With the exception of the last one, banning box cutters, requiring people to take off their shoes, and limiting explosives are actually quite reasonable for security. Current detection technologies can't really detect explosive substances, except for the X-ray machine, but the best it can do is tell you the density of the materials in question, not necessarily what type of explosive it is. So running your shoes through the machine makes sense, as enough dangerous solid explosive can be packed into the sole of a shoe.

Liquid explosives are extremely unstable. The explosive power is probably not enough to blow up a plane, but it can definitely injure or kill many people or set fire to the plane. Plus, no machine exists to distinguish explosive liquids from non-explosive liquids at the scale that an airport requires.

For the curious, I'll just briefly list some of the detection systems that are out on the market, and what their capabilities are:

- Metal detector

- X-ray machine - Densities of materials

- Ion Mobility Scanner (eg. Puffers or Swipers) - Trace explosives detection, but easily confused by other nitro compounds such as hand cremes, fertilizer, and uh..jet fuel.

- Millimeter Wave Scanner (eg. Whole Body Scanner) - Anomaly detection (eg. can penetrate clothing), but cannot identify explosives.

- Terahertz Imaging - Can penetrate clothing and can identify materials, including explosives, but can be blocked by water and hasn't reached commercialization.

- Other wavelength imaging - Our group uses UV light and can detect trace amounts of explosives, but does not penetrate clothing. It's only a surface technique.