Superposition is the idea in physics that matter can seem to be in two places at once. Strange stuff, yes, but real. More precisely, superposition is the idea that quantum matter can hold multiple possible states at once until an interaction, measurement, or observation produces one outcome, or until the wave collapses, which is what a physicist might say.
It is one of the remaining mysteries in physics. We observe its effects, and the math works beautifully, but it still feels impossible and definitely needs more and better explanation. We can calculate matter as existing in multiple potential positions until we measure it. Almost as if reality is holding several doors open at once—and when we look, only one opens.
The traditional explanation says a quantum object—like an electron—exists in multiple states at once. To me, that sounds magical, and as an empirical pragmatist, I don’t love that explanation. Until we have a better description, I like to think of superposition as potential. Imagine flipping a coin, but only seeing heads or tails when you look. Before that, it is both—or could be. When you actually flip it and look, it can be either heads or tails, but not both. Luckily for me, some theoretical physicists also view superposition this way. The problem is that it seems to be more than just ordinary potential.
The wave-particle duality of physical matter is still part of this mystery too. In the double-slit experiment, a single particle behaves like a wave of possibilities, as if its potential passes through both slits at once. But when measured, it shows up as a particle in one place. That’s what we see. The problem? We still don’t have a satisfying explanation for why.