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Many science-fiction universes depict exotic defensive shielding technology protecting their vessels from enemy attacks. The different depictions of defensive shielding have varying levels of realism.

For more information, please see the main articles for how the shields of each universe behave:


Even though many defensive shield technologies rely on exotic physics, any analysis of defensive shielding technology should take into account real-life physics as much as possible.

Conservation of Momentum
A shield should obey Conservation of Momentum. If a shield stops an incoming projectile, the projectile's momentum must be transferred to the shield system. If the momentum is greater than what the physical shield system (or its bracings) can withstand, then the equipment may suffer physical damage.
Some dispute this, citing Wesley Crusher's homemade tractor/repulsor beam[1], which didn't seem to transfer momentum to the device.
  • Wesley held it casually while lifting a chair with it, but this proves nothing, since Wesley could presumably lift the weight of the chair himself, allowing momentum to transfer all the way to the deck of the ship.
  • Later he left it sitting on a table projecting a force field keeping people out of engineering, and it wasn't knocked off when people tapped the force field. This objection ignores the possibility of the device being secured to the table in some fashion or transferring momentum directly to the doorframe that the field was blocking.
  • Wesley later modifies the Enterprise's own tractor beam to act as a repulsor, and it gives the Enterprise a "push off" from the USS Tsiolkovsky that buys the time the crew needs to save the ship. Far from contradicting the theory that their shields transfer momentum to the ship, this incident only confirms it.
Conservation of Energy
Any energy a shield draws or absorbs must go somewhere, typically--but not always--as waste heat. Many sci-fi defensive shields redirect incoming attacks and glow at the impact point when the energy from the attack is radiated away.
Sometimes depictions of shields suggest that simply having the defensive shielding raised will result in the shield system using a lot of power. However, the energy cannot magically disappear and must still go somewhere, so some rationalization may be required when analyzing those depictions in order for its behavior to make any sense.

Defensive Shields in Different Universes

Star Wars

Most starships in Star Wars use deflector shield technology to protect against enemy attacks and navigational hazards. Energy from incoming attacks that cannot be completely deflected is absorbed and stored in heat sinks. The shields can become overloaded if the system absorbs heat more quickly than it can be radiated away. The shield system must be shut down while the systems cool off, or the projectors will burn out and require replacement.

Some starships can make use of the energy absorbed from incoming attacks, at least to a certain extent.

Star Wars shielding technology can be miniaturized enough for use on some droids or onto personal shield devices that can be carried (similar to medieval shields). It can also be placed on platforms that can be carried onto a battlefield, allowing broad areas to be shielded against attack. The technology can also be scaled up to protect entire planets.

At least some Star Wars shields are permeable to slow-moving objects.

Please see the main article on Star Wars shielding for more information.

Shields flare when struck by phaser fire.

Star Trek

Most starships in Star Trek use shields to protect against enemy attacks. As the shields take damage, they weaken, and effects from enemy attacks bleed through with greater frequency.

Star Trek shields oscillate in some fashion at specific frequencies. Knowing the frequency of an operating shield will allow an attacker to tune weapons to penetrate the shield easily.

Please see the main article on Star Trek shielding for more information.

Mass Effect

In Mass Effect, shields known as "kinetic barriers" can be created using mass effect fields. These shields can stop or deflect incoming projectiles, but they are ineffective against directed-energy weapons. Barriers are scalable from personal defense shields to starship shields.

See Also


  1. TNG, "The Naked Now"