impossible ubiquity of force fields.
There are current theories that postulate a fifth force which is causing the universe to expand more rapidly than expected. But this can only be felt over galactic sized distances so you can rule that one out too even if it exists. How about plasma? That's what the original Star Trek always attributed such things to. But for a shield, the plasma must still be held in place by an EM field, and then you're back to square one again. What about momentum? You can stop matter if you shoot something at it in the opposing direction, but that is not a static shield. It requires a constant flux of matter or energy in the outward direction. For example, meeting an energy beam with an opposing energy beam would be enough to stop it, so why do our protagonists always need impossible energy shields? All they really need is computerized targeting technology and they are quite nicely protected while using a heck of lot less energy.
So the next time you encounter a story that features the use of force fields, just remember that although there is some wiggle room to work with, the existence of such a thing is extremely improbable even in the distant future. I have yet to encounter an author that has even attempted to come up with a merely plausible explanation. If anyone out there finds one, please leave a link for me.
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