I'm writing a science fiction story where the protagonist's experience requires long intervals between events. My plan is to put him/her/it on a spaceship between events at an appreciable percentage of the speed of light. I've found the formula, but I am not enough of a mathematician solve it. I'd like at least a 75 year difference between earth events.
At what velocity and time makes sense and what would the round-trip distance be?
Assuming you wrote a realistic SF story, you need to allow time to accelerate to the velocity of say, 95% of the speed of light.
As CuriousOne points out, that's asking a lot in the following areas:
in terms of time, (even if you could get to very high speeds, your characters would be dead , due to long acceleration times way before relativity effects occurred),
needing large amounts of energy (fuel),
food supplies and,
most of all probably, having to deal with pure boredom.
But it's entirely your story, so it's entirely your universe, realistic or not, and I absolutely love SF stories, pure SF, though, no dragons, goblins etc!!
Here is a link to a site that might do the calculations for you, I am sure there are plenty more sites offering similar calculators, if this does not do the job.