There’s actually no empirical difference between those two cases. There is if there was a true superposition, but there isn’t in the cat case. It’s been demonstrated with macroscopic objects, but under conditions which would kill any cat (such as being in a vacuum and almost 0°K). — noAxioms

:brow:

This is nonsense. You have a reference to such a crazy definition from a consensus physics reference from the last century? What even is uniform existence? That a body must be the same everywhere? A carrot cannot taper? I presume you to be an absolutist and maybe get your definitions from the sites supporting such, but this is not the consensus definition as used by physicists. — noAxioms

"Uniform existence" is having an unchanging presence, as in not being acted upon by forces; what is described by Newton's first law, which is commonly referred to as "the law of inertia". Check the Stanford article I previously referenced:

The laws of Newtonian dynamics provide a simple definition: an inertial frame is a reference-frame with a time-scale, relative to which the motion of a body not subject to forces is always rectilinear and uniform, accelerations are always proportional to and in the direction of applied forces, and applied forces are always met with equal and opposite reactions. — https://plato.stanford.edu/entries/spacetime-iframes/#QuasInerFramNewtCoroV\

No, not at all. I can for example reference the inertial frame of Earth when referencing the twins scenario. No duration is specified or necessary when identifying that frame. — noAxioms

That's a fictional "inertial frame", not properly formulated, so not an actual inertial frame. I could reference "the inertial frame of my right big toe", but unless it's properly formulated as an inertial frame, it's just fiction. Your terminology is not logically rigorous noAxioms. That's why I needed to point out your equivocation with "relative". Notice the above quote, "with a time scale". Any proposed inertial frame would be completely useless without a time scale.

There were lots of basic topics covered, down to interpretations of time near the bottom, but I didn’t see quantum interpretations mentioned at all, which requires probably a whole separate course — noAxioms

Might be hard to find a faculty member of a philosophy department capable of this. :cool:

I think that deep within the mathematical structure of QM is where superposition or assumed existence in two "separate" states simultaneously occurs, or a mixture of states. The process begins with Hilbert spaces and their inner products (in C these might be thought of as the power of combined vectors). States of a system are subspaces of these. A pure state is determined by a unit vector in the Hilbert space. Combining systems is interpreted as taking tensor products of two Hilbert spaces. The probability of a property when the system is in a pure state is given by the inner product.

It seems to be a mathematical thing and perhaps someday a different math approach will clarify this. Schrödinger's cat deserves a bowl of milk and gentle scratching around the ears.

But the issue is, what do these mathematical representations represent in the real material world? Or do we simply deny that there is a real material world? Perhaps our senses deceive us.

The problem with vectors is that they represent things (forces and movements) with one dimensional straight lines, when we know that in reality these things act in a multidimensional way. This produces a fundamental requirement of stacked vectors to represent a multitude of dimensions. Since the vector is a line segment, it fundamentally represents a relationship between points. However, through terms of usage which manipulate human thought, we come to believe that a vector represents properties at a point, forgetting that it really is a relationship between points.

So we get terms like "inner product' which appear to represent something which is internal to a point, when in reality it represents that point's relation to other points in a dimensional representation. Then there is a whole class of concepts such as "angular momentum" and "spin" which through the terminology used appear to represent something internal to a point, when in reality they are produced by relating that point to other points dimensionally.

The issue is, as I said at the beginning, the straight line of a vector does not accurately represent a multidimensional activity which has curves inherent within every infinitesimal point. So real movement from one infinitesimal space to the next is not accurately represented with straight vectors, and the longer the vectors are, the more the inaccuracy is magnified.

The problem with vectors is that they represent things (forces and movements) with one dimensional straight lines, when we know that in reality these things act in a multidimensional way. — Metaphysician Undercover

The issue is, as I said at the beginning, the straight line of a vector does not accurately represent a multidimensional activity which has curves inherent within every infinitesimal point. So real movement from one infinitesimal space to the next is not accurately represented with straight vectors, and the longer the vectors are, the more the inaccuracy is magnified. — Metaphysician Undercover

Oh my. This is dreadful, I fear. :gasp:

Whereas the simplest vector spaces (in R^2 or C) have vectors which can be represented by little arrows in the Euclidean or complex planes, most vectors in QM go far beyond this and cannot be so described. See Hilbert space. But, if I read between the lines you write I think what you may be getting at is the fact that linear maps are fundamental in applications.

Working with complicated functions in math one frequently tries to approximate little parts of these functions with linear functions, which are so much easier to work with. That's what happens, say, in finding a distance an object has traveled, D=Rt. If R is constant, we have R(t1+t2)=Rt1+Rt2. But if R varies we go to a definite integral, which, itself, consists of adding tiny parts of time with constant rates applied.

I've wondered about this linearity feature of QM and why it is so fundamental to the subject. But I am not a physicist. Here is a comment found in Wikipedia.

Oh my. Whereas the simplest vector spaces (in R^2 or C) have vectors which can be represented by little arrows in the Euclidean or complex planes, most vectors in QM go far beyond this and cannot be so described. See Hilbert space. — jgill

The basic principle of the vector remains the same, but the vector space described is more complicated. The complexity of these vector spaces is what gives rise to the idea of "inner products". The use of "inner" makes it sound like these are properties internal to the point. In reality they are how the point relates to other points (by means of vectors), therefore external relations.

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

The use of "inner" makes it sound like these are properties internal to the point. In reality they are how the point relates to other points (by means of vectors), therefore external relations. — Metaphysician Undercover

Inner product = input vectors into a form producing a real or complex number (scalar). Outer products are matrices or tensors. Word salad.

As soon as you get into the actual physics, then it's really better suited to Physics Forum. The question is not one about physics, it's one about meaning.

The question is not one about physics, it's one about meaning. — Wayfarer

Unfortunately, it may well be the meaning of the math as one follows that path to actualizations. When does that occur? Beyond me. The math is hard for me to follow. Not so young, anymore. :worry:

The issue is that you get dimensionless points (no volume of space occupied by the points), in space, which have properties, as point particles. Traditionally, a point could not be a body with properties, but its mass might be represented as a point, the centre of gravity. So the point became a very effect way to represent a body's mass for calculations in physics. The centre of gravity. You can see though, that the point does not provide a very truthful, or even accurate representation of a body, which really exists in the area around the point, though it provides a very useful representation of its mass for many practises.

But when we start to break bodies apart, getting down to smaller and smaller parts, the concept of "mass" breaks down as well, being a feature of a body's way of occupying space as a coherent whole. That's why "density" is an important concept in relation to "mass". So for instance, if you propose to break apart a massive particle, like a hadron (proton, neutron) into its composite quarks, it's mass cannot be accounted for. The mass is more like a property of the space that the combined quarks are existing in, as a coherent unity, and this is known as the strong force. So further particles, gluons, might proposed to account for the existence of this force, but these would be represented as points, therefore not properly representing the area.

You ought to be able to see that the strong force is a property of an area of space, the area within which the hadron exists. That area is responsible for the existence of the hadron and its mass. So when it comes down to the nitty gritty of providing a true and accurate representation of mass, the point, as the centre of gravity, fails badly. In reality, "mass" refers to how a body is extended in space, so when a physicist tries to break a massive hadron into its composite point particles, its mass cannot be adequately represented.

If the physicist does not respect this difference between theory (representing mass as a point), and practise (the experiments demonstrate that mass cannot exist at a point), then the physicist will continue into that theoretical fantasy land, a fictional world requiring the assumption of "virtual particles", in a pointless attempt to maintain the representation of mass at a point.

then the physicist will continue into that theoretical fantasy land, a fictional world requiring the assumption of "virtual particles", in a pointless attempt to maintain the representation of mass at a point. — Metaphysician Undercover

Lattice field theory avoids virtual particles, which are mathematical conveniences.

the point does not provide a very truthful, or even accurate representation of a body, which really exists in the area around the point — Metaphysician Undercover

I'm curious how you would express what you have said in the context of field theory.

Lattice field theory avoids virtual particles, which are mathematical conveniences. — jgill

Hmm, there seems to be an incommensurability between the lattice representation, and the continuum representation. Here's from Wikki' entry on lattice guage theory:

"When the size of the lattice is taken infinitely large and its sites infinitesimally close to each other, the continuum gauge theory is recovered...
...Such calculations are often extremely computationally intensive, and can require the use of the largest available supercomputers."

I think the issue with the lattice representation is that the designation of a quantum (discrete unit) of space is completely arbitrary, not based on any real attributes of space itself. Then it becomes just a matter of re-representing a spatial continuum as an infinity of spatial units. That an infinite number is required demonstrates the incommensurability. But when mass, forces, and motion are represented, there's probably no significant different from points and vectors, because it appears like they are just trying to reproduce this in a different form anyway.

I'm curious how you would express what you have said in the context of field theory. — jgill

I think that field theory gives properties to space itself, the electromagnetic field for example. But since the electromagnetic field is observed to react with massive objects like atoms, or even just electrons, through quanta, the tendency is to give the quantum of energy a particle-like existence, as a point. This is very similar to what I said above, that the object with mass is represented as a point (centre of gravity). It's a matter of simplicity, to interact with a particle with mass, which is represented as a point, the thing interacting is also represented as a point. As I said above, I believe this is inaccurate. So in as much as the representation of fields might in some way represent real spatial attributes, the points in the field, which are supposed to be particles are not adequate representations. But these points are needed to explain how the field interacts with mass which is represented as a point, a centre of gravity for the sake of simplicity. So what is needed is to get away from representing mass as a point. Then when the field interacts with mass in the way of quanta, it is not at a point in the field.

Consider the way that an electron interacts with a proton in an atom for example. The interaction does not occur simply between a point where the electron is, and a point where the proton is. The interaction is occurring everywhere within the orbital, so it is understood as the "electron cloud". It is not the case that at any moment, the electron is at some point in the cloud, it is the case that at every moment, the electron is everywhere in that cloud. This is because "the electron" does not exist as a particle at a point, that's just a representation which was made for simplicity sake, to show its mass as being at a point.

I think the issue with the lattice representation is that the designation of a quantum (discrete unit) of space is completely arbitrary, not based on any real attributes of space itself — Metaphysician Undercover

What are the "real" attributes of space?

I'm not an observer, no, I'm definitely not. — Numerius Negedius

This is very confusing. :chin:

Does the observer have to be conscious or are there non-living "observers"?

Does the observer have to be conscious or are there non-living "observers"? — Agent Smith

First of all, why do you engage consciousness with living/non-living? There are livings who are not observes because they are not aware of anything. So, being alive does not imply being observer if I am not conscious enough around my scenario of reality, mind, ideas, persons, etc...

For example: A living dog is not conscious of metaphysical and philosophical enquiries, but at the same time, he is a living animal...

I find your views on the matter are relevant but ... (deliberately?) misses the point.

what am I missing then? If you can explain me, I would be so much appreciated.

what am I missing then? If you can explain me, I would be so much appreciated — javi2541997

You're exploring a vital aspect of the issue, but I would rather not go down that road. I'm bad at it and your precious time, mon ami, your ever so precious time.

What are the "real" attributes of space? — jgill

I wouldn't know the answer to that question, nor would anyone else, I believe. The point though is that we can represent space in two fundamentally different ways, as being a real thing with real properties (though unknown), or as completely abstract, being a conceptual tool to help us understand the existence of things. In the former, we are constrained by a desire to know the real properties of space, and produce models accordingly. In the latter, we are constrained only by arbitrary principles, pragmaticism, according to what serves the specific purpose. This is the basic difference between absolutism and relativity. Absolutism dictates that there is an absolute truth to the way that motions are modeled, while relativity dictates that differing models of motion are equal.

So, in the case of "the continuum", continuity is a principle based in the Aristotelian conception of "matter". Matter is how Aristotle accounts for the continuity of sameness as time passes, consistency in existence. Continuity is a temporal concept. So Newton gave matter a principal property, "mass", and mass is proposed as the means by which temporal continuity is maintained, the first law, inertia. That is supposed to represent a "truth" about time, the continuity of mass, as inertia. But geometric representations of space, perfect squares, perfect circles, perfect triangles, the number of degrees in a circle, the number of spatial dimensions, etc., derived as mathematical axioms, are not supposed to be "truths". They are abstractions created for practise, and mostly derived from practise, containing the arbitrariness which pragmatism relies on.

Special relativity takes the arbitrariness of spatial abstractions, and assigns it to our conception of time. Under this conceptual structure there is nothing real, no "truth", to ground the continuity of time. Continuity is now based in spatial conceptions, and the true basis for the conception of continuity, time, is left as obsolete. Since the conception of "space" has no real continuity, being arbitrary because of its base in pure abstraction, and "time" has been subsumed under "space", we are left with no principles for a real or true "continuum". Our lived experience of temporal continuity, and the only access we have toward an understanding of "true" continuity, is made no longer relevant, by denying its bearing on our concept of "time". The "continuum" now is just another arbitrary spatial concept, guided by pragmatism, and our real experience of continuity which is the continued existence of material objects as time passes, is not allowed to have any bearing on this conception of "the continuum". So our conceptions of mass must be manipulated to be consistent with the arbitrary conception of "the continuum", instead of altering the conception of "the continuum" to be consistent with our true observations of mass.

Your replies are always entertaining, and frequently thought provoking. Continuous mappings, in the context of QM, can mean the topological definition applied to the Hilbert spaces of that subject. Inner products yield norms which give rise to metrics, within which continuity is defined. It's a long way from Aristotle.

I still think much of what is discussed in this forum concerning QM boils down to the problem of unitarity.

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? Maybe my prior example of the half life of carbon 14. I called it a representable number, and I suppose that if you can represent it, you can compute it as per your definition, and if you can’t compute it, you also cannot represent it, except I think I just did in my example.

Do we know that "the ratio of the half lives of two specific isotopes" isn't representable? — noAxioms

I think I just represented it with such words, so I’ll answer my own question.

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

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

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.
So commenting on specific things in the paper: https://arxiv.org/pdf/1902.05080.pdf

Does the observer have to be conscious or are there non-living "observers"? — Agent Smith

A.Smith, this is also relevant to your question. No, no experiment has demonstrated a living thing to have a special role.

The authors are Proietti, Pickston, Graffitti, Barrow, Kundys, Braniciard, Ringbauer, and Fedrizzi, so I’m going to refer to them as PPGBKBRF.
First of all, they put these little grey figures all over their diagrams, the observers. It makes it look like human interaction is somehow a critical step in a measurement, but no, near top of page 3 they define what these little guys are presumed to be:

let us first clarify our notion of an observer. Formally, an observation is the act of extracting and storing information about an observed system. Accordingly, we define an observer as any physical system that can extract information from another system by means of some interaction, and store that information in a physical memory. — PPGBKBRF

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. 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.
They go on to a description of the Wigner’s friend thought experiment where they blatantly violate their own definition:

According to quantum theory, the friend randomly observes one of the two possible outcomes in every run of the experiment. The friend’s record, h or v, can be stored in one of two possible orthogonal states of some physical memory, labeled either |“photon is h”> or |“photon is v”>, and constitutes a “fact” from the friend’s point of view. — PPGBKBRF

This is suddenly a relational wording of the situation due to the addition of ‘from the friend’s PoV’. Suddenly the ‘observation’ doesn’t make anything a universal fact at all, as evidenced by Wigner’s measurement:

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.

"Uniform existence" is having an unchanging presence, as in not being acted upon by forces; what is described by Newton's first law, which is commonly referred to as "the law of inertia". — Metaphysician Undercover

Thank you for the definitions. The article you referenced makes no mention of ‘uniform existence’, ‘unchanging presence’ so it helps to define these terms up front if you’re going to use them.

Check the Stanford article I previously referenced:
“an inertial frame is a reference-frame with a time-scale, relative to which the motion of a body not subject to forces is always rectilinear and uniform, accelerations are always proportional to and in the direction of applied forces, and applied forces are always met with equal and opposite reactions.”

A good definition, and it comes from the top of the article, not section 1.7 to which you linked. That section deals with pre-20th-century handling of what is now called accelerated reference frames. It even includes an early form of the equivalence principle as worded by Newton.

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.

That's the exact definition I copied above:

"Uniform existence" is having an unchanging presence, as in not being acted upon by forces; what is described by Newton's first law, which is commonly referred to as "the law of inertia". Check the Stanford article I previously referenced:

The laws of Newtonian dynamics provide a simple definition: an inertial frame is a reference-frame with a time-scale, relative to which the motion of a body not subject to forces is always rectilinear and uniform, accelerations are always proportional to and in the direction of applied forces, and applied forces are always met with equal and opposite reactions.
— https://plato.stanford.edu/entries/spacetime-iframes/#QuasInerFramNewtCoroV\ — Metaphysician Undercover

I think, "uniform existence" and "unchanging presence" are adequate descriptions. Notice that "uniform" is even used in the passage. "Motion" is taken for granted by me, for the reasons I gave already. Under the precept of the relativity principle any existing body is always in motion. "Existence" is how I described a body relative to a time-scale. The problem appears to be that you did not have a very good understanding of what an inertial frame is, so you did not recognize my description as a good one. You were trying to deny the importance of an essential aspect of the inertial frame, the time-scale.

So we have two features of the inertial frame. Firstly, what I called "uniform existence", which is the unchanging presence of a body not being acted upon by forces, and secondly, a time-scale relative to this body.

Would you agree that the inertial frame is just an ideal, and it does not actually represent anything real in the real world of physical, material bodies? In reality, whenever time is passing a body is subject to forces, and there is no body in the universe which is not subjected to forces at every moment of passing time. So the "inertial frame" is really just a convenient fiction, serving as a pragmatic principle to base mathematical calculations around. It really does not serve as a good representation of what is actually going on in the world. The use of "rectilinear" to describe the uniform motion which the inertial frame is based on, is a dead give away for revealing the fictitious nature of the concept. No true motion is really rectilinear, but this assumption supports the use of vectors (discussed above with jgill), in that fictitious misrepresentation of motion.

Rather a good series of 10 essays by Marcelo Gleiser on The Big Think about quantum physics and philosophy. Well-informed and level-headed.

https://bigthink.com/people/marcelo-gleiser/

So, coming back to this thread after many days away, Sean Carol has stated a solution to the Boltzmann Brain problem is that there won't be any observers in De Sitter space to cause decoherence under the MWI. Boltzmann Brains are thought to be the results of quantum fluctuations over an infinite amount of time after the heat death of he universe, but the wave function is deterministic, so as long as there are no decoherent branches, there's no sense of fluctuation.

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.

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.

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.

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?"