Mounting mirrors on the Moon and using them to signal across space could let ET know we Earthlings are here.
Ever since radio broadcasts began we've been trumpeting our presence to nearby parts of the galaxy, so far without reply. To improve the chances of being found, Shawn Domagal-Goldman and Jacob Haqq-Misra of Pennsylvania State University in State College reckon we should cover half of the Moon with mirrors.
When angled to catch the Sun's rays, the mirrors would increase the amount of light the Earth-moon system reflects by 20%, they say, more than enough to catch the eye of a vigilant alien astronomer. Domagal-Goldman proposes using a code of prime number flashes - just as aliens used to get in touch in Carl Sagan's book Contact. This will ensure the flashes aren't mistaken for natural variations in brightness.
As a bonus, the underside of the mirrors could be covered ...
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