A new study suggests an invisibility cloak can never hide an object from all observers.

The very best invisibility cloak would be able to hide an object from a stationary group of observers, but a secondary observation group moving in relation to the first would be able to see distortions.

Einstein's theories on space and time, called special relativity, help explain the impossibility.

Invisibility cloaks attempt to hide an object by redirecting light around the object. The fastest path between two points is the straightest. Light bent around an object would travel more slowly. Researchers say the time delay would cause visible distortions.

Today's invisibility cloaks typically avoid the time delay problem by hiding objects from just a single frequency of light waves. But these also have problems hiding an object if that object is on the move, thanks to the relativistic Doppler effect whereby light waves are bent by moving object.

The researchers -- physicists Jad Halimeh at the Ludwig Maximilian University of Munich, in Germany, and Robert Thompson at the University of Otago, in New Zealand -- also modeled a cloak that only hides the amplitude of light waves. These cloaks would only work if the object and observer were completely still.

As described by a phenomenon called Fresnel-Fizeau drag, light waves moving through a medium get dragged along by the medium. As the light wave amplitudes dragged, viewers would see distortions.

The researchers published their findings in the journal Physical Review A.

"Although our results may be disappointing for would-be wizards, understanding the limitations of cloaking devices is actually important in real life," Thompson said in a news release. "New technologies are beginning to emerge from cloaking research, and we're looking for effects that could either compromise the functionality of these technologies, or which could be exploited for some new practical purpose in the future."