In relation to this question, how could I create a force field dome over that city?

Specifications of the city:

  • 10 x 40 km on the surface
  • 1 km in height
  • has a city wall of 100 m all around the edge.

What the force field needs to do:

  • Hold against artillery barrages
  • block line of sight in the city
  • does not let anything in or out when activated
  • round edges and 2-3 km in height so inside flight is possible

What is, as far as science gets, the closest to real technological solution? I want technical buildings I need to do this, I am sure I need some sort of pylon but what else?

  • 1
    $\begingroup$ I don't think science and engineering can come close to what you're describing. Sadly. But we can dream. . . $\endgroup$
    – HDE 226868
    Oct 4, 2014 at 22:00
  • $\begingroup$ Do you want it turning on or off like a light switch or is there room for a deployable structure that creates it? $\endgroup$
    – mechalynx
    Oct 4, 2014 at 23:22
  • $\begingroup$ "does not let anything in or out when activated". What about light, magnetic fields, etc? Would be pretty dark inside... $\endgroup$ Oct 6, 2014 at 10:13
  • $\begingroup$ @ivy_lynx yes and no - i think of some time between switching it on and being at full strength. $\endgroup$
    – Fulli
    Oct 7, 2014 at 11:10
  • $\begingroup$ @KonstantinPetrukhnov yes this is one point i need - the dome would be darken the city completely $\endgroup$
    – Fulli
    Oct 7, 2014 at 11:11

7 Answers 7


If you're looking for hard science, you're out of luck: there are no known ways to create a non-material barrier that will stop projectiles or other large objects. You might be able to create a physical shield from steel or near-future composites that can be opened or closed as needed, but that would be a rather epic engineering project.

  • $\begingroup$ i am looking for "science based" not "reality based" it should be as close at it comes to real world. $\endgroup$
    – Fulli
    Oct 7, 2014 at 11:13
  • $\begingroup$ If it's technobabble you want, @Black's answer provides a good starting point. $\endgroup$
    – Mark
    Oct 7, 2014 at 20:28

As far as hard science goes the closest you'll come to repelling large objects is warping space. Something along the lines of an Alcubierre drive I'd imagine. According to the wiki page it would exert large tidal forces. Tidal forces can get pretty scary (see Roche limit, keyword disintegrate). Of course that doesn't stop energy weapons but if your atmosphere has a decent amount of Hydrogen I would assume the forces exerted by the force field would produce enough pressure to make it solid. As solid hydrogen is a metallic superconductor you get your energy shielding for free: Faraday cage, high reflectance, perfect heat distribution, etc. As for the specific differences of the field and whether you would indeed achieve enough pressure is more maths than I care to work out. I am imagining a rotating field so that your tidal forces occur even for a direct shot.

  • $\begingroup$ You would need a fairly large body for a Roche limit to make any sense. Also, how would an Alcubierre drive play into it? It would, in theory, warp space-time only for those inside the "bubble." $\endgroup$
    – HDE 226868
    Oct 7, 2014 at 2:31
  • $\begingroup$ I was using the drive as an example of using physics to warp space. The significance of that particular example being the fact that the tidal forces don't effect the flat region of space in the center. $\endgroup$
    – Black
    Oct 7, 2014 at 14:34
  • $\begingroup$ @HDE 226868 The Roche limit was the example of the effect of tidal forces acting on a body with mass (the satellite in the case of that page). Note that wiki page states the reason for there being a limit in the first place is because of the tidal forces generated by the gravity of the larger object (a gravity gradient). In this case you wouldn't necessarily need a massive object for the drive but it would generate tidal forces which would act the same. $\endgroup$
    – Black
    Oct 7, 2014 at 14:41

This seems impossible with our technological level, but there's ways around those limitations if we give ourselves a bit of artistic license.

Here's my picture of the dome shape from your other question:

dome shape

We need a force field to cover that are and be essentially impassable. I can think of two ways to do this - an actual plot-field, which would be hard to explain but actually is a "field" and an alternative approach using a combination of a physical dome, with gaps that it can fill with fields or material to give the same effect.


Since we need a force-field that would block everything, we basically need it to behave like matter. Very tough matter, but matter nonetheless. That means getting through the field should be no different than trying to break through titanium or something like that, albeit thick and tough titanium that can fix itself.

I don't know of any plausible physics that can create such a field - if we could do that, we could create matter out of thin air. However, we can assume you civilization has mastered the means of creating fields that mimic the way that matter works and can project them.

Considering the shape of the field, we'd have to place the generator very far underground to get the intended result (it's effectively a shell of a large sphere). We could alternatively place multiple generators across the city's foundations and create a composite field that would have approximately that shape, but that would be crude for someone who can already create matter fields and placing it underground gives the city the massive advantage of protecting itself from all directions even underground - in other words, enemies can't even dig a tunnel under it to get around the shield.

The problem is, we can't just have a spherical field underground because, to get this dome shape, it would have to be 72km underground:

spherical shield

link to plot

So we'll have to make it extend from a plane under the city, much like this:

underground source

link to plot

If shaped this way, we can place a circular plate under the city about 900m, to give us the needed field.

It doesn't end there though because if we just have a field, activating it will affect everything in its range - it won't just create a nice dome. If this was designed the repel matter and we power it, it would make the entire city explode around it. So we need to carefully shape it.

double dome

Since we're assuming exotic fields that don't exist yet, lets assume an exotic mechanism for them to work by: they turn electrical energy into tiny vortices and give them spin, making them matter particles (that's pseudoscience technobabble, but hey, it's a force field :P). The red and blue fields create spin in opposite directions, so they cancel each other out. To create matter, they need to reach a threshold, so if the field strength is too low somewhere, nothing happens. By adjusting the red field's strength and compensating with the blue field, we can adjust the thickness of the field's dome.

Using this arrangement, we can cancel the field out inside and prevent the city from exploding. What form would a generator of this kind take? Here's an idea:


The "4f" arrangement seems nice. Just make it spin really fast on all axes.

Physical dome

A probably preferable mechanical solution could be accomplished by having a deployable dome, that forms a mesh when deployed - then it fills the gaps with matter somehow.

I'm thinking of something like this:

Missouri botanical gardens

But of course, ours would be a lot larger and flatter. Your dome could deploy from the perimeter of the city and once in place, extend titanium panels to fill the gaps. Obviously, it would need some other form of exotic technology to expand so much - perhaps the city has huge stores of molten titanium under it and using some shaping trick it alloys the titanium with tungsten (to give it a higher melting point than plain titanium) and then funnels more titanium through the bars, suspending it through magnetism or something, until it cools into titanium plates.


Here you can use an actual physical field to act as a force field - in certain scenarios. Let's say you want to stop a single particle. What can you do? You can trap it. Not deflect it, in the sense you mean, but make it get stuck in a single point in space. Here's how:

If the particle has electric charge or is magnetic, you have a solution: Use an electric field to attract the particle. If this is the primary influence on the particle (discounting gravity), the particle will eventually stop moving in whatever direction it was going before and be "stuck" in the field. The same goes for magnetism - in fact, this might work better for larger objects.

Let's take a look at the requirements for the force field:

  • 10 km $\times$ 40 km
  • 1 km tall
  • Mounted on a 100-meter-high wall
  • Dome-shaped

Okay, that doesn't sound too hard. . .

Lets face it: a single magnet at the center of the city isn't going to do squat to an object at the edge of that big an area. 400 square kilometers is nothing to sneeze at here. So you definitely need multiple magnets. My first thought would be to stick a bunch of electromagnets on pedestals around the area, but that's a bit of an oversimplification. Your everyday junkyard car-picker-upper won't do the trick. So we'll take a step up and go to superconducting magnets. These are a lot more powerful. They're also fairly big and need a lot of energy, so maybe we'll change the "pedestals" to small buildings, complete with a power supply.

The bit about the dome is tricky. A magnetic field permeates throughout all of space, but the field gets weaker as it gets further away from the magnet. In other words, having a bunch of magnets lying in a plane won't produce a lot of force on a particle somewhere above the plane. So in order to create a dome-ish shape, the pedestals should slowly gain in height as they near the center of the field.

So to create this field, simply mount a bunch (hundreds, if not thousands - I should have mentioned that earlier) of superconducting electromagnets on some pedestals of varying height throughout the city, hook them up to an enormous power supply, and turn the thing on.

  • $\begingroup$ "General relativity says that a large object, such as a black hole, would create a strong enough gravitational field that it would deflect the path of the spaceships - saving the city!" Uhm, wouldn't it deflect towards the city, not away? $\endgroup$
    – PyRulez
    Mar 3, 2016 at 2:23
  • 1
    $\begingroup$ @PyRulez If the ships are between the black hole and the city, I would assume that the ships would be attracted towards the black hole and away from the planet on which the city resides, no? $\endgroup$
    – HDE 226868
    Mar 3, 2016 at 2:41

A Dome of Drones

I will go with a very big swarm of drones, interconnected and fully autonomous.

Not strictly a "force field" but give answer to all requisites:

  • can cover all city
  • can block vision: emitting strong light
  • can block unwanted objects: drones can be armed, or be explosives, and swarm towards any aggressive incursion/shell to block/destroy it
  • can let pass "identified" objects in/out

From the city, perhaps it will not even visible the composition of the dome or his nature.

Some kind of hangars should be placed somewhere (on top of the city wall?) to allow drones to charge / repair / replace losses.


Security experts recommend "Defense in depth", and "Multi-layer security". Plus, it makes the system more interesting to describe to the reader. So you could use several different devices or systems to achieve all your ends. While all a bit scifi-esque, here are some ideas at least a little less of a hand-wave than "forcefield".

Hold against artillery barrages

  • Automatic border-defense lasers (like Reagan's Star Wars strategic Defense Initiative)
  • Intercept missiles (like the Patriot missile defense system)
  • EMP and scramblers to mess with targetting systems for guided missiles
  • Strong electromagnetic fields to deflect rather than merely stop. Not necessarily redirecting entirely away from the city, could just direct into catchers or save zones every km or so.
  • A physical dome barrier shield, could expand out of rooftops, if the buildings are built at the right heights. Depending on its strength, could protect from minor shrapnel (a chickenwire dome) or the full bombardment.

block line of sight in the city

  • Charged particles could be held in a kind of smog over the city, which could, with the right kind of particles, be impenetrable to most or all wavelengths. Charging the roads differently or venting clean air out of regular vents could keep ground level clear and the smog at roof level and above.
  • A physical dome barrier, even if just black plastic.

Does not let anything in or out when activated

"Anything" is a big word. If you nuke it, will it let the blast through? The radiation? sunlight? Air?

  • Some defenses could be built into the buildings by building regulations. So, if all housing A/C is required to be tied into the municipal air supply, so to protect against airborne radiation, chemical or biological attack, just go indoors and close the door.
  • Vehicles, tunnels and covered roadways and walkways count as indoors, especially if tunnels have emergency doors that can close if needed.
  • Sturdy building construction can withstand reasonable blasts.
  • Retractable buildings provide full nuclear-bunker level protection. If the ability to retract exists, this might also be used during the winter, nighttime, etc, just for climate control.

Round edges and 2-3 km in height so inside flight is possible

Inside flight implies that the smog idea either won't work, or will require flying by instrument. Shouldn't be a problem at this level of tech. Also requires friend-of-foe identification, again not an issue.

All these layers of protection would give you a super-well-defended city, but I suspect would be too much of an urban redesign. But some of them might be usable to beef up the defenses you take from other ideas, at least.


I think the force field should be invincible and can be only activated only in times of war.my idea as an electrical engineer you can make a complete full circuit that half of the dome is a terminal and the other half of the dome another terminal that ends the circuit circle, when activated each terminal connects together,completing this force field circuit and form this force field that is highly strong and can defect anything

  • 2
    $\begingroup$ Can you be more clear on how would this be made? $\endgroup$
    – L.Dutch
    Sep 26, 2018 at 12:53

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