# QR3.1.3 Young’s Experiment

Wave-particle dualism is embodied in a simple experiment carried out by Young over two hundred years ago that still baffles physics 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 if so, why do light rays follow lines? Conversely, if photons are particles, how can they interfere like waves?

To find the answer, physicists sent one photon at a time through Young’s slits. Each photon gave the expected dot on the screen as a particle would but over time the dots formed an 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 an interference pattern. Since each photon can’t know where the previous one hit, how does “interference” occur?

In an objective world, one could just see which slit a photon went through before it hit 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, we see a photon particle but when we don’t, it behaves like a wave. It is as if a single skier set off, went around both sides of a tree on the way, then crossed the finish line as one skier (Figure 3.3).

The problem is:

1. If a photon is a wave, why doesn’t the photon smear over the detector screen as a wave would?

2. If a photon is a particle, how can one photon at a time give an interference pattern?

The problem 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).

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