Young's Double Slit Experiment

Thomas Young was a British polymath.

In 1801, he sought to prove Christiaan Huygens' theory that light was made of waves travelling through the invisible aether. Newton believed light was physical particles called photons. Young's experiment demonstrated wave-particle duality and laid the foundation for quantum physics.

A brief description of the experiment that shows the wave/particle duality:

A light source shines photons towards a slit.

The stream of photons meet another plate with two slits in it.

If photons are like particles - tiny balls - they go through either slit and hit the screen on the other side as two vertical lines.

If photons are like waves on a pond, the waves will interfere with each other after tasing through the double slits and create alternating dark and light patches: dark - where a crest and trough of the wave coincide and cancel each other out, and light - where two troughs or peaks coincide.

When it was turned on, an interference pattern was seen - so photons are waves.

However, when a detector was placed beside one of the double slits to observe it, a bizarre thing happened when it was turned on: two vertical lines appeared on the screen.

This is quantum wave-particle duality. The photon exists in both states simultaneously, and the observer affects whether it manifests as a wave or particle in the physical world. The photon was aware it was observed and showed a degree of consciousness.

Oops! This is a bit tricky for materialism which holds that matter is inert and lifeless.