Young presented his interference work in 1801 while Newton's corpuscular account of light remained influential. His fringes were not just pretty bands. They gave the wave theory a powerful experimental case, made with sunlight, cards and patience.

Light from two narrow openings can produce an interference pattern: alternating bright and dark bands. Classical wave theory explains the pattern through reinforcement and cancellation. Modern quantum theory retains interference while refusing a simple choice between a classical wave and a stream of independent classical particles.

For classical light, two openings produce overlapping waves. Where their phases reinforce, the screen is bright; where they cancel, it is dark. A classical stream of independent pellets would instead add two distributions. The fringes reveal interference, and quantum theory predicts them by adding probability amplitudes before calculating probabilities.

Run the experiment so gently that detections occur one at a time. Each event leaves a localized mark. After many events, the marks build the same interference distribution.

The result rules out a classical picture in which each object simply follows one unknown path while nothing else changes. Quantum theory represents the alternatives with amplitudes that can interfere. Saying that a particle literally travels both classical routes is one interpretation of that mathematics, not an additional measurement made by the experiment.

Add an interaction that can record which opening was used and the interference is reduced, disappearing when the path information is complete. The crucial change is physical: the alternatives become correlated with a detector or environment and can no longer combine coherently in the same way.

In quantum mechanics, "observation" means this kind of interaction and record. A person does not have to watch the apparatus, and consciousness is not the switch that changes the pattern.

Wave and particle are inherited classical pictures, each useful for part of what the apparatus records. Quantum mechanics does not require a tiny object to switch costumes when someone looks. It supplies a different predictive structure in which experimental arrangements determine which distinctions can be made.

The poetry here belongs beside the mechanism, not inside it. One arrangement preserves alternatives well enough for them to interfere; another leaves a durable path. The border is drawn by the interaction.

Feynman called the two-slit setup 'the only mystery' because every quantum oddity hides the same structure: alternatives combine as amplitudes, not as classical either-or events. The fringes are the grammar of quantum theory.