How do you find out what’s inside a building if you can’t go in? It’s a question that’s surprisingly relevant in areas like disaster relief, when being able to see inside a collapsed structure could mean the difference between life and death.
So how exactly do you find out what’s inside a building without going in? By detecting tiny particles called muons. Muons are produced in the upper atmosphere by collisions between air molecules and cosmic rays from the sun and other stars. These muons then make their way down to the ground, where specialized detectors can pick them up.
Muons can usually pass through walls, but sometimes they collide instead, meaning that a detector will see fewer muons if there are walls or other objects in the way. By measuring how many muons pass through a certain area—like a building—we can figure out exactly how much of that area is solid and how much is empty space.