Blinovitch Limitation Effect
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It is usually understood as having two aspects: firstly, that a time traveller cannot "redo" an act that he has previously committed, and secondly, that a dangerous energy discharge will result if two temporal versions of the same person come into contact. The first aspect is similar to a real-world physics conjecture, the Novikov self-consistency principle.
How the Blinovitch Limitation Effect works has never been made clear by any of the television programme's production teams. It remains a convenient plot device rather than an attempt to rationalise time travel in the Doctor Who universe.
 The original series
The Effect was introduced in Day of the Daleks (1972), when Jo Grant asks the Third Doctor why a group of time-travelling guerrillas on a mission to assassinate a diplomat cannot simply go back into the past and try again if they fail. In reply, the Doctor cites the principle and begins to explain, but is interrupted before he can explain further. The "Effect" was invented by script editor Terrance Dicks and producer Barry Letts to gloss over the plot problems inherent in the time travel premise of the serial. The interruption introduced by the writers meant that the explanation did not have to be expanded upon.
The Effect is next mentioned in Invasion of the Dinosaurs (1974), where the Third Doctor states that it "tends to limit research into time travel" but once again he does not go into detail as to why. The novelisation of the story reveals that Blinovitch was a "great bear of a man from Russia" who had reversed his own timestream, reverting to infancy.
The next time the Effect is mentioned on-screen is in Mawdryn Undead (1983), but there it is not a scientific principle that prevents people from redoing their actions (for whatever reason). Rather, it is a physical effect that occurs when two versions of the same person from different time periods make physical contact. This results in an energy discharge, shorting out the "time differential" between them. The Mawdryn Undead storyline establishes that the younger version of the character involved in the discharge, Brigadier Lethbridge-Stewart, is traumatized by the event and, for the next several years, loses his memory of the Doctor. It is possible, however, as Lawrence Miles and Tat Wood theorise in their reference book About Time 5, that the energy discharge is simply a side effect of the principle's operation. It also appears, given the number of times that the Doctor has met his other incarnations, that the Effect does not apply to Time Lords, or at the very least can be mitigated. The Doctor appears to show sensitivity and resistance to temporal distortions, notably in The Time Monster (1973), Invasion of the Dinosaurs and City of Death (1979) (where Romana does as well).
The effect does not appear to present a problem with characters interacting with their own ancestors, as Ace met her infant mother and worked extensively with her grandmother in The Curse of Fenric (1989).
The Virgin New Adventures novel Timewyrm: Revelation (1991) by Paul Cornell gave Blinovitch the first name of Aaron (and the title of his book as Temporal Mechanics). The Virgin Missing Adventures novelisation The Ghosts of N-Space (1995) by Barry Letts stated that Blinovitch formulated his principle in the British Museum's reading room in 1928, and although he was not the first to discuss it, he was the first to formulate it properly.
Miles and Wood suggest that the key word is "Limitation"; that is, the effect limits the amount of interference in the past as opposed to completely prohibiting it. This interpretation is supported by the Barry Letts-written Doctor Who radio play The Paradise of Death (1993), where the Third Doctor explains to Brigadier Lethbridge-Stewart that it is possible to leave a location in the TARDIS and arrive at a time before they actually left, since the Effect only stops someone from interfering with their own past. Although the First Doctor claimed that it was impossible to alter history in The Aztecs (1964), and the concept of unchangeable "fixed points in time" has become a recurring concept in the 2005–present revival, other stories such as The Time Meddler (1965), Day of the Daleks (1972), Genesis of the Daleks (1975), Pyramids of Mars (1975), Remembrance of the Daleks (1988), and A Christmas Carol (2010) imply that changing history is still possible.
 The new series
In the 2005 series episode "Father's Day", Rose Tyler crosses her own timestream and redoes her actions, saving her father from dying in 1987, despite the Ninth Doctor's warnings not to attempt to cross their timestreams a second time and not to touch her infant self. This paradox results in serious repercussions, dealt with in the story. The Effect does not behave as described in earlier stories, but the contact does create a paradox that summons the Reapers, which proceed to "sterilise" the resulting wound in Time by devouring everything in sight. When Rose briefly carries her younger self in her arms, there is no visible energy discharge, although a Reaper appears immediately. Speaking at the Gallifrey convention in February 2006, episode writer Paul Cornell said that although his script does not mention the Blinovitch Limitation Effect by name, it was in the forefront of his mind while writing the episode.
The loss of the Time Lords and their stabilising influence on time due to the Time War is hinted at in "Father's Day" itself, "The Unquiet Dead" (2005), and later in "Rise of the Cybermen" (2006). This fits in with Miles and Woods's suggestions of a cosmic observer effect imposed on the universe by the Time Lords, resulting in the creation of the Blinovitch Limitation Effect as a safeguard against tampering with causality. It is also consistent with the Doctor's observation in "Father's Day" that if the Time Lords were still around, they could have repaired the paradox.
In "The Parting of the Ways" (2005) and "The Girl in the Fireplace" (2006) the Ninth and Tenth Doctors respectively make reference to becoming "part of events". In the former episode, Rose asks the Ninth Doctor why he cannot go back in time and warn Earth of the Dalek attack that is happening and he replies that once he "lands in that second, [he becomes] part of events. Stuck in the timeline." The Effect is not mentioned by name, but the consequences stated appear to be similar to those in Day of the Daleks, where the guerillas become caught in a predestination paradox, doomed to create the very future they are trying to avert. Similar language is used in "The Stolen Earth" (2008), where the Doctor describes the events of the Time War as being "time locked" so that no one is supposed to be able to travel back to that period, though Dalek Caan was presumably able to make the transition and avert the consequences of the Effect.
In the Doctor Who Christmas Special "The Runaway Bride" (2006), Donna Noble remarks that she wishes to use the TARDIS to travel back to a point during her own wedding, as she had been transported inside the TARDIS before the ceremony could take place. The Tenth Doctor says he cannot allow her to cross her own time stream.
In "Time Crash" (2007), the Tenth Doctor attributes the aged appearance of the Fifth Doctor to shorting out the "time differential" (the same phrase used in Mawdryn Undead) between them, and stating that it will "snap back in place" when the Fifth Doctor is returned to his rightful moment in time.
In "Blink", elderly Billy Shipton tells Sally Sparrow that he often thought about contacting her prior to the time of his younger self's disappearance, "but apparently it would have torn a hole in the fabric of space and time, and destroyed two thirds of the universe." In the same episode, however, Martha Jones tells younger Billy that she and the Tenth Doctor attended Apollo 11's landing four times.
In "The Hungry Earth" (2010), the Eleventh Doctor discourages Amy Pond and Rory Williams from being in close contact with future incarnations of themselves, stating that it would "...make things complicated." but not specifically mentioning the Effect. Nevertheless, in "The Big Bang" (2010), at a point where Time itself is collapsing, Amy Pond makes physical contact with her younger self in an alternate history with no repercussions. At this point, there are no other species in the universe except those on Earth (the eye of the storm). Reapers do not appear. However, when the Doctor touches a present-moment sonic screwdriver to a future version of itself, sparks are emitted. This appears to confirm to the Doctor that the two identical configured screwdrivers are the same object at different points in the timeline. The Doctor subsequently directs adult Amy to give her 5-year-old self an icecream cone in "Good Night".
In "A Christmas Carol" (2010), older and younger versions of the character Kazran Sardis meet and touch without any problems at all, which reviewers have pointed out seems to violate the principle.
In the mini-episodes "Space" and "Time" (2011) which immediately follow the aforementioned "A Christmas Carol", two versions of Amy Pond, only minutes out of syncronisation from each other, interact with each other within the TARDIS without ill effect. Similarly minute-older Rory Williams and Eleventh Doctor give instructions to the trio as they pass through the TARDIS.
River Song counsels Amy Pond not to inform the Doctor of his 200-year-older self's death in "The Impossible Astronaut" (2011). When Amy argues that the Doctor has met his younger self before, her husband Rory Williams reminds her that the universe nearly blew up as a result, referring to the events of "The Big Bang".
 Popular culture
Blinovitch is mentioned a number of times (presumably as an in-joke for Doctor Who viewers) in the romantic comedy Happy Accidents (2000), which has a plot involving time travel. In the film, Blinovitch is said to have been from "Yugoserbia" and discovered how to bend spacetime. Two of his laws are invoked:
- "Blinovitch's Second Law of Temporal Inertia" apparently states that is impossible to time travel in your own lifetime. One can only time travel to the distant past, and only small changes in history are possible, which will "dampen out" by the time they reach the relative present.
- "Blinovitch's Fifth Law of Causal Determination" resolves (in an unspecified manner) all paradoxes involved with time travel.
In Supergirl Annual #2 (2010), Brainiac 5 of the Legion of Super-Heroes in the 31st Century complains how hard it is to maintain the timeline when heroes like Superboy and Supergirl visit their future, saying, "The Novikov and Blinovitch Effects alone take me hours to account for!"
- "The Blinovitch Limitation Effect was first mentioned in Day of the Daleks (and subsequently in Invasion of the Dinosaurs ..." Parkin, Lance (1996). Doctor Who: a history of the universe. Doctor Who Books. Retrieved October 16, 2010.
- Letts, Barry (2008). Who and Me (A Doctor Who Audiobook). BBC Audio.
- "Invasion of the Dinosaurs". BBC. Retrieved October 16, 2010.
- Miles, Lawrence & Wood, Tat (2005). About Time 5: The Unauthorized Guide to Doctor Who (Seasons 18 to 21). Metairie, Louisiana: Mad Norwegian Press. ISBN 0-9759446-4-9.
- "Mawdryn Undead". BBC. Retrieved October 16, 2010.
- "Aaron Blinovitch formulated his Limitation Effect in 1928, publishing it in Temporal Mechanics." Parkin, Lance (1996). Doctor Who: a history of the universe. Doctor Who Books. Retrieved October 17, 2010.
- Alex Westthorp (August 12, 2008). "Doctor Who audio: The Mutant Phase". Den of Geek!. Retrieved October 17, 2010.
- Vinnie Bartilucci (June 18, 2010). "Post Game TV Recap: DOCTOR WHO S5E8 - "THE HUNGRY EARTH"". Newsarama. Retrieved October 16, 2010.
- "Explaining Doctor Who: The Big Bang". Den of Geek!. July 5, 2010. Retrieved October 17, 2010.
- The Doctor states it was 1994; Amelia Pond was seven years old in the 1996 scenes of "The Eleventh Hour"
- Rich Johnston (December 25, 2010). "Ten Thoughts About Doctor Who: A Christmas Carol". Bleeding Cool. Retrieved December 25, 2010.
- Park, Ed (August 21, 2001). "Accidents Will Happen". The Village Voice. Retrieved October 17, 2010.
- Supergirl (vol. 4) Annual #2 (2010)