> And in a universe where different systems could be billions of years different in age, the UFOs have always seemed closer to a few thousands of years off from current technology at best.

I think this is something that doesn't get enough attention; in a universe like our's, almost all first contact events will consist of relativisticly expanding spheres of von Neumann machines impacting planets populated solely by nonsapient life. (Say three billion years before humans evolved versus a generous million years of human 'civilization' gives a three thousand to one ratio.) We should not expect to see anything remotely near our point in the developmental timeline.

(If "relativisticly expanding spheres of von Neumann machines" sounds unlikely, let me remind you that every star in the sky is a massive pile of valuable raw materials that has unfortunately caught fire and needs to be extinguished as soon as possible.)

"in a universe like ours, almost all first contact events will consist of relativistically expanding spheres of von Neumann machines impacting planets populated solely by nonsapient life."

You talk as if that's a certain fact. (Even estimating a probability) I'm no expert in the area, but why? Is that justified? What's that based on? (Not to mention the last sentence 'reminding me' of another odd 'fact'.)

I think it's like a big-o type argument about rates of growth of approximately spherical areas in 3d space

That combined with the aforementioned thousands-to-one odds that a life form that hasn't yet developed into a relativistically expanding sphere also hasn't developed sapience. (I'm not using the mathematical definition of "almost all" of course.)

There would (probably) also be a significant number of cases where two spheres run into each other, but it's debatable whether most of those count as first contact events (light speed lag versus "it's all the same civilization"). The main point was the enormous unlikelyhood of a nearby alien civilization being within a thousand years or so (out of multiple billion) of our developmental level.

The Fermi paradox is more a question of "Why hasn't anything eaten every star in the sky yet?".

I feel like I was given many more strange assumptions, without any of the things I was asking about being addressed. Your "of course", which I didn't understand, in a few different ways, makes me think I'm talking to experts. I understood almost nothing you wrote. But well, like I said, I don't buy all these assumptions. Reading this is like reading those unhelpful wikipedia pages that only make sense if you already understand what it's saying. And your way of talking as if it's all obviously true, (e.g. "let me remind you") raises my suspicions that it's not.

Can you time travel without being able to alter your location in space? I mean, the earth is moving pretty fast, if you popped back a few hours, wouldn’t you land in empty space?

Yes, you would need to adjust for that. Might be the explanation for why the guys from the future shows up in spaceships. I guess it would be dangerous calculating slightly wrong and instead end up inside earth or inside a building's wall.

Better to appear some distance out into space and then fly the last few hundred miles to earth.

The galaxy is rotating and moving through space. All estimated speeds are in relation to other objects.

I think relativity makes this impossible; you're essentially saying there's a particular trajectory that has an absolute velocity of zero. Relativity is founded on the idea that there's no such thing; any trajectory has an equal claim to be the zero velocity.

Maybe time travel would stay in the same reference frame, so if you're on the Earth's surface you end up in the same position on the Earth. On the other hand if you're on an object that experiences non-gravitational directional changes in the span of your time travel, you might be in trouble.

that presumes something like 'time teleportation'. Time travel based on changing / reversing your 'speed of time' doesn't have that.

Kind of like the difference between wormhole based FTL and warp drive based FTL. In one, you just teleport, in the other, you still travel in a continuous line.

This is how I've always imagined a fictional time machine working. You build the thing, it creates a field inside itself, and you can use it to move whoever/whatever is inside that field to other points in the timeline of the machine's existence.