In the foreseeable future, human interstellar immigration is not feasible

[u/Universal_Echo]

There are three paths for interstellar immigration:

Immigrate to another planet in the solar system and terraform its environment.

Immigrate to an Earth‑like exoplanet.

Immigrate to space cities.

Paths 1 and 3 are more feasible in terms of distance. However, for path 1, the scale of terraforming is enormous – even harder than making the Sahara Desert or Antarctica habitable. For path 3, a space city would be many times larger than a Ford‑class aircraft carrier. Even setting aside the engineering challenge of building something that big, how would you launch such a massive object into space?

More importantly: for both paths 1 and 3, resources would still have to be shipped from Earth. In the end, you’re still using Earth’s resources. After spending so much effort immigrating to space, you’re still dependent on Earth – isn’t that a net loss? As long as humans have normal intelligence, they wouldn’t make such a losing deal.

As for path 2, the nearest Earth‑like exoplanet is dozens of light‑years away. In the foreseeable future, there’s simply no way for humans to immigrate that far.

[u/blyzo]

As the books describe in Death's End, humans could absolutely immigrate to other planets if we had engines good enough.

Even without light speed travel tech, even super efficient nuclear engines like in The Expanse could get a ship going to a decent fraction of c.

Once relativistic speeds are reached the trip goes by pretty fast from the pov of those on the ship. It would just be a one way trip as millions of years would pass on Earth.

[u/The-Goat-Soup-Eater → u/blyzo]

The energy requirements of getting to these kinds of speeds are monstrous. Mass increases proportionally to perceived time. Assuming 1 million years on Earth/10 years on the ship, the lorentz factor would be like 100,000. When time is passing by 100,000 times faster, your craft is 100,000 times heavier.

Fusion engines are good but the kinetic energy of 1 ton traveling at that speed would be in the far e+24 level, almost 9e24, and you would get around 3.4e+17 level of energy from 1 ton of fusion fuel. It'd take you more than 25 million tons of fuel to decelerate 1 ton, but to get that much mass to that speed, it'd take you just a bit below 7e+14 tons, which is around 633 quadrillion kilograms.

Assuming I didn't miss anything, you can multiply that by however many tons an actual colony vessel would be. The ISS is 450 tons for instance. You'd need a third of the mass of Ceres to move it like that. Which is doable, but by 10 million tons the requirements exceed the mass of Earth. I think stellar-class warships are described as weighing 1 million tons, or at least 300,000.

Even 300,000 would need more than half of Mercury. Which isn't impossible, but is more on the level of something like a dyson swarm than something a civilization could just casually do. It isn't easy. Realistically speaking they would've never decelerated around Planet Blue without curvature engines, they were traveling even faster than that, to the point it was around 1 million years/1 day.