Does the inside of a brick exist?

In Surely You’re Joking, Mr. Feynman, Richard Feynman tells a story of sitting in on a philosophy seminar and being asked by the instructor whether he thought that an electron was an “essential object.”

Well, now I was in trouble. I admitted that I hadn’t read the book, so I had no idea of what Whitehead meant by the phrase; I had only come to watch. “But,” I said, “I’ll try to answer the professor’s question if you will first answer a question from me, so I can have a better idea of what ‘essential object’ means. Is a brick an essential object?”

What I had intended to do was to find out whether they thought theoretical constructs were essential objects. The electron is a theory that we use; it is so useful in understanding the way nature works that we can almost call it real. I wanted to make the idea of a theory clear by analogy. In the case of the brick, my next question was going to be, “What about the inside of the brick?”–and I would then point out that no one has ever seen the inside of a brick. Every time you break the brick, you only see the surface. That the brick has an inside is a simple theory which helps us understand things better. The theory of electrons is analogous. So I began by asking, “Is a brick an essential object?”

The way he tells the story (which, of course, need not be presumed to be 100% accurate), he never got to the followup question, because the philosophers got bogged down in an argument over the first question.

I was reminded of this when I read A Crisis at the Edge of Physics , by Adam Frank and Marcelo Gleiser, in tomorrow’s New York Times. The article is a pretty good overview of some of the recent hand-wringing over certain areas of theoretical physics that seem, to some people, to be straying too far from experimental testability. (Frank and Gleiser mention a silly article by my old Ph.D. adviser that waxes particularly melodramatic on this subject.)

From the Times piece:

If a theory successfully explains what we can detect but does so by positing entities that we can’t detect (like other universes or the hyperdimensional superstrings of string theory) then what is the status of these posited entities? Should we consider them as real as the verified particles of the standard model? How are scientific claims about them any different from any other untestable — but useful — explanations of reality?

These entities are, it seems to me, not fundamentally different from the inside of Feynman’s brick, or from an electron for that matter. No one has ever seen an electron, or the inside of a brick, or the core of the Earth, for that matter. We believe that those things are real, because they’re essential parts of a theory that we believe in. We believe in that theory because it makes a lot of successful predictions. If string theory or theories that predict a multiverse someday produce a rich set of confirmed predictions, then the entities contained on those theories will have as much claim to reality as electrons do.

Just to be clear, that hasn’t happened yet, and it may never happen. But it’s just wrong to say that these theories represent a fundamental retreat from the scientific method, just because they contain unobservable entities. (To be fair, Frank and Gleiser don’t say this, but many other people do.) Most interesting theories contain unobservable entities!