Discontinuities II

See the combox for Discontinuities.

Multiple timelines means many four-dimensional space-time continua coexisting in parallel along a fifth dimension. A mutable timeline means a single continuum changing along a second temporal dimension. Let us suppose that we inhabit a single immutable timeline.

Poul Anderson

Multiple timelines: the Old Phoenix multiverse

Mutable timeline: the Time Patrol universe

An immutable timeline: There Will Be Time and The Corridors Of Time

 

In our (supposed) single immutable timeline:

If we now learn that, in 1942, an assassin left Britain intending to kill Hitler, then we know that, whatever else happened to that assassin, he failed in his mission. Something, some mishap, prevented him from killing Hitler. The situation is no different if we now learn that, five minutes ago, a time traveler departed into the past intending to kill Hitler. Some mishap (a heart attack, a malfunctioning time machine, God knows what) prevented him from killing Hitler. However, if a thousand or more time travelers set out to kill Hitler, then the laws of statistics would have to change to allow for the necessary number of mishaps.

Someone killing Hitler in 1942 if Hitler lived till 1945 is a logical contradiction. If a logical contradiction is unacceptable (it is) and if changed laws of statistics are also unacceptable, then it follows that, in an immutable timeline, time travel:

is impossible;

or does not happen;

or is very rare;

or happens but with major restrictions, e.g., is possible only into the very remote past;

etc.