Sub-Light Speeds

Anderson's Future Histories That Presuppose Only Slower Than Light Interstellar Travel

The two Kith Histories

The Rustum History

Tales Of The Flying Mountains

The Harvest Of Stars Tetralogy

The Boat Of A Million Years

Genesis (only post-organic intelligences fly between stars)

How much inter-species communication is possible at sub-light speeds? If there is a technological civilization twenty thousand light years away, then it cannot detect our present civilization because it can see Sol only as it was twenty thousand years ago. At that distance, would it be able to detect a Solar planet, then oxygen in the atmosphere, vegetation on the surface and artificial light on the night side? (Not that there would have been artificial light back then.) If there is going to be a technological civilization at that distance twenty thousand years hence, then it might with difficulty detect our present civilization. Our brief burst of radio signals - now being replaced by fibre-optic cables? - would be weak and swamped by other cosmic noise. But, if such a civilization were to launch an expedition towards Earth, then, at sub-light speeds, that expedition would arrive here well over forty thousand years in our future. Would such an expedition even be launched?

I am summarizing from memory arguments presented by Brian Cox.

Starfarers is Poul Anderson's approach to the concept of detecting an extra-solar civilization and launching a slower than light expedition to it.