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Physical Nature of Shields

Written: 2002-05-04
Last updated: 2003-04-20


"Raise shields!" is a familiar order to sci-fi fans. We take it for granted that when they raise these "shields", they'll be safe. We take it for granted that when the shields are gone, they're in deep trouble. But we rarely ask: what, exactly, is a shield? And what's all of that "frequency" nonsense they're always talking about on Star Trek? There have been many lengthy treatises written on shields, but they usually bundle countless assumptions about shield nature into their conclusions, which tend to exist solely for the purpose of generating a strength figure of some sort. Rarely does someone make an attempt to examine their nature, or ask what conceivable scientific principle might allow for their existence.

Some shields are visibly obvious, such as the ones below:


You can clearly see the blue "soap bubble" around each droid. But others are more subtle; in the ending battle of TPM, for example, we kept hearing about how they couldn't break through the Trade Federation battleship's deflector shield, but we never saw this shield in any way. So we're still left with the question: what are they? What scientific principle could possibly account for a shield?

What do they do?

Before embarking on any speculation of mechanism, we should first determine what a shield does. Shields in sci-fi generally serve 2 purposes:

  1. Stop "energy weapons" (lasers, phasers, blasters, etc).

  2. Stop physical objects (bullets, knives, people, vehicles).

Fair enough, but what does it mean to "stop" energy weapons and physical objects? Physical objects tend to stop abruptly when they hit a shield, but not always: sometimes the "slow blade passes" concept from Dune seems to be in effect, and objects can slip through (see the Gungan dome shields in the Battle of Naboo). Sometimes incoming objects bounce, and sometimes they explode on impact. And what about "energy weapons"? Some ships (such as the Trade Federation battleship in the end of TPM) look as if they're taking every hit right on the hull, but their shields are said to remain up, as if they are coincident with the hull surface. Others (such as the Droideka shields shown above) project their shields out into space, and these shields light up in a multi-coloured display when hit.

Are shields forcefields?

The most popular candidate for shields is forcefields. In fact, shields and forcefields are often treated as interchangeable terms in the literature and dialogue. This is encouraging, because the term "forcefield" comes from real-world science, not science fiction. Unfortunately, the resemblance between real forcefields and sci-fi forcefields ends at the name.

The two types of forcefield you are most familiar with are electromagnetic and gravitational. Sure enough, those are the forces routinely mentioned in sci-fi. In the 1950's classic "War of the Worlds", the Martian spacecraft were said to be using an "electromagnetic blister", which easily warded off artillery shells and all other methods of attack. In Star Trek, the writers gleefully steal terminology from particle physicists and say that they're based on "gravitons" (the theoretical carrier particle for gravitational forces).

But electromagnetic and gravitational forcefields share an interesting characteristic: they are both long-ranged, and their effects weaken with the square of distance. So if you double your distance from the centre of the Earth, the force of gravity drops to one quarter of its original value. Simple enough, correct? Unfortunately, this creates a problem for our shields: you see, they typically have no effect whatsoever until you reach some invisible point. When a man runs into a forcefield on Star Trek, he feels nothing until he touches the invisible wall, which produces a sparkly effect like this:


Now, if this were a gravitational forcefield, he should have felt its effects from anywhere in the room, gradually increasing in strength as he approaches the window. A forcefield has a volume effect, hence the name "force field", not "force wall". But this is obviously not what we saw. Have you ever tried to force the positive poles of two magnets together? You can't do it, can you? And you will notice that the forcefield effect is gradual, not abrupt. It gradually increases as they approach, until it eventually becomes so large that you cannot force them any closer together.

And what about the fact that they wear down? We've all seen the displays:

Shield Status

In Star Trek, rather than simply being up or down, shields have a strength property ("deflector power", which is a bit over 70% in the image above). It wears down after multiple hits, and when it goes to zero, the shields are nonexistent. But why would a forcefield weaken after use? Does the magnetic field of an electromagnet get weaker each time you use it to pick something up? No, so why would shields get weaker? Is there a "fatigue" property? None of this is consistent with a forcefield.

Are shields made of energy?

Rather than imagining shields as forcefields, some people imagine them as a "wall of energy". That seems like an improvement (after all, there are no real walls of energy which we can use for comparison, so it's not as easy to say that it's wrong), but even if we disregard the question of how you would go about constructing this beast, some obvious questions leap to mind:

  1. What holds this energy in place? Pure energy is light, and moves at c. It does not sit in a particular spot, nor does it form walls of arbitrary shape. If you had some kind of mechanism which could control the energy and force it to move in a contour around the ship, why bother with the energy component? This mysterious energy-manipulating mechanism would obviously be capable of deflecting incoming energy weapons by itself if it can already manipulate a wall of energy to hold an arbitrary shape.

  2. Why aren't incoming objects destroyed by this energy? Not only can Picard touch one of these shields with his hand, but incoming objects such as Roga Danar's flimsy escape pod in "The Hunted" have been observed to bounce off a starship shield with no ill effects.

  3. Where does the energy go when they turn off the shields? If it's released, should it not be quite violent?

  4. Why would the energy necessarily interact with other energy?

The problems with the "wall of energy" idea are extremely difficult to resolve for many reasons, not least of which is the fact that the mechanism for manipulating the energy into a shield would perform the function of a shield all by itself.

What about frequency?

Star Trek shields have a "frequency" characteristic, which implies that they oscillate. It should be noted that this behaviour is unusual to Star Trek, and there is no reason to assume it is universal to all shield concepts. Many natural phenomena are frequency-based, but even a device based on a frequency-based principle need not be phase-coherent, so it would not exhibit an aggregate "frequency" characteristic. There are some general advantages and disadvantages of frequency coherence in shields:



Against a frequency-based attack, a phase-coherent shield could be theoretically optimized to give greater protection than a flat shield with the same average amplitude, by synchronizing with the attack.

Against a non-coherent attack, a phase-coherent shield would allow partial penetration even if it's working perfectly.

It should be possible for a ship to fire outgoing weapons out through its own phase-coherent shield by matching frequencies but being 180 degrees out of phase. You would have to open a small hole in a flat shield in order to fire through it, which requires fine control over shield geometry.

The knife cuts both ways. An attacker could penetrate a phase-coherent shield by matching frequencies and being 180 degrees out of phase.

Note that it's possible to oscillate with respect to amplitude or vector, although we would expect a vector oscillation would cause significant scattering effects with outgoing beams (turning a tight beam into more of a spray), and we generally don't see that with Trek weapon/shield interactions, or those of any other sci-fi series for that matter. A square-wave (as opposed to sinusoidal wave) would allow perfect penetration with a synchronized weapon, thus eliminating the scattering effect of a vector oscillation, but it would also allow 50% penetration from an incoherent weapon.

There are interesting theoretical possibilities for a shield which oscillates, but the disadvantages outweigh the advantages, depending on what kinds of weapons the enemy is using, what kind of control you have over shield geometry, etc. Worse yet, if the enemy has sensors which can detect the activity of your shields, it should be trivially easy for him to match frequencies, synchronize phase, and shoot through your defenses. Ultimately, the idea of a phase-coherent frequency-based shield seems more attractive for its script-writing flexibility than its tactical attributes.

Competing shield mechanism theories

It is difficult to determine what sci-fi shields are, but we can determine what they are not. They are not made of energy, and they are not forcefields. Complicating the matter is the fact that not all sci-fi shields operate the same way. However, a common paradigm seems to be that of the invisible solid wall. Most shield manifestations do in fact behave as if they were an invisible solid wall, so if we were to theorize about mechanisms, we would need to look for mechanisms that would produce this effect. There are a few explanations widely circulated:

  1. Shields are actually a circulating cloud of exotic matter (presumably non-interactive with visible light frequencies). Raising the shields would be a matter of pumping this material out into space and then activating the containment system which causes it to circulate in a controlled fashion about the ship. Lowering the shields would be a matter of simply turning off the containment system, which would cause this cloud of matter to harmlessly disperse. The containment system might even be able to maneuver this material to increase its density at points of attack. If the material is charged, it could be theoretically contained through electromagnetic means without too much difficulty even without the common sci-fi mechanism of a device for easily applying force to electrically neutral objects. This is the most realistic concept, and in fact, theoretical ideas for "plasma sheath" systems have been floated in real life.

  2. Shields are a volumetric forcefield with a sharply defined edge (note that this would not apply to a Trek-style "prison cell forcefield" which is planar when approached from both directions). There are certain configurations of forcefield which do cancel out beyond a boundary, although this boundary is normally defined by physical coils and does not simply float in space. The interior of this forcefield would behave like a normal forcefield, while there would be no effect outside its cancellation boundary. Incoming objects would hit the edge and be affected immediately, but if they achieved penetration, the effects would continue (unlike breaking through a wall, this would be more like pushing into a sponge).

  3. Shields are a two-dimensional force wall, although it is difficult to imagine what could possibly produce this effect, or why the resulting shield would demonstrate conduction/reradiation properties, both of which imply mass.

  4. Shields are a gravimetric effect. This is the official theory used by Star Trek's Technical Manual, as well as those of many other sci-fi authors who have similarly attached themselves to "graviton" as the god-particle to make everything sound plausible. However, gravity effects are insensitive to light frequency, so it is difficult to understand why a gravity-based shield would be invisible. Gravity is also much more effective on matter than on light, yet we have seen that most sci-fi shields are more effective on light than matter. The former problem can potentially be solved with amplitude frequency coherence (if it oscillates from 0 to 1 at a high frequency, it will "flicker" imperceptibly, just as a movie screen flickers imperceptibly even though you're actually seeing 24 still pictures per second interspersed with darkness). However, the latter problem cannot, and one would have to wonder why it would be so difficult to determine an enemy ship's shield frequency if it works this way, since you should be able to detect it optically.

Of those possibilities, the first one most easily explains the "strength" property common to most sci-fi shield schemes. After all, it would take a while to construct this circulating mass of matter around the ship, and if parts of it were literally blown away with each hit, this would explain the gradual degradation of shield function. The other models are forced to explain degradation by imagining damage to the shield generators themselves, and while such explanations might work in a different series, the damage would not be as predictable or linear as the strength reduction shown on Scotty's display panel in ST6, so they would not work for Star Trek.


Sci-fi is diverse, and not all shield systems do the same thing. But there are a few questions you can ask to narrow down what basic phenomenon a shield represents (note that this is different from trying to invent some technobabble explanation for how it works).

  1. Do energy bolts or beams "bounce off" the shields, still intact? If so, you are looking at reflection (for examples, see the trash compactor scene in ANH, or the battle droid shot which ricocheted off Anakin's Naboo starfighter in TPM).

  2. Do energy bolts or beams splinter, or break apart into a shower of smaller bolts? If so, you are looking at scattering (for an example, see the ISD turbolaser bolt that struck the blockade runner's shields in the opening scene of ANH).

  3. Do energy bolts or beams make a large area of the shield glow? If so, you are looking at absorption, conduction, and subsequent retransmission (for examples, see most TNG-era Trek shield incidents, as well as the incident in TESB where the Falcon was knocked about its longitudinal axis by a turbolaser hit).

  4. Does shield geometry affect the shield/energy interaction? If so, it may be a vector effect, deflecting the bolt in specific directions based on geometry (phrases like "angle the deflector shield" in SW or "continuously vary the shield geometry" in ST hint at this possibility).

  5. Does the shield completely block incoming energy, or does it allow a portion through even if it is still functional?

Ultimately, there are far more questions than answers when it comes to shielding, and one must be careful not to leap to facile conclusions.


Shielding is more complex than it may appear on first glance. There are more issues to consider, and more difficulties involved in evaluating strength than one may initially realize. However, this hardly means that the exercise is futile. On the contrary, a thorough and systematic examination of observed events can be used to determine realistic limits, given certain caveats:

  1. The fact that they often call it a "forcefield" does not mean it actually conforms to the description of one.

  2. There is no intrinsic need for a great volume of energy in a shield, so it is wrong to assume that shields must consume large amounts of energy. Do not become attached to a particular model of shields simply because everyone else seems to accept it. A solid case can even be made for the idea that shields have mass.

  3. Always remember to consider the weakest link in the chain, not the strongest link. This is an important lesson from real-life engineering which is often lost on sci-fi debaters, who tend to conceptualize sci-fi in a purely abstract theoretical sense in which they pick one particular phenomenon and concentrate on that phenomenon to the exclusion of all others (ie- focus on the non-physical shields and ignore physical constraints).

  4. Do not assume that all energy shields employ the same mechanisms. We can view the behaviour of energy shields in action and see that there is significant diversity in their operation, and this must be considered when attempting to synthesize a consistent model of their operation.

  5. At no point do any of these theories require that the shield must draw as much energy from the ship's systems as the incoming weapon carries with it. Yet I have often noted that virtually everyone assumes this energy equivalency to be the case, without making the slightest attempt to explain the logic. Why should a shield require energy equivalent to the weapon? Does a piece of armour on a tank consume energy when a shell bounces off its surface? Did you ever wonder why an air conditioner's rate of cooling can exceed its electrical power draw in watts?

Before making any leap in logic about how much power a shield must need or how it must work, just ask yourself whether your assumptions are coming from observation and logic, or from common practice. Because ultimately, common practice is a poor justification for anything.

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