To sum up, I understand fully that fuel is an issue, I’m talking theoretical.
In space there is little to no resistance because of how few particles floating about. But if you were always running and engine, pushing matter out behind you, youd have a constant acceleration. So it seems like we should be able to travel near speed of light (combining this constant acceleration with abusing gravity for slingshot maneuvers). Yet people always say nothing can travel the speed of light.
In: Physics
In short? A constant thrust force produced by an engine won’t accelerate you past the speed of light because as you approach that speed, the mass of the ship approaches infinity.
Fast things are more massive. This is the result of Einstein’s famous equation, as fast things have more energy, and increasing the energy of an object necessarily means increasing its mass. The speed of light is the speed at which the mass of an object made of matter would be infinite. As a result, getting there through traditional means would require infinite energy. Photons get to travel at that speed because they have zero mass. In fact, all things with energy and without mass must necessarily move at the speed of light.
Because of this infinite energy problem, all hypothesized methods of moving faster than light involve fucking around with space itself rather than moving through it faster. A wormhole, for instance, could theoretically have pretty normal space in the middle, but connect two points an arbitrary distance away through a shorter path. If this works, it would be because we managed to bend space itself so two distant points touched. Alcubierre drives (also known as warp drives) work by surrounding the ship in a bubble of space, then accelerating the bubble through normal space at faster than light. This works because there’s no rule stating space itself can’t travel faster than light, just objects in it. So-called hyperdrives would work by shoving the ship out of 3 dimensional space into higher dimensions where all points in 3d space can be accessed without technically moving.
All of these drives have serious issues. The most plausible one, which requires the least exotic physics thought to be likely impossible, is the Alcubierre drive. Unfortunately, warping space in the required way requires negative energy, which we do not know how to generate in sufficient quantities. We think negative energy is a thing, in the form of virtual quantum particles, but actually producing the stuff may well be impossible.
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