I never quite understood the security theater thing. Isn’t the fact that at each airport , you will be scanned and possibly frisked a deterrent and you can’t measure what dissent occur so the only way to know if it works is observe a timeline where it doesn’t exist?

For one thing the rules adopted vary and different countries do very different things. It struck me once on a flight where at one end liquids were restricted, but shoes were not checked, and at the other we had to take our shoes off but there were no restrictions on liquids.

So an attacker who wanted to use a shoe bomb would do it at one end, and one who wanted to use liquids would do it at the other.

There are also some very weird things like rules against taking things that look vague like weapons. An example in the UK were aftershave bottles that are banned - does this look dangerous to you? https://www.fragrancenet.com/fragrances?f=b-spicebomb

Then there are things you can buy from shops after security that are not allowed if you bring them in before (some sharp things). Then things that are minimal threats (has anyone ever managed to hijack a plane with small pen knife? I would laugh at someone trying to carjack with one).

> know if it works is observe a timeline where it doesn’t exist?

Absolute proof maybe, but precautions need to be common sense and evidence based.

>has anyone ever managed to hijack a plane with small pen knife?

Well, the 9/11 hijackers used box cutters. Might as well be the same thing.

A very small grenade, made of glass, and fill with liquid?

It looks like a grenade in the same way a doll looks like a human being.

In full color vision sure, but not to the machines used to scan the insides of bags. You pretty much just get a silhouette.