Fast-moving invisibility cloaks become visible

(Phys.org)—Physicists have found that invisibility cloaks that can achieve perfect invisibility when not in motion will become visible when moving at speeds of around thousands of meters per second. This is because invisibility cloaks appear invisible only to light at a certain frequency called the cloak's "operational frequency." When the cloak is moving at high speeds with respect to an observer, relativistic effects shift the frequency of the light arriving at the cloak so that the ligh