The idea of wave-particle duality has puzzled scientists since the beginnings of quantum mechanics, over 100 years ago. But what does it really mean? How can something be both a particle (like a ball bearing) and a wave (like the light from laser)?
The truth is, it’s not both at the same time; it depends on how we look at it. If we measure light as a wave, it behaves like a wave. If we measure it as a particle, it behaves like a particle.
A famous experiment called the double-slit experiment shows this clearly. Imagine shining light through two small openings (slits) and then looking at the pattern on a screen behind them. Instead of seeing two bright spots (like you’d expect from particles), we see a pattern of stripes, called an interference pattern — the kind of pattern waves make when they mix (interfere).
The really strange part is that even if we send the light one ‘particle’ at a time, the interference pattern still builds up! This occurs as if each tiny particle somehow goes through both slits at once like a wave and then becomes a particle again at the screen.
But if we set up something to watch which slit each particle goes through, the wave-like pattern disappears. We just see two lines corresponding to the two slits, like how we expect particles to behave.
It turns out, the interference happens because we don’t know which path the particle took. if we cannot know which of two possibilities happened, then we must consider that both were possible, and those two possibilities mix (interfere) like waves. While we can never know what is truly happening (as if we look it stops happening), is there some way we can use knowledge between two possibilities – or our lack of it – to our advantage.
