It’s said that Enrico Fermi wondered why we didn’t see signs of aliens since nuclear energy made interstellar travel possible. It’ s a worthwhile question and one which has been answered in a dizzying number of ways. Here’s my $0.02 worth…
Personally I think there’s two likely equilibrium states that answer the Fermi Paradox. Either we’re in the pre-Colonization era, before anyone ventures forth and colonizes the lot, due to Life starting late in the Universe’s life because of some recently changed astrophysical process.
Or the Galaxy has been colonized, in which case two sub-divisions seem reasonable:
(1) We’re an undeveloped patch missed by the last few waves of colonization as per Geoff Landis’s Percolation Theory.
(2) Or we haven’t been missed and we’re colonized, but just not how we usually imagine.
Consider: There’s so much room in the Edgeworth-Kuiper Belt, or the Opik-Oort Cloud, and if ETIs exist as star-travellers then that’s where they are. While the inner system seems warm and inviting, perhaps they avoid it because of coronal mass ejections, UV or solar flares or some such. Remember they’re used to living in space in their own habitats, not on wild, native planets.
Perhaps there’s a whole lot of deuterium lying around ready to be scooped up from the surfaces of cold Plutoes or Marses that might be Out There. Imagine a Mars mass planet that has chilled to ambient temperatures and all the hydrogen has frozen out – its atmosphere would be helium! Makes getting at the He3 easier. Assuming it formed with a primordial atmosphere that is, which is a fair assumption for the Outer Solar System.
Even if they only use fusion for propulsion and they use solar for life-processes, there’s sufficient materials for constructing huge solar collecting mirrors (“sollectors”) in most average sized cometoids. A sufficiently efficient closed-loop ecology can be sustained on a lot less sunshine than we experienced here on Earth.
Just how efficient is the Earth’s biosphere? About 100 billion tons of carbon is fixed by primary production in the land and sea biospheres – about 250 billion tons of carbohydrates – per year. If the energy content is ~480 kJ/mole and a mole is about 0.03 kg, then the total energy stored by the biosphere is ~4,000 exajoules per year. Earth absorbs from the Sun about 3,850,000 exajoules per year, so the biosphere captures and stores just ~0.11% of that energy torrent. So, in theory, we could get by on less energy than what drenches the Earth currently.
Of course a biosphere would have to stay warm, but with sufficient insulation the heat-loss can be brought into line with the biosphere’s own waste-heat, so the habitat is not radiating as a black-body at the biosphere’s preferred temperature, but at some lower value in line with its actual heat use. But I’ve covered similar ground in a different post. The main thing is that ETIs can lay-low in the OOC, if they so wish. And we’re not the cosmic backwater that Fermi scepticism might lead us to think…