Basically, length contracts and mass goes up as you approach light-speed, with mass approaching infinite as you approach the speed of light. Most of these effects aren't really noticeable until you're at ~99% or above of lightspeed. But as mass goes up, the energy required to accelerate it to even higher speed goes up exponentially. . .
If you’re not near any celestial body, then there’s no mass for anything. Sure, things have a measurable mass when they’re on the surface of our planet, but what happens to humans and things when they’re out in space? They float around because *!* there’s no gravity to hold them to any surface.
I didn’t take physics, but that seems pretty reasonable to me.
Now, instead of saying it will never be done in the whole history of humankind, how about we figure out a way for our children’s children’s children to visit about star, now? Then when the technology is available, they could leave this planet and solar system, and go to another one.