Heisenberg proposed that momentum and position should be thought of as observables—not quantities with definite values but questions we can ask by doing measurements. The question “What is the position of the electron?” doesn’t have an answer before you ask it. You can “measure the position or momentum,” but by doing so you are bringing the measurement’s outcome into existence, not revealing a preexisting truth. With this insight Heisenberg was able to correctly derive the way that light was emitted from atoms.
