Suppose we choose one Langevin observer and consider the other observers who ride on a ring of radius R which is rigidly rotating with angular velocity ω. Then if we take an integral curve (blue helical curve in Fig. 1) of the spacelike basis vector p → 3 {\displaystyle {\vec {p}}_{3}} , we obtain a curve which we might hope can be interpreted as a "line of simultaneity" for the ring-riding observers. But as we see from Fig. 1, ideal clocks carried by these ring-riding observers cannot be synchronized. This is our first hint that it is not as easy as one might expect to define a satisfactory notion of spatial geometry even for a rotating ring, much less a rotating disk!

In 1921, Langevin gives the appropriate (relativistic) explanation in a very short note to theComptes rendus.

this hypothetical medium, with its clearly very contradictory physicalproperties

Finally, the quantum mechanical phenomenon predicted by Y. Aharonov and D. Bohm [48] can also be perceived assome kind of Sagnac effect (by replacing the light beams by charged particle beams (charge q) in the experiment, formallysubstituting the rotation ω by a magnetic field B and doing 8πλc −→ q in the relationship (3)) [49]

A practical conclusion is that one cannot synchronize a collection of clocks distributed all along the Earthequator.

experiment carried out by Hafele andKeating in 1971 [44]. It consisted of the following. Two planes each equipped with an atomic clock flew around the Earth,one in the direction of the Earth’s rotation and the other in the opposite direction (the two clocks are synchronized atthe start). On arrival, it is observed that the two clocks indicate different times

The subtlety is that the points D+ and D− reported in Fig. 4 are actually distinct and are identified only by abuse.

Fig. 4. Effect of no-synchronization of the clocks in a rotating frame

Following Langevin, this expression is called the metric of the rotating disk. It will be noted the presence of a cross termin dθ dt, source of the impossibility of synchronizing clocks, uniformly distributed around the periphery of the disk andconnected thereto (a relativistic phenomenon which is the essence of the Sagnac effect)

6 Also let us note that Paul Langevin is recognized as one of the first physicists to have defended the relativistic conceptions in France in numerousseminars; in particular the contributions of Einstein in the field, better known as they had been eclipsed in this country by the work of Poincaré on thesame topic.

Einstein considered this concept superfluous

but more complete

the observer O (inertial) now uses a coordinate system that accompanies the disk in itsrotation

Many derivations were in fact proposedall along the 20th century, issued from both special relativity and general relativity [41,42], even though the Sagnac effectis usually deemed to be a special relativistic effect [43].

Propagation of the light beams seen from the laboratory

Let us specify however that the Sagnac effect wasalready derived from special relativity by Von Laue one year before Langevin [40]

Eventually, in 1921, the mathematician (and relativist skeptic) Émile Picard asked Paul Langevin to demonstrate the effect inthe framework of the relativity, perhaps guessing it was impossible [39]. Again any such expectations and within a coupleof months, Langevin derived the effect from the General relativity [36]

It appears rather amazing that the correct relativistic interpretation of the Sagnac effect took eight years. A seeminglyobvious reason is that Sagnac’s experiment was not very much discussed in the scientific literature, even in France afterthe discovery of 1913. 7 Conscious of this situation, in 1919, Sagnac published five papers on his work in the Comptes rendus[37]. The paradox is that his ideas were nevertheless borne by a French group of strong antirelativists. In 1919, Sagnac waseven rewarded with the Pierson–Perrin Prize for his achievements on this topic (first for the experiment, seen as a rebuttalof the relativity principle, the constancy of light, and also for having proven the reality of absolute space and time) [38].

The first convincing explanation of the Sagnac effect, in the framework of relativity, was provided by PaulLangevin in 1921 [36]. 6

An experiment such as Sagnac’s was still used as a strong argument by the opponents of the theory of relativity, whowere still quite numerous at the time, including Sagnac himself.

experiment of Michelson and Mor-ley [5].

But in science, an hy-pothesis cannot be validated only if it allows one to explain all the experiments, not one in particular.

For Sagnac, this result proves the existence of the aether.

for an observer linked to the rotating disk the light speed propagates differently,according to whether the beam propagates counterclockwise (the direction of rotation of the disk by convention), or inreverse. It is then respectively c − v and c + v (Fig. 3). For the observer D, we have for the difference in time of course:tD = lc − v − lc + v  4 A ωc2 (4)which is in perfect agreement with the result found by the observer L

According to Georges Sagnac, the light has amedium of propagation which is the aether. In the reference frame linked to the laboratory (observer L) the propagationvelocity (celerity) is c

The difference in the duration of travel induces a phase shift between the twowaves, hence the appearance of interference fringes (Sagnac made a differential measurement by rotating the disk in onedirection and then in the other). The phase shift (expressed in wavelength) is given by the Sagnac formula: φ = 8π Aωλc

The duration of the path of the light ray

for the inertial observer, L linked to the laboratory

In spite of the fact that all these experiments, taken together, showed that the aether wasan illusive medium

This aether was automatically identified with the absolute space of Newton.But the Earth moves in this absolute space

t is less known that Sagnac was a strong opponent to the theoryof special relativity proposed by Albert Einstein. He set up his experiment to prove theexistence of the aether discarded by the Einsteinian relativity.

it originates fromthe closure of the two space paths as seen in the frame co-moving with the emitter/receiver and from relative motionbetween the emitter/receiver and the mirrors (or physical de-vice). Indeed, its foundations are related to the relativity ofsimultaneity.

CONCLUSIONWe have shown that the Sagnac effect is due to the closureof the path followed by light and to the relative motion ofthe observer with respect to the physical system that causesthe beam to bend and come back to the observer

Another aspect to clarify is related to the form of Eq. (2).The accidental presence of the area A suggested to somepeople (for instance, Refs. 25 and 26) an interpretation of theSagnac effect (at least for matter waves) as a sort ofAharonov-Bohm effect. However, this is just an analogy,which holds only at lowest relativistic order (see, e.g.,Ref. 23).

It is worth noting that formula (1) also appears in Ref. 24,where it is related to the relativity of simultaneity typical ofspecial relativity

No rotation, acceleration, or enclosed area appears in thisexpression. What matters is just the existence of relative(even inertial) motion and of a closed path in space

moving emitter/receiver we would instead have (withb ¼ v/c)Ds ¼ 2 ‘vc2 ffiffiffiffiffiffiffiffiffiffiffiffiffi1  b2p  2 ‘vc2 :

Eq. (1) is expressed fromthe viewpoint of an observer at rest with respect to the fiber(or array of mirrors);

Despite the widespread tendency to ascribe the Sagnaceffect to rotating systems, it is easy to show that rotation isnot essential. Consider a source/receiver of light in inertialmotion at speed v with respect to a set of mirrors rigidly fas-tened to one another or to an optical fiber so that they guidethe light beams emitted by the moving source along a closedpath in space.

The vertical straight line is the world line of an inertialobserver at rest with respect to the axis of the disk. Segment AB indicatesthe Sagnac effect as measured by the rotating observer

we obtain the following formulas for thecoordinate and proper time-of-flight differences:Dt ¼ 4 pR2xc2 1  x2R2=c2ð Þ ; (9)Ds ¼ 4 pR2xc2 ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi1  x2R2=c2p : (10)At the lowest approximation order in xR=c these two expres-sions coincide, and they are in agreement with Eq. (2),

If one prefers to proceed analytically

A purely geometrical argument, 21 applied to Fig. 3,leads again, to lowest order in v/c, to Eq. (2).

intersect the world-line of the observer at two different events, labeled A and B.The interval between A and B is the proper time differencemeasured by the observer between the arrival times of thetwo light rays

The vertical axis of thefigure is not the rotation axis of the turntable; it is the coordi-nate time axis.

An equivalent deduction can be made adopting the view-point of the rotating observer. In the observer’s referenceframe, light will be expected to have speed c – v in one direc-tion (forward) and c þ v in the other; now the path length isthe same ‘ for both (see Fig. 1). The times of flight are againdifferent and a trivial calculation reproduces Eq. (1).

The Sagnac effect can easily be described in classicalterms if one assumes that the speed of light is c with respectto a static ether.

the "fast" light given off during approach would overtake "slow" light emitted during a recessional part of the star's orbit. Many bizarre effects would be seen,

Albert Einstein is supposed to have worked on his own emission theory before abandoning it in favor of his special theory of relativity. Many years later R.S. Shankland reports Einstein as saying that Ritz's theory had been "very bad" in places and that he himself had eventually discarded emission theory because he could think of no form of differential equations that described it, since it leads to the waves of light becoming "all mixed up".[8][9][10]

the Sagnac effect. It is less known that Sagnac was a strong opponent to the theory of special relativity proposed by Albert Einstein. He set up his experiment to prove the existence of the aether discarded by the Einsteinian relativity. An accurate explanation of the phenomenon was provided by Paul Langevin in 1921.

Bohm: Yes. So you see, Feynman was quickly elevated and everybody knew he was a bit odd but then they said, that’s okay because he’s a genius, you see. And his unusual methods can now be accepted.

Wilkins: This was part of the whole thing about people feeling that Feynman didn’t know what he was talking about. Bohm: Yes. I think a year or two later, Feynman and Brandt talked together. Bohr agreed. At that time, Bohr felt that Feynman was just talking nonsense. It was only a couple of years later that when Dyson came along, a year or two later, Dyson came along and showed them mathematically equivalence of Schringer and Feynman. He was working at the Institute, Dyson. That was essentially the only thing he ever did in physics at that level anyway. And then from then on, everybody went on to Feynman because it was so much easier. That Schringer stuff was really difficult to calculate. To calculate an effect heaven knows how long it would take, 50, 60, 70 pages. Going that way, you see.

Feynman got up and gave his stuff. And Bohr got up and objected because he thought Feynman didn’t understand quantum mechanics, but Feynman was talking about tracks and so on.

Bohm: Yes. And Schringer came out with a very long talk that lasted about eight hours explaining his method of removing infinities by, in a relativistic we call variant way. It was what he called manifest covariance. Essentially, he had a way of doing it. He had calculated a certain effect. If you take the interactions of the field and the particle then the particle is no longer moving, if you want to think, in the line it would move. But because the field is fluctuating quantum mechanically, the particle fluctuates a bit in its movement. And this changes the energy level slightly, very slightly, say hydrogen.

Wilkins: But basically it’s a sort of brainwashing type of activity isn’t it to turn you into a machine.

stayed in Brazil for four years

Bohm: Well, I had several conversations with Einstein. After writing this book on quantum mechanics, which I wrote to try to understand it (based on my graduate course), I sent a copy to various scientists including Einstein. He wanted to discuss it with me, and we discussed it. He felt that the book was as good as you could present the ordinary point-of-view, but he still didn’t accept it. So we discussed it for a while, and meanwhile I myself had been feeling that it wasn’t all that clear, and that therefore these two things together made me feel that the interpretation of quantum mechanics was not satisfactory. So I began to think about it, and I produced another interpretation, which came out in two papers in Phys. Rev, in 1952, two papers, using a particle and a wave, the causal interpretation I called it. And I discussed all those things with Einstein; we also had correspondence afterwards when I was in Brazil.

I finished the quantum theory book, and I sent a copy to various scientists including Einstein and Pauli, who liked it very much. Einstein wanted to discuss it with me, and we had several discussions on this and on several other topics. But I’ll come to that later

used to refer to a problem which is solved by a practical experiment.

I observed, that though we are satisfied his doctrine is not true, it is impossible to refute it

Pope pidió a su amigo Jonathan Swift que usara su influencia en la Universidad de Dublín para que ésta le concediera el grado, pero Swift se negó a ayudarle.[6]​

su filosofía basada sobre todo en el sentido común

this limited knowledge is much more than the subjective beliefs of some agent

Our knowledge of the external world is only partial and based on the phenomena which we are able tocreate and/or observe by the devices which are available now. Therefore our knowledge of theexternal world (not a personal belief) is never absolute but relative28 because it depends on our presenttools of probing external world and it may change in time

Epistemic interpretation of quantum mechanics and Qbism were also criticized ,from a differentphilosophical perspective, by Louis Marchildon 25,26 and by Michael Nauenberg

Hans De Raedt , Mikhail I. Katsnelson and Kristel Michielsen introduced recently a mathematicalframework of logical inference which can be used as a basis for establishing a bridge between objectiveknowledge gathered through experiments and their description in terms of (mathematical) concepts,thereby eliminating personal beliefs. The authors showed how quantum description emerges from thisframework 23,24

he physics is notinterested in subjective experiences of various agents but it is trying to deduce the `laws of Nature’governing the external world

Qbists and Cbists forget that QT is much more than quantum information

Values of physical observables, such as spin projections of photons, are not predeterminedattributes of physical systems but they are created during measurements in particularexperimental contexts

For EPR experiment a reduced state vector (for the system II) deduced from an entangled stateof two physical system I+II describes only the sub-ensemble of the systems II being thepartners of those systems I for which the measurement of some observable gave the samespecific outcome

a contextual statistical interpretation 4-11 of QT, inspired by Einstein and Bohr writings, isfree of all these paradoxes and is able to explain the long range correlations in spin polarisationexperiments and the violation of Bell-type inequalities without invoking a mysterious quantumnonlocality12-22.

The discussion of quantum nonlocality in paper 3 contributes only to confusion. Statements such as:‘’quantum mechanics is local because its entire purpose is to enable any single agent to organize her

The validity of these laws does not depend whether some intelligent agents are there to verify them

We discovered that that the invariance with respect to space translations corresponds to theconservation of the total linear momentum. The invariance with respect to time translation correspondsto the energy conservation. The invariance with respect to rotations corresponds to the conservation ofthe total angular momentum etc

Due to ‘’ miraculous constancy of the speed of light in all reference frames’’

extremely useful idealization but as Mermin correctly pointed out there are no point –like events.The space –time loses its empirical foundations when we move from macro- to micro- world

A space –time is an idealized mathematical model abstracted from our subjective everyday observationswhich helped us to discover important laws of Nature

However by no means quantum wave functions (state vectors) should be interpreted as themathematical entity corresponding to the subjective belief of human agents.

The wave function is a mathematical entity not having any real existence and it is not an attribute of anindividual physical system. It is part of quantum mathematical model together with the operatorsrepresenting various quantum observables

particles prepared by some sources interact with themacroscopic devices and after substantial magnification produce clicks on various detectors

Using the Standard Model (SM) particle physicists were able toexplain quantitatively their properties and predict the existence of new particles which were discoveredlater. Of course many questions in SM are not answered and comparison of the model withexperimental data involves complicated Monte Carlo simulations containing many free parameters butit is the best model we have for now

Any measurement, we make, is not precise but it was believed that byincreasing the precision we could approach as close as we wanted the true value of a measured physicalvariable. This belief was proven to be incorrect when we started to study quantum phenomena ,discovered incompatible physical observables and Heisenberg uncertainty relations

The aim of physics is to discover ‘the laws‘’ of Nature governing objectively existing external worldPhysicists are not verifying their personal beliefs about their observations but search for themathematical abstract description allowing to explain and to predict in a quantitative way theregularities observed (and those to be discovered) in physical phenomena which exist independentlyof the presence of any agent.

In the paper3 one finds several correct citations and statements of famous physicists but thecorrectness of these statements neither prove the correctness of reinterpretations of thesestatements given by the authors nor the correctness of Qbist and Cbist positions.

The subjective view of probability promoted by Bruno de Finett

All such claims are incorrect because fathers of quantum theorywere convinced that quantum probabilities have nothing to do with subjective beliefs of agents

One may find also another claim in anonymous article in Wikipedia thatJohn von Neumann was the first Qbis

Reading their article one mayunderstand that Bohr, Schrodinger, Heisenberg and Einstein had not enough courage to accept asubjective view of probability

Marian Kupczynski

In a paper: An Introduction to Qbism with an Application to the Locality of Quantum Mechanics ‘’published in 2013 the authors3 claim that Qbism is the correct interpretation of quantum theory andthat it solves all the famous paradoxes discussed during the last 80 years

Qbism was proposed in 2002 by Carlton Caves, Christopher Fuchs and Rudiger Schack and became asubject of several papers and books.

We strongly disagree with this point of view and we give belowarguments against it

promoting

The distant long rangecorrelations can be correctly explained using a contextual statistical interpretation of quantum theory

Physicists arenot verifying their personal beliefs about their observations but search for the mathematical abstractdescription allowing to explain and to predict in a quantitative way the regularities observed (and thoseto be discovered) in physical phenomena which exist independently of the presence of any agen

In anothersense it seems obviously wrong: the laws of physical science can be cast in mathemat-ical forms having a reality that transcends the mere creatures that discover them. Butmathematics itself is a form of language, even more highly refined than science.16 Math-ematical Platonists will recoil from this in horror. The rest of us, however, might thinkabout Wigner’s surprise at the “unreasonable effectiveness” of mathematics in science.Why shouldn’t mathematics, the most highly refined form of language, serve as the natu-ral vehicle for expressing the most fundamental forms of science, another only slightly lessrefined form of language. Both mathematics and science are human creations. Even if youare convinced that I should not have said “creations”, but “discoveries”, I hope that youwill still be willing to consider that what we are comparing here are two representationsof those discoveries or creations in human language.

3.2.1. Niels Bohr always emphasized the importance of “ordinary language” because,he said, it was the only way to represent to others the outcomes of experiments. Bohrtook such outcomes to be objective phenomena in a common external world, and thisunambiguous reality of experimental outcomes has been taken for granted in all subsequentinterpretations of quantum mechanics.15

When I assign probability 1, I only mean thatI’d bet my life on it

Probability 1 indicates a particular intensity of belief: supremeconfidence — “I’d bet my life on it.” It does not imply the existence of a deterministicmechanism guaranteeing the p = 1 outcome

QBism holds that probability assignments are personal

With the advent of quantum mechanics, deterministic mechanisms disappeared fromphysics. Should this be qualified in a footnote: “Except when quantum mechanics assignsprobability 1 to an outcome”?

In the orthodox interpretation when a measurement has an outcome the state of thesystem is said to “collapse” to a new state that incorporates that outcome. In QBism anoutcome is the experience induced in an actor by the world’s response to an action. Thecollapse of the state is nothing more than the normal updating of the expectations of thatactor on the basis of new experience. The collapse of a quantum state is conceptually nodifferent from the updating of a probability distribution on the basis of new data

a widespread misunderstanding of probabilities that are often very close to zero or one.12

Laws of science are the regularities we have discerned in our individual experiences,and agreed on as a result of our communications with each other.

Language is the only means we have for trying to compare the personal outcomes ofall such users, and for trying to convey all those outcomes to users who were not watchingor otherwise experiencing those actions and their consequences.

The outcome of anaction, being an experience, is subjective. It is private to the person taking that action.But its representation in language can be communicated to other people. What I call theobjective world is built out of such linguistically shared subjective experiences

To eliminate theconfusion it is necessary to acknowledge that science in general, and quantum mechanicsin particular, is a tool that each of us uses to organize and make sense of our own privateexperience

53 Freud and Schr ̈odinger are rare exceptions

The boundary — John Bell’s “shiftysplit”55 — is different for each scientist using quantum mechanics.

But why should scientific laws never, under any circumstances, mention any user ofscience? Science is a human activity. The laws are formulated in human language. Asempiricists most scientists believe that their understanding of the world is based on theirown personal experience.54 Why insist that an understanding of science, which I use tomake sense of the world I infer from my experience, should make no mention whateverabout the role of that experience?

4.14.2 and 4.14.3, which express the very common53 insistence that laws ofnature should make no reference whatever to the people who have formulated those laws

4.14.4 There seems no way to locate the boundary between the realms in which

4.14.3 It is not that we object to thinking about humans. Rather, we want to under-stand the relation of humans to nature, not just assuming the character of this relation byincorporating it in what we suppose are nature’s fundamental laws, but rather by deduc-tion from laws that make no explicit reference to humans. We may in the end have to giveup this goal, but I think not yet.

4.14.2 I hoped for a physical theory that would allow us to deduce what happens whenpeople make measurements from impersonal laws that apply to everything, without givingany special status to people in these laws.

endorsement of the view that all minds are one,

In his 1950 letter to Einstein, 4.13.3, Schr ̈odinger comes close to articulating a viewof the world as the common residue of the experience of many. While he does not mentionthe important role of language in such sharing of experience,

scorn

4.13.3 The conception of a world that really exists is based on there being a far-reaching common experience of many individuals, in fact of all individuals who come intothe same or a similar situation with respect to the object concerned. Perhaps insteadof “common experience” one should say “experiences that can be transformed into eachother in a simple way”. . . . [This reality] comes about for us as, so to speak, the intersectionpattern of the determinations of many — indeed of all conceivable — individual observers.It is a condensation of their findings for economy of thought which would fall apart withoutany connections. . . . 50

4.13.2 Quantum mechanics forbids statements about what really exists — statementsabout the object. It deals only with the object-subject relation. Although this holds,after all, for any description of nature, it appears to hold in a much more radical andfar-reaching sense in quantum mechanics.49

4.11.1 Unperformed experiments have no results.46

If he had takencare to specify that when he said “we”, “us”, and “our” he meant each one of us, actingand responding as a user of quantum mechanics, he would have had it exactly right. Butit seems to me likely that he was using the first person plural collectively, to mean all ofus together, thereby promulgating the Copenhagen confusion

In quotation 4.10.2 from his Physics World essay, Peierls articulates the QBist view ofwhy it makes sense to apply quantum mechanics to eras long before people existed. Eventsin the remote past can influence our present or future experiences.

4.10.2 If there is a part of the Universe, or a period in its history, which is not capableof influencing present-day events directly or indirectly, then indeed there would be no sensein applying quantum mechanics to it.44

Peierls also refers in that essay to “the view of Landau and Lifshitz (and therefore ofBohr)”. This identification of Landau and Lifshitz with Bohr has to be taken seriously,since Peierls and Landau worked together in Copenhagen in the early days. But I havenever read in Bohr anything as literally dualistic as the remarks 4.8.2 of Landau andLifshitz on classical and quantum “objects”. Nor have I read in Bohr anything as close toQBism as Peierls’ 4.10.1, which I would take (perhaps wrongly) to be his own reading ofBohr.

Contrary to QBism, Peierlsalso takes it for granted,45 like most physicists, that outcomes that are assigned probability1 must be backed up by an explicit physically describable mechanism

4.10.1 In my view, a description of the laws of physics consists in giving us a set ofcorrelations between successive observations. By observations I mean. . .what our sensescan experience. That we have senses and can experience such sensations is an empiricalfact, which has not been deduced (and in my opinion cannot be deduced) from currentphysics.43

QBism agrees withPauli that the particular qualities of the user are not a part of the theory, but the factthat each user is particular, plays no role for Pauli, though it lies at the heart of QBism

4.9.1 Nevertheless, there remains still in the new kind of theory an objective reality,inasmuch as these theories deny any possibility for the observer to influence the results of ameasurement, once the experimental arrangement is chosen. Therefore particular qualitiesof an individual observer do not enter the conceptual framework of the theory

And the second sentence of quotation 4.8.2 wouldbecome this: By measurement, in quantum mechanics, we understand any interactionbetween a user of quantum mechanics and the world external to that user that results inthe world inducing an experience back in that user

So the QBist reformulation of quotation 4.8.1 from Landau and Lifhistz would be this:It is impossible to formulate the basic concepts of quantum mechanics without referringto the personal experience of each user. And the second sentence of quotation 4.8.2 wouldbecome this: By measurement, in quantum mechanics, we understand any interactionbetween a user of quantum mechanics and the world external to that user that results inthe world inducing an experience back in that user

While other versions ofquantum orthodoxy locate measurement outcomes in a vaguely defined “classical” worldcoupled to a quantum mechanical system,41 Landau and Lifshitz are the only ones explicitlyto insist that classical mechanics is therefore logically prior to quantum mechanics. Theyliterally distinguish between “classical objects” and “quantum objects”

40 It was suggested to me by Czech colleagues that such quotations are nothing more thanLandau and Lifshitz paying lip service to the Marxist doctrine of dialectical materialism.I’m unconvinced.

The quantum mechanics text of Landau and Lifshitz provides the most extreme andexplicit example of the broad reluctance of physicists to allow any trace of a human presenceto play a role in our understanding of quantum mechanics.40

4.8.2 It must be most decidedly emphasized that we are here not discussing a processof measurement in which the physicist-observer takes part. By measurement, in quan-tum mechanics, we understand any process of interaction between classical and quantum

4.8.1 It is in principle impossible. . .to formulate the basic concepts of quantum me-chanics without using classical mechanics.

less directly and more inferentially, the world thatexperience has led me to hypothesize

The object of my knowledge is the contentof my private personal experience

As discussed above, in a QBist reading “our” does not refer,as it usually does, to all of us collectively. It refers to each one of us individually. Some ofus may have such knowledge; others may not

What does it mean to “know” something?

o whom does “our” refer?

What does it mean to distinguish between “behavior” and “our knowledgeof this behavior”?

4.7.1 The conception of the objective reality of the elementary particles has thusevaporated in a curious way, not into the fog of some new, obscure, or not yet understoodreality concept, but into the transparent clarity of a mathematics that represents no longerthe behavior of the elementary particles but rather our knowledge of this behavior.38

But I prefer to put it the other way around: the successof QBism in clarifying the murk at the foundations of quantum mechanics is a compellingreason for physicists too to embrace the widespread view of probabilities as subjectivepersonal judgments

Freud was writing about religion. But for a QBist his remark applies equally well tophysical science, contrary to the current views of most physical scientists

One would have thought that the Born interpretation calledfor a searching reexamination of the nature of probabilistic judgments

one’sunderstanding of quantum mechanics should differ dramatically depending on whetherone subscribes to the objective (“frequentist”) view of most physicists, or the subjective(“personalist”) view of most statisticians

Quantum mechanics is, after all, the first physicaltheory in which probability is explicitly not a way of dealing with ignorance of the precisevalues of existing quantities

Physicists generally believe that probability isan objective property of events, baked into their very nature by objective features of theworld. But many statisticians, mathematicians, and philosophers take probability to be anumerical measure of the subjective judgment of the person who assigns it, indicating, forexample, the odds at which that person is willing to make or accept bets.

De Finetti, in the 1930s, was one of the earliest and most eloquent advocates ofthe subjective view of probability, common today among statisticians and inherent inQBism, but anathema to most physicists

4.5.1 The abandonment of superstitious beliefs about the existence of Phlogiston, theCosmic Ether, Absolute Space and Time. . . , or Fairies and Witches, was an essential stepalong the road to scientific thinking. Probability too, if regarded as something endowedwith some kind of objective existence, is no less a misleading misconception, an illusoryattempt to exteriorize or materialize our actual36 probabilistic beliefs.

The sentiment is pure CBism, which makes me even more suspicious of its authenticity

So many people today — and even professional scientists — seem to me like somebody who has seen thousands of trees but has never seen a forest. A knowledge of the historic and philosophical background gives that kind of independence from prejudices of his generation from which most scientists are suffering. This independence created by philosophical insight is — in my opinion — the mark of distinction between a mere artisan or specialist and a real seeker after truth

3%, bafflingly, said they would prefer whichever theory was proposed first, a chronological criterion

“the observer” with respect to the behavior of quantum systems. 37% responded that the observer is a complex quantum system (like a measurement apparatus in a physics laboratory); 31% that the concept has a fundamental role in the formalism of quantum mechanics, but no physical role in the real world (i.e., it’s useful as a calculation device, but does not correspond to something “out there”); 22% disagree, stating that the concept of observer does play a significant role in the real world; and 10% said the observer plays not fundamental role at all, either in the formalism or in the real world.

“Do you believe that physical objects [like cats] have their properties well defined prior to and independent of measurement?”

It is supposed to illustrate the kind of problems that one of the most famous interpretations of quantum mechanics, the Copenhagen one, runs into when it is applied to macroscopic objects (like cats). The cat is in what is known as a state of quantum superposition, because the box containing it is linked to a random subatomic event that may or may not occur.

famous Schrödinger’s cat, a thought experiment suggested by Erwin Schrödinger in 1935, based on an original idea by Albert Einstein.

As the philosopher Don Howard notes, the Bohr-Einstein debate: “… was not a clash between a dogmatic bully and a senile old man. Theirs was a clash between two determined seekers after truth. They both knew that a deep truth was to be discovered where separability and entanglement came into conflict.”

I encourage readers with an interest in these questions to go beyond the superficial treatments that can be found in some popular science books, and (alas) in student textbooks. Dig a little deeper. As Sivasundaram and Nielsen imply in their paper, become more familiar with the foundational concepts. Learn a little more about the philosophy of the subject, and make up your own mind. I believe your efforts will be greatly rewarded, not least through a better appreciation of one of the most dramatic debates in the entire history of science.

Like the great philosopher Han Solo, I’ve got a very bad feeling about this.

But in recent years I have developed real doubts

No doubt this failure will not shake the firm realist convictions of some commentators. For many years, on balance I have preferred Einstein’s realism and have championed Bell’s rejection of the Copenhagen “orthodoxy” (I still do).

The matter of interpretation remains undecided.

But we should not overlook the simple fact that all the experimental studies of the last 50 years have failed to establish the superiority of any realist interpretation or extension of quantum mechanics.

There can be no doubt that the slow-to-reawaken interest in foundational questions raised by Bohm and Bell (and others) has uncovered some remarkable phenomena that might have otherwise gone unnoticed, and has helped to establish the wholly new disciplines of quantum information and quantum computing.

There can be no doubt that the slow-to-reawaken interest in foundational questions raised by Bohm and Bell (and others) has uncovered some remarkable phenomena that might have otherwise gone unnoticed, and has helped to establish the wholly new disciplines of quantum information and quantum computing. But we should not overlook the simple fact that all the experimental studies of the last 50 years have failed to establish the superiority of any realist interpretation or extension of quantum mechanics. The matter of interpretation remains undecided. No doubt this failure will not shake the firm realist convictions of some commentators. For many years, on balance I have preferred Einstein’s realism and have championed Bell’s rejection of the Copenhagen “orthodoxy” (I still do). But in recent years I have developed real doubts. Like the great philosopher Han Solo, I’ve got a very bad feeling about this.

Just a few months ago Mermin admitted to me in private correspondence that his professors were simply indifferent to philosophy, and only in this sense did he view them as “agents of Copenhagen.” “They had no interest in understanding Bohr, and thought that Einstein’s distaste for [quantum mechanics] was just silly. … It was a very unphilosophical time.”

Here’s N. David Mermin, writing about his experiences as a research student studying quantum mechanics in the 1950s, in the journal Physics Today in 2004, in which he recalls:“… vivid memories of the responses my conceptual inquiries elicited from my professors — whom I viewed as agents of Copenhagen — when I was first learning quantum mechanics as a graduate student at Harvard, a mere 30 years after the birth of the subject. ‘You’ll never get a PhD if you allow yourself to be distracted by such frivolities,’ they kept advising me, ‘so get back to serious business and produce some results.’ ‘Shut up,’ in other words, ‘and calculate.’ And so I did, and probably turned out much the better for it. At Harvard, they knew how to administer tough love in those olden days.”

The Copenhagen interpretation had become a dogma not through acceptance of reasoned philosophical arguments, but by default. In a 1987 interview, David Bohm explained it this way: “… everybody plays lip service to Bohr, but nobody knows what he says. People then get brainwashed into saying Bohr is right, but when the time comes to do their physics, they are doing something different.”

And American scientific hegemony after the second world war ensured that these standard textbooks, such as Quantum Mechanics, by Leonard Schiff (greatly influenced by Oppenheimer) carried often-garbled versions of Bohr’s complementarity and the Copenhagen line on measurement. Schiff’s text would go on to inform the teaching of quantum mechanics throughout North America, Europe, and Asia, through three editions spanning twenty years.

but English physicists such as Paul Dirac were essentially unmoved (you’ll find no mention of complementarity in Dirac’s The Principles of Quantum Mechanics, first published in 1930)

DeWitt’s search for a quantum theory of gravity had led him to the Wheeler-DeWitt equation which, when applied to the entire universe, implies a “universal wavefunction.” He perceived a problem. The Copenhagen interpretation appears to place special emphasis on the process of measurement and the role of an observer (it doesn’t, but never mind) and, as there can be nothing and nobody outside the universe to collapse such a wavefunction, he rejected Copenhagen in favor of Hugh Everett III’s “relative state” formulation, which he re-invented (with Everett’s blessing) as the many worlds interpretation of quantum mechanics.Let’s be clear. DeWitt was seeking an interpretation in which the wavefunction is to be interpreted realistically, and the “measurement problem” is a real problem in need of a solution. That in itself should be sufficient reason to reject Copenhagen, but no. DeWitt invents what he clearly believes is a stronger argument by conflating Copenhagen with the collapse of the wavefunction: “According to the Copenhagen interpretation of quantum mechanics, whenever a [wavefunction] attains a form like that in equation 5 [a superposition of wavefunctions pertaining to measurement] it immediately collapses.” This sentence appears in a section titled “The Copenhagen collapse.”

The Copenhagen interpretation appears to place special emphasis on the process of measurement and the role of an observer (it doesn’t, but never mind)

So how did the conflation arise? I’m confident there isn’t a straightforward answer to this question. But I’d like to draw your attention to an article published in the journal Physics Today in 1970, by Bryce DeWitt. It is titled “Quantum Mechanics and Reality.”

What’s quite remarkable is how von Neumann’s theory would become conflated with the Copenhagen interpretation and summarized under the heading of “the measurement problem.” In the Copenhagen interpretation, all measurements are classical and the notion of “quantum measurement” doesn’t arise. There is no such thing as the “measurement problem” in the Copenhagen interpretation. No matter how deep you dig, you’ll struggle to find any reference to von Neumann’s theory in Bohr’s writings.

The collapse of the wavefunction is more appropriately associated with John von Neumann’s quantum theory of measurement, included in his text Mathematical Foundations of Quantum Mechanics, first published (in German) in 1932, although von Neumann doesn’t actually use this phrase. Instead, he distinguishes two kinds of quantum process. Process 2 is the smooth, continuous evolution of a quantum system as described by quantum mechanics. Process 1 is the discontinuous “projection” of the system into a single measurement outcome (such as a single spot on the screen). At the moment of measurement, von Neumann postulates that we abandon process 2 in favor of process 1.

But the seeds of confusion were now already sown. If the (realistically-interpreted) wavefunction starts out “distributed in space” and ends up at a single point then it is presumed to “collapse.” Whilst it is possible to find references to an equivalent phrase — the “reduction of the wave packet” — in Heisenberg’s 1929 Chicago lectures, this is used to critique a purely wave description. The “collapse of the wavefunction” was never part of the Copenhagen interpretation because the wavefunction isn’t interpreted realistically. The only thing that happens when an electron is detected on a screen in the context of Copenhagen is that we gain knowledge of the position of the electron

This would later become widely known as “spooky action at a distance.”

And this is what Einstein did in 1927. Electrons pushed one at a time through a narrow aperture will eventually produce a pattern consistent with the diffraction of a classical “electron wave.” But each electron is detected as a single particle, registering as a single point on a distant screen. Einstein worried that this: “… assumes an entirely peculiar mechanism of action at a distance, which prevents the wave continually distributed in space from producing an action in two places on the screen.”

And there lies the rub: for what is the purpose of a scientific theory if not to aid our understanding of the physical world? We want to rubberneck at reality.

The Copenhagen interpretation obliges us to resist the temptation to ask: But how does nature actually do this?

This and the variants of the Copenhagen interpretation due to Bohr and Werner Heisenberg are essentially anti-realist. This doesn’t mean that they deny objective reality or the reality of “invisible” microscopic entities (such as electrons). It means that the wavefunction shouldn’t be taken as a literal representation of the real physical states of the real physical things. Bohr referred to application of the quantum formalism as a “purely symbolic procedure,” and is famously quoted as saying: “There is no quantum world. There is only an abstract quantum physical description.”

Bohr’s principle of complementarity can be articulated in the language of classical waves and of particles, or as descriptions in which the physics evolves smoothly and continuously and energy is conserved (an electron “orbits” an atomic nucleus but we can’t say where it is at any moment in time), or discontinuously in a space-time framework (the particulate electron is “here”). These descriptions are not contradictory, they are complementary. According to Bohr what we cannot do is go beneath these descriptions and say what the electron is at all times in all circumstances.

where Ehrenfest much to his dismay had to side with Bohr's position in this great debate.

Entonces se destaca la lucha de Berkeley contra el materialismo que fundamenta el ateísmo, el cual supondría la negación de la religión. Berkeley trataba de triturar la Idea de materia porque la esencia y existencia de ésta trituraba a Dios

Lo primero que dice el diccionario es que Berkeley es un filósofo inglés, pero eso es inexacto porque se trataba de un filósofo irlandés. No le llames inglés a un irlandés.

En consecuencia, los objetos percibidos son los únicos acerca de los que se puede conocer. Cuando se habla de un objeto real en realidad se habla de la percepción del objeto

En su juventud, Berkeley propuso que no se puede saber si un objeto es, solo puede saberse un objeto siendo percibido por una mente. Por tanto, concluyó que todo lo que puede conocerse de un objeto es su percepción del mismo, y resulta gratuito suponer la existencia de una sustancia material que sustente las propiedades de los cuerpos. Los conceptos abstractos de Locke no existen para Berkeley, ni en la naturaleza ni en el espíritu, es una ficción. Las ideas siempre conservan su particularidad. No es la abstracción, sino el lenguaje, lo que hace posible extender observaciones particulares a lo general.[3]​

La filosofía de Berkeley es el empirismo llevado al extremo

Los sermones de Berkeley explicaban a los colonizadores que el cristianismo permitía la esclavitud, y en consecuencia los esclavos debían ser bautizados: «sería una ventaja para sus negocios (de los patrones) tener esclavos que deban "obedecer en todo a sus patrones desde las entrañas, no solo cuando les observan, sino de todo corazón, temerosos de Dios"; que la libertad del evangelio concuerda con la servidumbre temporal, y que todos sus esclavos solo serán mejores esclavos siendo cristianos».[2]​

This does not need any "conscious observer," as had been argued by John von Neumann and Eugene Wigner,

Jordan went further, arguing that there were times when a quantum system effectively observed itself, by collapsing into a specific state rather than remaining in a superposition of states.

"we ourselves produce the results of measurement" [Wir selber rufen die Tatbestände hervor] (Erkenntinis, 4, 215-252, 1934).

"the electron is forced to a decision. We compel it to assume a definite position; previously it was, in general, neither here nor there; it had not yet made its decision for a definite position.... If by another experiment the velocity of the electron is being measured, this means: the electron is compelled to decide itself for some exactly defined value of the velocity; and we observe which value it has chosen. In such a decision the decision made in the preceding experiment concerning position is completely obliterated."

According to Max Jammer (The Philosophy of Quantum Mechanics, p. 161), Jordan declared, with emphasis, that observations not only disturb what has to be measured, they produce it!

Mermin: One of the breakthroughs that had just come to my attention recently is high-temperature superconductivity.