In 1801, Thomas Young shone light through two narrow slits onto a screen. He expected two bright bands, one for each slit. Instead he got something the physics of his day said was impossible: an interference pattern. Stripes of light and dark, alternating across the screen, exactly what you get when two waves overlap and add up or cancel out. Newton had insisted light was made of particles. But particles fired at two slits make two bands. Waves make stripes. Young saw stripes, so he decided light was a wave. That held for over a century, until Einstein showed in 1905, with the photoelectric effect, that light also acts like particles. So which is it? Light behaves as both, and as neither. It is something our everyday words were not built to describe.
The modern version swaps light for electrons, and the result is even stranger. Physicists fire electrons one at a time through the two slits. Only one electron is ever in flight, so none of them can be interfering with each other. Yet after thousands of single electrons land, the interference pattern shows up anyway. Each electron seems to have gone through both slits at once and interfered with itself. Before it hits the detector, an electron is not a tiny dot in one place. It is a spread of possible positions, a probability wave. It only snaps to a single spot when it is measured. It was not secretly using one slit or the other. Until you check, it genuinely goes through both.
Wave-particle duality does not mean quantum mechanics is broken or confused. It is a clue that reality has a deeper layer our everyday words cannot reach, because those words were built for rocks and rivers, not for this. A quantum thing is not a wave one minute and a particle the next. It is a quantum thing, described by math called a wavefunction, and how it acts depends on what you measure. Ask 'where is it?' and you find a particle in one spot. Ask 'how does it travel?' and you find a wave spreading and rippling across space. The measurement you choose decides which face it shows you.
Richard Feynman called the double-slit experiment the only mystery of quantum mechanics. His point: understand this one thing and you have grasped the core strangeness that everything else in the quantum world grows out of. A particle's wavefunction is a piece of math that holds every state the particle could be in at once. It is not a real wave like a water wave or a sound wave. It is a wave of probability. When the particle meets a detector, when it gets measured, the wavefunction collapses to a single definite state. The cloud of maybes becomes one fact. What actually causes that collapse is the biggest open question in physics. And the answer you pick splits the whole field into rival camps with wildly different views on consciousness and reality.
The wildest part comes when physicists try to catch which slit the electron actually used. Put a detector at the slits, any detector at all, as long as it can in principle record which path the electron took, and the interference pattern vanishes. The electrons stop acting like waves. They act like ordinary particles, going through one slit or the other and making two bands on the screen. Just having a way to observe, just the fact that the which-path information exists somewhere, is enough to kill the wave behavior. That is the observer effect. And it does not need a conscious person watching. It only needs a physical system that can record the information. The lesson is startling: how a quantum system behaves depends not only on what it is, but on whether information about it exists in the world.