And now a few words about gravity.
Gravity is an innate property of matter. Everything made of matter has gravity. You, me, my coffee cup, this chair I'm sitting in, the Earth itself, the Sun; everything made of matter (and presumably antimatter as well) exudes a gravitational attraction towards everything else. The larger and denser the object, the greater the attraction it has.
[Insert obligatory flippant one-liner here: something like, "And now you know why fat and stupid people have active sex lives."]
As for what exactly gravity is, many theories have come and gone over the centuries, but today it's generally accepted that Einstein was basically right: gravity is a deformation of local space/time caused by mass. And I would insert a really spiffy illustration here, but they wanted money for the reprint rights, so instead I'll give you an offsite link: "Gravity Well Detection Tests" by Theophilus Britt Griswold, at Science-Art.com.
While I'm at it, here's another pretty neat bit of comparative gravity well visualization I found on xkcd.com. It doesn't have anything to do with this column. It's just really cool.
There is a hypothetical gravity particle called a "graviton," and gravity appears to propagate at the speed of light in a form resembling waves and in the usual accordance with the inverse-square law, but it's questionable whether it's even possible to detect gravitons and gravity waves with any instrumentality that can physically exist in this universe, much less to control them. So before you go equipping your mighty starship with a propulsion system that emits a stream of coherent anti-gravitons, remember: only Star Trek is permitted to use such galloping blatant pseudo-scientific gobbledygook.
Or at least, please think it through far enough to realize that if your fictional civilization has developed the ability to engineer gravity and anti-gravity at will, the ramifications go way beyond your hero's having a space-board with which he can surf the gravity waves, or a flying car that periodically requires touching-up the chips in the coat of Cavorite™ paint on the underbody.
Ergo, if classic sci-fi anti-gravity is pretty much out of the question, how do you work against the tendency of all things, in time, to fall down?
The first method has been known since the dawn of time, and has been well-understood at least since Archimedes jumped out of his bath and ran naked and dripping wet through the streets of Syracuse shouting "Eureka!" Buoyancy is the result of the combined interaction of gravity, mass, and density.
You must have gravity in order to have buoyancy. In space, hot air doesn't rise; in fact, it doesn't go much of anywhere. Put a person into free-fall and make him hold perfectly still, and pretty soon he'll be surrounded by a miasma of his own hot exhaled carbon-dioxide, flatulence, ammonia, and whatever. So you'd better equip your spaceship with a forced-air ventilation system.
But impose gravity, and in time everything stratifies very nicely. At the lowest level you get solid matter, composed of the heaviest elements; metals and the like. Next, you get a layer of liquids; for example, on Earth, water. Above that, the gasses, in layers of different densities ranging from only slightly less dense than liquids up to hard vacuum.
Buoyancy, then, is simply a matter of displacing a volume of something more dense with an equal volume of something less dense. Submarines work this way; a submarine is essentially a bubble of air, enclosed in a protective shell. It doesn't even need to be air. Some early bathyscaphes used tanks of high-octane gasoline for buoyancy, as the non-compressible nature of the liquid solved a host of engineering problems.
But we're not interested in submarines. Today, we want to fly. So how do we get a vehicle airborne? Well, if we approach it from a buoyancy perspective, all it requires is a container filled with a sufficient volume of something less dense than normal air. Hot air will do, for as long as the fuel supply holds out. Vacuum would be better, if you could come up with a container strong enough to resist implosion and yet light enough to fly. A purified light gas is the best compromise, as, much like the gasoline tanks in the bathyscaphe, it's buoyant, and yet has density enough to keep the container from imploding. But which gas to use?
Here on Earth, your choices are simple: hydrogen or helium. Beyond that, every other element is either a solid at room temperature or else a gas insufficiently less dense than ordinary air to serve the purpose. Perhaps a more complex gaseous molecule might work? Late 19th and early 20th Century fantastic fiction was full of miraculous balloon gasses, and there are some hydrocarbons and fluorocarbons that might conceivably work to a limited extent, but in truth, simple chemistry dictates that for pure lift, nothing beats hydrogen.
Excepting perhaps hot hydrogen, but that has it's own problems:
The other way to get a vehicle off the ground is by giving up on making it lighter than air, and instead using some mechanical means—rotors, ducted fans, turbojets, rockets, whatever—to displace a mass of gas downward greater than the mass of the vehicle. That, simply and sloppily stated, is what thrust is: a measure of the mass of gasses being pushed through the system. When the mass of gasses being pushed downward at any moment exceeds the mass of the vehicle, it lifts off. Provided the thrust continues, it stays airborne.
As anyone who's ever been around an operating helicopter or VTOL aircraft can tell you, it takes one hell of a lot of moving air to get the thing up into the air. And it's not even remotely close to being silent.
So why all the foregoing discussion? A few weeks ago, I made mention of one of Jules Verne's lesser-known books:
For the past week or so I've been playing host to some sort of undefined sinus and upper respiratory crud, and a couple of nights ago I was awake after midnight, sneezing, coughing, and watching the Insomniac Channel, when I chanced to catch:
Oh, boy, I'd forgotten all about this one! This 1961 epic, starring Vincent Price at his scenery-chewing best, co-starring a very young Charles "I've been in worse" Bronson, and featuring special effects that would embarrass a bunch of high school kids making a movie in the garage with dad's camera, is just so— so— so—
The script, by Richard Matheson (one of my writing heroes), is a mash-up of Verne's 1886 Robur the Conqueror, his 1904 sequel Master of the World, and some elements stolen from 20,000 Leagues Under the Sea. The character of Robur himself is the sort of megalomaniacal genius who would be a villain in a British or American story, and could only be a hero to a French writer, living in the Third Republic and still pining for the glory days of Napoleon III. Robur's mad quest, to bring about world peace by flying around in his mighty airship and bombing the living s**t out of anyone who isn't peaceful, is the sort of utterly insane mission that would only make sense to a half-crazed 19th Century French science fiction writer or someone from the U.S. State Department.
But the more I think about these books and this movie, the more I focus on Robur's amazing invincible armored dirigihelibawhattheheckisthat?, the mighty Albatross.
I mean, sure, it's clearly the great-grandaddy of all those airships flying around in contemporary steampunk fiction, but seriously, what the heck is it?
France's answer to the zeppelin? No, it's sufficiently armored to be impervious to rifle and cannon fire, so that rules out its being lighter than air. Some sort of incredible 1880's proto-helicopter? The forest of rotors on the topside suggests that (and also reminds me of all those wind-power turbines alongside I-76 in northeastern Colorado), but in that case, the downblast from those rotors would make it impossible to have a conversation on the bridge, much less for anyone to walk along the promenade deck without being blown over the side and plummeting to their death. (There are good reasons why we, even with all our modern technology, do not build helicopters the size of ocean liners.) And certainly, this thing could not possibly be silent, which is a gimmick used repeatedly to explain how this Albatross—truly an utterly apt name—could sneak up on and attack non-peaceful (and therefore presumably awake) people.
In the end, I can only shake my head and chuckle softly. The Albatross was silly when Verne first created it in fiction, 125 years ago, but he can be forgiven that because at the time aeronautics was not even in its infancy. It was sillier still when he brought Robur back for the sequel, twenty years later, because by that time Verne really should have known better. It was absolutely laughably ludicrous when it appeared on film, fifty years ago.
It's now 2011. If you must write steampunk fiction, at least take the time and trouble to recognize that we know things now that Jules Verne didn't know.
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