How do force field work in many sci fi fictional universes? I remembered the last time I checked the strength of the electromagnetic field a.k.a force field is proportional to the power output from the engine, so how does applying additional kinetic energy to the shield affect the power output? It is analogous to saying an overcrowded maglev train can only be run at half its normal energy consumption, makes no senses at all. Seriously Scotty, did you break my ship... again?

I should clarify the force field in this case is being treated like a rigid structure, meaning it can break like any solid object, so don't go harassing my science officer!

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    $\begingroup$ As I see it, force fields are not electromagnetic. They're as far beyond our science as nuclear fusion is beyond a 19th century thinker. (There was a guy who calculated the age of Earth based on the assumption that the sun was made of coal, and how long it could sustain the fire ...) $\endgroup$
    – o.m.
    Dec 23, 2016 at 7:37
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    $\begingroup$ Shields at 10% one more hit and… To find out, tune in next week to another exciting episode of "Trek Wars". $\endgroup$
    – M i ech
    Dec 23, 2016 at 8:11
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    $\begingroup$ I don't think that can be science-based unless we a/ accept that we are talking about science-based electromagnetic fields as shields and b/ we are wondering how to deactivate them by shooting at them. $\endgroup$
    – njzk2
    Dec 23, 2016 at 18:23
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    $\begingroup$ Am I the only one that don't feel comfortable with the tags in this question? $\endgroup$ Dec 23, 2016 at 23:48
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    $\begingroup$ @Mindwin The tags look good to me. $\endgroup$
    – Navin
    Dec 24, 2016 at 11:04

14 Answers 14


I can see a few ways that this could work without breaking thermodynamics too badly. But you'll never get "Shields at N percent" with any kind of magnetic or other field that we know of today. This is strictly handwaving.

  • The shield acts like a blob of heat-conducting, viscous liquid that slows down bullets and absorbs heat from lasers and other energy weapons. The rate at which absorbed energy can be dissipated depends on the size of the shield. (A "damaged", i.e. hot, shield will glow red-hot.) If it gets too hot, various bad stuff might happen: Generator melts, shield goes critical and dumps all the energy both inwards and outwards, your choice. (This is the approach used by Larry Niven in the Mote in God's Eye).

  • The shield consists of an array of targetable force-field projectors -- something like a CIWS with the Half-Life 2 gravity gun or something. If it has to deflect too many threats in too short of a time, they will run out of ammo / stored power / fuel / whatever and the threats will start getting through (gradually at first, hopefully the targeting system will prioritize.) (this is possibly the Mass Effect approach)

  • The shield substance behaves in some ways like a normal mechanical solid armor, only it's not made of of ordinary solid matter. It gets damaged much like real armor, dissipating the energy of incoming projectiles (by shattering) or directed-energy weapons (by vaporizing or heat-sinking). If a hole opens in it, you're vulnerable.

  • (possibly most realistic one) The shield consists of an actual liquid held in some kind of forcefield (ferrofluid, maybe?) and serves as self-healing / flexible inertia and ablative armor outer-layer. (This will be much heavier and thicker than real armor and probably work best on very large ships in zero-G). Getting hit vaporizes or splashes the liquid, and if too much of it gets used up, there isn't any more.

I think that the idea of shields having a percentage that goes down when they deflect damage is mostly a concession to video and tabletop gameplay where having hitpoints is useful. In movies and the like, they tend to be much more all-or-nothing, but tend to let at least some threats through... which can damage the shield generator... leading to the shield failing. Still not very realistic.

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    $\begingroup$ It also adds drama. It wouldn't be the same tension of it went "direct hit, we're okay. direct hit, we're okay. direct hit, we're okay. direct hit, shit, we're screwed!" $\endgroup$
    – user
    Dec 23, 2016 at 10:45
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    $\begingroup$ Larry Niven''s shield is convincing by not being obviously impossible. Incoming energy is conserved. The shield traps and stores that energy until it can be radiated outwards. If overloaded, the shield collapses and all the energy escapes at once, inwards as well as outwards, which destroys the ship it was protecting. $\endgroup$
    – nigel222
    Dec 23, 2016 at 13:18
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    $\begingroup$ I was recently reading some of the Honor Harrington novels, and was quite taken aback with the vulnerability of the spaceships in that universe. Travel times were non-trivial, enemies would mostly slug it out with missiles at long ranges, and even a glancing hit could result in the deaths of hundreds of crew members, and ran the potential of crippling the ship. Because that's the truth - we can make far more powerful weapons than armor. (Depleting) Shields are a nice way of adding tension to a situation, and keeping the ship out of the repair bay every other episode. $\endgroup$
    – AndreiROM
    Dec 23, 2016 at 14:31
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    $\begingroup$ Classical shields in movies/games usually imply they have a kind of battery and a generator. The generator can keep shields up indefinitely, and otherwise charges the battery to full, but taking hits depletes the battery. Battery dies, Shields have no energy to do anything to incoming projectiles, even if they are up. $\endgroup$
    – Ryan
    Dec 23, 2016 at 18:04
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    $\begingroup$ Niven's shields also opened up potential plot avenues for, e.g., expanding shields radiating away heat faster during a battle. It's a neat feature. Which then led to more plot when that "feature" came into play while the protected ship was in the wrong kind of environment for that expanding behavior. $\endgroup$
    – Ti Strga
    Dec 23, 2016 at 20:28

The shields that made most sense to me was a gravitational field, not electromagnetic. but that does not change anything, really.

Shield was not a constant field. It was actually a directional effect generated on demand to conserve power. When something relatively light "hit" the shield, it only used up energy from capacitors in the node that took the hit - and everything went back to normal in a matter of seconds. When capacitors were all empty and there was another hit, there was a need to supply power from central accumulator, not engines (because dynamo in engine room was still slower than central accumulator battery). This was replenished slowly - and this would be your percentage. When something heavy hit the shield, there was a risk of burning out "nodes", lessening the ability to stop multiple projectiles later to save ship now. When there were many projectiles, ship would use all nodes for short peak of omnidirectional field, at the expense of power.

Based on beer talks, Weber's books and few other resources.

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    $\begingroup$ This is a good way to envision shields $\endgroup$
    – kingledion
    Dec 23, 2016 at 14:11
  • $\begingroup$ Where does Weber do anything like this? The only book I can think of of his with traditional shields is The Apocalypse Troll and the shields are around only for a very short period of time and we saw almost nothing about them. In the battle against the Navy the shields were unavailable and in the final battle they were bypassed, not worn down. $\endgroup$ Dec 23, 2016 at 23:24
  • $\begingroup$ @LorenPechtel Based on and inspired by, not copied from. $\endgroup$
    – Mołot
    Dec 23, 2016 at 23:35
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    $\begingroup$ I think of it like a cheap battery in a camera with flash on. The battery drains really fast but if you let it sit a while or drop it the level comes back up. Hook it to a trickle charger and it's pretty much what you see in the movies. $\endgroup$
    – Josiah
    Dec 27, 2016 at 3:01
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    $\begingroup$ Exactly my thoughts. If nothing else, why have a constantly-running field at all? It would be a huge waste of energy. The only threat I can think of that would require continual activation would be laser (or similar) weapons, and even then you might be able to protect yourself if the shield activated quickly enough (the damage a laser does will be proportional to the length of the attack). $\endgroup$ Jan 10, 2017 at 15:53

Technobabble warning: I just made all this up. None of it has any scientific validity. It's all magic space wizards.

The shield is an active crystalline energy held into a specific form surrounding the ship. When the shield is hit this damages the crystals as the impact is absorbed and spread out through the shield matrix.

The shield generators have a reservoir of energy (essentially capacitors and batteries). The generators are constantly charging the capacitors but the energy required to stabilize and repair the crystalline energy matrix is far beyond what the generator can sustain. This means that when taking heavy hits the capacitors get drained and are then gradually recharged by the shield generators.

Shields at 10% means that the shield capacitors are at 10% charge. If they reach zero then the shields will start to destabilize and collapse. If the shield takes no hits for a while then the capacitors will refill as the generator's output is used to charge them.

In other words:

  • Shield Generator -> Shield Capacitor -> Shield Projector

Larger or tougher ships may have more powerful generators or larger capacitors, which means the damage they can take is larger. If incoming damage is less than the generator output then the shields will never drop below 100%.

  • $\begingroup$ "I just made all this up. None of it has any scientific validity. It's all magic space wizards." Did you miss the fact that the question is tagged [science-based], not [magic] or even [science-fiction]? $\endgroup$
    – user
    Dec 23, 2016 at 10:46
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    $\begingroup$ @MichaelKjörling To be honest I sort of did. I read the question which says "How do force field work in many sci fi fictional universes?". The answer is "magic". Star Trek and Star Wars are all fantasy in space. I provided a plausible technobabble explanation for the phenomenon and clearly described it as such. Force shields do not exist with our current science so it is impossible to be any more "science based" than that. $\endgroup$
    – Tim B
    Dec 23, 2016 at 11:10
  • $\begingroup$ Actually, if you presume that the shields are Suitably Advanced Technology that requires a large burst of electricity to operate correctly, this makes perfect science-based sense. Admittedly, there is that first step, but that applies to shields of any sort. $\endgroup$
    – Bobson
    Dec 23, 2016 at 18:46
  • $\begingroup$ "plausible technobabble" - Do such thing exists? $\endgroup$ Dec 24, 2016 at 14:26
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    $\begingroup$ @VictorStafusa At one point cell phones were plausible technobabble. Star Trek called them communicators. $\endgroup$ Dec 24, 2016 at 16:36

Shield is some sort of field. Engines power it. When a hit comes in, engines spin faster momentarily to repel it. But there's also a battery supplying additional oomph (technical term). The battery gets depleted. Once you get away from battle, the battery starts recharging. The percent is how much battery is left.

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    $\begingroup$ The technical terminology is strong with this one. $\endgroup$
    – AndreiROM
    Dec 23, 2016 at 14:23

The shield actually pushes back on the projectiles being launched at it. The energy to do this (which could be very huge, dependent upon the projectiles) comes from some central ship supply, which could be whatever you want (antimatter, hydrogen, plutonium, giant battery, plot device x, etc) as long as its capacity is limited. When shields are at 10%, that means that the central reserve of energy that powers your shields only has 10% of its energy supply left to use for stopping projectiles.

For other ideas, this might draw energy from the same source as (some or all) other ship components, resulting in raising the shields to block a hit being a choice to sacrifice the other components that need the same energy, which could be a minor inconvenience if you choose to connect communications or interior lighting to the same systems, or a big thing to worry about if critical functions like propulsion or life support are connected to the same supply.


I saw documentary about creating a real life force field they theorize that having a layer of hot plasma sandwich between two layers of cold plasma could create an impenetrable shield that would soak up any energy or projectile weapons. I've always imagined that Shields in science fiction would work something like this but would require an outrageous amount of power to maintain and that would eventually run out in a fight.


Perhaps the shields are formed as a combination of:

  • Electromagnetic fields to deflect particle beams and railgun/coilgun slugs.
  • An energy barrier which imparts kinetic energy on matter impacting it, which is used to counter said projectiles' current kinetic energy.

The electromagnetic field has a constant energy drain while active, and draws more power the further it's pushed up. The energy barrier has a lower constant drain, but drains a significant amount of energy on-hit, proportional to the stopped projectile's kinetic energy.

The shield's percentage rating indicates how much power it has directly available in its capacitors and/or batteries, without having to go through the ship's main power system; the ship can easily recharge capacitors while out of combat, but attempting to do so while in combat puts it at risk of an overload (due to already drawing significantly more power than usual, for weapons and evasive maneuvers); there's also the possibility that any feedback in the shielding system could damage the main power system, if you attempt to recharge while the shield is active. Therefore, shields will likely either not recharge at all during combat, or recharge more slowly (due to the main system actually recharging a secondary capacitor, which is then used to recharge the shield capacitors; this prevents any feedback from potentially crippling your entire ship).

Alternatively, if the shields are designed to absorb incoming energy instead of stopping it, the percentage can mean one of two things: how much power is left for stopping matter projectiles, or how much leeway there is for absorbing energy. If the system absorbs too much energy, it could overload, potentially damaging itself in the process. Conversely, if the power level drops too low, the field can't be maintained at all, leaving the ship vulnerable to any kind of attack.

In this situation, "Shields are at X%" could have one of two meanings:

  • If blocking physical projectiles, "We only have X% shield energy left."
  • If blocking energy weapons, "We only have X% more leeway before things start exploding."

...This could get confusing when dealing with mixed weaponry, but it's likely that shields would either remain more-or-less constant or just give out more slowly (due to the energy weapons replenishing the energy used up by the physical weapons).


First, a quick recap of what we're trying to accomplish:

  • Shield must be able to break
  • Shield "value" must be measurable
  • You can see the shield when something hits it (blue sparks or ripples or whatever you see in movies)
  • Unobtainium exists

First, we begin with the assumption that the majority of the ideas about how a force field really works (that it is a uniform shield projected around the entire ship) is wrong.

Ever watched a video of someone playing table tennis with a pair of nun-chucks?

Let's take that and expand it into a force field.

Consider that the ship can spin unobtainium threads at extremely high speeds, such that you can't see the movement of the threads, and that any projectile will be intercepted by the movement of the thread, either knocking it (the object) out of the way, or destroying it - perhaps even absorbing energy type weapons, if lasers exist in your world. (Think grass mower plastic thread theory).

The ship can have sensors which detect how much the unobtainium threads have degraded, and spit out new thread to replace old thread - as the ships shield takes more hits, the threads are lost faster, and eventually the ship runs out of thread (0% shields left). The ship may also be able to detect or predict what kind of projectile the shield is intercepting, thus allowing the prediction of "1 more hit and we'll lose our shields".

The unobtainium threads may also contain a property which allows it to emit blue (or other color) for a short time as it gets hit by projectiles. You'll also be able to explain away sparks created by the interception of projectiles from the thread(s).

With this system, we will also be able to explain why shields have to be "turned on" - the threads need a bit of time to spin up to speed. We also get to say that glancing hits from projectiles cause less damage than direct hits (if you want to in your story - I believe this works out from a physics standpoint, but if I'm wrong please point it out)

"But you can't cover the entire ship like this! You'll end up with circular spinning plate-type shields!" you say? No problem - what about an array of smaller plates, which cover up holes between plates with more plates? Think of a chain mail type arrangement, or a fish-scale type layout.

Now we can also explain away "redirect power to the forward shield generators" - all they're really doing is using more thread for the area that they want to redirect "power" to.

  • $\begingroup$ This seems a lot more convoluted than supposing they're made of energy. $\endgroup$ Dec 25, 2016 at 12:19
  • $\begingroup$ @immibis It may be convoluted but at least it's now more plausible than an "energy force field" $\endgroup$
    – Aify
    Dec 26, 2016 at 3:11

Hmm...The shields run on Handwavium (term borrowed from another post). They rely on a network of projectors surrounding the ship and their individual shields overlap, somewhat like scale mail. Each can deflect only so much energy before failing until recharged, which takes time and energy.

If one individual shield is hit until it fails, the load is taken up by nearby sheild projectors, but they are less efficient now. Enough failures in an are means a hole in the shield, but not the complete failure of the whole system.

If the network have 100 projectors and 90 fail, shields would be down to 10 percent.

This kind of network gives the additional advantage of being able to "re-route power to the aft shields" by reducing the amount of power going to fore shields.

Repair comes from replacing, repairing, or recharging damaged or depleted projectors and would give "Shields back up to 37%" a bit more meaning.

Sounds like fun!


If I were to make a shield for a space ship, I would try to use iron filings in orbit around the ship by a magnetic field. As objects or energy beams hit the iron filings, they would be scattered and no longer in orbit around the ship. With sheilds at 10%, only 10% of the iron filings are left.


Based on the technology in my answer in Hard Sci-fi energy shields:

The technology is based on a swarm of physical shield elements held in place and moved around using superconductive flux pinning.


There are two resources that can be depleted: the physical plates and the control system.


Plates that hit may break, or undergo stress that cause fractues to build up with use so it eventually breaks. They are designed to absorb more energy by ablating a passive coating so this wears off. It also absorbs energy by breaking along engineered seams so the smaller pieces can still be contolled by the system.

Plates may suffer a hit and absorb more energy than the flux pinning system can “catch” so it’s thrown out of the swarm.

In any case, fresh plates need to be deployed to continue to provide protection. It takes time to move them around and you may have a control capacity of only moving so many at one time. Bad ones still consume control capacity so must be removed before fresh ones can be put out.

control burden

When a plate is pinned in position, any force acting on it induces electric currents that set up magnetic fields that exactly counter the motion. When hit the plate is accelerated severely and thus causes huge currents. The superconductor has physical limits.

To reposition a plate requires active control over the currents, essentially making an electric hologram that changes over time. How many plates can be controlled at once, how quickly? There will be reql limits to the machine.

Imperfections will consume more power and cause heating.

executive summary

The boss wants a single number that indicates how much impact can be accepted, from a specified direction. You can see that after being used, plates need to be sent back to the correct positions or rotated out of service. Heat must be removed from the superconductor, and power added to carefully dampen/cancel disturbances and reposition the plates. Until that is accomplished, the system will have a reduced maximum ability for the next hit.

This is a summary, but is really an assessment of many factors. So it may indeed be inconsistent in how it really behaves. It’s not a simple hit-point scorekeeping, but a complex dynamic state and messy fracturing of physical plates. How it really handles the next hit can be quite variable.

failure modes

So when the capacity is exceeded, what happens?

If there is no plate that can take the hit, or deflect it completely, then the impact will hit the ship. This may be leftover bits and pieces that could not be slowed in time, so the effect can ramp up as the capacity is exceeded.

More spectacularly, what happens if the superconductor is overloaded? Look what happened at the LHC when it “quenched”. The heat build up caused it to exit the superconductive state, and then the entire energy stored as electric currents suddenly felt the material resistence.

The superconductor shell doing the control needs to surround the entire ship. It might be seggregated into individual units that have some isolation from each other. When one of them blows up, it’s like a grenade exploding right at the skin of the ship! So, this matches the TV trope quite easily.


Usual trope works as following :

1 - The ship has a finite capacity to generate energy. Meaning each second the system "generates" a finite ammount of energy.
2 - This energy generation capacity is divided between various subsystems - Engine, Weapons, Force Field and Life support.
3 - Each subsystem has a finite capacity to store energy.
4 - Everytime a subsystem "works" it drains a certain ammount of energy from its storage.
5 - Captain can decide the proportion of energy allocated to each subsystem. In other words how much of the total energy generated each unit of time a subsystem will receive.
6 - The ammount of energy in each subsystem is, therefore, the result of how much was stored in the previous unit of time, minus how much was used during this unit of time, plus how much was allocated from the generator.

So lets suppose the generator generates 10 Gigajoules per second (the unit of time in this example) and the captain allocates 5 gigajoules for life support, 3 for weapons, 2 for shields and 1 for engine, if you fire a very powerfull weapon that uses 10 gigajoules per shot, 5 will have to come from the subsystem storage, meaning that this subsystem will have 5 gigajoules less in the next second. If the weapons subsystem storage capacity is drained, the weapons subsystem will stop working, leaving the ship unable to fire until enough energy is stored. The same works for shields. Usually, the ammount of energy drained from the shields subsystem is proportional to how powerfully was the ship hit. So if your enemyy fires at you with a weapon that makes your shields use more energy than the generating capacity, the storage will be used. If hit again, it will drain it a more, and so on. So, if hit hard enough or fast enough, your storage will be exceeded and you will lose your shields, exposing your hull armour. The energy storage is usually expressed as per cent of total capacity.

In that situation, the captain will, usually, increase the amount of energy allocated for shields, leaving you with less energy for weapons and/or engines. Usually, in the tropes, you cannot decrease the ammount of energy to life support due to the risk of casualties, or this sacrifice is explored as a show of military honour etc...

So regarding the question, "Shields at 10% one more hit and ..." What ?

The next hit will hit the ships hull directly, totally or partially, dealing real damage to the ship, that might or not be fixed by the damage control parties.


Totally non-sense answer, you have been warned.

It always seems to me that shield needed a lit of energy to run, thus they had capacitors/battery banks. As the shields absorbed incoming energy they would deplete the capacitors. At some point, these capacitors would no longer have enough "charge" to function. This would give you a scale you could represent as percentage. The shields either worked or didn't, but the capacitors may have 10% charge left.

Of course at the same time, your totally amazing engine power would be refilling the capacitors. So if you could just dodge a few shots your shield capacitors would go up to 15%.

At the same time you wouldn't want to wire the emitters directly to your engine, cause if there was a power spike you would end up with a really big and loud space ka-boom.

As for how the shield actually work, well, we do have some shields today, but they don't work anything like sci-fi shields. They basically emit a wave or pulse that disperses another type of wave or pulse. Like "blocking" a flash light by turning on a overhead lamp.

I have noticed however, that sci-fi shields are usually good against energy thingies, and terrible at physical thingies. Perhaps a good example would be reading about Dune's personal shields, and their draw backs.


Your shields absorb energy from enemy attacks as heat, and store it in heat sinks until it can safely dissipate. Eventually, your heat sink will melt down into slag, and then you can't channel any more heat into it. The next attack won't be absorbed, and will mess your ship right up.

  • $\begingroup$ You might want to expand that a bit, and highlight hiw yiur idea is different from the other Answers. $\endgroup$
    – JDługosz
    Jan 10, 2017 at 9:07

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