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Researchers from the University of Munich and the University of Otago have found that invisibility cloaks that are effective even while they are motionless are able to become visible only when they touch speeds of about thousands of meters per second due to their "operational frequency." This is the frequency of light at which the cloak is invisible. At high speeds, the frequency of the light hitting the cloak moves, which changes its operational frequency and makes it visible.

Even as the study exhibits the "downsides of high speeds" for invisibility cloaks, it also displayed that such "harmful relativistic effects" get cancelled out if the light moves at the "right frequency in the right direction". Hence, the effective cloaking is possible.

In such a situation, the cloak enables people inside to clearly see people outside. Hence, it is "non-reciprocal."

"The non-reciprocal cloaking is actually quite an interesting feature, as people have been investigating it for quite some time in the context of transformation optics. Here, this effect just came about from the physics - we didn't really design it," Jad Halimeh, co-author of the study, said in a press release.

Length contraction takes place when a moving object gets condensed to a non-moving observer, and time dilation is what happens when time slows down for a moving object, compared to a non-moving observer. These two factors tend to indicate invisibility cloaks that become visible at high speeds.

Both the phenomena make up a relativistic Doppler effect, leading to a frequency shift in the light "through the motion of the source and the observer".

Still, invisibility can be restored to the cloaks.

"This is achieved by giving the light of some frequency the right direction such that the Doppler shift is canceled," Halimeh said. "The relativistic Doppler shift can be seen as an admixture of a longitudinal Doppler red-shift and a transverse Doppler blue-shift. For a given frequency, you can tune the direction such that these two shifts cancel each other out and the cloak ends up seeing the light as having its operational frequency, thereby cloaking it."

Scientists can thus harness the infinite combinations of "light frequency and direction" to cloak anything at any relativistic velocity.

"We are interested in such research for two reasons," Halimeh said. "First, it allows us to further investigate whether or not these invisibility cloaks are electromagnetically equivalent to vacuum. Second, it allows us to better understand the limitations of these cloaks when it comes to a relativistic world, where we have fast-moving objects that we need to cloak or cloak ourselves from, or in case we are in a strong gravitational field, for example, and we wish to have a cloak that works there as well."

The study was published in Dec. 14,2015 issue of Physical Review A.