Wave function

Harvey had a tattoo of Schrödinger equation for the wave function of the particle. (AUDIO: The Big Blue Book)

Behind the scenes
In Daybreak and The Vardan Invasion of Mirth, the First Doctor refers to points in time being made fixed once a probability waveform has collapsed. This is in reference to, as studied in real-world quantum mechanics, which occurs when a wave function in a (ie. with multiple possible ) is "observed" (measured), and thus collapses into one singular possibility.

, who first introduced the idea of a probability wave, personally maintained that he would "like to regard a probability wave, even in 3N-dimensional space, as a real thing, certainly as more than a tool for ", as a physical phenomenon, more than simply its descriptive wave function.