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 that would give rise to a paradox, 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.

1 Novikov's scenario
2 Potential implications for free will
3 Assessments of the Novikov self-consistency principle
4 Cultural references
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 free will

In another example, taken from an episode of The Twilight Zone, first referenced by Henry James, a person travels back in time to discover the cause of a famous fire. While in the building where the fire started, he or she accidentally knocks over a kerosene lantern and causes a fire, the same fire that would inspire him or her, years later, to travel back in time. This situation is entirely consistent — after travelling back in time the person "fulfills" the events in the "past" which "already happened" (from the perspective of the future). In this example the person lacked free will — it is impossible for him or her not to have set off the fire, as that would be inconsistent. Even if the person somehow knew that this would happen, he or she 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. 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 counterfactual definiteness which is the assertion that there is only one timeline and that multiple alternative timelines do not exist or are not accessible.

Due to these assumptions, some 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.

[edit] Cultural references

Precursors to and variants of the Novikov self-consistency principle have been used in many time-travel stories including some which significantly predate Novikov's conjecture: