed that Einstein's idea didn't work. It doesn't follow that the moon isn't there when no-one looks at it.

You’ve said that before, and even though I obviously agree, I don’t think it’s as obvious, nor as insignificant, as you make it seem. As you might know, one of Thomas Nagel’s books is called ‘The View from Nowhere’. His point is to critique the widespread understanding that science provides a ‘view from nowhere’, meaning a view that is uncontaminated by anything we deem ‘subjective’, the aim being to arrive at a view which is at once universal and objective. — Wayfarer

I don't think it's either obvious or insignificant. Nagel critiques "the view from nowhere" but he doesn't reject it. He instead proposes an additional subjective dimension (per the usual Cartesian subject-object dichotomy) that just entrenches the error.

The fact that observation has an unavoidably subjective dimension is the very thing that Einstein strenuously objected to - ‘does the moon continue to exist when nobody’s looking at it?’, he asked. He strongly believed that there was a reality that existed just so, independently of any act of observation, and it was science’s job to discern that. Insofar as it had to make concessions to ‘the method of observation’, then quantum mechanics was, to him, obviously incomplete. Wasn’t that the gist of the Einstein-Bohr debates? — Wayfarer

Sure, but the debates have moved on. There have been subsequent discoveries that would have informed both their views, and have informed ours. For example, Bell's Theorem and decoherence, just to name two.

So there's no question that the moon is there when no-one's looking at it, not least because its effects are ever-present in the environment, such as in the ocean tides. Similarly, there's no question that dinosaurs once existed, even though no-one has ever observed them.

Both qualify as a computable number. The diagonalization method used... — noAxioms

The computable numbers are countable since they be put in a one-to-one correspondence with the natural numbers. However the real numbers are not countable per Cantor's diagonalization proof. Thus there are some real numbers that are not computable.

"So, if one accepts the authors' definitions for an observer and measurement, then one of the assumptions of free choice, locality, and observer-independent facts must be false."

That just sounds like Bell’s theorem (old news). What in 2019 was added to that? — noAxioms

The addition is that the experiment tests a Bell inequality for a Wigner's friend scenario (which the paper terms a Bell-Wigner test).

Specifically, there are two types of measurement that each Wigner (i.e., Alice and Bob) can perform. Alice can measure with her beam splitter removed (A₀) or present (A₁). Bob can similarly measure with his beam splitter removed (B₀) or present (B₁). When the beam splitter is removed, the Wigner measurement is the same as their friend's measurement. When the beam splitter is present, the Wigner measurement is in the {|+>,|->} basis.

So one of the A₀ or A₁ measurements and one of the B₀ or B₁ measurements are performed on each run of the experiment. Over multiple runs the measurements, on a classical explanation, must obey the CHSH inequality

$S = \langle A_1B_1 \rangle + \langle A_1B_0 \rangle + \langle A_0B_1 \rangle - \langle A_0B_0 \rangle \lt= 2$

where Ax and By are the values -1 or +1, and <AxBy> is the averaged product over multiple runs.

Per the experiment, and as predicted by quantum mechanics, the CHSH inequality is violated.

Sorry, but I don’t see what the AI adds that any simple device (like the circuit on the camera) doesn’t. — noAxioms

It would be a macroscopic-scale experiment where the AI friends would exhibit human-level intelligence. It would thus be comparable to a human Wigner's friend experiment.

I like Carroll’s definition of observer, appropriate for something like MWI.

'As John Bell inquired, "Was the wave function waiting to jump for thousands of millions of years until a single-celled living creature appeared? Or did it have to wait a little longer for some highly qualified measurer—with a PhD?"'
— Andrew M

This does not seem to reference that definition, but more of the dictionary definition of observer. — noAxioms

Yes, which was part of Bell's point. It's well worth reading the full context of Bell's comment:

It would seem that the theory [quantum mechanics] is exclusively concerned about "results of measurement", and has nothing to say about anything else. What exactly qualifies some physical systems to play the role of "measurer"? Was the wavefunction of the world waiting to jump for thousands of millions of years until a single-celled living creature appeared? Or did it have to wait a little longer, for some better qualified system ... with a Ph.D.? If the theory is to apply to anything but highly idealized laboratory operations, are we not obliged to admit that more or less "measurement-like" processes are going on more or less all the time, more or less everywhere. Do we not have jumping then all the time?

The first charge against "measurement", in the fundamental axioms of quantum mechanics, is that it anchors the shifty split of the world into "system" and "apparatus". A second charge is that the word comes loaded with meaning from everyday life, meaning which is entirely inappropriate in the quantum context. When it is said that something is "measured" it is difficult not to think of the result as referring to some preexisting property of the object in question. This is to disregard Bohr's insistence that in quantum phenomena the apparatus as well as the system is essentially involved. If it were not so, how could we understand, for example, that "measurement" of a component of "angular momentum" ... in an arbitrarily chosen direction ... yields one of a discrete set of values? When one forgets the role of the apparatus, as the word "measurement" makes all too likely, one despairs of ordinary logic ... hence "quantum logic". When one remembers the role of the apparatus, ordinary logic is just fine.

In other contexts, physicists have been able to take words from ordinary language and use them as technical terms with no great harm done. Take for example the "strangeness", "charm", and "beauty" of elementary particle physics. No one is taken in by this "baby talk". ... Would that it were so with "measurement". But in fact the word has had such a damaging effect on the discussion, that I think it should now be banned altogether in quantum mechanics. — John Bell, Against 'Measurement'

Ever seen the Andrei Linde interview on Closer To Truth? He talks explicitly about the role of the observer. — Wayfarer

Yes, we've discussed Linde's comments a few times before. See John Bell's comment above, where he says:

When one remembers the role of the apparatus, ordinary logic is just fine. — John Bell, Against 'Measurement'

One could also say, "When one remembers the role of the observer, ordinary logic is just fine." Which is to say, there is logically no view from nowhere.

The key point for me, whether about observers or measurement apparatus, is that quantum mechanics predicts the result of the interaction between two systems (the observer and observed), not the pre-existing value of an isolated system (which Bell showed to be incoherent).

It has nothing to do with consciousness or intelligence (of course). An “observation” in quantum mechanics happens whenever any out-of-equilibrium macroscopic system becomes entangled with the quantum system being measured
— Squelching Boltzmann Brains (And Maybe Eternal Inflation) - Sean Carroll

That is an a priori assertion, but which really could only ever be validated by observation. — Wayfarer

Touché!

As John Bell inquired, "Was the wave function waiting to jump for thousands of millions of years until a single-celled living creature appeared? Or did it have to wait a little longer for some highly qualified measurer—with a PhD?"

It's still weird to me that the observer is a necessary component of making sense of MWI, since decohered branches are still in universal superposition, which is what infinite De sitter space will become, except without the decohered observers. — Marchesk

Carroll defines what he means by "observation" and "observer" in comments here:

... It has nothing to do with consciousness or intelligence (of course). An “observation” in quantum mechanics happens whenever any out-of-equilibrium macroscopic system becomes entangled with the quantum system being measured. It will then decohere (become entangled with the wider environment), which causes a splitting of the wave function into separate branches.

It’s key that the macroscopic device in question starts out far from equilibrium. Otherwise it would already be entangled with everything, and the measurement/splitting process couldn’t occur.

...

The informal notion of an “observer” requires a macroscopic system that is out of equilibrium. In de Sitter space, everything is in equilibrium. — Squelching Boltzmann Brains (And Maybe Eternal Inflation) - Sean Carroll

In the main post he says:

The standard story says that the inflaton field undergoes quantum fluctuations, which then get imprinted as fluctuations in density. What we’re saying is that the inflaton doesn’t actually “fluctuate,” it’s just in some calculable quantum state. But there’s nothing “observing” it, causing decoherence and branching of the wave function. At least, not while inflation is going on. But when inflation ends, the universe reheats into a hot plasma of matter and radiation. That actually does lead to decoherence and branching — the microscopic states of the plasma provide an environment that becomes entangled with the large-scale fluctuations of the inflaton, effectively measuring it and collapsing the wave function. So in our picture, all of the textbook predictions for inflation perturbations remain unchanged. — Squelching Boltzmann Brains (And Maybe Eternal Inflation) - Sean Carroll

So in this case, the observer is the hot plasma of matter and radiation.

I can still express the length √2 with two characters, a very finite state. Humans deal only with such representable numbers, and they’re countable.— noAxioms
Yes, computable numbers.
— Andrew M

OK, maybe, but it’s a very different definition. Is there an example of something that isn’t in this set? — noAxioms

Yes, one can use diagonalization to produce a number that isn't in the set. Another example is the probability that a randomly constructed computer program will halt (Chaitin's constant).

"That question seems relevant to the physical Church-Turing thesis (Church-Turing-Deutsch principle) which says that any bounded physical system can be simulated by a Turing machine to any desired precision."
— Andrew M

Does the statement above apply to non-classical physical systems? Can it simulate say a quantum computer to arbitrary precision? — noAxioms

Yes and yes (since a quantum computer is itself a physical system).

Another interesting note about the above statement is that a Turing machine cannot simulate itself, which is not a violation of the statement. — noAxioms

A universal Turing Machine can simulate itself by accepting, as input, a description of itself and running it. See Turing completeness.

OK, I said I’d get back on this one. I admittedly get lost in the complex examples, but I did at least want to comment on some of the assumptions the paper is making, assumptions which are very interpretation dependent. The topic here is about how MWI would handle it. — noAxioms

That's an easy one. The experiment matches the predictions of standard quantum mechanics, and thus also the predictions of MWI. So it doesn't challenge MWI on those grounds. But also, on an MWI view, it's disputable whether a measurement actually took place since no decoherence (and thus no world branching) occurred.

An observer is apparently a clerk, reacting to a measurement and putting into some non-volatile state. A digital camera for instance has a CCD (the measurement device) and an SD card (the persistent state) and a bit of circuitry (the observer) to move the data from the CCD to the SD card. — noAxioms

Yes, specifically the measurement information is stored in polarization states of the photons marked as α and β (per Figure 2). Also the detection of the photons marked as α' and β' provide a permanent record that a measurement (per the authors' definition) has occurred, though not what the measurement was.

This is nothing particularly special, but they give it a very special role in the paper:
The observer’s role as final arbiter of universal facts [1] was imperilled by the advent of 20th century science.
…
in quantum theory, all physical processes are continuous and deterministic, except for observations, which are proclaimed to be instantaneous and probabilistic.
— PPGBKBRF
It seems that they’ve given this clerical role some special metaphysical status, that of arbiter of what is fact or not, and also the only physical process which is probabilistic instead of deterministic. I’m not sure if they’re asserting these things and strawman arguments to knock down or they’re actually pushing this. — noAxioms

The authors have performed an experiment based on a no-go theorem by Brukner:

for the no-go theorem we tested here [4] it is sufficient that [the observers] perform a measurement and record the outcome. — Experimental test of local observer-independence - Proietti, et al., 2019

The experiment successfully demonstrates the violation of a Bell inequality as predicted by standard quantum mechanics. So, if one accepts the authors' definitions for an observer and measurement, then one of the assumptions of free choice, locality, and observer-independent facts must be false. As they say:

Modulo the potential loopholes and accepting the photons’ status as observers, the violation of inequality (2) implies that at least one of the three assumptions of free choice, locality, and observer-independent facts must fail." — Experimental test of local observer-independence - Proietti, et al., 2019

One may well reject their definitions for an observer and a measurement. But that is not a fault of their experiment. It just raises the question of what does count as an observer and a measurement, and what would be required to perform the equivalent experiment in that case. Which is why Deutsch's proposal to use an AI on a quantum computer would be an important and compelling experiment.

Wigner can now perform an interference experiment in an entangled basis containing the states of Eq. (1) to verify that the photon and his friend’s record are indeed in a superposition—a “fact” from his point of view
— PPGBKBRF
Rightly so. There are no facts, just points of view. The friend is measured to be in superposition of having recorded one fact and of having recorded a different fact, pretty much demonstrating a lack of universal facts. Establishment of those universal facts were the only apparent role of these observers, so with that neatly shot down, the observer plays no role at all.
This pretty much answers the topic title here, at least from that article’s description. Facts are relative to a system state, which makes it ‘observer dependent’ if you want to apply the label of ‘observer’ to a specific system state, but I see no point in the special label. — noAxioms

You're rejecting the "observer-independent facts" assumption, which is fine. Others may reject a different assumption or, alternatively, reject the authors' definitions for an observer and measurement.

In my view, the experiment has value because it has confirmed standard quantum mechanics for the simplest definition of an observer (or, at least, a prototype of an observer) in a Wigner's Friend scenario. Now presumably no-one expected it not to. So the next step would be to increase the scale of the physical systems involved in the experiment until it does test interpretations that people actually hold.

I can still express the length √2 with two characters, a very finite state. Humans deal only with such representable numbers, and they’re countable. — noAxioms

Yes, computable numbers.

Actual numbers in nature (such as the ratio of the half lives of two specific isotopes) are not in this countable set. I have a hard time with a model of the universe that requires only the former sort of number, such as one would get in a simulation. Actual numbers are more analog, like ‘so big’ with your hands held apart. — noAxioms

How would we know that such ratios aren't representable?

That question seems relevant to the physical Church-Turing thesis (Church-Turing-Deutsch principle) which says that any bounded physical system can be simulated by a Turing machine to any desired precision.

Going to get back to you on this one. Interesting read, but the introduction is already full of interpretation dependent assumptions, such as counterfactual statements. I will look at it from my relational perspective which doesn’t make those assumptions, but thus far I’ve not read enough to really comment on it. — noAxioms

The authors do list the assumptions of their experiment which they note can't jointly be true. Those assumptions are observer-independent facts (O), locality (L ) and free-choice (F). Also, they acknowledge the relational perspective in their conclusion:

Another option is to give up observer independence completely by considering facts only relative to observers [24], or by adopting an interpretation such as QBism, where quantum mechanics is just a tool that captures an agent’s subjective prediction of future measurement outcomes [25]. — Experimental test of local observer-independence - Proietti, et al., 2019

Ouch. It would really such if nature allowed such approximations. I’d always envisioned pure mathematics behind the physics, not digital mathematics where all numbers are representable with finite states. — noAxioms

We may still be able to have a precise geometrical representation. For example:

Geometrically, the square root of 2 is the length of a diagonal across a square with sides of one unit of length; this follows from the Pythagorean theorem. — Square root of 2

So a balanced beam splitter will have a 1/sqrt(2) amplitude for each path. Nature doesn't encode a digital representation of that number (either finite or infinite), but the information content is there by virtue of the beam splitter being balanced.

The discussion was about observer effect (the observer causing effects), not observed effects (effects merely noticed by the observer), Relativity effects seem to fall under the latter category, prompting my foul call. — noAxioms

Fair enough.

"A related test has been carried out at a microscopic level (using photons instead of AI's) where it was shown that physical collapse does not occur." - Andrew M

Not sure what this is. Got a link for this one? — noAxioms

It was in the earlier post:

In a state-of-the-art 6-photon experiment, we realise this extended Wigner’s friend scenario, experimentally violating the associated Bell-type inequality by 5 standard deviations. 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. — Experimental test of local observer-independence - Proietti, et al., 2019

What if the ratio isn’t rational? — noAxioms

Any irrational number can be approximated to an arbitrary degree of accuracy by a rational number. From the associated paper:

For any wave function with irrational squared-amplitudes there exist arbitrarily similar wave functions with rational squared-amplitudes (as the rationals are a dense subset of the reals). — Self-Locating Uncertainty and the Origin of Probability in Everettian Quantum Mechanics - Sebens and Carroll, 2015

"While the laws of physics are the same for all observers, they may describe things differently from their respective reference frames." - Andrew M

That’s quite different than the interaction (measurement) actually changing the system being measured, which is what this topic is about. — noAxioms

Yes, it is quite different. As is the effect you mention of a clock travelling fast towards you that appears to be ticking faster than it is. Perhaps we can call them (classical) perceptual effects, (relativistic) frame effects, and (quantum) measurement effects to disambiguate them for the purposes of this discussion.

Anyway, whether or not the interaction changes the system being measured is what the Deutsch quantum AI experiment would test. That is, whether physical collapse occurs and interrupts the unitary evolution.

A related test has been carried out at a microscopic level (using photons instead of AI's) where it was shown that physical collapse does not occur.

In a state-of-the-art 6-photon experiment, we realise this extended Wigner’s friend scenario, experimentally violating the associated Bell-type inequality by 5 standard deviations. 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. — Experimental test of local observer-independence - Proietti, et al., 2019

So I'll give you the gist in a more straight forward way. — Metaphysician Undercover

Unfortunately, I didn't find your comment straightforward - I'm now not clear whether you accept special relativity or not. Can I suggest focusing on a single point, and stating it concisely. It might also be helpful to provide a quote from an authoritative source (such as SEP or a peer-reviewed paper) that backs up your point (or, alternatively, that you're disputing).