Lets bring it down to earth a bit. The speed of light is a constant, in a vacuum. So lets define light. Basically its electromagnetic in nature. It travels in waves, doppler effect, longer red going away from you, short sharp blue coming at you. Light can be deflected, it can be bent or curved depending on its local environment. Now take away the vacuum and light exhibits different properties. You can't speed it up but you can slow it down. A single quanta, photon of light, has been 'seen' moving through a medium like gas or whatever was used. This photon was even stopped cold, held in place briefly and photographed by a really fast camera. I saw this demonstration in an article I found on the news a while back. I guess you can still Google it. The point being is that what we perceive as constants albeit irrefutable in some cases like the Planck length and Planck time, smallest length and smallest time that can be measured, are only the results of current knowledge.
Currently we can measure the effects gravity has on local environments in space. We've detected gravity waves, ripples in space/time, eerily similar to the waves created by dropping something in water. So what does that prove? That space, like air and water, share similar characteristics. Of the three water is the densest. There is a lot more for an object to push against, friction. In the air not so much but the faster you go in that medium friction becomes a real issue. It gets hot, very hot, hot enough to melt metal. Leave now for space and the environment changes drastically. In space the density is on average one particle per ten cubic meters. That may not sound like a lot but on the scale of the universe that translates to a whole lot! Locally that means nothing, there isn't enough to get in your way.
The fastest surface vessel, in this case a Norwegian combat ship, is approximately 70 mph. In the air speeds of mach 7 have been achieved, seven times the speed of sound or 4,900 mph give or take. In space velocity is limited, according to current theory, to the speed of light. In all three mediums you are limited to how fast you can go using conventional means. We can use an ion drive but it takes a very long time, years, to reach an appreciable speed. An ion drive is basically an electric engine. We could use nuclear power but that was scrapped, I think in the early sixties, because of the cost, the risk and the lack of sufficient knowledge to pull it off. It was Project Orion. It might have worked but again, it will take a long time, not as much as an ion drive, but enough to dampen any dreams of getting to even the closest star system in a reasonable amount of time. Next up is a matter/antimatter drive. Sounds good in theory, but making it a reality right now is in the realm of fantasy, almost. There is the Alcubierre drive. A theoretical warp drive proposed some years ago. In theory and on paper it looks good but that's where it stays for the foreseeable future. We've taken our theories of physics to the point where they begin to break down. Like trying to see inside of a black hole, it can't be done because physics doesn't work beyond a certain point. The only way IMO to get past that limit would be first, to generate enough energy to reach light speed and second, multiply that energy to 'go faster' as it were.
Simply put it takes an enormous amount of power to speed up a single particle close to but not at the speed of light, 99.9 something something something percent. Not accurate but you get the idea. A ton of power to move a particle, imagine what it would take to move a ship in space. I can't even think that high.
*end rambling*
