it is not possible for humans to come back to earth in a distant future while remaining young, by traveling in a theoretical super advanced spaceship. if the speed of spaceship is changed gradually, for humans to survive g-forces, it doesn’t matter how fast the spaceship will travel and for how long time, the entropy of human body will be almost the same on the spaceship and on earth — both will age with same speed. it is usually described that in fast moving spaceship travelers age slower. wrong. it is not the speed that matters but the difference in gravity, the g-force created by spaceship. perhaps a millisecond difference is possible to achieve only, over many years of constant acceleration and deceleration. to speed up and to slow down fast enough to have any significant effect on time, the local gravity (g-force) due to positive and negative acceleration must be so large that it would create an atom-thin pancake from human body. if on an advanced spaceship traveler will be protected in an anti-gravity capsule, then the traveler will age at the same rate as another one on earth, regardless that the spaceship itself will age slower, if it will survive the g-forces of such magnitude.
the only realistic way to travel in time is by slowing entropy of the body by freezing it deeply and later bringing it back to life, which is already successfully done with many small living organisms, like frogs, mice and others. some viruses have survived frozen for millions of years — those are the real time travelers and perhaps need counceling for a psychological shock after waking up in a modern laboratory. for humans, the only option for now is cryogenics with cryonics, in hope that the technology of the future will enable to bring them back to life by correcting damage done by today’s not so perfect technology, and wish that they won’t be then on the same developmental level as viruses are to us today.