Or if they entered a region of dust or gas that had a temperature differential to the sphere.
If fact, I just realized that if any such structures like Ringworlds existed, they would be trivially easy to spot if they were at the right aspect to us. It would look like a star with either a band across it, or like the star had a single ring. The ring on the far side of the star would also be tremendously bright (if angled to our solar system slightly) and much easier to spot with a telescope that blocked off the light of the central star.
Just an OT thought.
Stars are too bright and too far away and the ringworld too thin (at least I think there would be stability problems if they were built too wide, but I don't know enough). Worse, I thought the way we were trying to detect planets these days was by looking at the gravitational effect the planet has on the star. But wouldn't a ringworld balance out its own gravitation effect on the star?
An uplifting message as we enter the new year, quoted from Edge.org:
A few thoughts: when considering the heavy skepticism that the singularity hypothesis receives, it is important to remember that there is a much weaker hypothesis, highlighted here by Tegmark, that still has extremely counter-intuitive implications about our place in spacetime; one might call it the bottleneck hypothesis - the hypothesis that 21st century humanity occupies a pivotal place in the evolution of the universe, simply because we may well be a part of the small space/time window during which it is decided whether earth-originating life will colonize the universe or not.
The bottleneck hypothesis is weaker than the singularity hypothesis - we can be at the bottleneck even if smarter-than-human AI is impossible or extremely impractical, but if smarter-than-human AI is possible and reasonably practical, then we are surely at the bottleneck of the universe. The bottleneck hypothesis is based upon less controversial science than the singularity hypothesis, and is robust to different assumptions about what is feasible in an engineering sense (AI/no AI, ems/no ems, nuclear rockets/generation ships/cryonics advances, etc) so might be accepted by a larger number of people.
Related is Hanson's "Dream Time" idea.