The double-slit experiment is a simple test carried out over two hundred years ago that still baffles physics today. Young shone light through two slits to get an interference pattern on a screen (Figure 3.2). Only waves interfere like this, so light is a wave, but why then do its rays move in lines to hit at a point? Conversely, if light is made of particles, why do they interfere like a wave?
To explain this puzzle, physicists used lasers to send one photon at a time through the slits. Each photon then gave a dot on the screen, as a particle would, but over time the dots formed the familiar interference pattern, whose most likely impact was behind the barrier between the slits! The effect was independent of time, so one photon shot through the slits each day still gave the same pattern. Since a photon can’t know where the last one hit, how can interference occur?
It seemed that each photon was going through both slits to interfere with itself! In an objective world, one could just observe whether a photon went through both slits, but our world doesn’t work like this. Detectors placed in the slits to see where the photon goes just fire half the time, as expected. A photon always goes by one slit or another, never through both, so interference shouldn’t be possible. When we look, there is a photon particle, but when we don’t look, there is interference. It is as if one skier set off, then went around both sides of a tree, but still reached the finish line as a single skier (Figure 3.3).
In summary, the problem is:
1. If a photon is a wave, why doesn’t it smear over the detector screen as a wave would?
2. If a photon is a particle, how can one particle cause an interference pattern?
Even worse, this wave-particle duality doesn’t just apply to light, as electrons, atoms, and even molecules exhibit Young’s two-slit interference (M. Arndt, O. Nairz, J. Voss-Andreae, C. Keller, & Zeilinger, 1999).