The double-slit study additionally raises questions about what “matter” really is. Our everyday human experience tells us that matter is easy to define: It is simply solid material that makes up the universe. My arm, which is made of many atoms of matter, certainly feels “solid” to me.

What causes this collapse? The debate rages on today, but Heisenberg writes: “The path of the electron comes into existence only when we observe it.” The act of observation itself seems to be the key to unlocking a particularly “local” reality out of the “non-local” probability wave of where the particle might be.