What Is Real?: The Unfinished Quest for the Meaning of Quantum Physics

by Adam Becker

Bohm accounted for the strange results of the double-slit experiment by doing exactly what the Copenhagen interpretation said was impossible: he gave a detailed account of what happens in the quantum world whether or not anyone is looking. Photons, according to Bohm, are particles surfing on waves. While a particle can only pass through one slit, its pilot wave passes through both and interferes with itself. That self-interference, in turn, affects the motion of the particle, because it is guided by the wave. The wave pushes the particle onto a path ensuring the appearance of an interference pattern on the photographic plate after enough photons have been sent through the double slit (Figure 5.4). Putting photon detectors on each slit affects each photon’s pilot wave—no matter how ingenious the design, any photon detector must alter a photon’s pilot wave, as ensured by Heisenberg’s uncertainty principle, which in Bohm’s interpretation places limits on how much measuring devices can avoid interfering with the things they attempt to measure. The effect of these measurements on the photons’ pilot waves alters their trajectories, causing them to form a pair of clusters on the photographic plate rather than an interference pattern. In Bohm’s account, although measurement can influence a particle’s motion, all particles have definite positions whether or not anyone is looking at them.