[ExI] The Fermi Paradox

Ben Zaiboc ben at zaiboc.net
Sun Mar 8 23:07:05 UTC 2020

On 07/03/2020 20:42, Robert G. Kennedy wrote:
>> If a shell of dense rock was constructed, and a civilisation located at
>> the centre (uploads in some nano-structured substrate, say), how thick
>> would the rock need to be? Can anyone do the maths on this?
> .                            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> Yep.  We did.
> Roy, Kenneth I., Robert G. Kennedy, and David E. Fields. 2009. "Shell 
> Worlds: An Approach to Terraforming Moons, Small Planets, and 
> Plutoids." Journal of the British Interplanetary Society, volume 61, 
> number 1, pages 32-38. January 8, 2009.
> Roy, Kenneth I., Kennedy, Robert G., and David E. Fields. 2013. "Shell 
> Worlds". Acta Astronautica, volume 82, issue 2, February 2013, pages 
> 238-245. Available online 2 October 2012, ISSN 0094-5765, doi:  
> 10.1016/j.actaastro.2012.08.034. Keywords: megastructures; space 
> colonization-industrialization; circumstellar habitable zones; 
> terraforming; extraterrestrial resources; SETI.
> Roy, Kenneth I., Robert G. Kennedy, and David E. Fields. 2013. 
> "Colonizing the Plutoids: The Key to Human Expansion into the Galaxy", 
> Tennessee Valley Interstellar Workshop 2013 Special Issue of Journal 
> of the British Interplanetary Society, volume 66, no. 10-11, Oct/Nov 
> 2013.
> Roy, Kenneth I., Kennedy, Robert G., and David E. Fields. 2013. "Shell 
> Worlds: The Question of Shell Stability and Other Issues", Journal of 
> the British Interplanetary Society, volume 67, no. 10, Oct 2014, 
> pp.364-368.

I think that's a different concept, as far as I can tell (any links to 
the above? or conclusions about the thickness of rock needed to shield 
delicate biological tissue or electronics from supernovae?)

The 'Shell World' concept seems to be to build a roof over an existing 
small planet. A roof to keep in atmosphere and keep out normal levels of 
solar and cosmic radiation would be far thinner than one needed to 
shield against constant and very high levels of radiation (high enough 
to sterilise entire biospheres from many light-years away).

I was thinking more of something like a hollowed-out asteroid, where the 
hollow interior is small compared to the entire volume of rock. As this 
is a far-future (I hope!) scenario, I wouldn't worry about things like 
gravity or simulations of gravity, maintaining high biodiversity, etc. I 
would expect that sapient beings would probably all be uploads or 
otherwise non-biological anyway.

The thing I don't know, is just how much radiation the shielding would 
need to deal with, and how thick the rock would have to be. The Greg 
Egan story I referred to ('Diaspora', available to read online at: 
https://bookfrom.net/greg-egan/55803-diaspora.html) has the catastrophic 
event as something that I assume is not reailstic (some unknown aspect 
of physics allowing the centre of the galaxy to collapse, spawning a 
big-bang-type event that releases enough energy that even atomic nuclei 
wouldn't survive, anywhere in the galaxy), but the type of thing that 
John Clark referred to is worrying enough, and real.

The kinds of civilisation that have been discussed on this list in the 
past (Dyson swarms, Jupiter brains, etc.) would seem to be very 
vulnerable to extreme high-radiation events. It would be carnage on a 
scale to dwarf anything this little planet has ever seen, or ever could.

Ben Zaiboc

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