At the heart of quantum mechanics lies a profound mystery: particles like electrons can act as both particles and waves. This is most famously shown in the double-slit experiment. But what happens when we try to 'watch' which path a particle takes? This interactive sandbox lets you perform the experiment yourself and discover that 'observation' in the quantum world is a very physical act.
Let's fire single electrons, one by one, at a barrier with two vertical slits. On the other side is a detector screen that will record where each electron lands. When we don't try to measure which slit an electron goes through, it behaves like a wave. It passes through both slits at once and interferes with itself, like ripples in a pond. Over time, these individual electron impacts build up into a distinctive 'interference pattern'.
The interference pattern is strange. How can a single electron interfere with itself? It must be going through both slits. Let's test this by placing a detector at the slits to see which path each electron takes. CRITICAL: This isn't a person watching. Our detector is a device that physically interacts with the electron (for example, by bouncing a photon off it) to record its position. This physical interaction is what quantum mechanics calls 'measurement' or 'observation'.
You've just demonstrated the Observer Effect! When the detector is OFF, electrons act as waves, creating an interference pattern. When the detector is ON, the physical interaction of measurement forces the electron's 'cloud of probabilities' to collapse. It behaves like a classical particle, passing through only one slit, and the interference pattern vanishes.