How (theoretically) can metal detectors be defeated?

I’m only asking because I’m sure that if there is any possible combination of materials that could hide a metal object from metal detectors, the major intelligence agencies are using them. But from a physics standpoint, what would do it? A Faraday cage? A strong radio absorber like graphite or ferrous oxide? A diamagnetic like bismuth? Superconductors? Attaching the metal object to capacitors or resistors in some sort of tuned circuit?

Take a few rolls of (old) pennies through the detector on your next flight. Let us know the result. The old ones were copper if I recall, the new ones are zinc mostly aren’t they?

Don’t use metal

I’m not sure there is a way to actively/electrically hide metal objects.

Theoretically wormholes may work too

There would be little need for major intelligence agencies to defeat metal detectors as they would simply use bribes or guile to slip weapons past them. After all, why spending millions to get weapons past a checkpoint when you could spend a few hundred or thousand to bribe a security agent into allowing it through (under the guise that it is drugs that you are smuggling)? Or simply disassemble the weapon and conceal it in a locked toolbox or a shipping container?

Also, since intelligence agencies have access to their nations’s diplomatic pouches, why wouldn’t they simply place any needed weapons in them and ship them to an embassy or consulate? After all, no one gets to search diplomatic pouches physically and X-raying them will only confirm the presence of a weapon; it’s unlikely that the host nation would stop the ouch for that.

Finally, obtaining a weapon in-country isn’t that difficult. If you simply contact the local criminal organizations (which are likely to be information sources anyway), a weapon can be purchased quite easily and then disposed of without having to worry about taking it through security checkpoints.

The easiest way to disable a metal detector (not a hand wand unit) would be to simply walk over to a breaker panel and flip its breaker off. There have to be enough malfunctions in the system for this to occur and not cause too many disturbances.

You simply obscure it so that even though the metal detector goes off the inspectors think it is something else. For example you hide the metal object in a larger metal object.

Hack the metal detector or maybe use espionage to modify the design of the software so that you can build in a backdoor protocol.

Yes, the ol’ Kobayashi Maru technique can be quite effective if it’s undetected.

You could in principle do it with an active device that produced an opposing magnetic field. It’d have to work quicker than the detector, though, and the same technology used to make the detector-fooler could also be used to make an even better detector that wouldn’t be fooled. In other words, an expensive device could fool a cheap one, but you wouldn’t be able to fool an expensive one.

It wouldn’t be that hard to passively block a metal detector, but any way to do so would itself be detected by the metal detector (i.e., they might not see the gun in the metal suitcase, but they’ll still see the metal suitcase regardless of what’s in it).

A passive device that negated (rather than blocked) the signal a metal detector looks for would violate Lenz’s Law, and hence also conservation of energy.

All around, your best bet is probably just to avoid using metal parts in the first place, or to make the individual metal parts small enough that they won’t be detected.

A “metal detector” would detect an anomaly. The presence of the anomaly would suggest that a more thorough search be conducted.

A faraday cage, radio absorber(?), bismuth, capacitors, and resistors would register as an anomaly.

A Faraday cage wont be detected as a big solid lump of metal though, will it?

Is it possible to make one that is fine enough that it comprises very little metallic mass, but close enough weave toobscure mmetalinside it?

AIUI, metal detectors work by generation a fluctuating magnetic field (in practice, low-frequency radio waves) that induce a counter-current in metal that in turn generates a field detectable by a magnometer. Hence my wondering if something could either repel the external field altogether (Faraday cage, superconductors, diamagnetics) or absorb it so that no counter-field is generated (radio absorbers, tuned circuit, etc.)

Note to all: I’m more interested in the physics of the scenario rather than the practical outcome.

My understanding is that stealth aircraft work in part by being built mostly of graphite composite; and that the SR-71 Blackbird used ferrous oxide in it’s paint for it’s radar-absorbing properties.

Radar works very differently than a metal detector, though. In short, radar works at a distance, but is significantly easier to defeat, while a metal detector works only at close range, but is much harder to defeat.

Hm, OK, I was thinking at the level of the return signal, not the initial signal. Something along those lines might work; I’d have to give that some thought.

Pole vault over it.

Dress as a priest, and when the metal detector goes off, display your wrought-iron cross necklace and wait for the TSA agent to wave you through with a quiet “Sorry, Father.”

[ref: In the Line of Fire, 1993.]

If the field can’t be altogether countered, can it be either:

  • Averaged out across a wider area such that any detected change in field strength is below the triggering threshold?

  • Redirected to another place, such that a specially-constructed wire could “redirect” the field from a gun into a belt buckle?

First of all, a Faraday cage insulates the interior from a static or slowly varying electric field (by slow we mean <10[SUP]6[/SUP] Hz). It does so because the interior is a closed volume surrounded by a uniformly distributed charge such that there can be no net charge induced in the interior by the external static field analagous as to how the gravity induced within a uniform shell is net zero. It does not protect the interior from a highly transient field, radiation that is at a wavelength that is significantly less than the aperature size of the cage, or static magnetic fields. Simple metal detectors work by using an external coil through which a changing electrical field induces a magnetic response in the target object. They measure the change (e.g. the loss of energy or phase imbalance between two coils) to detect an object capable of carrying a current. A Faraday cage may protect the interior object against the inducing electrical field but only at the expense of creating eddy fields within its mesh; that is, since the field will not be balanced around the entire conductor simultaneously you will get streams or cells of currents which interfere and influence each other and induce magnetic fields which will be conducted, such that while the detector may not detect the target object it will detect the case, and reducing mesh size or metallic content to a minimum does not diminish this effect.

In the case of millimeter-wave scanning systems (such as full body scanners used in airports) they will scan right through a mesh cage. They will be blocked by a solid and continuous substrate of metal (even just aluminum foil) but obviously the substrate itself will show up. So wrapping a Glock 17 pistol in foil will just produce a Glock 17-shaped outline to the scanner, which obviously isn’t too useful. When airport scanners do find an impenetrably object they typically insist upon inspection (the primary reason why you have to take your laptop out at the airport). Such methods are even less effective on x-ray backscatter systems; in order to stop the x-rays you’d need to have a sufficiently dense material to absorb the radiation, which would again indicate a large and likely metallic mass.

I have read speculation about compartments lined with a combination of mesh (to block radiation) and mu-metal (nickel-iron alloy with high magnetic permeability which blocks static magnetic fields) which would effectively shield or at least produce ambiguous results in a simple metal detector. Whether such systems were actually developed and used to smuggle weapons or other neferous devices as portrayed in spy novels, however, is uncertain; back of envelope calculations indicate that a compartment large enough for a small pistol would be the size of a lunchbox or larger, which seems incongrous for such a task, especially when more prosaic methods for smuggling or acquiring weapons are available to would-be spies and illegal arms dealers. (One favored trick is simply to protect and bury weapons in a conex full of rotten food or fertillizer that port inspectors are disinclined to dig through.)

The easiest way to defect a metal detector isn’t to beat the detector by trying to shield the part; it is to beat the system by either saturating it with false positives, or find another way around similar to how an experienced burgler will check for open windows rather than start by picking or prying the locked front door.


I am not sure,maybe there is a kind of imitation metal that can escape from the metal detector.
However,I hope this technology will not be mastered by the bad person,they will abused it to hurt the innocent person.
On the other hand, the quality of metal deetctor used in public should be guaranteed.

The recent Mother Jones article from an undercover reporter working at a CCA prison got me interested in this, as one of the inmates, “Corner Store” was showing off how he was able to trick the detector by jumping through sideways. If you keep the cross section of the object small and move it through quickly, it apparently won’t hit the threshold. Carefully positioning and disguising of metal objects that pass through the scanner can work too, when you’re dealing with overworked employees, which I’m sure is everybody at the TSA.

Put the metal in your mouth and “sneeze” into your hands on the way through. If it still goes off, stick the metal behind your belt buckle, or even better, in your pocket behind your fake Air Marshal badge. Give the guy a wink and a pat on the shoulder for not blowing your cover.

This page seems to indicate you could do it with a ferromagnetic substance acting as a shield.

Ceramics… Simply having somebody toss you a weapon when nobody’s looking… bringing parts of a largely composite weapon through one at a time, and hide them in a bathroom ceiling tile… I’m sure there are legit reasons to carry a metal tube and a spring.

How about you carry a large bomb into the massive line of people that got there three hours early to ensure they could get through security?

I forget the term, but there’s a prison equivalent of a “fool’s chamber”, a fake stash to hide the real one. Bring something innocuous through (maybe some medals for valor in an American conflict), say “oops, I’m sorry, here it is…” Tired employees don’t always make you pass through a second time.

Try a wheelchair. War medals would help then too. Be sympathetic, tell them that you appreciate that they’re doing their jobs and keeping us safe. Bring a diaper to crap your pants if they’re being a bit too thorough.

If you can’t fool the machines, then play the people. They have an awful record among legit testers who wanted them to succeed. Still, I don’t blame the people on the ground. It’s an awful, unrewarding job, it doesn’t need to exist, and it’s impossible for them to succeed.

Back in WWII, most benefited from the boost in the economy. Now it’s very few. We’ve spent countless dollars on unnecessary security that a total amateur can list 20 ways to defeat.

You want to defeat metal detectors? Write to your representatives.

Edit: I see I resurrected an ancient topic. Sorry. Long time reader, excited to jump into an interesting forum. I won’t do it again.

I always thought metal detectors - beach toys or TSA portals - work by a simple process; a current through a loop produces a certain inductance (measured in Henrys). Introduce a conducting core instead of air near/in the loop, and the inductance changes. (Because the magnetic field is doing more work to induce a current in the conductor) (Which is why some coils in old radios were wound around metal cores) In the beach toys or wands the coil is part of a frequency generator tuned circuit, IIRC - when the inductance changes, the frequency tone of the output changes. So the only escape is a non-conducting material. AFAIK I’ve never heard of a non-conducting faraday cage - it’s counter-intuitive. They are for keeping inside signals inside and outside signals outside.

Stealth aircraft work by avoiding radar wave reflections. When the radar wavelength is, say, 4CM (little over 1.5 inch) then a 1cm non-conductive coating with metal particles suspended in it (a paint or foam with metal flakes) will reflect radar waves from myriad of continuous points between 1/4 wave apart - this should result in the various different reflections of the radar wave cancelling out. Flat facets help, because in theory this is like detecting a clear glass object by its reflections - unless you happen to be right where a reflection is aimed you won’t see it.