[aidenn0]

Thanks. I think Clark overstates things a bit (unless you take a very extreme form of what "impossible" means), so I prefer my version. It also seems possible that engineers are more likely to hedge their bets by calling things "unworkable" or "impractical" because what actually is and isn't possible in theory is more of the realm of the researcher than the practitioner.

[inglor_cz]

A lot of the impossibilites are relative to the current state of the art.

"X is impossible with current metalurgy". "Y is impossible with current best integer factorization algorithms".

Absolute impossibilities do exist too, but you basically need to prove that that particular thing goes against laws of physics/maths.