As a science fiction writer, I’ve generally tied to keep my “space drives” at least tenuously related to scientific reality as we currently know it, but that distance is likely more attenuated than many readers realize.
In doing some research for my next book, I came across the speeds of the fastest outbound human space craft, which is currently Voyager 1, at 38,610 mph, or roughly 10.7 miles per second. For those who are interested, the speed is measured outbound, because anything launched toward the sun will have its velocity increased by the sun’s gravity. As an example of that, when Comet ISON was discovered in 2012, between the orbits of Jupiter and Saturn, its inbound speed was calculated at roughly 10.5 mps, but by the time it reached perihelion, that speed was between 63 mps and 360 mps (or 1,300,000 miles per hour)[that depended on who was doing the calculating, because exact figures were not possible, given that ISON was either so close to the sun or actually on the far side of the sun from earth at the time around perihelion]. The fastest sun-grazing comets could theoretically reach a speed of over 400 mps.
If a human spacecraft could attain and maintain such a speed headed out of our solar system, it would still take over 118 days to reach the average orbital distance of Pluto from the sun… and something like 2,300 years to reach Alpha Centauri [those are rough calculations; feel free to refine them]. These figures indicate why SF writers tend to focus on interstellar travel through “jumps,” “warps”, wormholes or the like, especially since there’s the not-so-small problem of what happens when one encounters a comparatively immoveable object at that velocity. While space is largely empty, it appears as though most solar systems have all sorts of objects scattered around their periphery and approaching any other system with that sort of velocity would be problematical. So, of course, we writers come up with screens and shields, and never mind the energy consumption required for such powerful drive systems and the requisite shields.
Then, too, according to Einstein, any object approaching the speed of light would also gain a huge amount of mass, possibly infinite mass… and there’s not enough energy to push that mass. So… we’re back to getting around the speed of light and all those pesky relativity problems.
Theoretically, from what I’ve read, a so-called Hawking wormhole could be used to thwart the speed of light limitations. There’s just one catch – to create one would require the energy of a small black hole… each time it was used.
Is it any wonder that more fantasy and less hard SF is being written? And that the vast majority of all the space operas [including, I must admit, some of my own SF] ignore those facts?