[krisoft]

> People assume this and I never understand why.

Of course it is not a guarantee. It is possible that the alien invasion fleet arrives, they land, unload their main battle tanks and a passing puppy laps up their whole fleet accidentally.

What we know that by virtue of them being here they are either very good at faster than light travel, or they are good at traveling slow.

If they are good at FTL what else are they good at? We currently think that is impossible. What other things we think as impossible are practiced by them?

If they traveled slow, they must be also good at maintaining their equipment on crazy long timeframes. It also shows that they are the patient sorts who plan and execute things on the order of timelines our empires crumble. How long have they been with us then? How much preparation did they do beforehand? Did Pham Nuwen code the intel management engine?

But sure, it is possible that the Aliens arrive. They broadcast a TV signal threatening us, but unfortunately the sync is a bit off so basicaly no-one understands it. Then they enter our atmosphere. The high oxygen concentration rusts their equipment and they all die.

[throwanem]

Seeing something occur that we thought was impossible tells us our understanding of physics is incomplete, which we already know. Seeing how it is done would probably tell us more, but we haven't. Until we do, we're guessing, and to assume FTL mastery confers godlike powers is as much an assumption as any other. Turtledove addresses this in the story that a sibling commenter mentions; I can also recommend that story, which rewards the reader with considerable entertainment while making its point about what can reasonably be assumed in the total absence of information.

The same goes for the sublight option, only a lot more so. Unless they have FTL communications, which I believe we also consider effectively impossible, by the time they get here anything they think they know about us will be wildly outdated, in technological terms at the very least. Possibly also in terms of the dominant terrestrial species, but we can be generous here.

We can be generous about their information latency because that only matters if they want it to. Any species which wants us dead and doesn't care what state the planet's left in - a reasonable assumption, if we're talking about them popping our nukes at us - doesn't need to come close to landing, or even to orbit, to do it; a kinetic bombardment in passing will amply suffice to depopulate Earth to more or less any degree desired. For subluminal interstellar travel to be even remotely feasible, even for an individually long-lived species, implies access to the kind of delta-V budget where the only limiting factor in such a bombardment is the time it takes to accelerate impactors, which may be zero if those are released before or during deceleration to match velocity with our solar system.

In the former or FTL case, we don't know how FTL works or even could, and we therefore can assume anything we like - with all assumptions at equal risk of bankruptcy. In the latter or high-sublight case, they don't need to be more clever to kill us if that's all they're after, and it may be unreasonably charitable to assume we would even get a chance to see it coming.