Anything with mass requires energy to accelerate. The faster you go, the more you actually get a diminishing return on the energy you put in. This effect is tiny at first, but as you approach the speed of light, it gets so big that, for example, a particle accelerator has to put in gobs of energy just to get an elementary particle to go from 99.98% the speed of light to 99.99% the speed of light. And the energy required to get to 100% would be infinitely large, and thus impossible.
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