there 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
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