The duo suggests that alien-like ships traveling at relativistic speeds can easily intermingle with photons in the cosmic microwave background, which is dubbed CMB. This means that a spacecraft traveling at near light speed would leave a unique signature, and this means it would therefore be fully discoverable. Their research, which was just published in this month's MIT Technology Review, points out that the interaction with photons in the CMB “should create a drag that imposes specific limits on how fast spacecraft can travel.” And, “it should also produce a unique signature of relativistic spaceflight that ought to be visible...