The Elusive Object

29 March 2011

Behind the curtain

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

4 March 2011

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

1 March 2011

Ultimate 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

23 February 2011

Vortex of a Vacuum

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

28 June 2010

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

25 April 2010

Jigsaw Puzzle

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.

Unreality at the Local

23 March 2010

Quantum entanglement

Correlating Reading and Understanding
Chapter three of On Physics and Philosophy was a pretty frustrating read.

D’Espagnat seems to be drawing several threads slowly towards a core conclusion, but it often feels like an extremely meandering trail.

Working from the top down, here are some of the points he seems to be making.

The Basic Theme
Bell’s theorem makes predictions about how observations of two particles relate to each other.

Bell’s inequalities predict certain correlations at a distance when you assume locality and a belief in free experimental choice.

The locality condition (more or less) states that influences can’t travel faster than the speed of light.

The belief in free experimental choice lets the experimenter decide what to test, otherwise it’s hard to do empirical science.

If your results don’t match Bell’s inequalities at least one assumption (locality or free choice) is wrong.

A pair of photons created by the same atom will be entangled. We can measure a photon and and its distant partner.

Do that with enough photons and we get a pattern to compare with Bell’s inequalities.

Experiments by Aspect and others show that Bell’s inequalities are violated at the microscopic level.

Since we want to believe in experimenters’ free will, locality must be discarded.

Supplementary Points
A “supplementary theorem” devised by four scientists states that if there are faster-than-light influences between the photons in the pair, these influences cannot convey matter, energy, or usable signal.

If one rejects objectivist realism then locality or nonlocality is a meaningless distinction. But nonseparability remains in play.

If you try to add hidden local variables to specify the particles’ polarizations in advance (right at the source) then you get the wrong predictions.

Even More Points to Ponder
The violation of Bell’s inequalities is often assumed to imply the falsity of all hidden variable theories.

In fact the experimental data only disprove the falsity of local hidden variable theories.

Non-local hidden variable theories such as de Broglie’s or Bohm’s match the predictions of standard quantum mechanics (at least non-relativistically). A pilot wave controlling localized particles must still travel through both slits of a double-slit experiment.

By extension, the disproving of locality shows the limitations of Descartes’ “divisibility by thought.”

You cannot spatially divide the photon pair’s wave function and still get the predictions of quantum mechanics that Bell’s theorem and the Aspect-type experiments say you should.

Last revised 18 June 2010

A Mysterious Trajectory

13 March 2010

Mysterious Trajectories

Construction vs Deconstruction
Continuing on with my binary (but gentle) deconstruction of Bernard d’Espagnat’s On Physics and Philosophy, we can now proceed to chapter two, entitled “Overstepping the Limits of the Framework of Familiar Concepts.”

Here are some points I’ve gleaned, which I’ve repackaged into the black-and-white dichotomies I love so much.

Aristotle vs Galileo (Reprise)
Although Galileo believed in a priori mathematical concepts, d’Espagnat says Galileo’s two main scientific contributions — inertia and the relativity of motion — were based on sensory data derived from inclined planes and moving ships.

Primary vs Secondary Qualities
Galileo believed he could make observations of “primary” qualities to find out what “really is.” He could then reject the supposedly illusory “secondary” qualities.

Galileo vs Descartes
Descartes was the other “founding father of modern science,” but more of a philosopher than Galileo. He justified his realism from “cogito ergo sum” (I think therefore I am), an ontological argument for an infinite Being that could deceive us, self-evident truths, and finally fundamental notions of form, size, and motion, which are true because they’re clear.

He believed in “near realism”: nature can be described through basic notions of figure, size, and motion. Natural and manmade objects are similar but differ in size.

D’Espagnat says Galileo, Aristotle, and Descartes were all objectivist realists, even multitudinist ones. However, Descartes was more concerned with metaphysics and his system of thought could hardly have led to Galilean relativity, says d’Espagnat.

Petitot vs d’Espagnat
D’Espagnat rejects Jean Petitot’s view that Galilean relativity’s space and time are “desubjectized mental forms.” Galileo, responds d’Espagnat, kept the number of intrinsic properties to a few, but he still believed in their reality. Relative positions and motions are normally said to be real, and neither Galilean relativity nor Newtonian mechanics are inconsistent with absolute space.

Forces vs Fields
Galilean ontology led to the notion of forces, which could produce action at a distance. However, forces were still properties of objects. No object? No force. In the 19th century the notion of fields was introduced. For instance, an electromagnetic field can exist “in vacuum” devoid of electric “sources.”

Physics vs Realism of the Accidents
As physics joined with mathematics to overcome the old familiar concepts the other sciences looked for simpler notions allowed through a belief in the realism of the accidents.

A quantum wave is a function of many variables: the number of particles times three (for each particle’s x,y,z coordinates). So the wave has no value at a particular three-dimensional point. Unlike a field one can compare to gelatin, the wave is hardly realism-friendly.

Relativity vs “Universal Thing-ism”
Although relativity replaced objects with events, it didn’t reject realism. Realism of the events is allowed. Naïve realism of “universal thing-ism” is not.

Objects are sequences of events but the events (and geometry of space and time) still supposedly exist, even if they’re perceived differently by different observers.

Useful Notions vs Actual Existence
Physicists explain complex features that are visible by simple invisible ideas that work well. The temptation is then to think these “clear, distinct ideas” (as Descartes might say) also “exist.”

The notion of “electrons” is great for explaining things, but when you start to think of an electron as localized, one per orbit, the picture gets misleading.

To say each electron exists simultaneously on all “allowed” orbits is much less misleading.

Trajectories vs Quantum Probabilities
Put a bubble chamber where it can capture cosmic rays. Soon tracks will appear that we will be tempted to call “trajectories.” We imagine a particle started out in space along some trajectory and reached the bubble chamber, leaving a trail.

Quantum mechanics says there are no such trajectories. The liquid in the bubble chamber reacts with radiation, and the quantum probability that two adjacent atoms will get excited is essentially zero unless they’re almost exactly aligned with the direction of the radiation.

Quantum Field Theory vs Dirac’s Virtual Sea
Quantum pioneer Paul Dirac correctly predicted that every time a fermion (such as an electron) is created so should its opposite partner, an anti-fermion (such as a positron).

He visualized a sea of invisible and nonlocalized fermions. A particle is created when it escapes this sea, leaving a “hole” in the sea with opposite properties. It wasn’t a very credible theory.

Quantum field theory says the existence of a particle is just a state. Existence is a property of “something” — but everyday physicists are reluctant to commit to whether this “something” actually exists.

Hidden Variables vs Quantum Completeness
Einstein and others disliked that quantum theory doesn’t describe the world as it “really” is. Instead of the quantum wave only predicting probabilities people like Louis de Broglie and David Bohm theorized about specific particles on specific paths moving in a way that produced the same predictions.

These specific paths are determined through invisible factors not included in quantum theory.

Such “hidden variable” theories run into problems when paired particles grow distant and one particle ends up experimentally detected. Assuming a specific trajectory doesn’t eliminate nonlocality, as Bell’s Theorem shows.

Ontology vs Pseudo-ontology
Physicists avoid espousing “near realism” or “realism of the accidents.” They try to remain “open” about the concepts they use. Instead they’ve developed a pseudo-ontology using diagrams.

Feynman Formalism vs “Something’s” State
Feynman diagrams help physicists navigate through complicated formulas. An “H” diagram will show one particle on the move, emitting a virtual particle, absorbed by the second incident particle, and they continue on their way.

In this system there is no state of “something” that’s changed. If pressed, a physicist may say the elements of such a diagram are just a “way of speaking.” But the danger is we think they’re actual names of actual things.

Description of Experience vs “Reality Out There”
While some physicists feel empirical evidence gives us only a description of experience, other physicists feel some reality is “obviously” out there. These two positions are distinguished by three factors.

Experiential vs Realist Objectivity
In chapter four d’Espagnat will explore objectivity. For now he says the realist has more stringent standards for objectivity than the experiential proponent.

General Laws vs Specific Entities
A realist looks for specific objects with specific properties, and likely relies on counterfactuality (inferring that unobserved objects still retain their properties).

An experientialist may believe in an explanation of the world compatible with the realist’s, but works from general laws to account for specific observations.

Quantum Completeness vs Incompleteness
A realist may look for hidden factors to preserve the belief that localized particles and trajectories “really” exist. Founders of quantum theory retorted that quantum theory is a complete description of reality. Additional factors just don’t exist.

Strong Completeness vs Weak Completeness
Slight problem with saying hidden variables “don’t exist.” It makes definite statements about nonexistence in a similar way to a realist’s pronouncements on existence. This early position might be called “strong completeness.”

“Weak completeness” just says that no competing theory can make predictions about atomic phenomena that aren’t also correctly predicted by quantum theory (as Henry Stapp puts it).

Compatible vs Incompatible with Quantum Theory
D’Espagnat says weak completeness is compatible with his approach in the book. If another theory imagines reality’s structure with additional parameters or variables — but has no “false consequences” — then he’ll say it’s “compatible with truth.”

He says his approach will be one of “enlightened agnosticism.”

Dividing up the Wholeness of the Text

11 March 2010

Quantum Ripples

The Reader vs The Read
As I make my way through Bernard d’Espagnat’s On Physics and Philosophy (Princeton University Press, 2006) I’m taking notes to keep track of his often dense arguments.

Although I don’t think “ontological” reality is necessarily binary, I do find for my own study purposes that extracting an A vs. B division can be helpful when making summaries.

So in that spirit, here are some of the dichotomies that d’Espagnat points to in the first thirty pages or so of his book, including the front material and first chapter.

These are just my impressions of some of the issues d’Espagnat raises, so check the original text for all the juicy stuff.

French vs English Edition
D’Espagnat writes that no significant material on the philosophical issues he raises had been published in between the French edition (2002) and the English one four years later.

He says that for the English edition he made some of his remarks stronger and clearer, and he also had a chance to respond to critics of his latest work.

Philosophy vs Science
D’Espagnat says that philosophers need to go beyond their own “cogitation” and examine evidence from other fields.

Epistemology vs Physics
D’Espagnat says that in theory epistemology bridges science and philosophy, but all too often epistemologists think having a broad idea of the results of 20th-century physics is enough. He says they should pay far closer attention to the details.

The Right Answer vs Not the Wrong Answer
D’Espagnat notes criticism that science in the past has been wrong, and will likely be wrong in the future. Science’s positions change over time.

While acknowledging that scientific theories can evolve, d’Espagnat says that science can at least indicate to philosophers which positions are no longer viable, for instance: “Being is not this.”

Ontological Reality vs Empirical Reality
What is “really” out there is ontological reality, and different philosophies take different positions on whether we can “access” that reality. In other words, they have different views on whether our perceptions and our immediate inferences from those perceptions (our empirical reality) can lead us to an understanding of (the ontological) reality behind the veil.

Nonseparability vs Separability
D’Espagnat believes that quantum experiments (empirical reality) indicate that nonseparability underlies any notion of a “mind-independent reality” (ontological reality).

Nonseparability comes up in the way that two particles originally paired up can travel a great distance but measurement of one particle’s state then limits the possible results when measuring the other particle’s state later.

The opposite position, separability, implies that objects separated by distance are indeed distinct entities that cannot directly influence each other.

Nonseparability vs Action at a Distance
Nonseparability refers to a quantum connection between what we might think are distinct entities some distance apart. In classical physics there’s also an influence but that would be through action at a distance, such as gravity, called a force or (later) field.

Empirical Limitations vs A Mirage
D’Espagnat does not believe empirical knowledge is a mirage, even if it can’t lead us to direct knowledge of the underlying “ground of things” (which he says cannot be described analytically).

He says that the symmetries and regularities of our empirical evidence will presumably correspond “to some form of the absolute” even if this correspondence is obscure.

Quantum Predictions vs Descriptions of Reality
D’Espagnat says that despite appearances quantum theory makes predictions (which have been well verified) instead of describing a “mind-independent reality.”

Aristotle vs Galileo
D’Espagnat says that Aristotle’s wide-ranging data are collected by the senses, data that Aristotle regards as basically lying on the same plane.

Galileo (and the more philosophical Descartes) uses a hierarchy of concepts, some considered basic, which are then used to explain other concepts. It’s a mechanistic world view in which some concepts can be built up from smaller ones.

However, both Aristotle and Galileo view their fundamental ideas as “common sense” and so neither doubts they are talking about some kind of (what we’d call) an “ontological reality.”

Objects vs Properties
Two particles collide. In some situations instead of just continuing on their way or destroying each other they’ll survive and create new particles.

How did they create these new particles? Well, the original particles have motion, and it’s this motion that creates the new particles.

But this raises an interesting issue: a property of the particles creates more particles, which are objects.

It’s as strange, says d’Espagnat, as if the height of the Eiffel Tower managed to create a second Eiffel Tower.

Quantum Approaches vs Basic Ideas
D’Espagnat notes that general quantum rules lead to at least three different theoretical approaches for making predictions — predictions that are basically the same.

He says that this undermines the idea that there are “basic notions” out there acting as the “real” foundation for other ideas.

Creation vs Change of State
Classical physics might see the creation or destruction of particles, but quantum physics sees these transitions as changes in various states of “Something.”

Building Blocks vs Wholeness
This “Something” suggests a wholeness of some sort instead of classical physics’ multitudinist world view founded on localized atoms or particles as basic building blocks.

Physicists’ Practical vs Theoretical Concerns
Although a physicist might acknowledge that particles and trajectories don’t “really” exist, they are useful concepts that have a pseudo-reality, especially in Feynman’s approach to quantum calculations.

Idealism vs Realism
Idealism says our only knowledge of the outside world comes from our (often mistaken) senses. Realism believes there’s a mind-independent reality we can either gain access to or say something about.

Counterfactuality vs Observation
If we leave an office with books on the shelves we normally assume that the books are still there even if we’re not observing them. This is called counterfactuality.

Stability vs Realism of the Accidents
Some observations are relatively stable, leading some people to see them as pointing to stable features of the world.

Other sense impressions change frequently. A belief we could term realism of the accidents entails that these quickly changing contingent “accidents” also point to something real.

Realism of the Accidents vs Realism of the Events
Galileo appears to have believed in the realism of the accidents. Space and time are real (despite Galilean relativity) but relative positions are “accidents” and hence “paradigmatically true.”

Einstein emphasized events but still believed that there were elements out there that physics could determine were true or not. Hence he believed in what could be called realism of the events.

UPDATE (15 April 2010)
Princeton University Press offers the first chapter as a sampler.

Entropy on Hold

18 February 2010

Philosophy and Physics

I’m going to put the entropy book on hold for a bit and get back to it later. The basic theme of Arieh Ben-Naim’s book seems quite blunt and compelling: that entropy can be understood only through atoms or molecules — allowing for the distinction between micro- and macrostates — and that the apparently inexorable increase in entropy is the result of some macrostates having lots of possible microstates. (The author introduces different terms: “dim” vs. “specific” events, respectively.)

Ben-Naim presents various mathematical models using simple rules and shows how the system’s “entropy” rises to a relatively likely macrostate (or close range of macrostates), especially in systems with a large number of identical components. These more likely macrostates “hide” information in the form of their many microstates, which although distinct on the microscopic level produce no observable change in some all-encompassing macrostate. In many ways the book has a simple and direct message, but his side pleas against other interpretations and definitions of entropy (such as increasing disorder instead of increasing amount of missing information) can make for a confusing read.

In any event, more about that in the future when I have the energy to re-read Entropy Demystified. A few days ago I bought a book that’s even closer to my core interests. Bernard d’Esagnat, writing in his On Physics and Philosophy (Princeton University Press, 2006), states he will avoid discussing metaphysics in detail, but:

Still, in the present book the metaphysical domain has willy-nilly to be approached in one respect. For indeed one piece of information that contemporary physics clearly yields, as we shall see, is the absolute necessity of carefully distinguishing between two concepts of reality. One of them is ontological reality, that is, the notion referred to when “what exists independently of our existence” is thought of or alluded to. The other one, empirical reality, is the set of phenomena, that is, the totality of what human experience, seconded by science, yields access to. (p. 4)

D’Espagnat intends to describe the philosophical importance of quantum results on “nonseparability” with its nonlocality and holism, the universal applicability of quantum rules, and finally “quantum measurement theory and the immensely puzzling riddle of the nature of consciousness” (Schrödinger’s cat and all that). I’ll post comments as I make my way through the book.