Some time ago, I did a back-of-the-envelope calculation about magnetism, electricity, and graphene. I was (and still am) curious about developing a "deflector shield" based in known and viable science. I don't remember the exact numbers I used, but my calculations indicated that passing a unit charge through a certain length of graphene would produce a magnetic field capable of exerting enough force at a 30-foot (9.1-meter) radius to deflect or stop an inbound missile traveling at about Mach 2.5, which appears to be on the slow end of air-to-air missiles.

I don't have enough of a physics background to be confident in my quick numbers, so my question also involves someone being kind enough to check my math.

However, my real question is this: Is such a defense feasible and can it be scaled to satisfy the needs of a single soldier, a battalion in open terrain, and a spacefaring vessel? What impact would such a field have on people and things in or near it?

As mentioned here, employing magnetism in the extreme can have undesired effects, such as levitation and cellular disassembly. So anyone using a magnetism-based shield would need to have some form of protection against their protection.

Since magnetism only repels/attracts things with the same/opposite charge, and most things have a neutral charge (just don't walk across a carpet), anything that needed to be deflected by the field would need to be appropriately charged. Is there a way to manipulate the charge of an arbitrary object at distance, such as by firing electrons at it? (Note: this seems difficult and infeasible.)

To sum up, since this seems to be a little haphazard:

  • Is a magnetism-based deflector shield a possible construct?
  • If it is possible, what scale is feasible: soldier, battalion, ship, fleet?
  • What impact would such a shield have on people and things in and around it?
  • What protection, if any, would be needed by the user to defend against negative effects of the shield?
  • How could the effective charge of any arbitrary object be manipulated at a distance of about 30 feet (9.1 meters)?
  • $\begingroup$ If the projectile is ferromagnetic or paramagnetic, wouldn't it always be attracted, never repelled? $\endgroup$
    – KSmarts
    Feb 26, 2015 at 15:15
  • $\begingroup$ @KSmarts I'm not sure, but my understanding of magnets was always that some were repelled and some attracted. Perhaps objects would align themselves to be able to pass through. I honestly don't know. $\endgroup$
    – Frostfyre
    Feb 26, 2015 at 16:26
  • $\begingroup$ If that is the case, you could always heavily fortify the generator and use it like a lighting rod. (Assuming the machine is plausible) $\endgroup$
    – KSmarts
    Feb 26, 2015 at 16:33
  • $\begingroup$ What do you mean by unit charge? Coulomb, statcoulomb, elementary charge? $\endgroup$ Feb 26, 2015 at 17:39
  • $\begingroup$ @2012rcampion I believe I used a 1 Coulomb value, but it's been a while. $\endgroup$
    – Frostfyre
    Feb 26, 2015 at 18:36

2 Answers 2


Is It Possible?

In theory, yes! The methods through which a deflector shield would work is straight magnetism and the Lorenz force. If it's magnetic, it will be attracted to the shield (since it itself is a magnet, and likely a strong one!). If it has a charge, the Lorenz force will swing it off to one side. The faster it goes, the more the Lorenz force kicks in.

What about the non-magnetic and non-charged items? A magnetic field will not protect anyone in that case. Fear any missile made of ceramic, or a weak magnetic field producer!

What Scale is Feasible?

For A Soldier?

Bullets, as far as I can tell, are sometimes magnetic materials. (Lead interacts weakly with STRONG magnetic fields.) Given that many types of bullets are made of lead, I think this makes is unfeasible for your troops. Besides that, bullets seem to have a charge of 1 pC to 1 nC. That's tiny for such a large projectile. While they are traveling quickly, they would need a massive magnetic field to get any appreciable force on them. Here is a list of items which produce measured magnetic fields.

Unless we start shooting plasma or other highly charged particles, this is not going to work for bullets shot by infantry.

For Tanks or Ships?

It seems that most shells for tanks and other high-powered weapons of war use steel casings for their weapons. Steel reacts to magnets decently well, so we can not eliminate the shields based off of material alone.

We should note, however, that this would result in the deflector shield unit actually *attracting** the projectile, which presents the possibility that what would be a clean miss would be a hit. I personally prefer avoidance whenever I can, so I would not fund this style of deflector shield against modern arms. Deflector shields could by deployed at the ends of a craft, to attempt to steer projectiles away from the middle.

It turns out calculating the force between magnets and magnetized objects can be tricky. Assuming that $B=\frac{\mu_0 I}{2\pi r}$ and $F=\frac{B^{2}A}{2\mu_0}$ (this is possibly a wrong approximation), the force on the shell is: $$F=\frac{\mu_0 I^{2}A}{8(\pi r)^{2}}$$ where A is the surface area of the projectile closest to the deflector shield. For a 125 mm BK-14m HEAT round, that is about $0.04 m^2$ At 9.1 m away with 1 Amp you get about $7.6*10^{-12}N$ of force. That is a pathetically small force for an anti-tank weapon.

In effect, no, this is not feasible for a ship, a tank, or any atmosphere based item against modern weapons. Modern projectiles simply do not have enough charge, nor can we produce strong enough magnetic fields to deflect massive objects in this way.

If you opponents shoot plasma, then deflector shields may come into being, as plasmas are highly charged and the Lorenz force can easily dictate their movements.

Effects of Massive Magnetic Fields

Magnetic fields, especially massive ones, can cause problems. Such as:

  1. Levitation (first frogs, then other life forms, depending on diamagnetism and intensity of your field.)
  2. Pulling electrons off of you (from a massive Hall Effect gone bad)
  3. Wiping out information on hard drives
  4. Getting your keys stuck to the deflector shield generator
  5. Massive power bills

Where Are Deflector Shields Feasible?

In space, against highly charged particles going fast, you may effectively use a deflector shield. Space.com reported that several scientists were investigating such a field to protect astronauts against cosmic rays and other high velocity, electrically charged baddies in space. A more recent article from CNN talks about some improvements and more thoughts on this. There are other variations as well, such as the plasma-field, but that is out of the scope of your question.

  • $\begingroup$ So magnets are infeasible until we advance our technology to the point where we're using the power of stars for warfare. Thanks for taking a stab at what seems to be one of the hardest questions yet asked. $\endgroup$
    – Frostfyre
    Mar 1, 2015 at 5:54
  • $\begingroup$ @Frostfyre Which is too bad, because we could do AMAZING things with them. :( $\endgroup$
    – PipperChip
    Mar 1, 2015 at 7:54

Based on what has been said, a magnetism-based deflector shield doesn't sound practical at all. However, in my mind, this does raise two distinct possibilities;

  1. A strong enough magnetic field could potentially be used to draw projectiles away from a targetted object, and;

  2. ...a magnetic field could also potentially be used as a "projectile trap" a la the alien mech suit in District 9.

Wishful thinking, maybe, considering the power requirements needed, but it's given me something to think about.


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