A common trope across sci-fi and fantasy are immaterial shields which still manage to stop things from harming those within. Occasionally these will cause my suspenders of disbelief to slip a little, leaving me a little less engrossed in the story if not flat out embarassed to be enjoying it. In others they pass without comment. So I decided - let's talk about them, and at least some of the different types that can be found out there.
The most common type of shields are barely visible but able to withstand significant punishment before getting 'broken' somehow. Star Trek & Star Wars both have these types. Trek tries multisyllabic gibberish as an attempt to rationalize their functionality, and Wars pretty much just assumes they work (barring plot points indicating differently.) They're common enough to not be very offensive to sense, until you think a little too much about it.
The users of these shields - yes, even the Gungans - are able to see through them, both in and out. This means EM radiation is getting through. Many ranged weapons in both settings are EM radiation based. Whoops, slight contradiction. shrug If you don't think too hard, they're still fun and help explain how massive power sources capable of cratering the hardest physical materials in the setting.
Others, like Niven & Pournelle's ultimate battleship defense in "The Gripping Hand" or similar from Weber in "Path of the Fury," are EM-opaque. Great - that contradiction goes away. Unfortunately that makes it impossible for EM sensors (like eyeballs) to work through them. Oops. (To their credit, all the aforementioned authors do well using this technological limitation as an interesting plot point.)
Others use opaque shields which only partially cover the vehicle, such as in Weber's Honor Harrington series (a very enjoyable C S Forester Horatio Hornblower pastiche with its own charms.) These produce areas which are defended and others which are open to attack. This leads to interesting technical & tactical considerations. Examples of this are encountered throughout the series and are well developed, with few internal inconsistencies.
A further variation on opaque shields bubbles the object to defend, but allows for narrow channels through it to reach external sensor devices bound to the defended object. The obvious flaw here is that your eyes are outside your defenses, and depending on how easy or hard it is to open more portals to let new sensors through, this can be a significant tactical consideration.
Note up until this point all the mentioned shields are able to withstand EM, physical and (in some cases) gravitic energies by opposing them in one manner or another. There are other options.
One of the woefully underused techniques of shielding something from harm (IMO) is to preclude the damaging effect reaching the protected object. Admittedly, this is used occasionally, as in Neo's apophyosis in The Matrix where he stops bullets in mid air and eventually lets them fall to the ground when their momentum is spent. (OK, admittedly this is more his uber-hakz of the 'reality' within the Matrix, but the visual image from within the Matrix would appear to be this kind of thing...)
To quote the computer game Portal, "Now you're thinking with Portals." Other spatiotemporal alterations can lead to all sorts of intriguing options. The ability to shift damaging sources away from the spacetime otherwise shared with the target is at least different to the invisible-wall trope.
Some ways this could be justified/explained/rationalized:
Gravitic lensing - either move a heavy enough mass to tug the incoming damage out of line, or somehow make spacetime think there's now a heavy enough mass in place to do so.
Spatiotemporal harmonics - If space and time are waves of something, perhaps ensuring the damage sources' wavefront can be caused to either shift away from the targets' wavefront, or driven out of phase with the target's wavefront.
Alcubierre warp bubbles - a field effect where the spatiotemporal properties within the bubble are separate from those outside the bubble. This is one potential faster-than-light theory, but used defensively to wrap up incoming damage sources, might also be useful in defense.
Unexplained 'portal' generation - much like the aforementioned Alcubierre warp bubbles, portals like those found in the game with the same name could be massively effective in defense - as well as in offense (open a portal within a star, and its output beneath your target....)
Note all of these have been 'technological' in basis. If you should choose to look, you can find similarities in many magic systems, including for example D&D's Wall of Force spell.
What'm I missing here? What'd I get wrong? I look forward to your comments!
