Excellent question, ares1! I've given some thought to how an EV-type hyperdrive/jump system might work, but not to deflector shields. The best approach may be to consider what we need shields to do. I think that whatever the theoretical basis for shields, it must satisfy certain generally accepted conventions or assumptions:
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The shields have to deflect or repel matter (asteroids, dust, radiation, other ships).
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The shields have to deflect or repel certain types or levels of energy, but not others (harmful energies should be blocked, but light below a certain intensity must get through - otherwise the occupants of the ship would not be able to see!).
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The shields must either be degradable themselves (a sufficiently powerful bombardment must wear shields down) or the source of the shield energy must be somehow finite (under sufficient bombardment, shields must "run out").
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The shields must be renewable (given time, it must be possible to regenerate shield energy).
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The energy that makes up shields must not interfere with the ship's weapons.
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...must not interfere with the ship's propulsion.
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...must not interfere with the ship's communications.
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...must not interfere with the ship's sensors (or, at least, must interfere in a known fashion that can be taken into account when deciphering sensor readings).
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The shields should be manipulable (probably not essential, but useful if you want "extend the shields" to protect some nearby lesser vessel, or if you want to "alter the frequency" of the shields for whatever reason people do that).
These are all the assumptions I could easily think of, and they constitute a tall order indeed. Electromagnetics probably can't do satify our requirements, for some of the reasons that Low-Key has already described. Fortunately for us, Star Trek fans have had a lot of time to think about this stuff. The Trek canon has long held that deflector shields are generated by...oh, I'll let (url="http://"http://www.trekmania.net/")Trekmania.net(/url) and their Treknology page explain it:
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Force field grid deployed around starship hulls for protection against any number of threats found in space, from naturally occurring dangers such as radiation, debris, ion storms, nuclear, magnetic and thermal forces, and of course enemy weapons. Modern shield systems function via a heavily energized graviton field suspended within a layer of controlled spatial distortion. Shaped and refined in many preprogrammed configurations, mutations and frequency modulations, the shield grid around the ship is capable of deflecting and partially absorbing hazardous external energies and forces, from heavy particle bombardments, to objects of great physical mass. At impact, a localised object or force would appear to the eye to visibly 'bounce' off the shields, this is caused by the intense gravitational/spatial distortion.
Not a bad plan on the face of it; technology for the generation of 'artificial gravity' is pretty much a given, yes? But I don't much like the 'spatial distortion' aspect for reasons I described above. Fortunately, the USS Excelsior Technical Manual (NCC-2000-A) provides a way out of that fix:
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The shielding system makes use of graviton polarity sources whose output is phase-shifted through subspace field distortion amplifiers.
I like it! Let's use it...or most of it, anyway. I suggest that your deflector shield is an intense graviton field (or rather, several fields surrounding the ship) that is polarized to allow the ship to interact with the surrounding universe while still protecting the vessel from external forces. And if the graviton particles are both carried by/fixed in place by a subspace field (which is kind of in space yet not of space - I'm guessing that's what "phase-shifted" means), the tension between those two balanced forces may be tenuous enough that bombardment might disrupt that balance, and so weaken the shields.
Now all you need to do is know how subspace fields are generated! (I'm partial to the "running plasma through uridium coils" method myself.) I like the addition of the subspace field to the theory; it adds enough ambiguity that you could explain how shields can do just about anything you need them to. But you could probably get by without the subspace field bit as long as you remember to polarize those gravitons!
Last bit: if you wanted a real wonky-sounding description for your own technical manual, you might do a riff off of something like this:
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Each generator consists of a cluster of fourteen 42 MW graviton polarity sources feeding a pair of 675 millicochrane subspace field distortion amplifiers. On the Galaxy class starship each generator consisted of a cluster of twelve 32 MW graviton polarity sources feeding a pair of 625 millochrane subspace field distortion amplifiers.
Or something like that.
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"Your death will be the proof that Horus does not lie."
Roger Zelazny, Creatures of Light and Darkness
(This message has been edited by Phil Barron (edited 03-21-2002).)