In the late 1970s, legendary physicist John Wheeler proposed a radical question: Exactly when does the universe notice that we're paying attention to a quantum experiment? And does it really matter? The answer goes against everything we thought we knew.
Wheeler's thought experiment, which eventually became a real experiment, involved the famous double-slit experiment. Let's say you have a source of light and a screen with two thin, vertical slits. When you shine the light through the slits, the light acts like a wave. It interferes with itself, causing a ripple-like pattern on a far wall, with strips of brightness alternating with darkness. This is exactly how waves work, and if you ever find yourself in a harbor with two narrow openings, you'll see the waves washing up onshore with a similar pattern.
