Only mostly joking: I think the most likely way aliens could be in the solar system is a relict machine civilization from a life-bearing phase of Mars or Venus. Crazy? Yes. More likely than successful interstellar travel? Still yes.
I mean, interstellar travel isn't outside the realm of possibility. We see all kinds of weird junk in the deep background, quasars 13 billion years old. Even for a type 1+ civilization, let alone a dyson sphere, the energy requirements are not insane. Couple hundred thousand tons of antimatter and matter, specific impulse in the millions, get there. Time dilation solves all your "being alive to see the sights" issues, and as long as you don't leave anyone behind that you care about and bring enough antimatter all the other problems are solvable.
How does the mechanics of antimatter propulsion work? Is the idea that the momentum of the released photons is enough to push you in the opposite direction at relativistic speeds? And that you presumably somehow shield yourself from that radiation through a perfect paraboloidic reflector of some sort?
Yes, as far as I'm aware that's essentially the theory. A pion torch uses pions through a magnetic nozzle to achieve ~10^6 Isp with the downside that lots of prompt radiation (gamma mostly but I'm no expert) goes every direction.
It's not just about the energy, though, even if I think it's a bigger problem than you do. If you get up to a decent speed where time dilation helps, then the interstellar medium becomes a serious problem. I think there were some other weird problems, too.
I'm not saying interstellar travel is impossible, I'm just saying an early technological civilization in our solar system is more possible.
I'm not sure about that. Earlier life? Almost certainly I think, I tend towards panspermia on space rocks as being likely at least within the solar system.
I think the likelihood that there was some technological civilization that evolved here before ours and we haven't found any trace to be extremely low. Lower than "there are people with warp drives who visited our solar system at some point".
I see your numbers, but I still don't understand the reasoning. We know technological civilizations can exist, the only question is the timeline. We don't know that interstellar travel is feasible under any timeline at all. How can you rate the second one more likely?
(And as far as detecting relict machines: our astronomy is really not that thorough. They wouldn't even have to be hiding. Have we conclusively analyzed every little point of light in the asteroid belt? No.)
