Gestures in Empty Space

Physics vs Philosophy
In the second half of his book On Physics and Philosophy Bernard d’Espagnat explores more of the philosophy and less of the physics of quantum theory, and I’m taking notes to keep track of where he’s headed.

In chapter 13 (“Suggestions from Kantism”) d’Espagnat says that physicists nowadays aren’t entirely sure how their scientific theories and results are to be interpreted.

Science of Reality vs Science of Phenomena
Immanuel Kant steered science away from “reality-per-se” and toward the study of phenomena, so d’Espagnat thinks Kant’s views might be promising.

Kant did not reject the idea of an underlying reality, otherwise he would have discarded the “thing-in-itself” concept.

Kant did, however, say that Pure Reason would not be able to make any pronouncements on it. Beliefs and speculations are fine as long as we don’t call them scientific or rational views, he said.

The Table vs the Representations of the Table
Here is a basic problem: if you look at a table you not only have a representation of that table somehow inside of you, but you also believe there’s a table somewhere out there.

Can we check that this representation is accurate? Well, we can’t check what’s “really” out there since our senses give us just a representation of the table.

Space vs Experience
Kant argued that the very concept of space and spatiality was “a priori.” We’re more or less born with it as we need to peg various bits of sense data somehow in relation to each other.

D’Espagnat disagrees, saying the same argument could be made about riding a bicycle or learning to swim. He says Kant, living in a time ignorant of the evolution of species, would also not have considered “learning by apprenticeship.”

That’s where systems of neurons gradually favour useful gestures and discard the unhelpful ones.

D’Espagnat finds himself not just rejecting the realists who argue for the “absoluteness” of space and time, but also the idealists who claim Kant pinned down the foundations of the argument.

Not just evolution but also quantum physics was lacking in Kant’s approach, yet even today philosophers consider a “demonstrated truth” the view that spatiality is just the mind’s way of framing phenomena.

Modern physics shows that concepts of Euclidean space, universal time, and precise localization are misleading, even if we need those concepts to operate as humans in our ordinary macroscopic lives.

Kant’s vs Quantum Theory’s Objective Language
In any event, Kant sees science as addressing phenomena, so his version of science is “weakly objective.” So is quantum physics, which speaks of experimental setups and predictions of experimental observations.

But Kant uses the language of objective reality with little modification, while quantum physics uses terms such as electrons and virtual particles for reasons of reluctant convenience.

Kant vs his Followers
Kant’s followers became much more hostile to the idea of an objective reality. Kant talked about the “thing-in-itself,” even if inaccessible to science directly. But neo-Kantians rejected the notion except as a “limiting concept.”

D’Espagnat looks at Ernst Cassirer, a neo-Kantian writing around a century ago. D’Espagnat says he may be the “clearest” of all the neo-Kantians.

Cassirer’s Concept of Concepts vs Traditional Concepts
Cassirer was very interested in the process of coming up with concepts.

He noted that traditionally a big concept contains little information because so many distinctions get blurred.

For instance, we can move from an oak to a tree to a plant to a living being, a concept so broad we hardly have words to describe it.

But with mathematics the bigger categories combine all the qualities of the smaller categories that feed into it.

For instance, the concept of second-degree curves doesn’t mean we can’t tell a circle from an ellipse any more. We just have to plug the right values into the right parameters and we can get a circle.

Logical Necessity vs Quantum Results
Cassirer tries to describe science as a mathematical approach incorporating more and more concepts through some sort of logical necessity.

Reason and the universal scope of logical necessity form the basis for some sort of Being.

But d’Espagnat says that’s just an analogy. If we look directly at quantum physics we see it doesn’t link disparate impressions into some sort of logically and causally required arrangement of entities.

Cassirer’s views on the rules of knowledge and logical necessity put great emphasis on our mental powers to create order rather than some reality “out there.”

Yet d’Espagnat reminds us that experiments often refute one theory or other, so there is something “out there” that can derail some view and just say “no.”

Complete vs Ongoing Pursuit of Knowledge
Furthermore, d’Espagnat’s concept of a “veiled reality” won’t leave us exasperated and depressed, he says, because we will always be able to come up with better and better ways to deal to generalize about phenomena.

Mind-independent Reality vs Internal Consistency
D’Espagnat says some modern philosophers with “Kantian or empiricist inclinations” are less resistant to the idea of an independent reality than Cassirer was.

D’Espagnat says that Hilary Putnam’s “internal realism” sees descriptions evaluated by some kind of “ideal coherence” that keeps our beliefs consistent with each other and with experiences represented in those belief systems.

But d’Espagnat says Abner Shimony thinks Putnam gives up too easily in rejecting any kind of correspondence between our senses and a “mind-independent or discourse-independent ‘state of affairs.’”

Shimony and d’Espagnat think the concept of a “protomentality” has some similarity with quantum mechanical concepts.

Bas van Fraasen came up with a “constructive empiricism” that rejected scientific realism. He said it might be useful to consider structures and processes not directly accessible to an observer, but they’ll have no intrinsic reality.

On the other hand, Fraasen said the issue of what is observable and what isn’t should be left to science not philosophy: the “Grand Reversal.”

Empiricism vs a Knowing Subject
But can Fraasen stay an empiricist when he’s making use of a “knowing subject”?

Shimony thinks empiricism needs to be dropped for some kind of realism — with a strong mental component.

In the end d’Espagnat agrees with Shimony that there must be some way to “close the circle” even if there is a “dark cloud” making the project difficult.

That dark cloud is the impossibility of considering quantum states to have an ontological status.

D’Espagnat says that as long as contemporary philosophers stick to philosophy they have trouble radically casting doubt on veiled reality, or even realism in general.

Only when modern physics is considered do we see how impossible it is to be a conventional realist.

Background image: NASA/JPL-Caltech/P. N. Appleton (SSC-Caltech) via galex.caltech.edu.

Terms of Ontological Endearment

Material Witness
In chapter twelve of his On Physics and Philosophy Bernard d’Espagnat tackles three kinds of materialism: dialectical materialism (briefly), “scientific” materialism, and what he calls “neomaterialism.”

Ultimately… ultimate reality isn’t the same as “empirical” or “epistemological” reality, something materialists just don’t get.

At least that’s what he says, and I largely agree.

Here’s my summary of the chapter.

Dialectical Materialism vs Bohr
D’Espagnat says he’s not going to do a detailed analysis of dialectical materialism. He says it’s been sufficiently dismantled elsewhere. However, he warns against seeing too many parallels between Neils Bohr’s approach and this form of materialism.

Bohr’s thought and dialectics may share some general features, but that’s different from dialectical materialism. Bohr had a “human-centred” approach, which could be called materialism only if you radically changed the meaning of the word.

Scientific Materialism vs Atomism
D’Espagnat says “materialism” or “mechanism” doesn’t automatically refer to atomism. Descartes didn’t believe in atoms, and even in the 19th century ether and fields lay outside the realm of the atom.

Macroman on the Street vs the Microworld
The man on the street and even many scientists (particularly in the softer sciences such as biology) think of nature as composed of smaller and smaller grains or specks, eventually leading to atoms. This microworld has (roughly) the same nature as the macroscopic world we experience.

The problem with that idea is that standard quantum theory and the experimental results used to test it show conclusively that atoms, particles, and the forces emanating from them just aren’t like the world at large (as we experience it). This material reductionism doesn’t work.

Standard vs Non-standard Interpretations
Penrose (calling himself a physicalist) adds gravitational effects to the Schrödinger equation. Sokal and Bricmont rely on Broglie–Bohm. However, the first choice is more a research program than a fully fledged theory, and the second choice runs into some trouble with relativity.

The Sokal and Bricmont approach combines corpuscles with nonlocal entities or forces that have the same strength whatever the distance. This isn’t your grandmother’s materialism.

Empirical Reality vs Materialist Reality
Standard quantum mechanics rejects both approaches. At best these materialist approaches describe some “empirical” or “epistemological” reality, a product of how our “mind structure” divides and categorizes reality.

Positivism vs Materialism
Some materialist apologists say quantum mechanics is a product of its times: the 1920s, when positivism (and its emphasis on observation rather than underlying reality) reigned.

D’Espagnat rejects that objection. He says that whatever the origins of quantum theory, rival interpretations still need to be bolstered by evidence.

Research vs Traditions of Research
Michel Bibol and Larry Laudan offer subtler challenges by examining the higher-level assumptions that scientists use. Laudan calls them “traditions of research,” which Bitbol calls “values.” They’re what imparts meaning to a scientific quest.

Observations vs “Ampliative” Arguments
D’Espagnat acknowledges that when mainstream physicists reject Broglie–Bohm because its concepts are unnecessarily complicated or because “action at a distance” messes with relativity they are using “ampliative” arguments.

These are arguments that go beyond what the observations are telling us. After all, physicists could reject the relativity principle as long as they come up with some theory that uses other principles, but acts as if the relativity principle still works.

Bohm vs Materialism
However, even David Bohm rejected materialism. He first spoke of a wave function then later a quantum potential. Neither is localized, hardly what a conventional materialist would call real.

Although Bohm found a way to explain physics without specifying consciousness, he also noted that quantum physics suggests a “mental pole” exists.

Sophistication vs Atomic Materialism
Adding sophistication to atomic materialism doesn’t rescue it. Rather, its “atomism” disappears and its materialism looks increasingly doubtful.

Neomaterialism vs Matter
A third approach to materialism comes from André Comte-Sponville.

He acknowledges nonseparability, a concept that other materialists ignore. D’Espagnat calls this approach “neomaterialism.”

Comte-Sponville gets himself into definitional circles trying to define “matter.” It’s supposed to be everything (but a vacuum), yet also produces the mind. However, if thoughts are real then they’d already be part of “matter.”

Neutral vs Suggestive Terms
D’Espagnat also criticizes Comte-Sponville for using “image-carrying words” such as “matter.” D’Espagnat notes that he himself doesn’t use “matter,” “God,” or “spirit.” Rather he tries to use neutral terms such as “mind-independent reality.”

Nonseparability vs Neomaterialism
Comte-Sponville says the primary question is whether matter is idealist or spiritualist on the one side, or of a physical nature similar to what we experience on the macroscopic level. He’s not an idealist or spiritualist, so he clearly believes in a physical reality.

But as with scientific materialism the idea that reality bears any resemblance to our macroscopic experiences is blown out of the water by quantum physics.

Nonseparability—which Comte-Sponville says is a “mystery”—is an issue whatever theory you choose. It ensures that “ultimate reality” is nothing like our everyday experiences.

Utility vs Evidence
Comte-Sponville eventually acknowledges that if matter includes thought then matter can’t be defined as everything except thought.

However, he says that ultimately what the “natural sciences” say is less important than neomaterialism’s purpose: to explain mind from concepts other than mind, and to do all this to “defeat religion, superstition and illusion.”

D’Espagnat says this argument about the usefulness of neomaterialism just ends up being a circular argument. Deeply held convictions are not themselves an argument.

Empirical vs Ultimate Reality
Ontologically interpretable theories are not consistent with experiment. D’Espagnat says particles and their attributes have a well-defined existence only in relation to knowledge, hence the mind.

Our knowledge of particles and other micro-objects are just that: a kind of knowledge, hence pointing to elements of an empirical, not ultimate, reality.

D’Espagnat says that he and Comte-Sponville both agree that “existence” comes before “knowledge.” But d’Espagnat says mind comes from an “independent reality” not “empirical reality.”

This a materialism does not make.

Convenient Ontologies vs Creeds
Back to materialism in general, d’Espagnat agrees it’s a “tradition of research” as Laudan might put it.

These traditions use values that neither explain nor predict. They are not testable.

These research traditions may include contradictory theories under their umbrella. But some scientists attach a lot of meaning to this identity, and aren’t likely to give up on the term “materialism.”

On a day-to-day basis physicists are using and abusing terms from classical physics such as “particles.” Since physicists would find it hard to move ahead just pondering observations and equations, these concepts are convenient components of a “fabricated ontology.”

D’Espagnat warns these scientists that relying on this ontology to support their rationality may be useful from a practical point of view. Just don’t convert that choice into “an illegitimate doctrinal creed.”

Knowledge of Good and Banal

Philosopher’s Walk
A little past the halfway point in Bernard d’Espagnat’s On Physics and Philosophy he switches from a look at the relevance of physics to philosophy to the relevance of philosophy to physics.

If the first chapter of part two, chapter eleven, is any indication, the last half of the book should be a much easier read than the first, though perhaps less satisfying.

It was a huge challenge to wade through d’Espagnat’s descriptions of quantum theory and interpretation, hence I felt the need to write (and post) lots of notes to help me out. At least I felt a sense of reward whenever I finally grasped something of the physics.

But as far as I can tell I agree with d’Espgant’s philosophy anyway. Part two may make easier reading, but I already felt a lot of the modern philosophy, soft sciences, and cultural studies he critiques was just plain hokum. I don’t need more convincing.

In any event, I will trudge on, and I expect I’ll be posting updates to my dualistic summary much more often now.

Science vs Philosophy
Descartes, Pascal, and Leibniz were brilliant scientists and philosophers, but by the eighteenth century a huge breach developed.

Nature of Things vs Behaviour
Fourier refused to speculate on the nature of heat. Instead his heat propagation equation quantitatively predicted heat’s behaviour.

Intuitive vs Unintuitive Notions
Specialization works when concepts such as (in Fourier’s time) “hotter” and “colder” seem obvious, so don’t need to be defined by a theory.

But what’s a quantum field or space-time metrics? Then we do need to consider the nature of such concepts.

Ontology vs Operationalism
The physicist can give up his exclusive interest in behaviours, or can decide that “behaviour” is just a series of recorded observations.

The first option sounds like philosophy, while the second gets close to operationalism.

Physics-Aware Philosophy vs Philosophy-Aware Physics
In first part of his book d’Espagnat called on philosophers to pay attention to the physics. In this second part he calls on physicists to pay attention to the philosophy.

Epistemology vs Scientific Knowledge
D’Espagnat says epistemology, the philosophy of knowledge, is particularly important when considering scientific knowledge.

Logical Positivists vs Modern Sceptics
Epistemology forty years ago was dominated by logical positivists. Nowadays there’s more diversity.

Present-day epistemologists often combine extreme scepticism toward science with an “everything goes” attitude to knowledge in general.

They also talk of “paradigms” in a way that suggests an underlying belief in objectivist realism.

Stubborn Epistemologists vs Blasé Physicists
Physics has moved far, far away from realist attitudes to experimental data. Epistemologists generally ignore two points physicists find obvious.

The first is that equations, such as Maxwell’s, show remarkable power and longevity, even if interpretations of those equations have changed.

The second is that with the help of these equations physical science gets better and better at predicting phenomena.

Paradigm Change vs Continuous Change
D’Espagnat finds fault with much of contemporary epistemology, but he says Thomas Kuhn and others usefully pointed out that science doesn’t always change slowly but surely.

Kuhn sees a strong sociological basis in “paradigm changes”: it’s easier to cast doubt on the present “received” theory than to prove its replacement.

Therefore advocates of change must use more tools of persuasion than just the data.

Experimental Choice vs Outcome
D’Espagnat appreciates that funding and fashion might influence choice of experiment, but he strongly doubts that they affect the results of those experiments.

Short-Term Chaos vs Long-Term Progress
D’Espagnat says epistemologists probably act like historians, seeing short-term upheaval during science’s most productive periods.

But in the long term d’Espagnat strongly believes the “winner” theory will explain not just new facts, but the old ones the previous theory took care of.

Huygens’ Waves vs Newton’s Corpuscles
D’Espagnat acknowledges how Newton’s corpuscular theory of light replaced Huygens’ wave theory even though Huygens’ explained double refraction a lot better.

Does that mean explanatory power is sometimes lost as science “progresses”?

D’Espagnat emphasizes that today’s theory of light is quantum electrodynamics, which improves upon both Newton’s and Huygens’ theories.

Universal Physics vs The Rest of Science
D’Espagnat says epistemologists might dispute the universality of this argument. Instead of physics maybe their claims apply to some other sciences.

He believes in the universality of science in principle, but he admits maybe sciences less dependent on technology may suffer a loss of craft as one theory replaces another.

However, d’Espagnat still believes such a loss would be temporary. Epistemologists again confuse loss of predictive power and a (temporary) lack of interest in some field.

Paradigms vs Reality
Kuhn-like epistemologists are so fixed on objectivist or constructivist realism that they see a change in concepts as a radical change in physics’ view of reality.

As a result some epistemologists speak of the “noncumulative” nature of physics.

Allegory vs Equations
D’Espagnat points to the remarkable stability of equations despite changes in “wordings and outward interpretations.”

A change in concepts doesn’t destroy the old theory, it just generalizes it and provides a new allegorical picture.

Kuhnian vs Other Viewpoints
D’Espagnat notes that in the past fifty years other approaches to scientific knowledge have developed that don’t rely on Kuhn.

Professional Language vs Sloppy Thinking
D’Espagnat says most professional languages help prevent misleading shifts in meaning, but philosophical language actually encourages it.

If you apply critical thinking to philosophical texts you’ll often discover ambiguous meaning and mannered style replacing sound arguments.

Context vs Scientific Purpose
D’Espagnat says some epistemologists delve deeply into the psychology or sociology of scientific discovery, yet remain near silent about what science is really concerned with.

Ideas vs Evidence
Jean-Jacques Rousseau decided humans are good by nature, but forgot this was an idea of his not a piece of evidence.

D’Espagnat says many philosophers of science act the same way, clinging to an idea that is ultimately just part of their dogma.

Empiricists decided a priori that evidence comes from the senses, while positivists have their verification principle.

Relativity vs Quantum Theory
Epistemologists have started taking into account relativity theory but don’t realize how damaging quantum theory is to some of their views.

Positivists vs Realists
Some realist epistemologists speak of entities as having an unconditional individual existence, or naturally assume that particles travel on continuous trajectories.

Realists point to positivism’s failings on philosophical grounds, but the physics points to the failings of realism.

Science vs Cultural Fashion
Some epistemologists think the positivism of the 1920s led to the “weak” objectivity of standard quantum theory, while today’s attitudes are friendlier towards realism.

D’Espagnat calls this argument “valueless.”

If it were just an issue of social psychology and today’s fashion then physicists should now have solved the quantum interpretation problem.

However, as he’s already explained in detail, other quantum interpretations that make the right predictions cannot be interpreted ontologically, and vice versa.

Language vs Thought
Throughout the twentieth century many philosophers paid attention to language, thinking it had to mirror—even mould—the logic of thought.

The problem is various languages have very different structures. Do we think if a group speaks a different language it thinks differently?

D’Espagnat believes “language creates thought” is a Rousseau-like assumption. Aristotle came up with the concept of potentia not so he could think in a new way, but to accommodate new data.

New language is convenient and helpful, but springs from a need to explain new evidence.

Quantum vs Classic Logic
Quantum theory muddies the distinction between concepts of objects and predicates.

Some people have put forward a quantum logic to remedy that situation, but this new logic isn’t a necessary part of quantum theory.

Metalogic vs Specific Rules
D’Espagnat believes that the metalogic used to speak about logic is a universal logic, while specific thinking rules might apply to specific situations.

He notes with approval Bohr’s “basic truth” that everyday language is the only clear means of communication that we have.

Sociologism vs Science
D’Espagnat condemns the idea that “anthropological situations” determine scientific results.

He asks, for instance, if the Heisenberg uncertainty principle would have failed had German and Danish culture been different.

He says this is sheer absurdity and calls the attitude “sociologism.”

Sokal the Anti-Sociologist vs Sokal the Realist
D’Espagnat applauds physicist Alan Sokal’s exposé of sociologists’ fuzzy thinking (by submitting an incoherent, jargon-filled paper to a humanities journal).

However, d’Espagnat regrets how Sokal “drifted to the other extreme” by clinging to physical realism.

Certainties vs The End of Certainties
D’Espagnat disagrees with the phrase “the end of certainties,” which is often used to describe the loss of certain knowledge in modern times.

He rejects this idea, whether it refers to challenges to determinism or physical realism.

Predictive rules, whether of events or probabilities of events, do work. Once experimentally verified they keep on working.

D’Espagnat thinks this is “certain” knowledge, though he agrees that “illusively simple” certainties may prove deceptive and short-lived.

So Say We All

Quantum States of Confusion
The weeks or months between entries do not reflect a lack of desire to post, or to read, or to learn. I’ve just found Bernard d’Espagnat’s On Physics and Philosophy a tough slog.

His rather involved prose and the often bewildering translation combine to produce some very indecipherable moments, and my merely rudimentary understanding of quantum theory doesn’t help much either.

Here’s the latest of my chapter summaries, presented in the black-and-white dichotomous style that seems to match my thinking. Things are looking up, though, as he’s now getting into more philosophical territory.

Up to now he’s spent a lot of time justifying his interpretations by reviewing the experimental evidence and pointing out flaws in other approaches to the physics. It’s all perfectly legitimate, but as a lay reader I don’t always feel competent to judge who’s right.

So here’s a look at chapter ten of Bernard d’Espagnat’s On Physics and Philosophy.

Laws of Classical Physics vs Quantum Physics
D’Espagnat says that the laws of classical physics were “objectively interpretable” while in quantum physics the laws merely predict observations and the mutual agreement of observers.

Prediction vs Justification
D’Espagnat says modern physics prefers to come up with laws to explain patterns of observation rather than explain what’s “objectively” going on. Even so, can we justify why we choose these rules and why they work?

Quantum Laws vs Macroscopic Predictions
Quantum laws lead to paradoxes such as the Schrödinger cat, not observed in real life. Can we justify the laws behind these paradoxes?

Quantum Laws vs The Alternatives
But more generally, how do we justify (as in “explain”) that these laws work better than any others we might invent?

Ensembles vs Individual Observations
In chapter eight d’Espagnat tackled the “and-or” problem: quantum laws seem to suggest the mixing of two quantum states (aA + bB) whereas our world looks classical (either we detect aA or we detect bB).

Decoherence theory shows how observing a whole ensemble of cats will show each one is either dead or alive in the proportions quantum mechanics predicts. Quantum laws are thus reconciled with observation—except that we generally observe individual cats not ensembles of them.

Mere Predictions vs Hidden Variables
If quantum mechanics merely predicts observations rather than says anything about the “reality” behind what we see then simultaneous states are just a way of calculating probabilities.

On the other hand, Broglie-Bohm’s pilot wave (see chapter two) has a particular probability of being in one of two regions—really being in one of two regions (equivalent to aA or bB).

Standard quantum mechanics says whatever the “Real” happens to be is what produces results that match quantum predictions. The pilot wave approach gets more ontological and suggests what the “Real” must look like to produce those observations.

The two approaches are compatible, though that doesn’t solve various problems with the hidden variable approach (as d’Espagnat has already pointed out).

Outside vs Inside Observations
D’Espagnat then moves beyond our vaguely disembodied, collective observations of instrument pointers, which is what decoherence theory can explain.

He ponders conscious beings that can make predictions about themselves—making observations from within.

Consciousness vs Matter
D’Espagnat says both scientists and “the enlightened general public” look upon states of consciousness as states of matter.

Matter vs Sense Data
D’Espagnat says equating consciousness and matter is hasty, as the argument depends not on the concept of matter but rather on the concept of awareness, as in “registering sense data.”

Sense Data vs Reality
D’Espagnat says Berkeley, Kant, and the neo-Kantians followed the “great principle” that things take place “as if” our senses and classical physics give a true picture of reality.

In other words, the belief was that our sense data give us “an access to ontology.” However, quantum mechanics makes it clear that this principle is a faulty assumption.

Schrödinger’s Cat vs Wigner’s Friend
D’Espagnat says Schrödinger’s cat presumably has a viewpoint on whether it’s alive or not.

Furthermore, Wigner suggested substituting a human. Can we still justify quantum predictions?

Can quantum mechanics agree that an outside observer and “Wigner’s friend” have states of consciousness at the same time?

Improper vs Proper Mixtures
A “proper” mixture of quantum states would be just like a classical sum of states, typically seen in the ordinary macroscopic world.

An “improper” mixture (or “pure case”) is typically seen in entangled quantum systems more or less undisturbed by the outside environment.

The kind of mixture can be determined by measuring a large number (“ensemble”) of identical systems and seeing which states are observed.

Pointers vs Bacteria
Let’s imagine an electron is in the state c = a+b. Some bacteria come along, creating a large number of electron-bacteria quantum systems.

This microscopic electron-bacteria ensemble should be an improper mixture of states aA and bB.

But a problem arises if definite states of consciousness imply definite states of matter.

If the bacteria have states of consciousness then they would be aware if they’re in state A or state B.

The bacteria’s (supposed) self-awareness implies a proper mixture of quantum states, but orthodox quantum theory says there should be an improper mixture.

If orthodox quantum theory holds true then our “conscious” bacterium will lack any power to make predictions.

Pilot Waves vs Schrödinger Wave Function
In the Broglie-Bohm model the “total wave function” doesn’t “collapse” or get “reduced.”

The Schrödinger wave function just gives the probability that the (unorthodox) pilot wave is in some region of the system. No contradiction.

But in orthodox quantum theory the quantum state of the bacterium is an improper mixture of various states, so we get an apparent contradiction.

Microscopic vs Macroscopic Subjects
Bacteria are microscopic but humans are macroscopic. Decoherence theory kicks in, so in practice we can consider electron-human systems to be a proper mixture of two reduced wave functions. Therefore in practice their state of consciousness has predictive power.

Predictive Consciousness vs Predictive Science
D’Espagnat says he’ll explore notions of consciousness in chapter 18, but says the “scientific” definition of consciousness is limited to one of predictive power, and that’s because science itself limits itself to predictive power.

Predictive vs Non-predictive Consciousness
The bacterium or Schrödinger’s cat who thinks it’s alive cannot influence the outside observer if this state of consciousness is non-predictive, says d’Espagnat.

If you could somehow ask the bacteria what their state had been you’d find the results were the same as for a proper mixture.

But we’d assumed the quantum ensemble was initially a pure case (in other words, in an improper mixture).

Asking a question is like taking a measurement. It disturbs the system. Hence no contradiction.

Group vs Individual Observations
D’Espagnat says the generalized Born rule is “impersonal.” It makes predictions about the correlations among observers not what a particular individual observer will see.

So “Aspect-type” experiments measure correlations among observers, whose states of consciousness are correlated (because they’re observing correlated readings).

But if the Born rule is applied to individual observers this correlation would be cut.

D’Espagnat is not comfortable with the “quite inordinate strangeness” that private states of consciousness would differ in this situation, and hence be deprived of some of their predictive power.

He does not, however, rule out this possibility.

Individual vs Group Observations
Although in theory the overall wave function must be the basis for predictions, in practice decoherence lets macroscopic observers rely on their own states of consciousness to make predictions.

Can this reliability of individual predictions be extended to group predictions?

Two friends are looking at the same pointer. By the conventional Born rule they will get the same “impression” of the pointer and hence build up the same kind of wave function to make future predictions.

The different waves that correspond to different impressions of the pointer mostly don’t overlap. That fact and decoherence assure us that the two friends’ predictions will coincide too.

Physics vs Philosophy
D’Espagnat notes a “turning point” in the book. He feels he’s shown the amazing predictive powers of quantum mechanics and its universality. Now he wants to explain why these rules work and where they came from. This takes him further into philosophical territory.

Instrumentalists vs the Kantians
D’Espagnat says the “diehard instrumentalists” think such questions are meaningless, while those sympathetic to a Kantian viewpoint think some explanations are in order but would be found “within the human realm.”

Naive Realism vs Open Realism
D’Espagnat looks for an explanation he calls “open realism.” He believes an explanation can acknowledge “what truly exists” without descending into a simplistic “naive realism.”

Paying attention to the physics dissuades one’s outlook from becoming too naive, he says.

Scientists vs Philosophers
D’Espagnat says most scientists point to the obvious, common-sense appeal of some kind of objective reality.

Philosophers who lean to Kantianism or radical idealism feel there are no “objects-per-se.” We build up a representation of the world using our senses and intellect.

Most scientists say our body of knowledge has increased immensely since Kant’s time, which they say makes obsolete his reasons for doubting human knowledge as a reliable account of what’s out there.

Most philosophers would say Kant wasn’t daring enough. They say he should have rejected the very idea of a “reality-per-se.”

Kant vs d’Espagnat
D’Espagnat agrees that Kant’s arguments were faulty, though justified at the time considering he knew nothing of evolution or mathematical physics.

However, updated arguments lead to a conclusion roughly similar: science doesn’t seek knowledge of “the Real” but rather just of phenomena.

This is not a rejection of some human-independent reality. In this d’Espagnat agrees with Plato and Kant.

Empirical vs Objective Reality
D’Espagnat does not believe physics describes an objective reality. Rather, it describes at most some kind of “empirical reality.”

He is, though, impressed by the consistent nature of Maxwell’s equations and other major laws, leading him to think physics provides “some not altogether misleading glimpses” of the underlying structure of reality.

“The Real” vs Phenomena
D’Espagnat puts out there the idea of an “extended causality” in which “the Real” imparts some kind of non-quantitative influence on phenomena. He adds that this is just a supposition of his.

Unreachable vs Veiled Reality
D’Espagnat does not believe “the Real” is “radically unreachable.” Rather, it is “merely veiled.”

He emphasizes that this doesn’t mean there’s even a “vague similarity” between our perceptions of reality and what’s behind the veil.

“The Real” vs Space-time
Bell’s theorem and Aspect’s experiments (among others) connect space-time with nonlocality. D’Espagnat hence doesn’t think “the Real” lies in space-time.

Mirroring Kant, d’Espagnat believes space-time is not “noumenal.” It’s just a “reality-for-us,” hence “phenomenal.”

D’Espagnat vs Mohrhoff
Another approach comes from Ulrich Mohrhoff. He thinks some aspects of phenomenal space may be considered strongly objective as long as other aspects are discarded. He suggests that physical space does not intrinsically exist independent of the objects that are in them.

In a footnote d’Espagnat says the suggestion “is certainly worth studying.”

D’Espagnat vs Modern Philosophers
Compared to his disagreement with science’s “physical realists,” d’Espagnat says there’s a “high degree of convergence” between his views and many philosophers.

There’s still some difference of opinion. D’Espagnat believes in a veiled reality while many philosophers think reality is “radically unreachable.”

He also notes that many philosophers feel his “open realism” postulate is arbitrary. D’Espagnat admits it’s unprovable (he does call it a postulate, after all) but feels there are serious arguments to support it.

Existence vs Knowledge
D’Espagnat’s first argument for his postulate is that “existence” comes before “knowledge.”

He admits that the existence of a particular something may logically depend on the possibility of our knowing it.

However, he agrees with Plato and Kant that the very notion of existence can’t depend on our possible knowledge of this existence.

Beautiful Theories vs Falsifiability
D’Espagnat’s second argument for his postulate is that beautiful, consistent theories can be struck down by experiment. Theories predict consequences that can be contradicted by observations.

We can’t be totally in control of what we perceive or not.  In d’Espagnat’s words: Something says “no.”

Realism of the Accidents vs External Influence
D’Espagnat’s third argument for his postulate refers back to chapter five’s “no-miracle” and “intersubjective agreement” arguments.

He says they do not support realism of the accidents or anything close to it. However, they do show, in his opinion, that physical laws depend at least partly on something that is not “us.”

Present vs Past Building Blocks of Knowledge
D’Espagnat’s fourth argument for his postulate is that a priori concepts of human knowledge change over time. Kant could rely on Euclidean space, universal time, and determinism to reject reality-per-se as the source of such concepts.

Nowadays science’s building blocks include curved space, space-time, and indeterminism. It’s hard to believe that we can rely on these basic concepts of human knowledge when they keep changing.

Also, Kant’s building blocks were very intuitive. Physics today uses unintuitive concepts that are so unintuitive it’s hard to believe they sprung from some innate concepts we hold.

So we’re left with either physical realism or contenting ourselves with science as prediction.

Physical realism has (in d’Espagnat’s view) been thoroughly demolished, leaving just the empirical/predictive option—with the caveat that there’s something out there that prevents us from being entirely arbitrary in our perception of reality.

Partial Knowledge vs Veiled Reality
D’Espagnat also mentions some misunderstandings he feels others have of his work.

He says Roland Omnès takes his concept of veiled reality and calls it some kind of “weak realism” in which only partial knowledge of reality is possible.

D’Espagnat says he never uses the term “weak realism,” and thinks Omnès doesn’t appreciate the huge jump from “independent reality” to “empirical reality.”

Veiled reality is no more partially knowable than Kant’s phenomena “are knowable bits of noumena.”

D’Espagnat repeats, though, that the mathematics of some physical laws may “vaguely resemble” some of the “great structures of ‘the Real.’”

D’Espagnat vs Esotericism
Although d’Espagnat says he’s mostly disappointed with the “esoteric visions” that have pointed to his writings, although there are some “intermediate cases.”

D’Espagnat appreciates Thierry Magnin’s approach to “levels of reality” but feels he got it wrong when he sees veiled reality related to “unpredictability and chaos,” the “constructive role” of time, and science as a social construct.

D’Espagnat does note that he’ll explore the objectivity of science in the next chapter, but his concept of veiled reality does not spring from that issue.

Veiled Reality vs Anomalous Phenomena
D’Espagnat says it’s understandable that the views of anyone not agreeing with a “mechanistic conception of nature” would be used to explain various “phenomena seemingly defying the laws of science.”

However, d’Espagnat believes nonlocality in no way explains supposed influences at a distance.

Neither does d’Espagnat doubt the “robustness of the physical laws,” but rather believes them likely to be “correct and universal.”

Finally, the concept of veiled reality addresses a philosophical understanding of reality, not the particulars of observed phenomena.

The Elusive Object

The Reformed Realist
Some of Bernard d’Espagnat’s best and dearest friends might be realists.

Chapter nine of his On Physics and Philosophy, entitled “Various Realist Attempts,” describes with a perceptible tinge of sorrow how the conventional realist’s goal seems doomed to failure.

If not certainly doomed, they are at least misguided, he feels, no matter how much he sympathizes with the impulse to believe in a knowable physical reality beyond the appearances.

These attempts have some difficult hurdles to jump. A successful theory should—

1. Make the same (or almost the same) predictions as conventional quantum mechanics
2. Respect the results of Aspect-type experiments and the Bell Theorem
3. Show that the interpretation is more than just a calculating convenience
4. Be more than just a reassuring linguistic reconfiguration, and
5. Keep its conceptual building blocks pretty faithful to its roots in realism.

The last criterion isn’t absolutely necessary, but if the only way a realist theory can work is by defining common terms (such as particles) in curiously non-realist ways then the project seems a bit dubious.

Add to that the requirement to respect the Bell Theorem and (more or less) match conventional quantum theory’s predictions, which mandate nonlocality if you want physical realism, and these efforts look increasingly futile.

In greater detail…

D’Espagnat’s Realism vs Near Realism
D’Espagnat says he very much sympathizes with realists, and says his own views don’t depart too radically from theirs. His disagreement, he says, developed not on a priori grounds but after he pondered the evidence of physics.

Proof vs Sentiment
Physical realism is an unprovable metaphysical stance, one among many. But “nobody” believes the moon disappears when we don’t look at it, says d’Espagnat. Commonsense arguments even convinced Einstein.

Giving Up Physical Realism vs Locality
John Bell (of Bell’s Theorem fame) continued to believe in a physical reality even after his theorem and experimental data shook the foundations of physical realism.

He could have given up the idea of a physical reality knowable in principle, but instead he chose to believe this reality is nonlocal.

Description vs Synthesis
D’Espagnat makes up “Jack,” a physicist who’s a hardline physical realist. Jack believes science has succeeded magnificently on so many levels. Theories aren’t just some synthesis of observations. They are more-or-less accurate descriptions of reality (as d’Espagnat calls it, “reality-per-se”).

Senses vs Reality
Philosophers like Hume would counter that our knowledge of reality depends on our senses, yet we have no guarantee our sensations correspond with reality. Jack might call this argument overly broad as it applies to any piece of knowledge, including our ordinary experiences that we could hardly doubt.

Words vs Reality
The sceptic might then say that the results of experiments are communicated by words, but how do we know these words correspond to the building blocks of reality? Again Jack points to everyday experience and the concepts we seem to know instinctively works: objects, their positions, their motions, and so on.

The hardline realist says an experiment described using these simple concepts surely must say something true about physical reality.

Strong vs Weak Objectivity
Jack the hardline realist might then lament all those physicists who claim to be realists but use standard quantum mechanics. Don’t they realize this theory is only “weakly objective”? In other words, it describes observations but doesn’t claim to describe reality itself.

Standard vs Broglie-Bohm Interpretations
D’Espagnat says Jack would be further perplexed because the Broglie-Bohm interpretation offers predictions identical to the standard interpretation (in the non-relativistic domain) and claims to be an explanation. It doesn’t just predict observations.

It also may offer a (partial) way out of the “and-or” problem with mixed quantum states. We’d like to show why the pointer dial doesn’t indicate multiple values at the same time.

Standard vs Broglie-Bohm Predictions
D’Espagnat notes that Broglie-Bohm’s predictions match the standard model’s. The good news is that Broglie-Bohm’s predictions aren’t wrong. The bad news is the standard model uses simpler mathematics and predicts so much more.

Superficial Realism vs Nonlocal Results
Though not a critical deficiency, it’s definitely odd that Broglie-Bohm starts off with concepts intuitively familiar to us such as corpuscles and trajectories but ends up predicting a nonlocal reality.

This doesn’t mean the theory is wrong, but it does mean the realist’s agenda is somewhat frustrated.

Real vs Abstract Particles
Broglie-Bohm replaces boson particles with abstract quantities (fields or their Fourier components). Photons are only “appearances,” somewhat undermining the realist model. The jury’s still out on how to deal with fermions.

Measured vs Secret Properties
Broglie-Bohm says momentum is really the product of mass and velocity even if quantum measurements show something else (see chapter seven). Also in this model detectors are sometimes “fooled,” acting as if a particle hit them even when it didn’t.

Finally, a “quantum potential,” which doesn’t vary by distance, means “free” particles don’t really travel in straight lines.

So some aspects of reality remain experimentally out of reach, yielding only illusions, an odd position for a realist model to take.

Realism vs Observer Choices
Consider two entangled particles, one going left and one going right. The Broglie-Bohm model says in some set-ups you’ll consistently get the same result if you measure the left-moving particle first, and a different result if you measure the right-moving particle first. Since the particles are entangled, the first one you measure matches the result of the other one you measure.

The problem is that this doesn’t sound like it describes the world “as it really is” but rather just our observations. Our choices as observers seem to affect what’s “really” going on. This does not fit in very well with the realist agenda.

Relativity vs Observer Choices
It gets worse. Depending on who’s checking, the “time order” of these measurements may differ if they’re “spatially separated” (that’s when you’d have to travel faster than the speed of light to get from one measurement to the other). Since the instruments are showing the same result to any observer, are they simultaneously telling the truth and lying?

It appears you can choose a privileged space-time frame that somehow still matches the predictions of special relativity but is consistent with Broglie-Bohm too, but again we end up with all these illusory appearances and an explanation that can’t be verified (or at least distinguished from competing theories).

Bohm #1 vs Bohm #2
D’Espagnat (in a footnote) says difficulties with the Broglie-Bohm model led David Bohm to devise his “implicit order” theory, which does not rely on corpuscles. The problem is that the “implicit” order of what’s really happening is separated from the “explicit” order of appearances, and it’s hard to turn that distinction into an “ontologically interpretable” theory.

Standard vs Modal Interpretations
Borrowing modal logic’s use of intrinsic probabilities, Bas van Fraassen initiated a different approach to realist quantum mechanics that led to various related interpretations.

Wave Function vs Finer States
Standard quantum mechanics says the wave function is the best description of a quantum system. “Modal” interpretations say sometimes there are “finer” states governed by hidden variables (d’Espagnat prefers to call them “supplementary”).

Standard vs Intrinsic Probabilities
In “modal” interpretations the wave function describes the probability of various measurements but not necessarily what is “really” happening. The use of supplementary variables rescues these interpretations from the problem of proper mixtures and ensembles (see chapter eight). A system is in state A or state B even before a measurement, even if the quantum state is A + B.

Wave Function vs Value State
A system’s wave function describes observational probabilities. In a “modal” interpretation the system’s “value state” uses supplementary variables to describe what’s “really” happening.

Broglie-Bohm vs “Modal” Interpretations
“Modal” interpretations are indeterminate and Broglie-Bohm is determinate, but they share the need for supplementary variables that are experimentally undetectable–and they produce predictions identical to the standard interpretation’s.

These realist approaches also seem to violate special relativity. Since their predictions are consistent with the standard interpretation’s they end up being nonlocal, which special relativity isn’t really equipped to handle.

Also, in some cases (say some authors) the “modal” interpretation implies the measurement dial will somehow show a value different from the predicted “observed” value. It’s as convoluted as the measurement issues in Broglie-Bohm (such as detectors’ getting false hits).

Unlike Broglie-Bohm the “modal” interpretations also get into difficulties about properties of a system and its subsystems. A subsystem can have a property even if the system itself doesn’t.

Language vs Ontology
D’Espagnat wonders if the “modal” interpretations are basically just offering a different language convention. The terms make it sound like something is “really” going on, but this alleged reality is inaccessible to observers, and “modal” interpretations make the same predictions as the standard interpretation of quantum mechanics.

Schrödinger vs Heisenberg Representations
Yet another approach makes use of the Heisenberg representation. Its equations are supposedly more realism-friendly than Schrödinger’s wave function.

Time-dependent vs Time-independent Equations
In both representations dynamical quantities (position and velocity, for instance) are represented by “self-adjoint operators.”

The Schrödinger wave function is time independent until a measurement is made. The wave function does double duty, describing states then knowledge.

The Heisenberg representation does things differently. Its self-adjoint operators are time dependent–so maybe they describe “real” states that are evolving through time.

Heisenberg Representation vs Contingent States
The problem is that the self-adjoint operators in the Heisenberg representation, though designating dynamical quantities, refer to all possible values of those quantities. You have to specify initial values if you want the measurement to be a “mental registration” rather than a “creation” of those values.

Just as bad, the best way to specify those initial conditions is by using the wave function.

Heisenberg vs Schrödinger Operators
D’Espagnat says that in the end the self-adjoint operator has too modest a scope in the Heisenberg representation. It does not label contingent states.

In the Schrödinger representation there’s the opposite problem. The self-adjoint operator’s role there is too ambitious. It labels the initial state as it “really” is, which leads to the problems of the measurement collapse.

Feynman’s Reformulation vs Physical Realism
D’Espagnat says high-energy physicists mostly see physical realism as self-evident. Richard Feynman’s “fabricated ontology” greatly eases their calculations, and apparently eases many philosophical doubts too.

Probabilities with Detectors vs without Detectors
In standard quantum mechanics the probability amplitude indicates how likely one would find a particle (for instance) at a particular spot if there were a detector there.

Feynman’s leap was to interpret it as how likely a particle would “arrive” at a certain point–whether or not there was a detector there.

Being vs Calculating
So is this “arrival” (which means that it “is,” however briefly, at that point) an ontological claim or is it just a calculating convenience? D’Espagnat says Feynman knew quite well the problems of interpreting quantum mechanics but was “absolutely reluctant” to talk about them.

Since fringes in a double-slit experiment show up, clearly this way of speaking is just for predictive purposes. If a particle “really arrived” at one slit or the other there’d be no fringes on the detector screen. In fact, the older quantum field theory and the Feynman diagram approaches “are quite strictly equivalent.”

This means they both support the nonlocality hypothesis.

Standard vs Non-Boolean Logic
Quantum mechanics’ formalism uses Hilbert space. This infinite-dimensional abstract space leads some to suggest a non-Boolean logic would rescue objectivist realism.

Formalism vs Experimental Facts
However, d’Espagnat says that this reformulation has no more ontological significance than Feynman’s approach. Nonseparability and nonlocality remain as issues since these are experimental facts not dependent on the formalism. Using a kind of quantum logic can’t on its own describe microsystems in realist terms.

Standard vs Partial Logics
Griffiths, Gell-Mann and Hartle, and Omnès have tried using “partial logics” and “decohering histories.” D’Espagnat says that this approach (like the non-Boolean approach) reformulates quantum mechanics but doesn’t change its predictions. The experimental facts remain a barrier to objectivist realism.

Macroscopic Reality vs Microscopic Unreality
Because of experimental results (such as Aspect’s combined with the Bell inequalities) it’s clear that the microscopic arena is not going to yield to some “strongly objective” form of realism. The challenge then becomes figuring out how “real” macroscopic entities could possibly be made up of “unreal” microscopic constitutents.

Existence vs Meaning
One approach is to deflect the question. Decoherence describes a mechanism by which macroscopic objects have a certain (physical-looking) appearance—but not existence as such. Maybe we can create Dummett-like criteria (see chapter seven) for determining just the meaning (“signification”) of statements about macroreality (but not microreality).

Entities vs Observability
If you’re going to make meaningful statements about macroscopic reality then it would help if you could define macroscopic entities. This is surprisingly difficult. One attempt uses statistical mechanics’ concept of “irreversibility” because human observational skills are limited.

D’Espagnat says this approach doesn’t necessarily sit well with a realist. After all, the general goal of realist approaches is to describe reality (to some degree of accuracy) through our own observations.

Schrödinger’s Cat vs Laplace’s Demon
Decoherence theory says that our inability to make precise measurements of complex systems creates the illusion of macroscopic reality. So what do we do about this limitation? We could imagine some version of Laplace’s demon who’s able to make precise measurements of all physical quantities in the universe.

We could then try to determine if he sees Schrödinger’s cat as simultaneously dead or alive—or just one or the other, as humans do because of their limited observational acuity. This would tell us what’s “really” going on.

But how powerful should this demon be? Let’s assume he can’t use an instrument made up of more atoms than the universe possesses. Some physicists then calculate that even Laplace’s demon couldn’t observe the complex quantum superpositions theoretically observable in macroscopic objects.

The “meaningful” conclusion is that these complex quantities are “nonexistent” and therefore the Schrödinger cat problem disappears.

Realism vs Human Decisions
But can a supposed reality depend on the capabilities of an observer (human or otherwise)? Even more fundamentally, mathematical representations of quantum ensembles (see chapter eight) are compatible with an infinite number of physical representations. Why is just one representation chosen?

In the end it seems this kind of realist argument ends up describing an empirical reality, not a meaningful approximation of an observer-independent reality.

Linear vs Nonlinear Terms
You can trace the “conceptual difficulties” of quantum mechanics back to the mathematical linearity of the formalism. Unsurprisingly, some realists might consider adding terms to make the mathematics nonlinear.

These new terms have almost no effect on observational predictions but allow a profound conceptual leap when it comes to macroscopic objects. Their centre-of-mass wave function will now collapse frequently and spontaneously, so there’s no more “measurement collapse.”

Relativity vs Nonlinear Realism
Nonlocality is still an issue, even though we’re talking about faster-than-light “influences” instead of signalling. The realist might retort that standard quantum mechanics runs into the same problem, but d’Espagnat says it’s the demand for realism that prevents relativity and quantum mechanics from being compatible.

Decoherence vs Nonlinear Realism
Decoherence theory and approaches based on nonlinear terms are making essentially identical predictions. However, decoherence theory says macroscopic objects are just phenomena. We share this knowledge and call it “empirical reality.” Nonlinear realism believes these objects are “real.”

D’Espagnat wonders why we even need nonlinear terms considering that according to conventional (that is, linear) quantum mechanics any macroscopic object with quantum features quickly goes through decoherence and ends up showing classical features.

Appearance vs Reality
So you don’t need nonlinear terms unless you want macroscopic objects not just to “appear” the way they do but also “really” to be like that.

Verbalism vs Reality
D’Espagnat is unimpressed by these ontological manoeuvres. He rhetorically asks if this is “some kind of a poor man’s metaphysics” amounting to little more than “pure verbalism.”

Open Realism vs Commonsense Realism
Yet D’Espagnat is not prepared to abandon realism altogether. He believes in a “veiled reality” that can be gently prodded through an approach he calls “open realism.”

But for realism to be consistent with the results of quantum experiments the reality that’s allowed is far different from the “commonsense” reality of the man in the street, or even that of many hard-nosed physicists.

Measuring the Decoherence

Realistically Speaking
Chapter eight of Bernard d’Espagnat’s On Physics and Philosophy is entitled, “Measurement and Decoherence, Universality Revisited.”

In some ways it was a very dense and difficult chapter to read (and summarize). However, in the end the main points seemed pretty reasonably clear:

1. Quantum universalism and our perceptions of macroscopic reality at first appear to clash
2. A macroscopic object easily shifts between numerous and narrow energy bands under the slightest influence from their environment
3. Therefore it’s almost impossible to measure the exact quantum states of macroscopic objects
4. Our lack of knowledge about large-scale systems in “decoherent” states leads to the apparent stability of the macroscopic world
5. However, on the microscopic level a “realistic” interpretation of superpositions only works if a system includes unmeasurable components or we restrict what measurements we’ll make.

There’s a lot of material in this chapter so one could easily come up with some other highlights. In any event, here are my impressions of the chapter in greater detail…

Realist Statements vs Realist Philosophy
Instead of saying “I see a rock on the path” one could say “I know if I looked on the path to see if I would get the impression of seeing a rock there, I would actually get that impression.”

That would be cumbersome so we use “realistic” statements even if we don’t believe in hard-line realism. If we switch back to the microscopic realm realist-like statements might mislead.

Macroscopic Realism vs Quantum Universalism
If we assume quantum formalism is universal, then why don’t we see a rock in two places at the same time?

Macroscopic realism says macroscopic objects have mind-independent forms located in mind-independent places. So even before we look at it, a measuring device’s pointer will point to one and only one part of the dial.

A macroscopic state-vector therefore can’t be a quantum superposition A + B, and hence we can’t see a rock in two places at the same time.

Schrödinger Equation vs Macroscopic Realism
The problem is that the Schrödinger equation will often demand such a superposition. Realists respond by using something other than state-vectors to describe macroscopic objects.

D’Espagnat says that he showed (in 1976) that such attempts will fail, and a somewhat more general proof was found by Bassi and Ghirardi (in 2000).

Antirealism vs Macroscopic Realism
A different approach is to follow Plato and Kant. The senses are unreliable and deceive us. There’s no distinction between Locke’s reliable “primary” qualities and the less reliable “secondary” qualities.

The only thing certain are the quantum rules that predict our observations. All else is uncertain.

Probability vs Determinism
However, we don’t experience the world as a sequence of probabilistic predictions. We picture objects with definite forms, and we can predict the behaviour of these objects using classical laws that are deterministic.

Textbook Realism vs Quantum Predictive Rules
Part of the problem is that textbooks talk about the mathematics (including symbols for wave forms) as if they represent physical states that “exist” whether or not we’re taking a measurement.

D’Espagnat notes the same old difficulties of realist interpretations will  then reappear. He says symbols for the wave forms and other values should instead represent “epistemological realities.” They signify possible knowledge once the observer makes an observation.

In other words, the quantum rules predict observations, they don’t describe unobserved realities.

Absorbed vs Released Particles
In chapter four d’Espagnat assumed that a measured electron gets absorbed by the measuring instrument. In practice this rarely happens.

If the electron gets released, then the instrument and the electron form a “composite system.” Instrument and electron are “entangled” (in the quantum sense).

Composite States vs Measurements
If an electron is in a quantum superposition of two states, the instrument dial shows just one of those states (which you can confirm by using a second instrument to measure the first instrument).

If you test an “ensemble” of identical states all at once then some of your instruments will show one state while others will show the other state.

Note that the measurement points to the state of the electron after it’s measured, not before.

Measurements vs Quantum Collapse
Some physicists who won’t accept “weak objectivity” or mere “empirical reality” see the measurement process as “collapsing” a “real” wave function.

Quantum Collapse vs Quantum Universality

A quantum collapse is a “discontinuous” transition from the (differential hence continuous) Schrödinger equation.

If the quantum laws are universal, then what’s so special about a measuring instrument to produce this collapse?

Moveable Cuts vs Realism
Using the “von Neumann chain” idea, one can predict observations by placing a “cut” between observer and observed at various points. There’s nothing special about one particular instrument.

The cut may be placed between a measuring instrument and the particle, or between a second instrument (measuring the first instrument) and the first, or between a third instrument and the second, and so on.

Von Neumann showed that the results will be the same no matter where this cut is placed.

The problem is that the realist believes in a mind-independent reality, so presumably this cut should be in one and only one place. The collapse of a quantum system shouldn’t be at the whim of the observer (and his mind!).

Longing for Realism vs the Practice of Operationalism
D’Espagnat says a lot of physicists suffer from a kind of logical “shaky balance.” They want to believe in realism but in their working methods they use “operational” methods (which therefore don’t require a belief in realism).

Schrödinger’s Cat vs Quantum Superposition
Getting back to the composite system of instrument and electron, if the electron was prepared by a superposition of two states, then the composite system is represented by aA + bB. The small letters represent the “states” of the electron, and the big letters represent the states of the instruments.

But the measuring instruments will point to A or B on the dial, not both at the same time. Schrödinger imagined a cat that’s dead or alive depending on the results of the experiment.

We don’t see an instrument pointing to two parts of the dial simultaneously, nor can we imagine the cat is both dead and alive simultaneously.

Quantum Superposition vs Probabilities
The measuring instruments will show one result each time. Quantum rules predict the probability that a particular result will be seen, not that several results will be seen at the same time.

Probabilities vs Ensembles
To test probabilities we can create a really large ensemble of identical conditions and see what results we get. Imagine we create a whole lot of composite systems with an entangled electron and measuring instrument.

On each of those instrument dials we’ll measure one result or another, not both, and not something in between.

Identical States vs a “Proper” Mixture
Staying with the electron that was prepared as a superposition of states, we calculate a percentage probability that we’ll measure that electron as “being” in one specific “state” and another probability it’ll “be” in another “state.”

What if instead of a large number of identical states and identical measuring instruments we prepare some electrons in one state and some others prepared in the other state? We’ll determine how many of each by the predictions for the superposed state.

If we then just measure, say, position, we’ll get (approximately) the same results as predicted for the superposition of states. But if we try measuring something other than position our results may violate these predictions.

So unless we ignore everything but position, measurements on our ensemble of electrons in superposed states will differ from our proper mixture of electrons in pure quantum states.

Coherent vs Decoherent Measurements
Imagine we measure an entangled system of an electron (with states in superposition) and an atom. Then an ensemble of identical superposed states cannot be approximated by a “proper mixture” of separate pure states.

But if the atom and electron interact with a molecule that is too complex to measure, our measurements of the electron–atom system will be the same whether we measure an ensemble of identical states or a proper mixture.

The system has become “decoherent.”

Electron–Instrument vs Electron–Instrument–Environment Systems
It’s already hard enough to measure the “state” of an electron using an instrument. If we try to measure the “state” of the electron and the instrument in relation to the environment then we have a big problem.

Macroscopic vs Microscopic Energy Levels
A macroscopic object’s energy levels are very close to each other, so a very small disturbance from its environment (or its internal constituents) will shift its energy level.

Measurement Imprecision vs Quantum Precision
There is thus so much environmental influence on an instrument that we cannot measure the “state” of the instrument and electron as a system in the same way we were able to measure just the “state” of the electron.

That’s why we can’t perform an experiment similar to our earlier one that found differences between measurements on the ensemble of superposed states and the proper mixture of separate pure states.

Therefore an instrument pointer, which is a macroscopic object, will act like it’s in a single state, not a superposition.

Ensembles vs Double-slit Experiments
In the “Young slit experiment” we imagine a particle source, a barrier with two slits, and a detector screen (see chapter four). Normally the screen would show fringe-like patterns because of the quantum system’s wavelike nature.

However, if you add a dense gas to the area between the barrier and the detector screen then you’ll just see two “blobs,” therefore showing no evidence of wave-like interference.

The molecules in front of the screen are analogous to the molecules that are near an electron–atom system. The molecules form part of a system but are not themselves measured. In both cases we lose the effects of superposition.

Independent vs Empirical Reality
Because the insertion of unmeasurable molecules prompts us to infer distinct beams with distinct states (corresponding to the “up” or “bottom” slit), this shows how decoherence creates the illusion of a macroscopic reality.

D’Espagnat acknowledges it’s a bit artificial to make this distinction since we know about the particle source. But it reminds us that decoherence is what provides the illusion of an independent reality, although it’s really just an “empirical” reality.

Entanglement vs Reduced States
If one system gets “entangled” with another (such as an electron with an atom) then each system loses its own distinct wave function. There’ll now be a wave function for the combined system.

But the quantum formalism allows some information about the original system to be recovered if we imagine a large ensemble of its replicas. The mathematics that represents this is called a “reduced state.”

Quantum Prediction vs Decoherence
Imagine an ensemble of grain sands or dust specks. They’re small but still macroscopic. The quantum formalism predicts these small objects would be enough to produce the macroscopic effects in the Young slit experiment.

And the quantum formalism also predicts that these objects will act macroscopically, supporting the role of decoherence in creating the illusion of a macroscopic reality.

Reduced State vs Localization
The matrix mathematics used to describe the reduced state suggests the reduced state can stand in for an infinite number of proper mixtures of pure quantum states, which threatens the idea of locality. Fortunately at least one of those proper mixtures is composed of quantum states that are localized.

Experimental Superposition vs Decoherence
In experiments by Brune et al. a “mesoscopic” object is put into a superposition of states. In the brief time before environmental interactions introduce decoherence, the object’s quantum properties can be observed.

The experiments therefore provide evidence both for decoherence and for the validity of quantum laws in objects larger than microscopic.

Quantum Universality vs Classical Laws
Brune’s experiments support quantum universality, but it would be good if we could also show how to derive the laws of classical physics from the rules of quantum prediction.

Classical Numbers vs Quantum Operators
In classical physics various properties of an object (such as a table’s length) are represented by numbers governed by classical mechanics. In quantum physics these properties are represented by (Heisenberg) operators and obey quantum equations.

Roland Omnès has proved that the observational predictions of both approaches coincide (in classical physics’ traditional domains).

Quantum Laws vs “Reifying by Thought”
Because classical physics and their predictive formulas are so reliable in the macroscopic realm we naturally infer that past objects and events have “caused” present ones, and present ones will “cause” future ones.

Counterfactuality vs Quantum Mechanics
Counterfactuality depends on locality, but Bell’s Theorem combined with the Aspect-type experiments show that nonlocality, and hence counterfactuality, is violated (relevant if we’re realists).

If we want to show classical and quantum predictions are the same in the macroscopic realm then we’re going to have to figure out how to “recover” the counterfactuality we imagine macroscopic reality possesses.

Is there action-at-a-distance with macroscopic darts? It turns out their orientation is a macroscopic variable that “washes away” microscopic variations.

In fact orientation is one of the “collective variables” that includes length, mass, and other classically measurable quantities. We’ve already noted that Omnès showed their values are consistent with quantum formalism.

Macroscopic Certainty vs Microscopic Uncertainty
Measuring a “complete set of compatible observables” will give you the state vector that “exists” after all the measurements were made, but that doesn’t help you figure out the state vector that “existed” before you made any measurements.

The idea of a measurement is usually that it measures something previously existing. By that standard you can’t figure out a state vector for sure no matter how many measurements you make.

By contrast, the mathematics behind a macroscopic ensemble’s “reduced state” will tell us which physical quantities may be measured without disturbing the system. We can therefore recover the “state” of a macroscopic member of that ensemble.

D’Espagnat says this ability helps shed light on our intuition that the properties of something must have been the same before we looked at it.

Realism vs Semirealism
D’Espagnat will discuss those who still cling to realism in the next chapter. However, he says there are “semirealist” approaches that manage to stay faithful to the quantum formalism.

A and B vs A or B
The “and–or problem” arises because when we measure a system of superposed states aA + bB we see it as either in state A or in state B, not in both states A and B at the same time. This shift from “and” to “or” is nowhere suggested in the equations. D’Espagnat suggests this is a conceptual not a mathematical issue.

One vs Many Realities
The mathematics of quantum formalism does not require there just be one and only one reality. Everett’s “relative state theory” interprets this formalism to suggest that the universe “branches off” when a superposed system is measured.

In a given branch only one of the superposed “states” is measured, but the overall multi-branch system is still represented by the same expression that combines superposition plus entanglement: aA + bB.

Common Sense vs Formalism
Some physicists are attracted to Everett’s branching universes because it agrees with the quantum formalism. They believe that following the formalism first rather than common sense could bring in a revolution similar to relativity’s own repudiation of common sense.

Zurek vs Reality
Zurek showed that the “reduced state” of a macroscopic ensemble is stable under certain measurements. He goes further and defines “reality” as whatever is out there that remains stable under such measurements.

Quantum Universality vs Classical Foundations
Decoherence theory tips the balance away from thinking classical physics is somehow more foundational than quantum physics. Decoherence theory shows how the rules of classical physics may be derived from quantum rules.

Physics vs Chemistry, Biology, and Other Disciplines
Decoherence theory can’t let us predict the structure of other disciplines though. The quantum formalism has to be simplified “by hand.” Quantum theory is still universal, but our human choices, our human ways of conceiving things, will crucially guide our perceptions.

The Antirealist’s Reality

The Invisible Hand
Chapter seven of Bernard d’Espagnat’s On Physics and Philosophy is a kind of grab bag, entitled: “Antirealism and Physics; the Einstein-Podolsky-Rosen Problem; Methodological Operationalism.”

D’Espagnat’s points in this chapter seem to boil down to this:

1. Physics (and science in general) is about predicting observations not describing some kind of reality
2. Operationalism (which concentrates on methodology) increases the reliability of science as it counters critics who complain scientific theories (which they say should describe and explain reality) keep changing, and
3. Although measurements (of “empirical” reality) depend on the observer, physical laws seem to be constrained in various ways (by the structure of an “ultimate” reality that’s scientifically indescribable).

This chapter feels a little scattered as d’Espagnat pre-emptively defends himself against a bevy of incoming realist missiles.

In the end, though, he’s an antirealist in terms of empirical reality, and a realist in his belief there’s an ultimate reality that’s (probably) beyond our direct knowledge but nonetheless influences the shape of our everyday reality.

Here’s some more detail…

Unconscious vs Conscious Antirealism
D’Espagnat says modern physicists (ever since Galileo) generally use an antirealist approach in their methods even if they don’t explicitly embrace antirealism as a philosophy.

Mind-independent Realism vs Pythagorean Ontology
Objectivist realism claims there’s a mind-independent reality whose contents resemble our observations.

A Pythagorean Ontology (capital “O”) claims there’s a mind-independent reality that is reachable through deeper mathematical truths.

Unlike either of these approaches, modern physics emphasizes instruments and measurements. It’s not very interested in saying what’s “really” out there in the “world,” whether physical or mathematical.

Meaningful Statements in Classical vs Quantum Physics
While done more intuitively in the past, physicists nowadays can more formally apply “meaningfulness conditions” to statements.

Also, quantum systems are so peculiar that certain distinctions need to be made. Antirealist statements have to be expressed and tested in special ways.

Facts vs Contingent Statements
D’Espagnat is concerned here not with general “factual” statements such as “Protons bear an electric charge” but rather with satements about physical quantities. A value is assigned to the speed of a particular object, for instance.

True/False Statements vs Meaningless Statements
Based on Dummett’s approach a statement about an object’s speed would be meaningful only if we can measure (at least in principle) that physical quantity at some specified time and place.

Necessary vs Sufficient Grounds for Meaningfulness
D’Espagnat says Dummett’s criterion is necessary, but that doesn’t mean it’s sufficient. Other conditions may need to be fulfilled.

Imagining vs Measuring a Quantity
It’s possible that we can conceive of a physical quantity that has no meaning. However, if we can measure it then that quantity will definitely have meaning.

Classical vs Quantum Measurements
In classical physics it’s intuitive to think a measurement reflects the “true” values of an object, but in quantum systems the measurement of a particle (depending on your model) either creates or changes the values that you’re trying to measure.

In quantum physics we’re not simply “registering” some pre-existing value when we take a measurement. So the “truth value” criteria will need to include more than just measurability.

Disturbing vs Non-disturbing Measurements
In the spirit of antirealism D’Espagnat introduces a test: for a statement to have a truth value “it should be possible” (at least in theory) to measure the required physical quantity without disturbing the system.

The Einstein–Podolsky–Rosen trio claimed in 1935 that in some cases there are indirect ways to make non-disturbing measurements, admittedly only on correlated systems.

Correlated Darts vs Photons
If you throw a pair of correlated darts (see chapter three) they originally have some identical orientation. Measuring one dart’s value after they become separated will tell us the other dart’s value. As a bonus, the measurement won’t even change that other dart’s orientation.

If instead of darts you use correlated photons, and instead of measuring orientation you measure the polarization vector’s component at some angle, then you run into a problem.

Consistent vs Broken Correlations
If you measure one photon’s component at a certain angle then you can be sure if you measure the other photon’s component at the same angle you’ll get the same value (which will simply be “plus” or “minus”).

Because we are capable of making this measurement then by our meaningfulness test we can tell if a statement about those values is true or false.

But quantum formalism says the system of these two photons can have just one value at a time. We can’t measure one photon at a particular angle, then measure the other photon to measure another angle’s polarization component.

Multiple Values vs Bell’s Inequalities
At least we can’t then claim the second photon has simultaneous values at two different angles. The first measurement destroys the original correlation.

Because Bell’s inequalities have been disproved experimentally, we know that these multiple values don’t exist simultaneously.

And because our original meaningfulness test implied such a simultaneity we know that test is flawed.

Actual vs Possible Measurements
If we instead require that measurements are available rather than merely could be available then we get a stricter test. By phrasing our requirements in the indicative not the conditional we end up with a sufficient condition, not just a necessary one.

Possible Measurements vs Observational Predictions
Dummett’s meaningfulness test is a very general antirealist approach. It doesn’t look at the factual data actually available in a microscopic situation. It just considers our ability to make measurements in principle.

D’Espagnat says the tighter requirements he’d impose take an approach even further along the antirealist path as they speak of observational predictions not measurements. This also takes us further down the path of instrumentalism.

Operationalism vs the Value of Science
D’Espagnat says if you understand operationalism properly then you’ll realize operationalism confirms the value of science and makes its statements more reliable.

Description vs Prediction
D’Espagnat says critics of science believe scientific knowledge is easily influenced by social and cultural factors, and is frequently throwing out old theories for the sake of very different new ones.

Superficially this makes sense. Einstein’s curved space-time replaced Newton’s gravitational force. They’re radically different approaches.

But science isn’t trying to describe reality. It’s trying to make predictions about observations. Newton’s approach makes good predictions in its own domain, but in other domains Einstein’s predictions are the only ones that work out.

Sometimes the predictions and domains can be identical. Fresnel’s and Maxwell’s theories of light make the same predictions. D’Espagnat says the value of Fresnel’s theory was independent of whether the ether was really out there.

If you drop the naïve realism and its concern for description, then science as a method for synthesizing and predicting experience is not so inconsistent.

Now we can see steady progress as science gets better and better in its power of prediction.

Scientific Knowledge vs Practicality
D’Espagnat says science is mainly knowledge. Even if science is  concerned with prediction and not description, don’t confuse science with the various practical uses it’s put to (such as technology).

Descriptive vs Instrumentalist Knowledge
Science brings together an account of human experience that can be communicated: “If we do this, then we observe that.” Just because it’s not trying to describe “reality” doesn’t mean it’s not imparting some kind of knowledge.

Instrumentalist vs Theoretical Knowledge
These methods of making observational predictions are at the core of science. Coming up with a theory to define certain terms and describe certain entities can be useful, but that’s something added onto this predictive foundation.

Operationalism vs Instrumentalism
D’Espagnat doesn’t try to distinguish the two terms. He says the most important aspect of any theory that conforms to this approach is that it’s an instrument of making observational predictions. He says mathematical physics is a prime example.

Open Realism vs Endless Possibilities
In chapter five D’Espagnat talked of his preferred approach of “open realism.” Certainly our view of “reality” (specifically its physical laws) depends on us, including our ability to make observations. But there seem to be “constraints” on what kinds of theories are valid.

Describing vs Acknowledging Constraints
This “something else” that lies beyond our observations but somehow constrains them may not be directly accessible by us, but D’Espagnat says our inability to describe the constraints does not mean they don’t exist.

Ultimate vs Empirical Reality
An elusive, indescribable “ultimate reality” may still shape the physical laws that we describe. In turn the laws we infer are shaped from our observations that contribute to our sense of “empirical reality.”

Explanations vs Theories
D’Espagnat quotes one critic of operationalism, Mario Bunge, who says that the main role of a theory is to provide an explanation. Therefore a theory must provide at least a “rough sketch” of reality as it is.

D’Espagnat replies that the explanation would actually lie in the ultimate reality that constrains our physical laws, but this ultimate reality is not scientifically describable. Therefore what Bunge desires is impossible.

Unless we grant that “miracles” happen all the time there appear to be constraints on our physical laws. But the ultimate reality producing these constraints can’t be scientifically described because of the problems with objectivist realism noted before.

Physics vs Physical Objects
D’Espagnat says that Bunge considers a value in physics attached to something that is not physical is meaningless. If the value doesn’t refer to something “real” then it’s pointless.

D’Espagnat points out that many physical laws refer to values that are not attached to existing physical objects. Probability is a concept referring to either imaginary objects or is a thought not subject to physics.

Particles vs Waves
Also, wave functions are useful, in fact, essential for quantum physics. So are wave functions real? If so, then particles would have to be real too. If waves and particles exist simultaneously then we’d have to accept the Broglie–Bohm model with all its problems (see chapter nine).

Also, a ground-state electron in a hydrogen atom would seem to have zero momentum because it’s not changing state (quantum potential is balanced by Coulomb force). But the Compton effect shows momentum is non-zero. We have two different versions of momentum. If they were both “real” then we get into pointless difficulties, says d’Espagnat.

Other possibilities: waves change into particles (but the collapse of the wave function has lots of problems attached to it) or only waves exist (but then nonseparability and measurements cause problems).

So D’Espagnat says Bunge’s objections seem pretty “dogmatic.”

Circular vs Practical Definitions
Another objection notes (correctly, d’Espagnat acknowledges) that operationalists place a lot of emphasis on precise definitions, but Bunge says some concepts will remain undefined (just like a dictionary uses some undefined words to define other words).

D’Espagnat replies that operationalism is a methodology, not an “a priori” philosophical system. We want efficiency. Dictionaries are useful despite their undefined terms. Some concepts we just seem to naturally know (whether they’re born with us or not).

These undefined concepts (though neither certain nor absolute) let us operate a measuring instrument, for instance, which then lets us define other concepts.

Sometimes concepts considered “primary” in the past get defined explicitly, such as Einstein’s replacement of “absolute time” with a time that’s partly relative to the observer.

Measurement vs Change
The act of measurement seems to change the quantum system. If, as Bunge’s approach would suggest, this change is “real” then we’d have the difficult problem of explaining this change.

But the quantum approach is “weakly objective” so it refers only to measurement. In the end theoretical entities are useful for helping to make predictions in modern physics. Just don’t regard them as self-contained and “real.”

Einsteinian Hope vs Descriptive Failure
Einstein and those of a similar optimistic bent believed reality would be increasingly describable. This view does not seem consistent with the reality that the quantum framework paints.

Universal Appeal

Confessions of an Open Realist
Like a slow-moving detective novel various suspects of an epistemological and ontological inclination have been eliminated chapter by chapter.

Bernard d’Espagnat, writing in chapter six of his On Physics and Philosophy, starts honing in on his favoured if still rather vague suspect, which he’s identified as “open realism” in previous chapters.

In the first chapter he defined the position as a “starting point” for further investigation. It was compatible with any approach save for “radical idealism.”

There is “something” out there that’s independent of the mind, he says, but whether that’s God, the Platonic Ideas, or something else, he’s not letting on.

So here’s a summary of chapter six, entitled “Universal Laws and the ‘Reality’ Question.”

Theoretical Frameworks vs Ordinary Theories
D’Espagnat believes pure physics has two kinds of theories: “theoretical frameworks” and “theories in the ordinary sense of the term.”

Newtonian Mechanics vs Law of Forces
Newtonian Mechanics was believed to have universal applicability. It could accommodate new forces such as electricity and magnetism. Hence it was a “universal theoretical framework.”

Newton’s theory of universal gravitation precisely specified various laws of forces. It concerned itself about details of a specific domain, and hence is a “theory in the ordinary sense of the term.”

Complete vs Partial Universality
What about modern physics? D’Espagnat asks if there are genuine theoretical frameworks out there, a set of laws with complete universality.

Classical Physics vs Modern Physics
Classical physics looked like the foundation of all sciences. Unfortunately it made some wrong predictions.

Quantum mechanics yields correct predictions whenever it’s used, so it’s the only candidate for a universal theoretical framework. Its specific applications such as non-relativistic quantum physics and quantum electrodynamics are ordinary theories.

Hard Sciences vs Soft Sciences
D’Espagnat notes that some thinkers in the soft sciences rightly point out the horrors that result when universality is applied to political and social realms. The difficulty arises when philosophers extend that criticism to the hard sciences.

Evidence vs Convenience
In the soft sciences objections to universality comes down to how useful or not, and how convenient or not the concept of universality turns out to be. This isn’t a logical argument that can be applied to the hard sciences.

Karate Blows vs Disc Galaxies
However, Scientific American runs articles on karate blows and disc galaxies. Can science really be so universal that it can apply to such a diverse range of topics?

Extreme vs Moderate Universalism
The objection isn’t convincing. We can imagine the electric field of an atom guaranteeing the stability of atoms in muscles and in galaxies.

Strictly speaking, says D’Espagnat, we can’t even discount extreme universalism in which everything is predicted from various general laws.

A less ambitious version of universalism (such as Hans Primas’s) says one could choose which laws to use from a larger set depending on the problem at hand.

Naive Realists vs Universalists
Even “naive realists” don’t always accept universality. The “vitalists” felt special rules applied to living beings.

Realists about Theories vs Realists about Entities
Realists about theories generally support universalism, otherwise what would the theories apply to?

Realists about entities (when not realists about theories too) move away from universalism. Despite the evidence of modern physics they feel an individual object has properties possessing an “existential primacy.”

Movable vs Unmovable Real
Even if we can’t move a ghost, flying saucer, or quasar, we can move a rock or an electron beam. Aren’t they real? Well, quantum field theory says a particle is not a reality in itself.

Real Individuality vs Correct Predictions
If you assume an electron is really an individual entity then you’ll predicts results different from modern physics. Remember that quantum predictions have never been contradicted by the evidence.

The Broken vs Unbroken Stick
On a macroscopic scale imagine a stick that’s partly immersed in water. It looks bent. We can move the stick up and down, and therefore move the “break.” That doesn’t make it real.

The Broken Stick vs The Atomic Microscope
Not only can we move a supposedly broken stick we can also move atoms with a tunnel-effect microscope, but that doesn’t prove the atoms exist as localized individual objects.

Objectivist Realism vs Logical Positivism
D’Espagnat tries to steer a course halfway between objectivist realism and logical positivism.

Existence vs Measurement Statements
Different ways of thinking produce different kinds of questions. “Is the stick broken or not?” is asking for a statement about what is “really” happening rather than the results of an observation.

Appearances vs Reality
Philosophers, especially the popularizers, like to point out physical appearances can be deceiving. That table is mostly empty space, for instance, not something classical physics would admit.

Facts in Old vs New Physics
Many thinkers stress the importance of facts. In “old-time physics” the microscopic level was real and precisely defined, serving as the foundation for the macroscopic. Many say that in modern physics there are no real facts as such.

No Boundary vs Fuzzy Boundary
In classical physics there’s no boundary between microscopic and macroscopic. In the new physics there’s a boundary that is rather fuzzy and depends on our observational abilities. The boundary is therefore “weakly objective.”

Classical Microcosm vs Broglie-Bohm
Classical physics saw the microcosm ontologically. A minority view in modern physics, the Broglie-Bohm interpretation of quantum mechanics attempts a microcosmic ontology but runs into difficulties.

Near Realism vs Collective Experience
Near realism (see chapter one) thinks we can ask questions about “reality-per-se.” It’s similar to “realism about entities” and doesn’t fit with the experimental data.

Another approach says our discursive knowledge springs from a “synthetic ordering” of our collective human experience. It’s a form of positivism.

Partial vs Total Positivism
But we don’t have to be total positivists. The Vienna Circle of early twentieth-century positivists confined scientific statements to observations. We’re not logically obligated to agree with this position.

Positivists vs Working Physicists
Most working physicists are intuitively realists. Unlike positivists, physicists changed their mind about total realism because of observational data.

Near Realism vs Objectivist Language
If we stop believing realism about entities (hence reality-per-se) then we can use objectivist language and Carnap’s linguistic framework to ask questions about existence or attributes.

We answer those questions through empirical investigations. We check the stick in the water and we (usually) answer that it’s not broken.

Human Skill vs Robot Fingers
A robot with less skillful appendages might only be able to move the stick up and down rather than carefully checking it from top to bottom.

Carnap would say we’re right to say the stick isn’t broken, and the robot is right to say the stick is broken. Different abilities let us assert different things.

Realism about Entities vs Realism about Theories
If we discard realism about entities we can still embrace realism about theories, which is much more universal than realism about entities.

Pythagorism vs Einsteinism
“Pythagorism” reminds us of how much modern physics looks for symmetry and symmetry-breaking. Espagnat’s term “Einsteinism” is the variant of physicists who miss Cartesian mechanism and search for the “true” concepts supposedly contained in mathematics.

Einsteinism vs Positivism
Einsteinism doesn’t restrict itself to observations of pointers and gradated scales. It’s close to an Ontology (big “O”). Einstein later in life felt general relativity offered genuine descriptions of structures really out there.

Bundled Realism vs Individual Concepts
But Einstein also refused to point to this or that concept as indispensably real. One had to verify the whole array of concepts taken together: physical reality, the outside reality, and the real state of a system, for instance. The verification step would show which concepts were needed and which weren’t.

Pre-arranged Ontology vs Consistency Quest
This “Pythagorean” ontology isn’t set up in advance but results from a successful consistency quest. Einstein believed he’d largely completed that quest, so felt confident in his realist stance.

Kant vs Einsteinism
Einsteinism’s Ontology/ontology (see chapter five) relies on contemporary physics’ mathematical entities. Kant would have disliked the ones that don’t correspond to an “a priori mode of our sensibility.”

This approach gives Einsteinism an advantage over moderate or radical idealism (see chapter 13).

Physics vs Einsteinism
The big challenge to Einsteinism isn’t philosophical but rather the results of modern physics.

In chapter two we saw how the ontological pictures of Feynman formalism made up just a pseudo-ontology.

In chapter three we saw how instrumentalism reconciled relativity and faster-than-light influences (in a realist interpretation).

In chapters four and five we ran into difficulties trying to fit quantum mechanics’ mathematical symbols into an ontological framework.

Platonist Intuition vs Quantum Data
Some physicists still embrace the intuition of pure mathematical beings waiting to be discovered in a world more real than our own.

The intuition is shattered by the experimental data showing the quantum framework’s mathematical formalism can’t access a mind-independent reality.

All “theories in the usual sense” based on this framework, such as supersymmetry and superstring theories, will encounter the same problem.

Independent Reality vs Research Guide
D’Espagnat believes that Pythagorism’s search for symmetry is still the best approach for physicists to take in their research even though mathematical physics can’t truly describe an independent reality.

However, great mathematical laws may still reflect “something” of this reality.

Naive Realism vs Modern Macrorealism
Physicists know that most people’s spontaneous realism is unjustified. Even Broglie-Bohm theory adopts a different kind of realism. But some thinkers want to rescue realism at least in the macroscopic realm.

Realism of the entities and classical mechanics are both correct, they’ll say, on larger scales. On the smaller scale there are two approaches.

Microscopic Measurements vs Partial Logics
Some advocates of macrorealism will say quantum physics describes measurements of the microscopic world but doesn’t describe it “as it is.” Everything we know about the world comes from our senses, but these “empiricists” don’t question the intrinsic reality of this observed world.

Other advocates assume we know the world (more or less) at it really is, but introduce different kinds of logic. Omnés’s “partial logics” are even quantitative. All this is instructive but not a “realist” position strictly speaking.

Quantum Rules vs Universal Frameworks
When it comes to a universal theoretical framework the quantum framework is the only plausible candidate. It has great predictive powers, but is it universal?

Atomicity vs Nonlocality
The “atomicity argument” says quantum mechanics successfully describes particles and fields, atoms are composed of particles and fields, and everything else is composed of atoms. Therefore quantum mechanics must be universal.

But this argument depends on the Cartesian principle of divisibility by thought. It imagines a mind-independent external reality with interacting but distinct parts. Quantum nonlocality (more specifically, nonseparability) disproves this approach in principle.

Atomicity vs Quantum Predictions
We could try to fashion together a compromise: pretend atomicity works, but when it doesn’t then use quantum mechanics for the rest of the predictions. This empirical argument doesn’t help show the quantum framework is universal.

Atomicity vs Born Rule
Another problem with the atomicity argument is that the Born rule says “orthodox” quantum mechanics makes predictions about observations. It doesn’t say whether an event takes place. That makes quantum mechanics “weakly objective.”

Atomicity vs Instrumentalism
Instrumentalism reconciles Aspect-like experiments and relativity theory (see chapter three), so again basic physics seems to be a source of mainly observational predictions.

Macroscopic vs Quantum Physics
The atomicity argument’s internal inconsistency suggests a different approach. With quantum mechanics’ predictive powers so impressive, can we derive macroscopic physics from the quantum framework?

In chapter eight we see recent evidence that it can. Universality of the quantum framework seems established.

Quantum Rules vs Quantum Theories
This quantum universality recalls Newtonian mechanics’ three great laws, which were considered to possess a universal scope. In our arguments we’re concentrating on fundamental questions about the quantum framework, which consist of the rules of quantum prediction.

We’re not really concerned about the specific ways this framework is applied to “theories in the usual sense” such as quantum field theory.

Dummetian Realists vs Antirealists
M. Dummet says realists and anti-realists differ in how they evaluate certain kinds of statements such as class L of general laws and class F of contingent facts (which is what most concerns d’Espagnat).

Knowledge-Independent vs Knowledge-Dependent Truths
Realists will believe a statement has an objective truth value whether or not we have a way to confirm it. Anti-realists believe a statement can be true only if it concerns something we could possibly know.

Imagine the late Mr. X. He led a sheltered life and never had to show cowardice or courage. How do we react to a statement, “Mr. X was a brave man”? A “Dumettian realist” will say it’s a meaningful statement, while a “Dumettian antirealist” will say it’s not.

Obvious Statements vs Complicated Concepts
A problem with Dumett’s approach is it assumes parts of statements are obvious so disputes concern whole statements. In modern physics the mathematical formalism and observational data mean we have to more carefully define “realism” and “antirealism.”

Small vs Large Domains of Definition
“Operational definitions” are discussed in chapter seven, but briefly philosophers debate how far a word’s meaning can be extended. A “consequent antirealist” says a concept depends on the factual data it was designed to describe. D’Espagnat mostly agrees.

However, d’Espagnat notes there may be exceptions. The English empiricists said, “Nothing is in the mind that has not passed through the senses,” but we can’t prove this rule is universal.

Antirealism vs Necessary Ideas
D’Espagnat believes the notion of existence is a “necessary idea” despite the English empiricists (see chapter five). He says one can believe in a necessary idea and still be a kind of antirealist.

Antirealism vs Metaphysical Realism
Citing Lena Soler, he says an antirealist can accept or reject metaphysical realism as long as he doesn’t claim a correspondence between theory and referent.

Constraints vs Correspondences
An “extra-linguistic referent” may still constrain scientific theories, perhaps by indicating that some possibilities won’t work out, even though we can’t describe it directly.

Open Realism vs Metaphysical Realism
D’Espagnat supports “open realism,” which he says is very close to metaphysical realism in a broad sense.

Open Realism vs Soler’s Antirealism
He concludes by saying his views are also compatible with antirealism in the way Soler presents it.

Getting Real

Chapter five of Bernard d’Espagnat’s On Physics and Philosophy is entitled “Quantum Physics and Realism.”

D’Espagnat attempts to demolish various arguments for conventional realism even as he pokes holes in anti-realist arguments, finally settling on a kind of unknowable realism — unknowable except indirectly through the patterns predicted by the laws of quantum physics.

The last few sections of the chapter feel somewhat disjointed as he discusses related issues but kind of runs out of steam.

Here in more detail are some of the dichotomies (and similarities) he raises.

Physical Realism: Instinct vs Argument
D’Espagnat says scientists and “laymen” generally support physical realism, not just because it’s “instinctive” but because of some explicit arguments.

Practical vs Counterfactual Definitions
Some philosophers consider what is real to be what we can act on. But we can’t act on stars. So other philosophers speak of what would be the results if one performed an action. This is a counterfactual.

Classical vs Modal Logic
When we bring in the conditional we leave classical logic and move into the realm of modal logic.

Actual vs Counterfactual Measurements
Quantum formalism says little about counterfactuality. We can anticipate what information we’d gain if we actually performed a measurement. But this expectation doesn’t guarantee anything about the system if we perform a different measurement instead.

Disturbing vs Not Disturbing the System
The uncertainty over a system’s state persists even if we perform measurements that couldn’t possibly “disturb” the system. If we perform one measurement we can’t say it has the state that some other measurement would reveal — unless we actually make that measurement.

Conceivable vs Actual Tests
A realist who uses so much counterfactuality faces a strict litmus test for any strongly objective statements: no consequence of such a statement can be false if a test is actually performed.

Intuitive vs Rigorous Realism
It might seem self-evident that at least on the macroscopic level realism works. However, these arguments end up failing.

Predictions vs Proof
The “no-miracle argument” (or “inference toward the best explanation”) says a theory that makes lots of successful predictions is likely to be correct.

Realist vs Quantum Predictions
The problem is that quantum theory makes macroscopic predictions that match those made by realism of the accidents, so there’s no proof that objects exist with attributes the way our common sense tells us they should.

Realism vs the No-Miracle Argument
If the “no-miracle argument” fails to prove something as “obvious” as the existence of objects, can it be rescued or does it fail entirely? D’Espagnat considers two counterarguments.

“Equivalent” vs “No Equivalent” Option
You can try eliminating the quantum option by removing the “equivalent theory” option from the “no-miracle argument.” But some philosophers say the “no-miracle argument” still fails, because realism of the accidents doesn’t explain enough.

Minimal vs Generous Explanations
These philosophers say a scientific theory has to prove more than the problem at hand. Newton explained planetary orbits but in the process also explained gravitation, the Moon’s motion, and the return of Halley’s comet.

Realism of the accidents “explains” how we make predictions in our daily lives, but appears to offer no corroboration beyond this domain. D’Espagnat sympathizes with this argument but notes it doesn’t offer an alternative to the realist position.

Raw Observations vs Constructed Entities
A second anti-realist argument is that observations have to be interpreted: the sun just doesn’t sink into the western sea. But scientific revolutions can replace some entities with totally different ones (Newton vs. Einstein’s theories of gravitation, for instance).

If a theory can junk old entities, or offer two equivalent but very different mathematical formalisms, how is a realist to know which interpretation to trust?

Again, d’Espagnat says this argument should be taken seriously, but doesn’t undermine quantum theory as a possible replacement for realism of the accidents.

Realism’s Flaws vs Disproving It
Acknowledging these flaws in the case for realism of the accidents, d’Espagnat says these problems don’t prove realism is entirely wrong.

Descriptions vs Open Realism
D’Espagnat advocates an “open realism.” He says the counterarguments attack realism’s “power to describe,” but it might still be “a miracle” if there didn’t exist a mind-independent reality beyond words.

Laws of Physics vs Whimsy
The “no-miracle argument” comes in handy when considering the laws of physics. We can’t just decide the electromagnetic field is a scalar. Something constrains our imagination as we discover such laws.

D’Espagnat says the no-miracle “postulate” can’t prove conventional realism, but it justifies a kind of “open realism” with its mind-independent reality.

No-Miracle vs Intersubjective Agreement
We’ve looked at the “no-miracle” argument based on successful predictions. Now we look at an argument based on agreement between observers.

Contingent vs Non-contingent Facts
The intersubjective argument looks at agreement between observers about “contingent” facts. These are statements about how things are in reality rather than as a logical necessity.

Reality vs Mental Organization
A contingent fact might be that there’s a teapot on the table. If two people agree that’s the case then the simplest explanation is there’s really a teapot there.

One could also argue that the concept of “teapot” just mentally organizes our sensations, but then (d’Espagnat says) it would be hard to see how two people could agree on what they’re seeing.

Phenomena vs Noumena
Some anti-realist philosophers object that the concept of causality applies to phenomena, not noumena (such as Kant’s). They refuse to assume a relationship between a person’s mental images and the real world.

Phenomena vs Ad Hoc Objection
We’ve just seen the anti-realist objection about phenomena. There’s another objection that the realist argument based on intersubjectivity is too ad hoc.

Minimal vs Generous Explanations (Encore)
The realist’s explanation for the intersubjective agreement (so the objection goes) only explains the agreement, nothing more. The claim is that you should be able to apply a good theory to more than just the initial problem.

Noumena vs the Objectivist Realist
D’Espagnat says both the phenomena and ad hoc counterarguments rely on the concept of noumena (some reality not evident in the phenomena).

He adds that the objectivist realist would reject the idea of a noumena.

Objections vs Alternatives
Also, neither counterargument offers a better explanation of intersubjective agreement.

Objections vs Disproof
So neither objection delivers a knockout punch against our intuition that objects exist because we mutually agree they exist.

Realist Expectations vs Verification
A more detailed scenario is this: Alice predicts that whenever she writes in her notebook that she sees a teapot, Bob will write a similar prediction if he’s in the same room.

If Alice didn’t believe in objects’ existing independently then she’d be surprised to learn Bob agrees with her so consistently.

Conventional Realism vs Quantum Non-realism
But if Alice knew about quantum mechanics then she’d know you can believe in non-realism yet still make predictions that both she and Bob can agree on.

The quantum formalism predicts probabilities of observations that all observers will make.

But it doesn’t claim a pointer or teapot is “really” there, at least not before the measurement.

Disproving Realists’ Proofs vs Any Explanation
D’Espagnat says quantum mechanics shows philosophers’ objections to realists’ proofs are valid.

But quantum formalism provides an alternative “explanation” despite philosophers’ doubts than any explanation is possible.

Open Realism vs Radical Idealism
D’Espagnat reiterates that physical laws don’t exclusively depend on us, so radical idealism doesn’t work.

When you combine intersubjective agreement and quantum mechanics, he says, you end up with a reality beyond what the human mind creates, but this reality is also beyond description.

Classical vs Quantum Broglie-Bohm
D’Espagnat looks at a Broglie-Bohm model that is conceptually classical but makes quantum predictions.

The Broglie-Bohm model imagines “real” physical particles guided by a wave function, but this function ends up having to be non-local.

Classical vs Non-local correlations
It turns out you can’t load up the two particles at the source with supplementary (commonly called “hidden”) variables to predict the correlations.

“Bell’s calculation” (named after John Bell) shows that Bob’s measurement of one particle depends on Alice’s earlier measurement of its twin.

Classical vs Non-local Correlations
So even when you assume classically physical particles, if you want to make predictions compatible with standard quantum theory then you need to accept non-locality.

Fact vs Law
The correlation between the particle measurements depends on quantum law rather than any facts (such as the additional variables) that you add on.

Contingent Features vs Deep Structures
So the predictions depend not on “contingent” aspects of reality but rather its “deep structures.”

The deep structures of reality are mind-independent, and cannot be described except through the laws that predict our observations.

Experimental Data vs Contextuality
In order to match the experimental data, any theory you want to interpret ontologically will need to incorporate “contextuality.”

This just means that the measurement of one quantity depends on whether another quantity is simultaneously observed, and what that quantity is.

Contextuality vs Nonseparability
Besides contextuality, an ontologically interpretable theory must take into account the nonseparability of one part of a quantum system from any other part.

These two considerations derail objectivist realism’s attempts to interpret quantum phenomena.

Personal vs Impersonal Probabilities
The Born rule requires that anyone — and everyone — viewing a particular measurement will get the same “impression.”

D’Espagnat points out his way of adding a “personal” rule.

Physical Observer vs State of Mind
The measurement is finally “registered” not by the observer as a physical system but by the observer’s state of mind.

His model, later revived by others, suggests Alice and Bob could measure entangled particles and end up with different mental states.

Measuring State of Mind vs Neurons
But quantum mechanics demands a strict correlation between these measurements.

D’Espagnat says that when Alice asks Bob for his measurement she is measuring Bob’s physical state of neurons, vocal mechanisms, etc., not his state of mind.

The quantum formalism will apply to this kind of physical measurement and therefore will guarantee a correlation, although d’Espagnat does ask if quantum physics could really be so peculiar.

Relativity of Knowledge in Theory vs Practice
Starting a long time ago various philosophers have acknowledged they might have to give up on objectivist (or “transcendental”) realism.

Science would be allowed to examine our experience rather than what is “really” out there. But later philosophers blurred the language and the “empirical” distinction got lost when they talked of “reality.”

Scientists in turn felt justified in taking the intersubjective agreement of shared observations proves that what’s seen is really there.

Macroscopic Reality vs Quantum Superposition
But once you go beyond the macroscopic world to quantum states in superposition “reality-per-se” breaks down and no longer matches empirical reality.

Pure vs Quantum Philosophy
D’Espagnat then speaks of how in the twentieth century various philosophers developed theories without paying attention to quantum theory, yet their conclusions show some parallels to the more scientifically aware.

Wittgenstein vs Carnap
Wittgenstein spoke of the world as a set of facts, not things, but d’Espagnat finds Wittgenstein’s language ambiguous, with “fact” used either in a realist or mind-centered fashion.

D’Espagnat finds Carnap much clearer with the notion of “linguistic framework,” or Quine’s similar “relative ontology” (or just “ontology”).

World of Things vs Sense Data
Carnap said that in ordinary life we might use the “world of things” as our linguistic framework, but philosophers might use the framework of “sense data.”

Carnap vs Quine
Quine said the question of whether an object or attribute exists is answerable only in the right linguistic framework.

Carnap similarly spoke of “the ontology to which one’s use of a language commits him.”

Relative vs Classical Ontology
Carnap used “relative ontology” to describe a linguistic approach without meaning ontology’s classical meaning of “Reality as it really is.”

Big vs Small Range of Linguistic Frameworks
Carnap (and maybe other philosophers) could believe in a free choice of linguistic framework, but a quantum physicist has to take nonlocality into account.

In the basic version of Broglie-Bohm a pilot wave depends on the coordinates of all particles in the Universe. This “thing” is nonseparable and therefore is nothing like our ordinary concept of a thing.

Knowledge Through vs Beyond Language
D’Espagnat says a philosopher may say the lack of any (strongly) objective knowledge about “reality-per-se” means the concept is meaningless.

Scientists generally believe there is some real “outside stuff” so they in turn think there’s more to the world than language.

Abstractions vs Ontic Systems
Despite the holistic nature of quantum systems we tend to look at just part of it through “abstractions.” The partial systems are called “ontic” by physicist Hans Primas.

Intersubjective agreement exists because people who use the same abstractions come up with the same ontic approximations.

Exophysical Ontologizations vs Endophysics
When we get more ambitious than just simple statistical interpretations we develop versions of reality called “exophysical ontologizations” or “contextual ontologies.”

Primas also conjectures a reality-per-se he calls “endophysics,” but this cannot be described directly.

Reality vs Its Forms
D’Espagnat concludes the chapter by saying that “unquestionably” some reality exists on its own, but what form it takes depends a lot on ourselves and the abstractions we perform.

There is No Path

Ploughing along through the quantum fields, I present my summary of some issues Bernard d’Espagnat raises in chapter four of his book On Physics and Philosophy (see publisher’s listing).

Holism vs Multitudinism
Previously D’Espagnat had been making the case that violation of Bell’s inequalities shows that localized particles cannot be making up the universe.

Double-slit Gas vs No Gas
Now he asks us to imagine a double-slit experiment where you can fill the room with gas and then measure the interference pattern.

Interference vs Non-interference
As a mind experiment, at least, one would stop seeing interference patterns when the space between light source and detector screen is filled with gas.

Full Particle vs Fraction of a Particle
In the original experiment with no gas the detector shows full particles not fractions of particles. This suggests each particle went through one and only one of two slits.

But then there’d be no reason for the interference pattern. D’Espagnat notes two solutions.

End Points vs In Between
The Broglie–Bohm model suggests a point-like particle is guided by a non-localized field or wave that interacts with both slits to indicate the interference pattern.

A different approach is to consider a concept’s “domain of validity”: it might be valid to think of a particle at the start and finish but not in between.

Just Waves vs Particles as Waves
The quantum wave function encodes information about the particle starting at the source. In between source and detector are the particles these waves or wave functions?

D’Espagnat says we should avoid overobjectifying. Keep in mind the concept’s domain of validity.

Path vs No Path
Whatever the formalism says, most physicists somehow imagine there is a real particle travelling from start to finish. And indeed if you apply the gas then the particle seems to have a definite path between source and detector.

Classical Particles vs Classical Gas
But if the transmitted particles are now acting classically then so should the gas particles. However, related experiments show a kind of interference pattern called “phenomena B,” so the gas is not acting classically.

Probability of Measurement vs Being
The wave function gives the probability a particle will be observed at various spots on the detector.

If you talk of the likelihood of its “being” somewhere then the particle would have to be pointlike, and hence there’d be no interference patterns.

Strongly Objective vs Weakly Objective
D’Espagnat proposes calling direct statements about attributes, “strongly objective,” and procedural statements about what you’ll find, “weakly objective.”

The Born rule takes the quantum wave function and yields probabilities of observation. It doesn’t describe “position” in a strongly objective way.

A “weakly objective” statement implies that there’s an observer (so it sounds subjective), but that the observations will be the same for any observer whatsoever (doesn’t sound quite so subjective now).

Observations vs Descriptions
The traces of “microblobs” in a cloud chamber are easily interpreted as well-defined trajectories.

However, quantum theory relies on “weak objectivity” and assigns probabilities that a microblob will be observed at some position.

Instead of “trajectories” we have “traces”: alignments of bubbles or microblobs. A theory that predicts observations can therefore compete with a descriptive theory.

Wide vs Narrow Domain of Validity
Quantum mechanics textbooks rarely discuss “domain of validity,” leaving the reader with the impression that a measurement “reduces the wave” and turns it into a pointlike particle.

This causes “ontological incongruities.” Neither “Heisenberg representation” nor a modified logic could restore strong objectivity to “orthodox” quantum mechanics.

Bohr vs Strong Objectivity
Some supporters of strong objectivity thought Bohr’s “intersubjectivity” of quantum description would rescue them. (But see below…)

Bohr’s Intersubjectivity vs Sociologists’ Intersubjectivity
D’Espagnat notes that the “intersubjectivity” used by philosophers and sociologists are at a higher level than the “raw, unanalyzed” sensations of quantum observation.

Only the second can be universal—and it’s not even assuming ontological reality.

Copernicus’s “Big” Revolution vs Quantum Mechanics’ “Small”
Some people say Copernicus’s revolution was much greater than quantum mechanics. But using quantum physics to explain classical physics undermines the standpoint of a strong objectivist.

Innovative Probabilities vs Innovative Objectivity
D’Espagnat also claims that the quantum physics’ main innovation compared to classical is that its statements are weakly objective.

Less innovative is quantum physics’ use of intrinsic probabilities.

Weakly Objective Knowledge vs No-influence Signals
As previously noted, the “supplementary theorem” says faster-than-light influences between particles cannot convey matter, energy, or usable signals.

But how can you have influences without signals?

Since quantum mechanics is so successful, why not apply its weak objectivity to special relativity?

If you do so, then the emphasis on knowledge rather than realism means you don’t have to worry about signals.

No signals… no influences.

Philosophers vs Reality “As It Really Is”
Philosophers know there’s no proof that our representations of reality show “reality as it really is.” So they’re ahead of most physics textbooks.

Physics vs Realist Language
But philosophers still use a realist language “as if” we could describe reality-per-se. Physics tells us to give up this language, or at least let go of its universalism.

Ordinary vs Quantum Complementarity
In ordinary life two separate photographs of the same object provide more information than just one photograph.

Bohr’s complementarity principle says you can’t do that with quantum systems.

Human-independent vs Weak Objectivity
Bohr adds that experimental conditions are an “inherent element” of our descriptions.

The experimentalist chooses those conditions, so “physical reality” cannot be “human-independent” reality.

Therefore reality cannot be strongly objective. The hopes of the strong objectivists were dashed.

Praise vs Use
Bohr admitted his thoughts about micro-objects were weakly objective.

Physicists formally praised the principle, but rarely used it on the measurement problem or anything else.

D’Espagnat won’t be using the principle either.

Multiple States vs Single Pointer
Imagine a group of electrons that have three possible states: state a, state b, and state c.

Let state c be the sum of the other two states. Then shouldn’t a measuring instrument’s pointer be in state A and state B at the same time?

Theoretical vs Practical Measurements
Decoherence theory tells us that quantum objects easily interact with their immediate environment—often before they can interact with a measuring instrument.

An ensemble of electrons in states a and states b produce such complex values that it’s almost impossible to measure them.

Therefore you don’t have to worry in practice about a fuzzy pointer, though there are other (rare) experiments that show quantum superposition among macro-objects.

Similarly it would be almost impossible to measure correlations between photons and gas particles in the double-slit experiment.

Simple Subjectivity vs Intersubjectivity
In the double-slit experiment without gas it’s often said you’ll get no interference fringes if you try to measure which slit the particle passes through.

That sounds like simple subjectivity. But it actually depends on the instrument you try to use. If it’s set up incorrectly the fringes show up. If the instrument has a fault the fringes show up.

Since you’re depending on the instrument, you’re measuring a “public” property, hence it’s intersubjective, which is to say, weakly objective.

Strongly Objective vs Epistemological Truth
If you exclude practically impossible tests then you can make statements that are empirically true or false such as talking about macroscopic objects and classical fields.

They’re weakly objective statements that are “epistemologically” true or false.

Someone who doesn’t know quantum mechanics or doesn’t believe in its universality might think they’re strongly objective.

Observing vs Observed Instrument
Properties of a physical system are encoded into a “wave function” or “state vector.” The Schrödinger equation and Born rule can then calculate probabilities of certain observations.

The mathematical formalism says we can ignore details of the observer’s eye, optical neurons, and so on.

John von Neumann said various elements can be assigned to either the quantum side or the macrosystem side of a calculation.

Because of this “von Neumann chain” an instrument can either be an observer or the observed.

It’s usually easiest to put the instrument on the classical, perceiving subject side.

Born Rule vs Wave Collapse
It can be hard to distinguish using the Born rule to come up with probabilities of observations on the one hand and talk of a “reduction” or “collapse” of the wave function.

Descriptive vs Predictive Method
Although useful in practice, this “reduction” is not required by quantum theory. For a simple photon pair the descriptive approach may be replaced by the predictive method.

Realism vs Non-realism
D’Espagnat says these concerns about the conceptual foundations of quantum mechanics make the issue of realism even more pressing.