It is nevertheless curious that the mass increases with speed, because speed is relative to a point, but which when we are in the middle of nothing, hold for example in the intergalactic space
How do we check that?
Everything therefore depends on the "observer", and no one has the same "time"
So the guy in his machine at 212 km / s would have doubled his mass, but as he is in a vacuum, he does not care, roughly with a thrust of 000 / 1G at that moment, artificial gravity would seem to him. "normal", but time "elsewhere" will pass less quickly;
especially when you know that it takes about 1 year to reach this speed ...
it looks like our reasonable speed limit, just good for walks in the solar system.
Finally, at constant thrust, the acceleration decreases in proportion to the increase in mass and we do not feel the effects, nature is well done
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To play, the distance from land to March the distance varies between 56 million kilometers (perigee) and 400 million kilometers (peak). How much real and apparent time to go there with a constant thrust to create an artificial gravity of 1G?