Archive for March, 2010

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.