Quantum physics

Quantum physics, (AUDIO: ScrapeJane, Dead Media) quantum mechanics, (TV: The Ark in Space, The Day of the Doctor, WC: Tia Kofi Enters the Time Fracture!) or quantum theory, (PROSE: Losing the Audience) revealed that particles changed their behaviour when observed. Ng theorised that the experience of "shoulder creeping" was related to quantum phenomena. (AUDIO: ScrapeJane)

Quantum theory was one of the subjects studied by Susan before she and the First Doctor settled at Totter's Lane. (PROSE: Losing the Audience)

In the 20th century, the standard way of thinking about quantum mechanics held that things did not definitively exist in the universe, there were simply probabilities of things that may or may not be the case, and may or may not be measurable. Only the presence of a conscious observer, it was held, could collapse the possibilities until one of them is perceived to be the case. R. B. Nevitz speculated that the primacy of observers entailed that it was meaning that was fundamental in the universe rather than matter, leading to the creation of Biodata theory. (PROSE: The Book of the War)

The Wirrn were keen on gaining human knowledge, including quantum mechanics. (TV: The Ark in Space)

While waiting for Clara Oswald to finish her day of work at Coal Hill School, (PROSE: The Day of the Doctor) the Eleventh Doctor read a book on Advanced Quantum Mechanics, (TV: The Day of the Doctor) which bored him after he failed to find Wally anywhere and had to resort to reading the text. (PROSE: The Day of the Doctor)

The Twelfth Doctor thought it was "adorable" that quantum physics was taught in universities on Earth. (AUDIO: Dead Media)

Doctor Errol Courtney was frustrated when Tia Kofi confirmed that she didn't have any knowledge of astrophysics or quantum mechanics, but tried not to show it. (WC: Tia Kofi Enters the Time Fracture!)

Behind the scenes
The 2019 Companion Chronicles audio stories Daybreak and The Vardan Invasion of Mirth establish that fixed points in time are only created once a probability waveform has collapsed. This is a reference to, a concept in quantum mechanics. This is when a wave function in a (ie. with multiple possible quantum states) is "observed" (measured), and thus collapses into only one possibility.

Ng in ScrapeJane is stretching this meaning of "observation", in the context of quantum mechanics. In the real world, a is not usually taken to be the same thing as mere observation through sight. The, treating the entire experimental apparatus as a single quantum state, would have an equivalent usage of the term, but this is the one outlier.