The question I have is whether these post-9/11 attempts were actually intended to destroy an aircraft. If that were the case, the terrorist would have been instructed to prepare and use the device while in the washroom, and not while in his seat. I suspect that the real objective was to cause the very security escalations that we have seen. These "security" procedures further cripple air transport (and indirectly handicap many other forms of business), and divert resources from other potentially productive activities into large bureaucracies and security organizations which have negative economic value. More insidiously, they undermine the very basis of Western society, removing or reducing freedoms and grossly expanding powers of state organizations (police, DHS, border patrols, etc.) to resemble those of totalitarian states.
We've blundered several steps down this slippery slope already. We went from mild and fairly unintrusive security procedures (to prevent tragedies such as the bomb on an Air India 747) which largely accomplished their purpose, to heavy-handed and extremely intrusive "security" procedures, which have no chance of achieving theirs, and are, in fact, utterly unnecessary and ultimately self-defeating. Here are some further steps down this slope, as I see it, and the reasons they are futile:
1a. Well, terahertz scanners can see through clothes to the skin revealing every little detail (ooh, privacy violations, creepy pedo images, blah-blah outrage, etc.), somewhat like a "hands-off" strip search. But once they are installed at every airport and used on every passenger, it would be impossible for a passenger to hide explosives/flammables/nerve gas/etc. on their person. WRONG. Ever seen a fat person naked? There are often rolls of flesh, with nice deep folds which the terahertz scanner does not see into - it can't penetrate skin, remember. Ladies with pendulous breasts also have nice hiding places which are not seen by the terahertz scanners. Men might even manage to hide a small item with creative placement at the scrotum. Perhaps armpits could be used, if one is not required to perform gymnastics while in front of the scanner.
1b. Before long, some terrorist takes advantage of having rolls of fat, and smuggles nasty things onto an airliner in them. But that terrorist gets overpowered by fellow passengers as he/she prepares the nasty things at his/her seat, and before the nasty things can be used (assembled/detonated/activated/whatever). Note that I expect the preparation to be at the terrorist's seat on the plane, where other passengers can spot what's happening, since the real objective is to escalate the security procedures.
2a. Next, passengers are required to raise limbs and adopt lewd postures etc. while being scanned, so that their entire surface is visible to the scanner. Those whose flesh folds are incompletely observed are required to step to the side so a security officer can have a grope inside those folds. This is tantamount to a "hands-on" porno strip search for those who are not particularly lean. Now this is quite intrusive, and some people would simply stop traveling by air. It would also require a certain type of security person to do the job correctly at low wage - exploring dusty crevices of human blubber with his/her fingers (indeed, the best candidates for this job might be in jail). But it's for the security of you and your fellow passengers, so you can't complain! Surely nobody can smuggle anything nasty aboard with these procedures. WRONG. How does contraband get into prisons? That's right, some of it travels in smooth containers shoved into body cavities. Every human has one such cavity, and ladies are blessed with a second suitable cavity.
2b. Soon enough, some terrorist passes through the scanner, with or without the extra grope-search, with one or more plastic containers concealed in a body cavity. These containers are retrieved in a washroom trip during the flight, but the terrorist is overpowered by fellow passengers as he/she prepares the nasty things contained therein at his/her seat, and before the nasty things can be used (assembled/detonated/activated/whatever). Note that I expect the preparation to be at the terrorist's seat on the plane, not in the washroom, since the real objective is to escalate the security procedures.
3a. Obviously, we need to add body cavity searches to the security procedures to prevent this sort of attack. Either it's full rubber gloves & lube, or an equivalent manufactured probe (presumably sterilized between uses), or some kind of ultrasound scanner that you can sit on (is ultrasound able to distinguish between a tampon/IUD and some other object?). This kind of examination would be required for every passenger. Of course, this would be an outrageous violation of civil rights, but how else to keep you and your fellow passengers safe from attack? After all, with this procedure added to the others, it would be impossible to get anything nasty on board an airliner! WRONG. How do high value drugs get smuggled via airliners? Some travels inside swallowed condoms, which would not be detectable via cavity search or ultrasound of the area around body cavities. Drug mules commonly swallow multiple condoms containing pure drugs.
3b. As soon as these procedures are widespread, some terrorist swallows one or more condom bombs before getting on a flight. There are numerous ways this could be arranged to cause an explosion or equivalent in-air catastrophe, which I will not go into. Here's one: two latex condoms are partly filled with binary components which will spontaneously ignite/explode on contact; they are placed inside a silicone condom, which also contains a small amount of a material which will corrode or dissolve latex; the terrorist swallows one or more of such silicone condom bombs. Note that in this method, no external device is required, and there may not be any indication that there is an explosion coming (unless the airline makes the terrorist puke via food poisoning). In this case, the other passengers do not overpower the terrorist, and the device presumably explodes (messily), but it is not certain that the airliner would be destroyed in every attempt. In some attempts the airliner would survive, albeit with a very messy and somewhat devalued interior, and the modus operandi would become evident, provoking further posturing by security agencies.
4. Citizens, rejoice! To protect you against terrorists who have swallowed bombs or other nefarious devices, new security procedures have been instituted. But what could they be? Any option I can think of (using existing technology) is appallingly intrusive and/or costly in time and money. Here are a few possibilities: expensive and time-consuming MRI scan of all passengers; compulsory stomach pumping; administer fast acting emetics and purgatives (and provide a sufficiency of toilets at security). The MRI scan would see most things, but not confidently distinguish among them: an apparent prosthetic implanted in the abdomen might need to be examined laparoscopically or even excised for proper identification. What of partly metallic implants to replace a damaged hip bone? Any sort of implant would completely pass by the stomach pump or emetic/purgative approach. And yes, there are ways to remotely detonate implanted explosives without using metals on the inside or outside. Short of MRI scans coupled with exploratory surgery as standard security procedures, a determined organization could still get a suicide bomber and bomb onto an airliner.
And where have we descended to on this slope? While airliners have been discussed in the steps above, other modes of mass transport and places of mass gatherings could also be targeted. Would trains and concert halls also be encumbered with such ludicrous pseudo-security antics?
I submit that there is actually no sure-fire defense against some forms of terrorist attack. In particular, it is impossible to prevent all possible ways to get a bomb or nerve gas on board an airliner. Note that this analysis even assumes that all of the security procedures are carried out competently, which is perhaps not the case. Protecting against a subset of those ways merely ensures that the unprotected methods will be employed. The steps outlined above can be viewed as a classical "arms race" between cannon and fortresses. The fortress never wins the race in the end, however many steps and twists the race contains.
I also submit that the best defense of an airliner is the self preservation instinct of its passengers (similarly for trains, concert halls, and so forth). By all means, employ the security procedures we had before 9/11, as they will catch the egregious large bombs and obvious suicide bombers. They were much less intrusive and much less expensive, and were a much lighter imposition on civil rights and freedoms. And that's what we should be preserving - our traditional liberties and civil rights, and the duties that accompany them. The slippery slope we are descending is rapidly eroding those rights and liberties, at great expense and without providing the security it pretends. It is also eroding what little is left of the notion of a citizen's duty to defend the society around him/her, replacing it with more encouragement to be a passive and powerless victim or bystander, while the "security" theater attempts to fulfill that duty on his/her behalf but inevitably fails.
This slippery slope of "security" theatrics leads nowhere that we should wish Western societies to go. It has us in lock-step with terrorists all the way, marching to the same destination; a destination of the terrorists' choosing. Every step down the slope is an act of surrender, including the steps already taken. How can that message be brought to those who are steering us downwards? And how can we reverse the process, and recover some of those lost liberties?