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When a ship is under attack, the shields wear down over the course of the attack.

What is the mechanism behind this? Aren't the shields powered using energy? While the ship has energy, shouldn't it be able to indefinitely defend against attacks the shield can absorb?

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  • To have tension. Also then all enemy ships would have the same rules applied.
    – kleineg
    Jun 9, 2015 at 12:26
  • Fascinating question! In comparison, in Star Wars shields do not "wear out", right? If the shield generator is up, the shield is up and that's it, right? That's why they try to destroy the generator in Hoth, when attacking a Star Destroyer, etc.
    – Andres F.
    Oct 26, 2023 at 15:03
  • The answer concerning graviton fields and distortions is obviously the straight goods. For me, I always simply assumed it was analogous to my phone's battery; a few texts don't budge the charge but a full video call drains it precipitously. Even with antimatter power sources, the cataclysmic forces impacting the shields take their toll in short order.
    – Blaze
    Oct 28, 2023 at 3:24

1 Answer 1

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According to the Star Trek: The Next Generation Technical Manual (by Okuda / Sternbach), Section 11.8, pg. 138 ("Deflector Shields"):

Like most forcefield devices, the deflector system creates a localized zone of highly focused spatial distortion within which an energetic graviton field is maintained. The deflector field itself is emitted and shaped by a series of conformal transmission grids on the spacecraft exterior, resulting in a field that closely follows the form of the vehicle itself. This field is highly resistive to impact due to mechanical incursions ranging from relativistic subatomic particles to more massive objects at lesser relative velocities. When such an intrusion occurs, field energy is concentrated at the point of impact, creating an intense, localized spatial distortion.

This zone of spatial distortion around a ship is created by the so-often-mentioned shield generators:

The deflector system utilizes one or more graviton polarity source generators whose output is phase-synchronized through a series of subspace field distortion amplifiers.... Heat dissipation on each generator is provided by a pair of liquid helium coolant loops with a continuous-duty rating of 750,000 MJ. Four backup generators are located in each hull, providing up to twenty-four hours of service at 65% of nominal rated power.

It is clear then that several things may go wrong when an "intrusion" through the shields is attempted. First of all, the distortion of space around the ship (what we call "shields") is designed to adjust itself rapidly from a diffuse state to a localized one, to provide maximum resistive force at the target site of the intrusion. This is an enormous drain on energy supplies. Volleys lobbed at disparate points in a short time would cause the biggest drain of this kind.

Secondly, the shield generators produce considerable amounts of heat, which need to be dissipated. This heat output increases when the shields localize. If the generator coolant loop fails on a particular generator, that generator must be taken off-line until diagnostics and repairs can be carried out, further compromising the integrity of the shields.

In short, stressed deflector shields cannot function "indefinitely".

What does it really mean to suggest that the shields can continue to function "indefinitely" because they can absorb energy? There is no reasonable or efficient way to transfer this energy back into the shield generators. The shield generators are located at precise points along the ship. When the shields absorb energy, this energy is dissipated in various forms, e.g. mechanically (by disturbing the shape of the field generated by the shields and/or the position of the ship) and as light and heat. There is no way to direct this energy coherently back into the shield generators, or into any collection point along the hull, I'm afraid.

Note: The Technical Manual is (almost entirely) canon. Okuda and Sternbach were designers and technical consultants on TNG, and the introduction for the manual was written by Roddenberry himself.

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  • 2
    Good answer, but on the second-to-last paragraph about absorbing energy, I think you may have misunderstood what the person was asking. The question doesn't say anything about absorbing energy from attacks, I think "attacks the shield can absorb" just means attacks that wouldn't break through a shield at full strength, and "While the ship has energy" refers to the power systems that supply energy to warp/phasers/life support etc., asking why these systems can't continually recharge the shield (the shield generators overheating and needing repair seems like the most plausible answer).
    – Hypnosifl
    Jan 18, 2015 at 20:55
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    ...although actually, if overheating is the issue, why don't they just design the ships with backup shield generators that are kept offline until the primary shield generators nearest them overheat, and then they can boot up and return the shield to full strength as long as there's enough power from the matter/antimatter reactor? (which supplies power to all the ship's systems according to p. 57 of the TNG Technical Manual, not just the warp engine) It actually seems kind of difficult to a come up with an in-universe explanation for this plot contrivance.
    – Hypnosifl
    Jan 18, 2015 at 21:06
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    @Hypnosifl: I think the part of the OP's question regarding absorption is ambiguous and open to interpretation, at least as it is currently worded. That being said, I believe that I have given a canonical and sound answer to the primary question: "Why do shields in Star Trek 'wear out'?"
    – Praxis
    Jan 18, 2015 at 21:12
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    Got it.. At the end all the energy absorbed needs to be dissipated, so its basically limited by energy dissipation capacity...
    – Akash
    Jan 19, 2015 at 5:40
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    @geewhiz : The graphic on Reliant's console when the shields go up might be purely symbolic. Anyway, there are too many "takes" on what shields can or cannot do, depending on which episodes or films you consult. That's why I just went straight to the manual.
    – Praxis
    Jan 21, 2015 at 17:38

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