Some things are impossible because they violate fundamental laws of the universe, as far as we know. The theory of relativity says that neither matter nor information can travel faster than light. Matter because an object reaches infinite mass at the speed of light. (Though the recent measurement of neutrinos apparently traveling faster than light remains to be explained, most physicists suspect it reflects a subtle error, not an overthrow of the theory of relativity.) Information because that would reverse the order of cause and effect for some observers, effectively enabling time travel and violating everything we think we know about how nature operates. Other things are impossible, or at least extremely difficult, because of practical or engineering limitations rather than fundamental ones.