Time dilation and space contraction only matter if you can reasonably achieve speeds of a significant portion of the speed of light. AFAIK nobody has even come up with a reasonable way to achieve this for lightweight probes, let alone for hundred-ton ships capable of carrying humans. And let's not forget the practical problems like all photons incoming from the front being blueshifted into ultrahard radiation that would make a point blank nuclear bomb seem like a small candle.
Realistically even getting to the nearest star in less than 400 years experienced time is way way WAY out of reach for now.
Laser accelerate a lightweight probe, probe lands on alien planet and self replicates a receiver and basic robot body. Send mind in the form of information at speed of light and download into robot body.
Something roughly along these lines was believable enough for the Altered Carbon universe.
Landing from relativistic speed would be a massive engineering problem, since you won't have a laser de-celerator on the other end. And landing on a planet would seem to require a rocket, which cannot be lightweight.
Not necessarily insoluble, but a massive unsolved problem.
"lightweight probe" and "self replicates" don't go together. Nanobots are just as much fantasy physics as FTL is.
So what? Dilithium + antimatter + magic space warping was enough for the Star Trek universe. The sky is the limit for science fiction.
Just in that first paragraph:
- How do you stop at the other end? There won't be a large laser array at the receiving end and a laser probe will not have enough stored energy to decelerate itself.
- How exactly do you download a mind to be transmitted? We can't do it right now to be sure, and it's not clear we could ever accurately do that depending on how finely detailed a human brain is.
- How do you transmit it reliably over several hundred light years? Background radiation alone is enough to drown out any signal after a few dozen light years no matter how good your transmission is. Also, when do you start sending? You cannot possibly know which probes survived. (you DID send out at least a few hundred probes right? Don't forget to multiply laser energy requirements by the amount of probes)
- How does the receiving end download a mind into a robot body? We can't even begin to do that on Earth, not even with worms or flies. Humans are right out.
- How do we power the lasers? Conservative estimates have put required laser power at several gigawatts at least. Current laser systems can do that in pulsed mode but only with extremely low duty cycles. Getting enough power together to supply millions of homes would be tricky to say the least. (and see the note above about needing multiple probes just to be on the good side of probability)
- How does the probe survive decades of ultrahard radiation? What about dust it will encounter at high-subluminal speeds, also for decades? The shielding for that won't be lightweight, but the heavier the probe gets the more difficult it will be to accellerate.
- The satellite which is light enough to be powered by lasers also contains the most magical 3d printer anyone has ever seen. You can't just pull the molecules for advanced processors and energy generation equipment out of the air, such a probe would need to set up significant mining industries all on its own without any human interaction.
- A basic robot body. Keep in mind that "picking up a keychain and choosing the right key out of it without dropping the whole keychain" is already a challenge for modern robots.
In short, it'll be several centuries before humanity even gets close to such a project. I'd like to be wrong, but it seems extremely unlikely anyone of us will see such a thing in our lifetime.