Novikov self-consistency principle

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The Novikov self-consistency principle, also known as the Novikov self-consistency conjecture, is a principle developed by Dr. Igor Novikov in the mid-1980s to solve the problem of paradoxes in time travel. Stated simply, the Novikov consistency principle asserts that if an event exists and that would give rise to a paradox, or to any "change" to the past whatsoever, then the probability of that event is zero.

Time loop logic is an application of this principle to hypothetical computers capable of sending information back through time.

Philosopher Paul Horwich, who has written a number of papers on time travel, made a similar argument prior to Novikov: that autoinfanticide did not present a problem for time travel - it merely showed that if you went back in time you would find you would not be able to kill yourself. Horwich also argued that it was possible to affect the past, but not to change it.

1 Novikov's scenario
2 Potential implications for paradoxes
3 Potential implications for free will
4 Assessments of the Novikov self-consistency principle
5 See also
6 External links

[edit] Novikov's scenario

Rather than consider the usual models for such a paradox, such as the grandfather paradox in which a time-traveller kills his own grandfather (and, thus, prevents his own birth), Novikov used a mechanistic model which was more amenable to mathematics: a billiards ball being fired into a wormhole in such a way that it would go back in time and collide with its earlier self, thereby knocking it off course and preventing it from entering the wormhole in the first place.

Novikov found that there were many trajectories that could result from the same initial conditions. For example, the billiard ball could knock itself only slightly astray, resulting in its going into the past slightly off course, which winds up causing it to knock its past self only slightly astray; this "sequence" of events (actually a causal loop) is completely consistent and does not result in a paradox. Novikov found that the probability of such consistent events was nonzero, and the probability of inconsistent events was zero, so no matter what a time traveller might try to do he will always end up accomplishing consistent non-paradoxical actions.

[edit] Potential implications for paradoxes

The Novikov Principle is able to circumvent most commonly-cited paradoxes which are often alleged to exist should time travel be possible (and are often claimed to make it impossible). A common example of the principle in action is the idea of preventing disasters from happening in the past and the potential paradoxes this may cause (notably the idea that preventing the disaster would remove the motive for the traveller to go back and prevent it and so on). The Novikov self-consistency principle states that a time traveller would not be able to do so. An example is the Titanic sinking; even if there were time travellers on the Titanic, they obviously failed to stop the ship from sinking. However, this does not necessarily prevent an individual from, say, rescuing people from a disaster and replacing them with realistic corpses seconds before the catastrophe occurs. (See Millennium.)

[edit] Potential implications for free will

In another example, first referenced by Henry James, and featured in an episode of The Twilight Zone, a man travels back in time to discover the cause of a famous fire. While in the building where the fire started, he accidentally knocks over a kerosene lantern and causes the same fire that would eventually inspire him, years later, to travel back in time. This situation is entirely consistent — after traveling back in time the man "fulfills" the events in the "past" which "already happened" (from the perspective of the future). In this example, the man may be interpreted as lacking free will — it is impossible for him not to have set off the fire, as that would be inconsistent. Even if he somehow knew that this would happen, he would be somehow bound to "follow" history by the self-consistency principle. Note that there are other equally plausible series of events for this case. For example, the fire could have never happened, and the person would then never travel back in time to discover its cause and make it happen. This is also entirely consistent (unless, of course, it creates a grandfather paradox, when one of his ancestors rushed out of the blazing building to meet his own future wife whose descendant is the man). Thus we see that under this principle there may be many valid "solutions" to the same initial conditions. For the same reason the reduction of free will is minimal: only paradoxes are prevented, all other choices count.

[edit] Assessments of the Novikov self-consistency principle

The Novikov consistency principle assumes certain conditions about what sort of time travel is possible. Specifically, it assumes either that there is only one timeline, or that any alternative timelines (such as those postulated by the many-worlds interpretation of quantum mechanics) are not accessible.

Due to these assumptions, it is possible to consider the Novikov self-consistency principle merely as a tautology, a self-evident truth; that is, it is as a principle that cannot be false by definition and does not need a justification.