According to special relativity mass tends to infinity at light speed. This is quite valid when the exhaust velocity is small compared to the speed of light which is the case for ion thrusters made to date exhaust velocity is on the order of 10 4 c. Light travels at a blistering 670 million mph a speed thats immensely difficult to achieve and impossible to surpass.
So if it takes you less than half a second to move your hand that distance then that shadow will have broken the speed of light.
Quantum Xchange has developed the first fiber optic network in the US. Scientists suspect magnetic reconnection is one way that particles are accelerated to nearly light speed. Whereas the Large Hadron Collider accelerates particles here on Earth up to a maximum velocity of 299792455 ms or 99999999 the speed of light cosmic rays can smash that barrier. According to special relativity mass tends to infinity at light speed.