It’s no secret, however, that light does act like a wave as well – which is very much where the questions start. And, in fact, wave-particle duality exists not just as a phenomenon of light, but, seemingly, of all matter as well. One of the most famous experiments in all quantum theory is the double slit experiment. It relies on diffraction, which most of us are familiar with from school. Pass a water wave through two slits close to each other and ripples will propagate from them both; as the two sets of ripples encounter one another you will see a pattern of constructive and destructive interference between them, as they either enhance or attenuate the oscillations. The same is true of other at least nominally more corpuscular things. Fire a beam of electrons towards two slits and you will also see peaks and troughs spread out on the wall behind them.
Moreover, if you fire one electron at a time through the slits, so that they cannot interfere with any other particle, you still get peaks and troughs at the other end. An interference pattern emerges over time as you fire electrons one by one through the slits, though individually they arrive on the other side of the slits in a defined place. We are forced to conclude that electrons can interfere with themselves like waves. They pass through both slits at once.
But this isn’t the strangest thing about the experiment. Every attempt – every attempt – to measure an electron passing through the slits has destroyed the interference pattern, and produced a definitive answer to which path the electron took. Without a measurement there is concrete evidence for a wave-like nature; but as soon as one is made, however it is made, all evidence of it disappears and we are just left with particles.