Well, I didn't notice the bit about a "noisy period" - I took the sphere to be the surface of a volume, not a shell. I think the idea was that once you start em

Well, I didn't notice the bit about a "noisy period" - I took the sphere to be the surface of a volume, not a shell. I think the idea was that once you start emitting detectable information, you don't stop. It's all hypothetical. All the stuff about probes: it doesn't matter whether the aliens are interested in talking to us. It doesn't matter whether they are interested in space and stuff. The authors define this sphere as the space wherein a hypothetical alien civilization COULD have detected our existence, and COULD have signalled or sent a probe in response. Also, they choose to define their sphere in terms of the beginning of the radio era. That's arbitrary. There might be other signals that planets carrying life emit, that we don't know about. SETI searches for radio signals like a drunk searches for his keys under the street-light. Note: I only read the abstract.