Anders Sandberg (
06 May 1998 20:45:31 +0200

Hal Finney <> writes:

> 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.

Yes, but that happens all the time now (superfluids, EB condensates,
gamma ray bursts, spacetime, self-organization, the Banach-Tarski
paradox...). Common sense and intuition doesn't fare to well in modern
science :-)

> 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.

Actually, the universe doesn't have to be very smart. Novikov makes a
careful analysis of the action principle on a billiard table with a
time-travel wormhole and a single billiard ball. In quantum mechanics
it turns out that the wavefunction for the inconsistent solutions
becomes identically zero, while some weird but consistent solutions
involving the billiard ball having interactions with
itself-from-the-future becomes possible. So the inconsistent stuff
doesn't happen, just weird but consistent stuff.

Anders Sandberg                                      Towards Ascension!                  
GCS/M/S/O d++ -p+ c++++ !l u+ e++ m++ s+/+ n--- h+/* f+ g+ w++ t+ r+ !y