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Hypothetically, suppose we have the ability to accelerate a spaceship to near speed of light (lets say 0.9c). From what I remember of relativistic kinematics, the energy required to accelerate the ship to that speed be given by:

$$ KE = mc^2 (\gamma -1) $$

If so, let us suppose the spaceship is to travel to the nearest neighbouring star. What would be the energy for the spaceship to maintain that velocity of 0.9c.

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    $\begingroup$ The problem is not coasting but decelerating at the destination. $\endgroup$
    – G. Smith
    Mar 1, 2021 at 23:04
  • $\begingroup$ I see! Hypothetically speaking, what forces could cause a deceleration of such spaceship? Would the gravitational potential of a nearby blackhole cause such disturbance? $\endgroup$
    – HeisenbergCat
    Mar 2, 2021 at 2:14

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Maintaining a constant velocity requires no energy.

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    $\begingroup$ But don't forget that it takes energy to slow down at the destination. And then there's the return... $\endgroup$
    – hdhondt
    Mar 1, 2021 at 23:20
  • $\begingroup$ @hdhondt The OP did not ask about that. $\endgroup$
    – G. Smith
    Mar 1, 2021 at 23:49

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