The Crystal Spheres is a podcast on StarShipSofa. David Brin takes on a theme that occasionally crops up in SF and colloquial contexts--why are we alone? Why have no alien civilizations contacted us, or conquered us? Earth has been prime real estate for billions of years, and yet no credible evidence of visitation (UFOlogists aside) has been found. Life should be common, but it isn't. Stephen Baxter's Manifold Time touches on this as well--see my earlier review.
Brin's take is that Earth has been surrounded by an undetectable sphere, which we broke accidentally from within. We explored outward, and found all other habitable planets similarly protected, and we cannot break in. The sadness and loneliness of future humanity is palpable.
This one is going on my all-time favorite list--I highly recommend it. Four stars.
I also listened to Brin's Tojours Voir. A very tiny piece on epilepsy as a space travel device, cute if you're a Brin fan. I have seen this theme before, but cannot remember where--will catch you up if it comes back to me.