Sunday, September 4, 2011

Reality versus realism

Quantum phsysics theorist David Deutsch writes on Einstein the Realist:
Likewise, the mid-twentieth century “Bohr-Einstein debate” about quantum theory is often misinterpreted as a personal clash between wizards. So counter-intuitive are quantum theory’s predictions that, under the leadership of one of its pioneers, Neils Bohr, a myth grew that there is no underlying reality that explains them. Particles get from A to B without passing through the intervening space, where they have insufficient energy to exist; they briefly “borrow” the energy, because we are “uncertain” about what their energy is. Information gets from A to B without anything passing in between – what Einstein called “spooky action at a distance.” And so on.

What these paradoxical interpretations have in common is that they abandon realism, the doctrine that a physical world, existing in reality, accounts for all of our experience. Anti-realism remains popular and appears in various guises in textbooks and popular accounts of quantum theory. But Einstein insisted that physical phenomena have explanations in terms of what he called “elements of reality.”

Fortunately, a minority of physicists, myself included, likewise side unequivocally with realism, by adopting Hugh Everett’s multiple-universes interpretation of quantum theory. According to this view, no particles exist where they have insufficient energy to be; it is simply that in some universes they have more energy than average, and in others, less. All alleged “paradoxes” of quantum theory are similarly resolved.
Einstein denied that he was a realist. His most widely praised work, his famous 1905 special relativity paper, is credited primarily for its anti-realist approach. He later abandoned that anti-realism, as partial explained in my book, How Einstein Ruined Physics.

The term realism means that ordinary things exist and have an objective meaning. In science, it often means that atoms are real, even tho they are only measurable with special instruments. I am not sure that it is a useful term, if means that there are alternate universes that are not measurable by our universe in any way.

Bohr was really a positivist, and not a reality denier, in the Bohr–Einstein debates. There are textbooks today that say that quantum mechanics is contrary to local realism, and hence is somewhat spooky.

Depending on whom you read, realism is supposed to be opposite to positivism, and also to idealism. Yet both of those seem more realist to me than Deutsch's multiple universes.

Here is the best argument for Deutsch's many worlds. In Young's double-slit experiment, light goes thru two slits and forms a diffraction pattern, just as you would expect from light being a wave. But if you assume that light consists of particles called photons, then the diffraction is explained by each photon going thru both slits at once. If you also assume time symmetry, then it seems reasonable to say that if a photon can be two places at once in the past, then it can also be two places at once in the future. But we never observe a photon being in two places at once, as that would violate energy conservation. So we say that the alternate places for those photons in the future exist in alternate universes that we cannot see.

Deutsch also says that a quantum computer would prove the existence of the many worlds, because the parallel worlds would be the answer to the question about where the mysterious parallel computation is taking place. No one has ever made a quantum computer, and no one knows whether it is possible.

Does any of this convince you of many worlds? Not me. It only convinces me to stop calling myself a realist.


  1. Has anyone ever detected a single photon?

  2. Yes, individual photons have been detected. Devices like digital cameras can do so. Light is absorbed by semiconductors in discrete amounts. But whether or not a photon is going thru two slits at once is a matter of interpretation, because measuring the photon going thru the slit destroys the interference pattern.