What with this being a science fiction website, and what with the embarrassingly poor science education afforded people in our country, it's easy to overlook the fact that a few starships have *actually* been designed. Nothing's built, obviously - NASA can't even be bothered to put people in space anymore, so good luck with that whole "Alpha Centauri" thing - but there have been several concepts developed to a greater or lesser extent.
The most nuts-and-bolts of this (Going as far as actual blueprints) is the British Interplanetary Society's "Daedalus" design, which I hope to talk about at a later date in more detail (Bad name, good idea), but the one that tended to get most scientists and hard-science SF types all ready for love is the "Bussard Ramjet." Personally, I never found it all that sexy, but I will admit it is an elegant idea. Nutshell:
Space is not a vacuum. There's crap thrown off by the stars in solar flares, times a trillion stars within 30,000 light years of us. That's a lot of flare...uhm...stuff? Bits? Whatever you call it, there's a lot of it. Not to mention gas that never coalesced into stars and is left over from the formation of the galaxy, not to mention nebulae and other detritus from exploded or forming stars. Bottom line, the space between stars isn't empty, it's estimated that the interstellar void has, on average, about 3 hydrogen atoms per cubic centimeter.
Yeah, that doesn't sound like much, but stack 'em up: There's 161,ooo centimeters in a mile, six TRILLION miles to the light year. Go find Alpha Centauri in the night sky. This will necessitate a trip to the southern hemisphere, as it's not visible from the US, but I'll wait for you to get back. I'm a science fiction blogger. I've got nothing else to do.
Hey, you look good! Well, yes, I know it was Black Water Fever, but really, now that you've kicked it, you're a bit slimmer. It looks good on you.
Anyway: assuming you were looking at Alpha Centauri and not some bacterial-infection-based-hallucination, you were looking at light from 4.2 light years, or 26 TRILLION miles away. Which means that in a straight line between you and said star were about 78 TRILLION hydrogen atoms, which, even as tiny things like atoms go, is a heck of a lot.
In 1960, a man named Robert Bussard had an idea: What if we use all that free hydrogen as fuel? He concocted a plan whereby you use an insanely huge electromagnet to scoop hydrogen out of interstellar space, funnel it into a fusion reactor, y'know, *Fuse* it, and use that for thrust. Shiny! Unlike all the goofy-assed doubletalk devices in SF to gad about between the stars, this one actually seemed like it'd work.
Granted: Slow as [profanity], but slow and going is better than not going at all, right? Assuming suspended animation or some kind of medically-induced longevity, and some (eventual) relativity effects, it seemed doable. For the first time ever, interstellar travel became more than a plot device. It became plausible!
Of course it ended up getting worked into all kinds of Science Fiction. The early stories in Niven's "Known Space" revolve around Bussard ramjets, it even wormed its way (Eventually) into the relatively dumb Star Trek, though it took until the 90s to do so (At which point the Blueprints guys decided the glowey bits on the front of the Enterprise's engines were ramscoops).
Alas: there is a serious, serious problem with the entire concept. One that *should* have been obvious from the gitgo, but wasn't, and which couldn't actually be *proved* to be a dealbreaker until the advent of modern computers.
Care to guess what it is? No, it's not the problem of accelerating the thing up to ram speeds, no it's not the weight of the electromagnet, no it's not the fact that we don't actually know how to build a fusion reactor and/or engine, and no it's not the insane power demands. Guess.
Nah, that's not actually fair. I made you go all the way to the southern hemisphere, I've already asked too much of you. OK: here's the problem:
In order to work, the magnetic field - the scoop - must be hundreds of thousands, or millions of miles across. This isn't theoretically a problem, since it's a magnetic field, it's not a physical structure, and we regularly see ones that big in nature. You can theoretically make 'em as big as you want, no problemo, assuming you've got the power.
The problem is friction!
Yeah, see, when the hydrogen hits the scoop, it's like wind resistance, and as it turns out, this resistance is commuted by the magnetic field itself to our theoretical starship. Over 100,000 miles or so, the resistance from 3 hydrogen atoms per cubic centimeter turns out to be GREATER than the thrust you get from fusing the hydrogen.
Sigh. And just like that, a zillion dreams go up in smoke.