A followup on Novikov

So the upside to the earlier theorem — if you can call it an upside — is that time travel is unlikely to be invented in the first place. Macroscale time travel involves significantly more paradox-possibilities; the butterfly effect leads to almost inconceivably many paradoxical (inconsistent) event-sequences that the Novikov principle disallows. So time travel "causing"`{::}^1` global thermonuclear war is no more likely than global thermonuclear war occurring anyway: if one interprets probability distributions as possible universes, there are very, very few possible universes in which time travel is invented, and in most of those it doesn't stay invented for very long. Thus it's more likely that we're living down a branch of the Trousers of Time that involves time travel simply not being invented.

The downside to the upside follows from the fact that FTL + GR = time travel. If we ever get warp drives, we probably need to start building them immediately. (And then destroy them once we get wherever we're going.)

There are a couple of loopholes for us to fit through here: first, we could make relativistic causality irrelevant by, e.g., only opening wormholes to locations that are otherwise causally disconnected from us (not in our future light-cone, nor us in its); the universe is expanding quickly enough for such locations to exist. The second is merely potential, via the anthropic principle: if FTL and/or time travel are relatively easy to discover, enough so that any sufficiently advanced civilization eventually would, we can take comfort in the thought that the universe has already tried and failed to kill us all off. (Or, to be less anthropomorphic, if intelligent life is sufficiently easily formed, it might actually be more improbable that all the species that would invent time travel happen to die off before doing so than it would be to swallow the NSP-induced improbability of nonparadoxical time travel; thus our continued existence would imply that we're the trillion-sigma outliers, just because somebody has to be.)

---

`{::}^1` Whether this really is causative, from a philosophical standpoint, is not terribly relevant.