Originally Posted by

**grandi**
Ah, the situation in the inertial frame of reference is not simple...

What if the ship was accelerated by some device in the inertial frame; using a large laser beam, kind of like a solar sail type of thing but in a much larger scale. Acceleration would be much less, maybe 10 m/s^2. Would the acceleration of the ship remain constant as seen in the inertial frame?

If the power source for the laser beam remained constant then the acceleration would be seen to decrease with time.

if you try to maintain a constant acceleration, then you have to keep increasing the power supplied to the laser.

As the ship increases in speed, its kinetic energy as measured in the inertial frame increases by the equation:

KE = mc^2(1/sqrt(1-v^2/v^2)-1)

So for example, the additional energy per kg of ship mass needed to go from 0.1c to 0.2 c is ~1.4e15 joules, and the energy needed to go from 0.2c to 0.3c is ~2.5e15 joules or 1.8 times more. The energy needed to go from 0.8c to 0.9c is 5.16e16 joules, or 40 times that needed to go from 0.1c to 0.2c

It gets even worse as you go from 0.9c towards c. So it would take 132 times as much energy to go from 0.98c to 0.99c than it did to go from 0.1c to 0.2c.

Put another way, the same energy that increases the ship speed to 0.2c from 0.1c, would only accelerate you up to 0.9014c from 0.9c

The amount of energy needed increases towards infinity as the ship speed approaches c.