system
Given the following post, respond as if you're commenting on Reddit:
human
[Star Trek/Star Wars/General] How do Shields work?

I'm just wondering how these things are supposed to work? What kind of energy creates an invisible physical barrier that can stop physical objects, and particle beams, yet still lets sound and light through? Since sound and light go through, does that mean they can develop weapons that go through shields as well?

Also, what exactly are they running out of when they run out of shields? Presumably it's energy of course, but (in the case of Star Trek) don't they get all of their energy from their warp engines which would be creating a near infinite amount of energy? If that's the case, then how are they running out, and how do they replenish it? When the shields go completely out, how do they get them back? 

Also, when the Enterprise gets hit, and the tactical officer goes, "Shields are down to 60%. Damage on decks 3 through 6!", what's happening there? Do they only have 60% of the energy reserved to power their shields, or are the shields only blocking 60% of the damage? And if it's the latter, how does that work?

And in the case of Star Wars, would a light saber be able to cut through a shield, or would it bounce off, and hit you in the eye?

Edit: Added the part about 60% shields.
gpt
Different universes use different methods for energy shields.

The simplest is usually based on simple electrostatic repulsion without a proton-based nucleus. You know how solid matter is solid, as in you have to apply a lot of force to go *through* it? Well, a significant component of that is electrostatic repulsion. Electrons repel each other, and when the electrons of some atoms, say, a bullet, encounter electrons of another, say a sheet of iron, they repel. If the energy of the bullet is enough to transfer the energy *through* the electrons to break the molecular bonds of the actual matter, then you penetrate the sheet of iron.

But if you *remove* the protons (and most neutrons) from the equation and simply "hold" the electrons in similar "orbits" that they would occupy in normal atomic matter, a few funny things happen. One, it acts as solid (well, more a compressed fluid with high density, which is functionally solid) matter with respect to other matter. This is your basic force field. Now, seeing as how transparency to EM radiation is a property of how photons interact with matter, and photons normally interact with - you guessed it - electrons, the way you configure your electron containment is going to determine how your "matter substitute" reacts with regard to EM radiation. You absolutely *could* make it opaque, but if you *can* make it transparent, you're not *blind*...in combat. Fighting blind is usually a bad idea.

Now, with regard to "shields at x%" this is simple. The maximum capacity of your shield is 100%. Let's say your shield is acting like a bubble of iron. As in, you "simulate" an iron nucleus and the electrons in your field behave like the electrons would in a bubble of iron 1 foot thick. Your total "volume" at whatever density is 100%. As you take hits to your bubble, the amount of iron you have gets damaged. Electrons get thrown off, bonds get broken, just like chipping away at solid matter. You can reorient on the fly, keeping the bubble uniform, or you can shift the density to be thicker in certain parts, but your *total* amount of "simulated iron armor" has been depleted.