Pine, ch. 8: "Our Time: Quantum Physics and Reality

[Quantum mechanics - begins with the fundamental premise, based in observation (for example, of spectragraph lines), that energy at the atomic/subatomic level is distributed in "packets" or quanta, discrete units rather than smooth continua.]

Outline:

What is an electron? (on the particle/wave duality)

I. The double-slit experiment

(see also: Drury alumnus' David Slaven's account of the two-slit experiment and the Copenhagen Interpretation)

II. The beam-splitter experiment

III. Solution? Mathematics - Schroedinger's Wave Equation

IV. Ontology, epistemology - and Kant

V. Bohr-Einstein, EPR, and the Aspect Experiment

What is an electron? (on the particle/wave duality)

First thought experiment: is light or electrons a wave (with a non-specific location, existing as a disturbance of a medium, etc.) or a particle (with a specific location independent of any medium, etc.)?

Our experiment:

• a laser through a diffraction grating
  a laser through the door
  a laser through a razor-blade split

I. The double-slit experiment:

A. When projecting a beam of photons towards the two slits:
a) if light consists of particles, they should distribute themselves closely together in two "piles" behind the two slits;
b) if light is a wave, the wave should diffuse behind the two slits and create an interference pattern (where wave crests intersect and reinforce one another)

Result: WAVE EFFECT - both "piling" of individual particles and distributed in wave/interference patterns occurs.

--> how can something behave as both a particle and a wave simultaneously?
--> when sending light through a vacuum - how can light qua wave travel without a medium in which to travel?

B. How about one open slit?

Result: PARTICLE EFFECT - a "piling" effect related to a single open slit

C. Low-intensity radiation towards two open slits
--> "If we assume that the radiation consists of particles, then if only a single particle is passing through at a time, it can go through only one slit or the other." (219)

Result: WAVE EFFECT - piling of individual particles distributed in wave/interference patterns

Pine comments: "So the wave effect shows particle characteristics, the individual hits on the film, and the particle effect shows wave characteristics, the diffraction pattern [of particles as "spread" behind a single slit]." (221)

These results are contradictory IF we expect the subatomic world to follow our macroatomic distinction between waves and particles. But what it the subatomic world does not follow this distinction?

As Pine notes:

• Many of the great names of science, however, who were working at this baffling microscopic realm, also had a background in philosophy, and it was immediately apparent to them that there was something major at stake here. Since the time of the ancient Greeks and the fledgling beginnings of scientific exploration, we have assumed that we are dealing with one world, one consistent reality." (221)

These results, however, seem to say that the concepts appropriate to one level of reality are not consistent with or applicable to another level of reality.

A last variation of the double-slit experiment: instead of photographic film - use particle detectors behind the slits - project one unit of radiation. If light is a wave, it should smear itself out behind the two slits - both detectors should go off.

Result: PARTICLE BEHAVIOR - only one detector goes off at a time.

Two consequences:

a) our macroatomic wave/particle distinction does not work at the subatomic level;

b) "reality" appears to change - indeed, how we as observers set out to measure and detect "reality" seems to shape how reality appears to us. (Kant would be pleased...)

-- that is, if we set out to detect light as a particle, we can detect light as a particle; if we set out to detect light as a wave, we can detect light as a wave.

II. The beam-splitter experiment, in which light is passed through a beam-splitter, makes the point most dramatically:

• When we set up the experiment to "observe" photons as particles - by sending out a single photon, and allowing to land at either one of two particle detectors - light behaves accordingly.
  When we send out a single photon, and replace the particle detectors with mirrors which reflect the light back to the same point - thus "observing" light as a wave - we get interference fringes.
  Indeed: under the second arrangement - i.e., where the mirrors work to recombine the light - if we interfere with the light by blocking one of the paths - then light will "suddenly" behave like particles again: instead of interference fringes, we're back to a "diffused piling exposure." (224)
  

HOW CAN THE "SAME" RADIATION ISSUE IN BOTH PARTICLE AND WAVE BEHAVIOR?

(Pine, figure 8-2, p. 223)

III. SOLUTION? MATHEMATICS....

-- specifically, Schrödinger's wave equation:

The equation "explains" the preceding results but with a high epistemological and ontological price.

• As we would expect from the name, the equation literally portrays the radiation as a wave, but a very strange wave. According to the equation, in our two-slits-open configuation as soon as the radiation leaves its preparation point, it begins to spread out in a strange mutidimensional "hyperspace." As it encounters the slits it splits, as any real wave would, passing into the chamber and interfering with itself. As the radiation touches the photographic film, however, all of it collapses to a single unpredictable point? We can never predict at what point the radiation will be received, but we can always, with a remarkable consistency, predict the probability of where it will strike and the overall statistical pattern, not only for this particular arrangement for all others as well.
  Few physicists, however, accept this literal interpretation; most have been taught to think of the equation as a calculation device. The special mathematical function used is thought to represent only a "probability function" - that is, given initial conditions, the probability of finding a hit, or a pattern of hits, at a particular location. Thus the only waves that exist are said to be "probability waves."
  But wait: What happened to reality? What is a probability wave What is an electron? What is a photon? Are these questions no longer meaningful?" (222)

Using Schrödinger's equation as a wave equation -

• ...the half-silvered mirror splits a wave packet into two 'hyperspatial, virtual/real, probability' waves. At the exact moment that the energy reaches the detectors, some sort of strange decision is made, and the entire unit of energy is received at only one point, at either A or B? The wave packet collapses. If a whole unit is received at A, then the energy that was approaching B has jumped over to A. In addition, the equation predicts this will happen even if the two detectors are separated by many light-years, even if one detector is much closer to the half-silvered mirror than the other. The latter case implies that the energy that is approraching the one that is closer, say B, waits (?!) until the energy approaches A and then either jumps to A or the energy that was approaching A goes "backward in time" and collapses at B. The mathematics always works, but what it describes literally seems impossible....According to the mathematics, there is an instantaneous collapse of potentiality located multidimensionally to a three-dimensional localized spot.....So physicists explain that we must not think of the split wave packets as real, but only as a description of the probability of where photons will go. (224)

IV. The ontological and epistemological implications: the Copenhagen Interpretation - and a Kantian reprise...

As Pine observes, since the beginnings of Western science, we have assumed a particular ontology: "The cosmos consists of one distinct, complete reality full of details."

Accompanied by a specific epistemology: "The details, whever they might be, can be known, and knowing these details does not affect what the details actually are independent of the knower." (225)

Pine doesn't say it here - but in other terms: this ontology and epistemology are specifically at work in

• a) Locke and the Anglo-American tradition which stresses an empiricist approach to reality and knowledge (reality = material reality; "knowledge" is an accurate "picture" of reality - first of all, as mediated to the mind (a tabula rasa or "blank tablet") which only passively receives information), and, more generally, in
  b) the Cartesian/positivist assumption of a sharp dichotomy between "subject" (the human knower) and "object" (the external object of knowledge we seek to know)
  - and thus between "subjectivity" and "objectivity"
  (where "subjectivity" = "individual, arbitrary, less than desirable knowledge," while "objectivity" = "universal, necessary knowledge" - i.e., the knowledge preferred or valued.)
  

As we have seen, Kant radically overturns this epistemology and ontology (in response to Hume's equally radical destruction of Newtonian science at the hands of the empiricist criterion of meaning) - precisely by developing

• a) an epistemology which sees "knowledge" as the result of an ongoing interaction/relationship between "subject" and "object" - especialy as the subject contributes the universal frameworks of time/space and categories (causality, substance, etc.) which make human knowledge of an external "object" possible, resulting in
  b) an ontology which sees "reality as it is in itself" always hidden from such knowing subjects.
  

Just as Einstein's relativity theory provides a confirmation of Kant's point in terms of our experience of time and space -
so quantum mechanics and the Copenhagen Interpretation provide a further confirmation of Kant's point in terms of the interaction between knowing subject and object -
especially as this interaction means:

• the instruments we use to "know" the world change the world at the very moment they reveal it to us.
  --> "Reality" as it is in itself will always be hidden
  --> our scientific accounts, however successful as instruments for prediction and control of the objects of our experience (in Kant's terms, the phenomenal domain), in principle cannot be "true" as correspondance "pictures"of "Reality."

If the "traditional" Western epistemology and ontology are correct - a photon, as an entity independent of the human knower, cannot "be" both a wave and a particle. The apparently contradictory results of these experiments with light suggest, then, that this epistemology and ontology are mistaken.

(If A --> B, ~B, so ~ A)

Rather, a major epistemological shift is required:

• Descriptions such as 'particle,' 'wave,' 'position,' 'mass,' and 'spin,' are human concepts. These concepts involving assumptions of space and time work for us at a normal macroscopic level and will always be indispensable for describing the results of our physical experiments. But nature is now making it clear to us that we have reached a barrier in our attempt to describe it in terms of human concepts derived from ordinary experience.
  Wave-particle duality is nature's way of informing us that we have no right to impose our human concepts on the subatomic level. Just as Einstein had discovered that we have no right to impose our normal assumptions of space and time to all levels of reality, so quantum physics reveals to us that we have no right to impose our most basic thoughts about the nature of reality on the subatomic realm. The idea of an extended thing sitting in a three-dimensional space, waiting for us to discover it, is revealed as another human projection, a limited image of reality, more of an echo of the way our minds work than of reality itself. According to Bohr, nature reveals this to us by showing that we can have only complementary views of reality. If we set up an experimental arrangment that allows for a wave manifestation of subsatomic pheonomena, then that is what we will observe. If we set up an experimental arrangement to view subatomic phenomena as particles, then that is what we will observe. According to Heisenberg...what we observe in our experiments is not nature itself but nature exposed to our methods of questioning nature. In short, an electron is not a thing until we observe it! (226)
  

Instead of single, coherent "picture" of the universe as the goal of science - quantum mechanics seems to say that complementary views of reality exist. As Pine points out, this liberates science in an important way: in our terms, the positivist insistence on reducing all of knowledge to a single kind (e.g., biology to physics) no longer makes sense. Rather, each discipline may offer its own complementary perspective on "reality." As we have seen with the example of different maps (highway, topography, etc.), this more instrumental approach to natural science issues in a pluralistic/complementarity understanding of knowledge. Aristotle would be pleased...

V. The Bohr-Einstein debate, EPR, and the Aspect Experiment

But, as Pine goes on to explain, the Copenhagen Interpretation was just too much for Einstein, especially as it violated his (classical Newtonian/Western ontological and epistemological) assumptions of single, ultimately knowable "underlying reality." Despite his own reliance on Kant as he developed his first through experiments regarding the relativity of space and time - Einstein was strongly opposed to the metaphysical idealism established by Kant and apparently recovered and validated by quantum mechanics.

Pine's way of putting this point is right on:

• Most scientists have always viewed this [idealist] metaphysics with disdain, as more of a symptom of despair of the sometimes harsh realities of the physical world, as primarily a religious view associated with those who find the physical universe threatening and who desire a more perfect but duller world. Does quantum physics validate this philosophy? How embarrassing for Western science if it does. Imagine that after thousands of years of struggling to know the details of Democritus's atom, Western science shipwrecks into a religious philosophy it thought it had left behind at a more primitive time! (228)

Accordingly, Einstein struggled to demonstrate the validity of traditional assumptions of nature as a "local reality," one which retains its independence of the human knower - in part, by conceiving of a series of thought experiments which, by showing that the assumptions of quantum mechanics led to ostensibly absurd results, would thus show that these assumptions were untenable.

(If A [quantum mechanics] --> B [absurd results], and ~B [because absurd results cannot be true], so ~A - modus tollens.)

--> the "killer" argument: the Einstein-Podalsky-Rosen (EPR) paradox

Briefly, EPR seems to issue in an incredible result - but one that, initially at least, cannot be confirmed or disconfirmed through experiment. So it remains "only" a philosophical argument.

EPR leads, however, to Bell's theorem, which does point to a way of empirically determining whether Einstein (and the traditional assumption of an independent, local reality) or the Copenhagen Interpretation (and the more Kantian/idealist/instrumentalist epistemology/ontology) is correct.

The Aspect experiment demonstrates "that the measurement of a subatomic particle at one finish line instantaneously determines the state of its twin at another finish line, regardless of how far apart the two finish lines are." (233)

The short and dirty answer: reality seems to confirm the Copenhagen Interpretation and its assumption of nature at the most fundamental level as a "nonlocal" reality.