Hi All

I agree entirely Mark. The "Star Trek" galaxy is utterly implausible, but I wasn't factoring in things like warp-drive, more technologies like fusion rockets, laser-sails and the like. I'd be very surprised if civilizations were closer together in time than ~ 10,000 years. Since it seems to me that city-building could have occurred anytime in the last ~ 100 - 50 thousand years and might therefore last about as long again, then civilised species might last ~ 100 - 200 thousand years or so. But the time of low GRBs or whatever may have allowed a spread of appearance of complex life spread over the last 0.1 - 0.5 Gigayear.

Co-existence of intelligence will be a narrow window in time.

Adam