Since time decreases in equal and direct proportion to the speed of light it would take only 8 years to reach the closest star (Alpha Centauri) but on return the earth would be over 400,000 years older than when we left.
Time slows as one approaches the speed of light. Traveling at the speed of light it would take a lot less than eight years to make this roundtrip from the perspective of the traveler. Don't have the math at hand.
But from the perspective of someone here on earth it would be eight years, not 400,000. Same time it would take to send a radio signal to AC and get a reply.
It is highly unlikely anybody will ever approach the speed of light. Too many issues. Hopes for faster than light speed involve "folding" space by getting a better understanding of what advanced physics thinks may be up to 12 dimensions, only three (or four, counting time) of which we can presently detect. If such ever becomes possible, we may be able to travel between planets, stars and even galaxies in no time at all.
Plus, Number 9 is so laughable that any response to enlighten the author is a waste of time.