Young’s double-slit experiment is a simple result discovered over two hundred years ago that still baffles physics today. Light shone through two slits gives an interference pattern on a screen (Figure 3.2). Only waves interfere like this, so light must be a wave, but why then do its rays hit at a point? Conversely, if light is made of particles, why do they interfere like waves?
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 result. Since each photon doesn’t know where the last one hit, how they interfere with each other?
Was each photon going through both slits to interfere with itself? In an objective world, we could just observe whether a photon went through both slits, but our world doesn’t work like this. Detectors placed in both slits to see where the photon goes just fire half the time as expected, so one photon always goes by one slit or another, never through both. How then is interference possible?
It seemed that when we look at the slits, we just see a photon particle, but when we don’t look, they interfere. It is as if a skier set off, skied around both sides of a tree, but still arrived at the finish line as one skier (Figure 3.3).
In summary, the problem is:
1. If a photon is a wave, how can it hit the detector screen at a point not a smear?
2. If a photon is a particle, how can going through one slit give an interference pattern?
And 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), so they also act like waves.