The first one has an excuse. Barely anybody knew you had to actually be accurate at the time, they just thought it would be cool to not be seen on the radar.
If we are going to have an actual arguement on the actual theory behind tachyon then ok.
Tachyon is technically a particle that is said to travel and relay information faster than the speed of light. It is true in theory that the tachyons speed increases as the energy of the particles decreases though. Since the particles move at such high speeds and are akin to that of light they are considered to highly condensed energy.
In movie terminology tachyons are viewed as merely pure energy that is used to disrupt the barriers between dimensions.