alt Hacker Newsthere is actually a science change that happened, and it's not (entirely) just politicians changing their mind.
The big thing going from X-ray (2d) to CT (spin an X-ray machine around and take a ton of pictures to recreate a 3d image) did a lot to let security people see inside of a bag, but the hitch is that if you see a blob of gray is that water, shampoo or something else?
The recent advance that is letting this happen is machines who will send multiple wavelengths of X-ray through the material: since different materials absorb light differently, your machine can distinguish between materials, which lets you be more sure that that 2litre is (mostly) water, and then they can discriminate
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There's a whole ton of people taking about MRI -- MRIs are a completely universe than CT/X-rays
it has been such a godsend flying out of Frankfurt where they have the new scanners and you don't have to empty out your bag anymore. So much smoother. Then I fly back and get all annoyed at the other airports. I was told Oslo airport is holding out until it becomes regulation to use the new scanners. Security-Theater is still what it is. It is super weak imho, despite never having seriously attempted a heist or trying to get contraband on a plane. I miss the good old days where you handed your luggage to a guy just before boarding the plane.
Dual energy x ray has been around forever though, like decades.
These machines don't really detect what kind of materials stuff is composed of, much of that is just a crude classification based on density. True identification requires broadband x-rays emission with spectral analysis.