Hal Finney (
Wed, 6 May 1998 10:29:46 -0700

Anders Sandberg, <>, writes:
> Recently I have begun to move away from classical causality towards
> Novikov's consistency principle instead: the universe is consistent
> (i.e. no paradoxes) but there is no a priori ban on time travel. This
> means you can get "causal loops", but no physical laws are broken at
> any point and you won't get any paradoxes.

You don't get contradictions, but you do get paradoxes in the sense of
things which are so contrary to intuition that you almost can't believe
that they happen.

There was an article in Extropy many years ago about electronic circuits
which depended on time travel. It looked like it would be possible to
do significant amounts of computation in zero time. You set up a system
which will be inconsistent unless a certain equation is satisfied, and
presto, the non-contradiction principle automatically means that the
system satisfies the equation.

Time travel computers would put quantum computers to shame. Any NP
problem (that is, any one whose answer can be checked easily, like
factoring) can be solved in constant time.

There is a classic sci-fi story in which (to simplify slightly) the
man who will be known as the inventor of the time machine didn't invent
it, he was given it by a future version of himself. This would seemingly
be consistent and has no contradictions, but it is paradoxical that
a complex device like this would have no inventor.

You also have to ask how the non-contradiction principle would prevent
the actions of people who want to produce events in the past which are
inconsistent with history. Seemingly the universe must interpose some
obstacles which will inevitably thwart their efforts. The universe
is forced in effect to act as a smart opponent, always coming up with
something which defeats the clever tricks people try to use.