In his Reference Frame “What’s Bad About This Habit” (Physics Today, May 2009, page 8), N. David Mermin discusses what is real and what is abstract in physics—but without first defining what he means by those terms. The lack of definition is another bad habit that diminishes his otherwise interesting comments. In physics, we can give a concrete definition of “real”: phenomena or events that can be recorded by a device. The process of recording involves an irreversible transition that generally requires an interaction with a macroscopic device. The device can be our brains, but with some reservations, because sometimes we see or remember imaginary events that never happened. But “acquisition of knowledge or information,” pace Werner Heisenberg, although it occurs, is not relevant. Or as Richard Feynman remarked, “Nature … behaves the way she is going to behave whether you bother to take down the data or not.” 1
By the above definition of reality, Mermin is correct to point out that operators in Hilbert space and quantum wavefunctions are abstractions. I also believe that one should not lose any more sleep over the collapse of a wavefunction than over the change of any probability function after one of the possible outcomes has been recorded. But it does not follow, for example, that because electromagnetic fields are operators in Hilbert space, the manifestations of those fields are also abstractions or that the spacetime continuum where those fields are located is devoid of reality. Ironically, Mermin’s article appears in an issue of Physics Today whose cover and associated article show photographs of the spectacular magnetic field lines located in space (the Sun’s corona) and recorded in time in a UV image taken by a NASA spacecraft. The fields are made visible by the radiation of charged particles, in a similar way that planets and their orbits are observed by the reflection of solar radiation or that subatomic particles are seen by tracks left in a cloud or a spark chamber. Likewise, in the two-slit experiment, the effect of quantum interference with single photons can be recorded; consequently, the manifestations of the associated quantum wavefunction that predicts the interference are also real.
Mermin claims that “spacetime is an abstract four-dimensional mathematical continuum of points that approximately represent phenomena,” and that it “is nothing more than an extremely effective way to represent relations between distinct events.” But spacetime points do not represent phenomena; instead, they represent the locations of phenomena, which are determined by measurements of the relative distances and the time intervals between events. Mermin remarks that his point of view “may also be easiest to see in quantum physics, where time and space refer ultimately to the time and place at which information is acquired or, if you prefer, at which a measurement is made.” But in both quantum and classical mechanics, the location in spacetime is also obtained by measurements, as are other characteristics of the event. Hence, in accordance with our concrete definition of reality, spacetime is real, although special relativity tells us that our measurements of position and time are frame dependent.
Mermin is also concerned with the habit of attaching reality to “spooky actions at a distance” associated with the Einstein-Podolsky-Rosen effect. But what is measured in the associated experiments are correlations that are predicted by quantum theory to occur at any distance of separation for the entangled particle states. That includes the case in which the distance becomes vanishingly small, and then such concerns as “nonlocality” and “faster-than-light influences” are not relevant. What would be really spooky is if those correlations depended on distance, because it would show that quantum mechanics is flawed. However, measurements demonstrate that the correlations do not depend on distance; thus they confirm again that the manifestations of quantum entanglement are not abstractions.
Bad habits in the interpretation of such quantum phenomena usually originate from attempts to impose on the microscopic world views of reality learned from classical physics.
