a) the kind of ship required to travel interstellar at a reasonable pace, such as a nuclear termal rocket or antimatter rocket or the like, is pretty well understood in theory. It will glow like a galactic beacon and would be visible from vastly further away than 100 light years. Unless we're somehow technologically ahead of all other existing civs in our light window, should see somewhere, anywhere, anywhen, a single ship. But we don't. There just ain't ships out there.

(b) I agree. We have all reasons to believe it is not possible to accelerate something with mass to 'c'. We know this with astronomical levels of certainty. Anything that changes location without acceleration is either pure energy or pure fantasy.

(c) I agree. Even Dyson spheres don't have the raw energy capture to move an Earth of resources out of the sun's gravity well. Perhaps an Earth has rare and valuable resources that are quickly expended, with high value per kilogram. But there's no reason to believe an Earth would be special. The only real reason aliens have to contact us are the same reasons they'd try to immediately genocide us to get us out of their way anyway.

(d) this is an entire field of xenospeculation, but high energy rockets are really bright and very probably the only way to get to a different place in space. You pretty much have to talk microbe scale to go unnoticed in space.

[Edit, misread point b]

The brighter a craft is, the less efficient it is. Every photon it emits in a random direction is energy that could have been better used to push it toward its destination. So, if any civilization out there really is super advanced, their ships may be extremely hard to detect. Also, a situation like Star Wars or Star Trek where beings in space vessels just cruise around the galaxy for personal reasons is highly unlikely at any technological level. We shouldn't expect to see a lot of ships out there.

The brighter a craft is, the more thrust power (watts) it emits. Both rocket efficiency (exhaust velocity) and thrust (newtons) contribute to thrust power multiplicatively if I remember correctly.

Any craft capable of moving things in our medium size realm or larger will glow. If you want to direct your exhaust along a cohesive line, you're basically talking lasers. And those can't really provide much thrust power.

If the hypothesis is that there are interstellar civs out there that don't exchange beings or materials, it's pretty hard to call them interstellar.

If you want to travel back and forth between inhabitable stars, forget rockets. They're just never going to get you moving fast enough.

I'm basically talking about a propulsion system that we haven't imagined yet. Because nothing we've (realistically) imagined would make an interstellar civilization possible.

I didn't say they wouldn't exchange beings or materials at all. I'm just saying that individuals will not be hopping in their space cars and cruising off to the crab nebula for the weekend.

> It will glow like a galactic beacon

There's no reason to believe it would have to accelerate for all its curse.

> You pretty much have to talk microbe scale to go unnoticed in space.

There are myriads of stars we know nothing about, planets being discovered; and dust clouds only detectable because they are unbelievably huge.