[hgdsraj]

Wait - didn't Newton say that a body will not accelerate nor change direction unless a force enacts upon it? Once you set an initial velocity in void space with no air, you should never decelerate unless you accelerate in the direction inverse from your motion.

[emiliobumachar]

Humans are very fragile to sudden acceleration. Unless we assume really out there, indistinguishable-from-magic tech, manned spaceflight cannot "set an initial velocity" suddenly, nor a sudden breaking at the end. To get to any other star in years, not centuries, the only way to go is to accelerate on the whole first half of the way, and then decelerate on the remaining half.

[Eupolemos]

Yes, that was indeed implied for the line "For interstellar travel to make sense..." to make sense :P

I don't mean to be annoying, but I don't think I understand why you ask?