A new study by researchers from the University of Munich and the University of Otago has found that invisibility cloaks that are effective while motionless become visible when they reach speeds of approximately thousands of meters per second due to their "operational frequency," which refers to the frequency of light that the cloak is invisible at. When at high speeds, the frequency of the light hitting the cloak is shifted, changing the light's operational frequency and thus causing it to become visible.
While the research shows the downsides of high speeds for invisibility cloaks, the team also showed that these harmful relativistic effects can cancel out when the light is moving at the right frequency in the right direction, meaning effective cloaking can still be achieved. However, during this situation, the cloaking becomes "non-reciprocal," which means that observers within the cloak can still clearly view people on the outside.
"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.
The study found that in particular, length contraction, which is when the moving object appears condensed to a non-moving observer, and time dilation, when time slows down for the moving object in comparison to the non-moving observer, are two of the main factors that lead to invisibility cloaks becoming visible at higher speeds. This is because each of these phenomena create a relativistic Doppler effect, which causes a frequency shift in the light through the motion of the source and the observer.
Despite these problems, there is always a way of restoring invisibility in these cloaks.
"This is achieved by giving 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."
Using this knowledge, scientists can use 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 findings were published in the Dec. 14 issue of Physical Review A.
