Over two hundred years ago Young did an experiment that still baffles physicists today. He shone light through two slits to get an interference pattern on a screen (Figure 3.2). Only waves diffract like this so light must be a wave but how then does a ray of light hit the screen at a point like a particle? Conversely, if photons are particles how do they interfere like waves?
To find out, physicists sent one photon at a time through Young’s slits. Each photon gave the expected dot but then the dots formed an interference pattern whose most likely impact was behind the slit barrier! The effect was independent of time, so shooting one photon through the slits each year gives the same pattern. Since each photon can’t know where the previous one hit, how does the interference pattern occur?
In an objective world, one could just see the slit a photon went through before it hit but our world doesn’t permit 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 at once, so the interference pattern should not be possible. The evidence suggests that a photon is a particle when we look but a wave when we don’t. This is like a single skier setting off, on the way sliding around both sides of a tree, then still crossing the finish line as one skier (Figure 3.3). The problem in a nutshell are:
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 it give an interference pattern?
And the same applies to every quantum entity, as electrons, atoms and even molecules show Young’s two-slit diffraction (M. Arndt, O. Nairz, J. Voss-Andreae, C. Keller, & Zeilinger, 1999).