by Chet Raymo
"I have just finished reading Brian Greene's book, “The Fabric of the Cosmos”. As with his earlier book, “The Elegant Universe”, he does a damn good job explaining the almost unexplainable - string theory, braneworlds, multiple universes, and all that. None of this stuff has an empirical basis, and is not likely to for the foreseeable future. So does it qualify as science? Well, yes, barely. Because in principle at least experiments are possible. We should value the wild speculations of the theoretical cosmologists precisely because they are pushing the limits of what is imaginable.
We live in an imagined world. Some parts of that imagined world are so tightly bound to sense perceptions that we call them "real." The chair I'm sitting in is real. Atoms are real. The common ancestry of humans and raccoons is real. Strings and branes and multiple universes are not yet real, but they spring from the same storytelling tradition. Democritus and Lucretius told stories of atoms long before atoms were real. It is ever for us as it was for the singer in a famous poem by Wallace Stevens:
So, what is the real? My own views on the matter were given shape when I was young by the poet Wallace Stevens. More influential was a book I read as a graduate student, the physicist-philosopher Henry Margenau's "The Nature of Physical Reality" (1950). Margenau uses a simple diagram to illustrate the conceptual maps we make of the world. Down the middle of the page he draws a vertical line that he calls the "perception plane." It is the locus of our immediate sensations of the world - sights, tastes, odors, touches, sounds - the interface between the world as it is and the world as we know it. To the left of the line is the world "out there," which we know only through the windows of our senses. To the right of the line Margenau draws circles representing "constructs" - names, descriptions, or ideas we invent to make sense of our perceptions. The more abstract the construct, the farther the circle from the line.
Immediately adjacent to the perception plane are constructs that correspond to direct sensations: "blue," "bitter," "pungent," "brittle," "shrill." The construct "dragonfly" is a bit further from the perception plane, but not very far away. I feel a sensation on my finger ("tingle"), I see a color ("blue"), a quality of light ("iridescent"), a shape ("long and narrow"). I name this ensemble of sensations "dragonfly,"
As my experience of the world increases, the construct "dragonfly" becomes enmeshed in a web of other constructs at varying distances from the plane: "insecta," "Jurassic," "mitochondrial DNA," etc. Resilience and interconnectivity of the web are the defining characteristics of the real. "Atom" is bound to the perception plane by a dense and sturdy web of constructs. "Cosmic strings" and "branes" are way out there, far from the perception plane, dangling by a gossamer thread.
Perception and cognition are hugely complex processes, endlessly debated by psychologists, neurologists and philosophers. Margenau's simple schematic of connected constructs is itself only a construct, a useful way of describing the devilishly complex business of perception and cognition. The important thing is to realize that our ideas about the world are not the same as the world itself (a point often missed by true believers). Nevertheless, only the most obtuse idealist would hesitate to call "dragonflies" or "atoms" real."