two hundred light years

We may be only one of millions of advanced civilizations. Unfortunately, space being spacious, the average distance between any two of these civilizations is reckoned to be at least two hundred light years, which is a great deal more than merely saying it makes it sound. It means, for a start, that even if these beings know we are here and are somehow able to see us in their telescopes, they’re watching light that left Earth two hundred years ago. So they’re not seeing you and me. They’re watching the French Revolution and Thomas Jefferson and people in silk stockings and powdered wigs – people who don’t know what an atom is, or a gene, and who make their electricity by rubbing a rod of amber with a piece of fur and think that’s quite a trick. Any message we receive from these observers is likely to begin ‘Dear Sire’, and congratulate us on the handsomeness of our horses and our mastery of whale oil. Two hundred light years is a distance so far beyond us as to be, well, just beyond us.

From A Short History of Nearly Everything
by Bill Bryson