Observer Effect

The Observer Effect, (PROSE: Going Once, Going Twice) also spelt observer effect, (PROSE: The Eyeless) was the ability to give something a fixed reality where, before the act of observation, there had only been unanchored, chaotic possibility. (PROSE: Going Once, Going Twice)

Nature
Quantum theory and its more developed successor, biodata theory, stated that simply by being present to observe an event, any conscious being could make the meanings of that event more fundamental than matter. (PROSE: The Cosmology of the Spiral Politic) This was echoed in quantum-locking, the principle by which the powers of the Weeping Angels were constrained: they could only move when unseen, and collapsed into lifeless stone whenever they were observed by another living thing. (TV: Blink)

Through the anchoring of the thread, the Great Houses guaranteed that the entire Spiral Politic could be observed by the Homeworld; (PROSE: The Cosmology of the Spiral Politic) due to this, Time Lords had a specially close and intense relationship with history. (TV: The War Games, et al.) They naturally refined and reshaped space and time around them through their perception. (PROSE: Christmas on a Rational Planet, The Infinity Doctors) Gallifrey maintained a non-interference policy because, if they were to interact with the universe rather than just observing it, they would collapse its infinite possibility into a single certainty. (PROSE: The Gallifrey Chronicles) A mad Time Lord could even have the power to infect the past and future with insanity. (PROSE: The Infinity Doctors)

The Rassilon Imprimatur played an important role in Time Lords' ability to travel through time, (TV: The Two Doctors) mapping their biodata onto the Time Vortex as pure mathematics (PROSE: Interference) and tethering them to their homeworld through what Sabbath called "the Great Eye". (PROSE: The Adventuress of Henrietta Street) Without this tether or the protection of the TARDIS, humans had difficulty traveling in Deep Time: (PROSE: Camera Obscura) in his journals, Sabbath described his experience there as

The very act of not being observed by the Homeworld's eye could cause a devastating attack of ignorance. (PROSE: The Book of the War) A similar mode of attack could be used to curtail the power of an unanchored being trapped in the anchored universe, who would devolve into meaningless chaos if left completely unobserved by either Houses or loas. (PROSE: Going Once, Going Twice) According to Littlejohn, the density of tardigrades throughout the universe threatened the Great Houses' observer criticality. (PROSE: The Short Briefing Sergeant's Tale)

The Eleventh Doctor told Amy Pond that Time could only be "rewritten" so long as "you hadn't read it": once one had viewed with certainty what the future was going to be, it became fixed in stone. For this reason, the Doctor tried to avoid reading from Melody Malone: Private Detective in Old New York Town to avoid restricting his own freedom of action in changing the future. (TV: The Angels Take Manhattan) However, on another occasion, he had admitted that to some extent, "knowing your own future" could be "what [enabled] you to change it", especially if one was "bloody-minded, contradictory and completely unpredictable". (TV: The Girl Who Waited)

History
After the anchoring made the universe subservient to the perception of the Great Houses, (PROSE: The Cosmology of the Spiral Politic) Mappers were loomed to "map out the meridians of Time" using the Observer Effect. The later Faction Paradox Godfather, Auteur, was claimed to have originally been one of these Mappers, who had discovered the ability to abuse the Observer Effect to will whatever he wanted into reality. (PROSE: Going Once, Going Twice)

The Yssgaroth "taint", which allowed Yssgaroth biomass to corrupt ordinary biomass by contact, (PROSE: The Book of the War) and whose most impressive incarnation, the Great Vampires, (PROSE: The Pit) were felt by the Kotturuh to be alien to the order of their universe, (PROSE: All Flesh is Grass) was speculated to be the effect of observation on the Spiral Politic by the actual natives of the Spiral Yssgaroth. (PROSE: The Cosmology of the Spiral Politic)

The Time Lords dealt with the "renegade Presidency" on Dronid by "ignoring them", (TV: Shada) with the ensuing attack of ignorance nearly tearing the planet to shred, although whatever scavenger society remained on the planet once the attack ended had access to a variety of Gallifreyan artefacts. (PROSE: Alien Bodies) During the War in Heaven, which originally began on Drornid, the Homeworld similarly attempted to defeat the Enemy by failing to observe them. However, this led to the discovery that the Enemy had their own, equally-powerful Eye. (PROSE: The Book of the War)

Behind the scenes

 * The Time Lords' observer effect would have played a major role in the unproduced novel Mentor.