Scouting the spectrum for interstellar travellers
  Advanced civilizations capable of interstellar travel, if they exist, are likely to have advanced propulsion methods. Spaceships moving at high speeds would leave a particular signature which could be detected from Earth. We propose a search based on the properties of light reflecting from objects travelling at relativistic speeds. Based on the same principles, we also propose a simple interstellar beacon with a solar sail. 
Juan Carlos Garcia-Escartin and Pedro Chamorro-Posada
physics.pop-ph physics.space-ph