Wheeler's delayed-choice experiment

Wheeler's delayed-choice experiment is a fun experiment that helps us understand some of the odd things about the universe. It asks if light (a small particle made of energy) is more like a wave or more like a particle. To do this, scientists shoot a few particles towards a wall with two doors in it. Right before the particles hit the wall, they're given a choice to pick a door. Depending on which door they pick, they'll go in different directions. When it's over, the scientists look to see what happened to the particles.

Surprisingly, whether the particles went through one door or the other could tell them whether the particles were waves or particles. It seemed like the particles knew what the scientists were going to decide right before they made their choice.

Even though this was strange, Wheeler's delayed-choice experiment showed us that light can be both a wave and a particle. It also showed us that particles from the past may influence the decisions we make in the present.