system
Given the following post, respond as if you're commenting on Reddit:
human
[Star Trek] What happens if two ships with their shields up collide?

Would their shields prevent their hulls from touching? Or would the shields pass through one another?
gpt
It helps to understand how shields work first-

Starship deflector shields are basically layered force fields- gravitons held unidirectionally in a shell that push back against any object or energies that enter their vicinity.

The direction and shape of them are managed by shield emitters, which use magnetic fields and exotic particles to shape the graviton fields and project them at a certain distance from the hull. The massive energy expenditures for those devices are handled by shield capacitors, basically huge temporary batteries that allow energy to be accumulated and then dished out to the shields.

The emitters function at a specific frequency. This means there are always certain frequencies of energy and matter than can pass through the shields, because the emitting ships needs to be able to emit particles and allow them to be reflected back to sense the outside universe, and fire weapons back at enemies. Shields are also programmed to flicker momentarily to allow physical objects like shuttles, missiles or torpedoes to pass through them. 

Now all this talk about how shields function is important because there are several different results to a shield clash, especially in a collision:

Your collision may describe shield vs. shield strike- lets assume the same sized ships, and that both shields hold. The ships collide, both ships' emitters are capable of handling the backlash of energy through the system, the capacitors hold their charge, and the frequencies clash. The two ships will basically divert one way or another off each other, according to whatever vector they had when they approached. it's a physics problem, and put basically, they bounce in some way.

This almost never happens except for smaller ships. Why? the inertia and mass involved with full-sized starships is staggering. Shields are not meant to repel objects of such mass, and when the average system is taxed to those limits, the emitters usually explode as too much energy is forced back through their circuits, or the capacitors run dry long before the object is diverted, and the shield runs out of juice.

If you change any of the variables listed above the result is usually the same- somebody's shield fails and that object runs into the opposing ship's shield or hull at speed. In stellar terms that is almost always a fatal event as we're talking percentages of the speed of light in terms of velocity, which involves a *lot* of potential energy. 

For most collisions it would be fatal to both ships. Some ships with extremely superior shields could survive such an impact, but they would likely be devoid of shield emitters or capacitors, their bridge crew likely burned or dead from plasma backlash, and their hull and systems significantly damaged. if one of said systems is life support, then they have only delayed an instant explosive death for a more horrific death by deprivation.

Unique shield collision effects are possible, though, even though the results are always the same. There is the possibility that the ships will collide at the instant there's a shield flicker on each side for some reason.

Also the shield frequencies could be set intentionally or accidentally be complementary, in which case the same effects ensues-

Both ships collide, shields up, but act as if the shields were not. Obviously that is always fatal to both ships. This actually is used as a traditional last-ditch tactic for death-above-dishonor type captains: "Match frequencies and ramming speed!" It's all very dramatic, but ends the same way- death and debris.

This really has been a lot of words to say something simple: If you direct a ship under your command into the shield envelope of another craft at anything but the barest of acceleration, one, usually both of you, will die.