Saturday, December 30, 2006

Smells Like Success

A recent article in Scientific American cites a paper accepted for publication in Physical Review Letters, positing a quantum basis for the olfactory sense:

The question: What property of an odor molecule (or odorant) do the receptors in our noses pick up? The reigning but still unproved explanation of smell supposes that the shape is the thing, with receptors fitting like a lock into the molecule's key. But the shape theory doesn't explain why some nearly identically shaped molecules smell vastly different, such as ethanol, which smells like vodka, and ethane thiol (rotten eggs).

Turin's more controversial theory, put forth in 1996 and now the subject of two popular books, holds instead that odorant receptors sense the way a molecule's atoms jiggle. The shape of the molecule still comes into play, Turin says, because it determines the odorant's overall vibrational frequency. But he didn't know how all the details fit together.

Physicist Marshall Stoneham and his colleagues at University College London report they have constructed a specific mechanism based on the properties of so-called G-protein coupled receptors, which project from olfactory cells inside the nose.

The researchers imagined that the odorant fits into a spot between a site that donates an electron and one that receives the electron. In this model, the receptor switches on when an electron hops from donor to acceptor. The group calculated that an electron could "tunnel" through the barrier imposed by the odorant, an effect made possible by quantum mechanics, they wrote in a preprint accepted for publication in Physical Review Letters.

I have not yet read the paper, but predict that some such mechanism will be established and that the "odorant's overall vibrational frequency" will be replaced by a vector in an EPR-complete Hilbert space, where by "EPR-complete" I mean a theory wherein all "elements of reality" are represented.

As I have long argued, QM is prima facie incomplete in that it does not explicitly represent the secondary properties or qualities, as Schrödinger tell us in the case of color:

If you ask a physicist what is his idea of yellow light, he will tell you that it is transversal electromagnetic waves of wavelength in the neighborhood of 590 millimicrons. If you ask him: But where does yellow come in? he will say: In my picture not at all, but these kinds of vibrations, when they hit the retina of a healthy eye, give the person whose eye it is the sensation of yellow.

Clark makes the more general case in his admirable book on Sensory Qualities:
The world as described by natural science has no obvious place for colors, tastes, or smells. Problems with sensory qualities have been philosophically and scientifically troublesome since ancient times, and in modern form at least since Galileo in 1623 identified some sensory qualities as characterizing nothing real in the objects themselves...

The qualities of size, figure (or shape), number, and motion are for Galileo the only real properties of objects. All other qualities revealed in sense perception — colors, tastes, odors, sounds, and so on — exist only in the sensitive body, and do not qualify anything in the objects themselves. They are the effects of the primary qualities of things on the senses. Without the living animal sensing such things, these 'secondary' qualities (to use the term introduced by Locke) would not exist.

Much of modern philosophy has devolved from this fateful distinction. While it was undoubtedly helpful to the physical sciences to make the mind into a sort of dustbin into which one could sweep the troublesome sensory qualities, this stratagem created difficulties for later attempts to arrive at some scientific understanding of the mind. In particular, the strategy cannot be reapplied when one goes on to explain sensation and perception. If physics cannot explain secondary qualities, then it seems that any science that can explain secondary qualities must appeal to explanatory principles distinct from those of physics. Thus are born various dualisms.

I wrote above that QM does not explicitly represent colors and so forth. Yet physicists often speak of such things as monochromatic light, red giants, white dwarfs and so on.

Mendeleev, in composing the periodic table, was guided by the secondary properties reliably exhibited by the chemical elements.

Feynman, In his terrific little book on QED, speaks of "blue photons." And we all know what he means — until we start to think about it.

If pressed, scientists have traditionally fallen back on the dogma received from Galileo and Newton, who had it in turn from Democritus and the Greek atomists, that the secondary properties are produced in the mind... er, somehow or other. Yet the mind is yoked to the brain, a physical thing, and mental states are dependent on physical states.

The essence of the solution I have put forward flows from Mach, who wrote in his landmark work on Contributions to the Analysis of Sensations:

The traditional gulf between physical and psychological research... exist only for the habitual stereotyped method of observation. A color is a physical object so long as we consider its dependence upon its luminous source, upon other colors, upon heat, upon space and so forth. regarding, however, its dependence upon the retina... it becomes a psychological object, a sensation. Not the subject, but the direction of our investigation, is different in the two domains.

Mach's views are wholly compatible with mind/brain identity theory, as expressed in Chalmers:

The abstract notion of information, as put forward by Claude E. Shannon of MIT, is that of a set of separate states with a basic structure of similarities and differences between them. We can think of a 10-bit binary code as an information state, for example. Such information can be embodied in the physical world. This happens whenever they correspond to physical states (voltages, say); the differences between them can be transmitted along some pathway, such as a telephone line.

We can also find information embodied in conscious experience. The pattern of color patches in a visual field, for example, can be seen as analogous to that of pixels covering a display screen. Intriguingly, it turns out that we find the same information states embodied in conscious experience and in underlying physical processes in the brain. The three-dimensional encoding of color spaces, for example, suggests that the information state in a color experience correspond directly to an information state in the brain. We might even regard the two states as distinct aspects of a single information state, which is simultaneously embodied in both physical processing and conscious experience.

And before him by Feigl:

The solution that appears most plausible to me, and that is consistent with a thoroughgoing naturalism, is an identity theory of the mental and the physical, as follows: Certain neurophysiological terms denote (refer to) the very same events that are also denoted (referred to) by certain phenomenal terms. The identification of the objects of this twofold reference is of course logically contingent, although it constitutes a very fundamental feature of our world as we have come to conceive it in the modern scientific outlook. Using Frege's distinction between Sinn ('meaning', 'sense', 'intension'), and Bedeutung ('referent', 'denotatum', 'extension'), we may say that neurophysiological terms and the corresponding phenomenal terms, though widely differing in sense... do have identical referents. I take these referents to be the immediately experienced qualities, or their configurations in the various phenomenal fields.

I am pleased to announce that my recent paper on "The Unification of Mind & Matter," which treats these points in greater detail, has been accepted by a journal for a series on Men Who Made a New Science. I will supply a link at the appropriate time.