Traditional rocket enthusiasts can tell you what everyone already knows, which is that overcoming gravity requires a propulsive force in the opposite direction of the gravity field you wish to overcome. Hovering in mid air for an extended period of time can be accomplished by either 1) blasting matter in the downward direction or, 2) pushing against the air as with wings or helicopter like blades. If there happens to be a vertical electro-magnetic field available, there are ways to push against it as well. However, science fiction is filled with spacecraft that seem to hover without using any of these methods. The most ubiquitous examples are the flying saucers that appear in so many of the older and sometimes lesser quality sci-fi films. But it appears just as frequently in more recent films, like the huge mother ship hovering over South Africa in District 9, or the land speeders in Star Wars. In Close Encounters, we see a ship behind Roy Neary's truck hover and then move slowly overhead, and a huge alien mother ship hovering over Devil's Tower. No explanations are given as to how these feats are being accomplished. It's not just ships either, for we often see entire cities suspended in mid air like Cloud City in The Empire Strikes Back, or the city in the the Star Trek episode entitled The Cloud Minders. It often makes me cringe when these things first appear on screen.
Now, I know that when it comes to the future or even alien worlds, writers have some freedom to present some pretty wild technological breakthroughs. But no one in this universe, whether from the future or some alien planet, can escape the laws of Physics, and gravity is one of the most foundational of them all. If you thought simulating gravity was tough, defying it turns out to be an even greater challenge. This entry is dedicated to the impossible ubiquity of anti-gravity devices.
The fact of the matter is simple, gravity cannot be turned off, shielded, or otherwise directly counteracted. For any physics buffs out there, Einstein did describe a theoretical anti-gravity device consisting of a ring of neutrons traveling close to the speed of light. The frame-dragging effect would render the center of the ring gravity-less. Of course, it would be impossible to build and would not even be self-levitating, so it's just a detail. By contrast, electro-magnetic fields can be generated, turned on and off, and shielded with various materials. I think it is the analogy to EM fields that cause many people to accept the possibility of finding ways to do these things to gravity fields, but it just ain't so.
The recent film Avatar featured a range of spectacular floating mountains, claiming that this was possible due to the planet's unusual magnetic field. Well, planetary magnetic fields fan outward at the poles. Any balancing act between gravity and magnetism would be highly unstable, especially while still in the planet's atmosphere. And since the field must weaken as you move farther from the planet, those huge rocks would be lined up in order of size, biggest on the bottom to the smallest up high. So much for the floating stairways. Hey, at least it looked real pretty!
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