>>13155439>>13155443There is no force
First you need to know something called the lorenz factor: . This is a function of the square of the fraction of the speed of light an object is traveling at (beta). Notice that in the limit that beta goes to 0, gamma is about 1, and in the limit that beta goes to 1 (which is the maximum value it can be), gamma goes to infinity. Then, the energy of the rocket is . This means that as velocity gets closer to c, then beta gets closer to 1, and gamma gets closer to infinity. So there is more and more energy, approaching infinity, required to go faster at large fractions of the speed of light.
