# Deflective properties of a forcefield

I have always though of three basic ways that force fields function.

The first way is that the force field projects an indestructible shell that oncoming objects can never break. Further, the velocity of the oncoming objects is irrelevant. In this case, the rock harmlessly rolls off.

The second way is that the force field applies a force to the oncoming objects accelerating them in the opposite direction. In this case, the rock is slowed down and then repelled by the force field.

The third way is that the force field projects an ultra-powerful wall of doom that instantly annihilates any oncoming object. This model uses immensely more energy than the first two.

My question is:

What is the most physically accurate model for a force field and which one would be best fitted for protecting a basic human civilization? If none of these types are realistic, are there any other ways one could work?

• Just to confirm...are we talking a solid force field that stops all matter, or are we talking a magnetic force field that only effects charged particles and metals/magnetic substances? Apr 23 '15 at 0:41
• A solid field that stops/repels/destroys all oncoming matter. not just a big electromagnet Apr 23 '15 at 0:43
• @Twelfth There are questions on this site about floating cites hovering over the earth, I did not expect to actually get a Real force field as an answer, that is awesome!!! Apr 23 '15 at 0:58
• @sumelic, pretty sure they all violate conservation of momentum. Apr 23 '15 at 1:11
• I'd like to compliment the OP for the beauty of the question's illustration.
– user6415
Apr 23 '15 at 7:39

## The Three Types One Type of Shield

You denote three types of shields1, describing them by their effects:

1. 'Instantly' reduces the velocity of incoming objects to zero.
2. Accelerates incoming objects outward.
3. Destroys incoming objects (and propelling their pieces outward)

However, we can actually describe Types 1 and 3 in a way that they all fit under Type 2.

Type 1 reduces the velocity of incoming objects. This change in speed is an outward acceleration, just a very fast one. This is exactly what Type 2 does.

Type 3 destroys an object and propels the remaining pieces outward. Same as before, we have a change in velocity corresponding to an outward acceleration, like Type 2. The acceleration is so large that it destroys said object.

All three of these types could be said to use the same mechanism, just at different magnitudes.

## Trigger Warning: Words

(If you take my word that conservation of momentum is a thing, then you can skip this part.)

I'll paraphrase an excerpt from my answer to Kinetic energy reflection:

Conservation of momentum is a result of Newton's three laws. If we define momentum as the product of an object's mass and its velocity, we can state the laws in this way:

• If no force is applied to an object, its speed (and therefore momentum) does not change.
• The rate at which momentum is added to or removed from an object (rate of change of mass times speed, or mass times acceleration) is equal to the force applied.
• When two objects apply a forces to each other, those forces (and therefore the objects' changes in momentum) are equal and opposite.

The result of all this is that momentum is a conserved quantity:

• Objects cannot create or destroy momentum, only trade it between themselves.
• The total momentum of a system is constant when no external forces are applied.

If it helps, you can think of momentum like charge, where force plays the role of momentum current. The crucial difference is that momentum has a direction that is also conserved.

You all know that Newton has been supplanted by Einstein, so you might argue that there might be some super-physics where the three laws, and therefore conservation of momentum, only hold as an approximation. However, there is a hugely important result in physics called Noether's theorem. It basically states that every spatial symmetry has an associated conserved quantity. In our case this means that if the laws of physics don't change from one place to another, then momentum (the quantity associated with motion) must be conserved. (In general relativity it's actually the 4-momentum that's conserved, which includes an object's 'speed through time;' but that's not important for this discussion.)

## Shielded from Physics

We can use the same argument from Kinetic energy reflection to show that all three types of shields would violate conservation of momentum. Basically, it boils down to: the shield doesn't change speed, so its momentum remains the same, however the incoming object does change speed, so its momentum changes. Since those are the only two objects in the system, the total momentum changes, which is not physical.

## Forcing it to Work

In order for shields to be possible, we'll have to make a key adjustment: the momentum from the object is transferred to the shield generator. From here it can transfer the momentum to a large mass, like the Earth. (This is exactly what a wall does, transfer momentum into the ground via a momentum current, i.e. a force.)

The upshot of this is that we need a way for the shield generator to apply a force to the object that it wants to stop. Well, as it turns out, there are only four forces for us to choose from:

• Gravitational force: not really an option since we haven't solved gravity yet (although we're working on it!).
• Electromagnetic force: This one is pretty promising, I'll go over it in detail in just a second.
• Strong force: only relevant on the scale of atomic nuclei.2 It decays exponentially with distance.
• Weak force: only relevant in interactions between certain subatomic particles. As the name suggests it is very weak and is also short-range.

It looks like the only good candidate is the electromagnetic force. This encompasses most of the forces we deal with every day though, so we need to break it down a bit:

• Near-field: this includes stuff like interatomic and intermolecular forces.
• Far-field: interactions of electromagnetic waves with matter.

We want far-field interactions, since we want the generator to be far away from the stuff it's repelling!

## Frickin' Laser Beams

In order to deliver the large amounts of energy required to the objects we want to stop/destroy, we're going to need some sort of powerful electromagnetic wave. Essentially we've just described a laser.

The only problem is, laser beams are not bubbles. This is disappointing, but inevitable. Basically you can't have some sort of force or energy hanging around somewhere without some stuff to keep it there.

This means that we need to shoot out our lasers at exactly the right time to stop incoming objects. Our 'shield' has basically turned into a directed-energy point-defense system.

Note that although light does carry momentum, we don't actually need to apply external momentum to stop an object. The laser beam will vaporize the surface of the target, and the resulting tiny explosion of plasma will push the object away like a rocket. This is good for stopping objects with high kinetic energy but low momentum, like bullets and missiles, but will be useless against slow-moving objects or ones that can redirect the momentum into the ground (like a tank rolling along); "The shield turns the fast blow, admits the slow knife!". There are some exceptions: some things are going too fast to be stopped,3 while most slow-moving objects will probably be destroyed by your high-energylaser beams.

1 I'm going to use the term "shield" instead of "field" to distinguish the science-fiction meaning of "force-field" from the mathematical and physics notions of 'field'.

2 Although the amount of energy it contributes to neutrons and protons it so high, it gives them almost 99% of their mass (through $E=mc^2$)! Cool, huh?

3 I always like to point out that the glowing trail you see behind the projectile is not propellant: the projectile is smashing the air apart into plasma it's going so fast.

• This answer is awesome, I'd vote it up +100 times if I could! I did have a slightly Sci-fi basis in mind when I made the .gifs for the question, but you provided links showing research being done to make stuff like that exist! Amazing! but as far as reality goes, an "InstaStopAll Scy-Fy force-field is impossible. (as of now)? Apr 23 '15 at 16:34
• @X-27 when you have enough reputation, you can come back here and assign it a bounty ;)
– o0'.
Apr 25 '15 at 16:10
• I just might :-) Apr 25 '15 at 18:03

If you're looking for a physically-accurate force field, the second model (soft deflection) is the closest to what you'll get.

A real-world force field would work by (as the name implies) applying a force to an incoming object, causing it to accelerate in a direction away from the area being protected.

Because of the inability to generate repulsive gravity, a real-world force field would need to act through electric, magnetic, or electromagnetic means. Electrically-neutral, non-magnetic objects (or reasonable approximations thereto, such as a human) can freely pass through such a force field; it's only useful for stopping things such as charged particle beams and metal projectiles, with a field strong enough to stop the latter likely to cause collateral damage.

Real-world examples of force fields would be the magnetic confinement fields used in particle physics research, or on a very large scale, the Earth's magnetic field acting to deflect solar winds and cosmic radiation.

• I hadn't even though of an electromagnetic system, but that would be great for repelling energy based attacks. Apr 23 '15 at 2:11

The fundamental problem you have here is that there is absolutely nothing in physics as we know it that would allow a force field to be generated.

There are a few things we can do like magnetic fields that will work on certain materials but a universal "shield" that blocks incoming things would be based on some form of physics that at the moment we have no concept of at all.

Since the current most unexplained part of physics is gravity, then the most likely place we will find a way to do something like this is going to be by manipulating gravity. Imagine a field that warps spacetime and causes anything coming into it to bend away, like a black hole but in reverse.

By modulating the effect correctly you would get a result similar to a force field. It would work like your type 2 but as the other answers have said actually type 1 is just type 2 turned up to 11, and type 3 is a type 2 turned up to 100.

A realistic force field could simply vaporise all incoming objects, which can be achieved by a thin layer of high energy plasma contained by an electric (edit: I mean, magnetic) field. The plasma will glow brightly though, and is transparent, so it would allow laser like weapons to pass (you can stop that by adding some sort of photochromic layer, or just a opaque layer). Alternatively, we could theoretically warp space, and either get the projectile to pass straight through us, deflect it or make it vanish into a singularity. This method would also have the advantage of being invisible and also works for radiative weapons like lasers.

Note: The second option isn't possible with current technology, but the first one is (only theoretically though, currently not very practical).

• The first option actually isn't possible with current tech. Earnshaw's theorem tells us that you can't use an electric field alone to contain a plasma. You need a magnetic field for the containment to be stable, and an electric field to keep it hot. Our two best methods (1) (2) so far require surrounding the plasma with superconductors to generate the requisite fields. Exposing the plasma to the environment is going to be really hard. Apr 25 '15 at 3:19

## Laser Ablation

On a planetary scale, if you don't mind doing a lot of damage to the secondary object, you could utilize a set of powerful satellite-mounted lasers to propel the object away in a quite efficient manner. Unfortunately, this must be done from a fair distance before the reflected radiation becomes non-apocalyptic. Most like the second option.

## Flux pinning

From a smaller point of view, flux pinning seems to be promising for many smaller or alternate variants of shielding technology. One method (cannibalized from another answer somewhere around here) would be to line up superconducting plates "glued" together by quantum locking. Maybe the object in question could merely be sprayed with adhesive superconducting nanobots. Most like the first option.

• I actually don't mind annihilating the secondary object (hence my third model in my question) If there is an attacking ship, an enemy missile, Etc, getting vaporized upon impact should not be objectionable :-) Apr 23 '15 at 2:13
• @X-27, destroying incoming objects is generally the realm of point-defense, not force fields.
– Mark
Apr 23 '15 at 2:15
• The flux pinning (fictional) invention is mine, BTW. Apr 25 '15 at 16:24