The Elusive Object

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.

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