[giantrobot]

You can only get so much power before something vaporizes. Some sort of ship emitting enough power to be detected at a non-trivial distance from Earth (see inverse square law) would vaporize itself.

Just because stealth in space doesn't work doesn't automatically mean you can see everything in space at all times. We need to build giant telescopes to see stars thanks to the inverse square law. The bigger our telescopes with better light collection the narrower their field of view. An antimatter spaceship could fly past Pluto tomorrow and we could easily miss it since we can't monitor all 4πr^2 of the sky at once with powerful telescopes.

[jhgb]

> Some sort of ship emitting enough power to be detected at a non-trivial distance from Earth (see inverse square law) would vaporize itself.

Then you can say goodbye to those robotic spaceships. Note that I'm not claiming that they're possible -- I'm just saying that if they do exist, they're very detectable.

> An antimatter spaceship could fly past Pluto tomorrow and we could easily miss it

We can detect Voyager from way past Pluto, even though it emits only several dozen watts. I'm not sure how you'd miss an antimatter-powered spaceship at a comparable distance.

[dotnet00]

To be fair, we know exactly where Voyager is and know to look for it. It would be pretty hard to detect otherwise.

[jhgb]

And if it were emitting 10¹² times as much power, like even a small interstellar vessel would, we wouldn't even have to know exactly where it is.