Interstellar Travel and the Light-Speed Barrier: Could UFOs Reach Earth?

To the point

Adam Frank notes that if UFOs were alien ships, travel would be limited by space’s vastness and the light-speed bound: a light-year would take about 20,000 years to cross, Alpha Centauri is a few light-years away (around 80,000 years at that pace), the Milky Way is about 100,000 light-years across, and nothing with mass can exceed light speed, so any interstellar visit must fit within known physics.