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On 07/03/2020 20:42, Robert G. Kennedy wrote:<br>
<blockquote type="cite"
cite="mid:mailman.86.1583613724.23370.extropy-chat@lists.extropy.org">
<blockquote type="cite" style="color: #000000;">If a shell of
dense rock was constructed, and a civilisation located at
<br>
the centre (uploads in some nano-structured substrate, say), how
thick
<br>
would the rock need to be? Can anyone do the maths on this?
<br>
</blockquote>
. ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
<br>
<br>
Yep. We did.
<br>
<br>
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.
<br>
<br>
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.
<br>
<br>
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.
<br>
<br>
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.
<br>
</blockquote>
<br>
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?)<br>
<br>
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).<br>
<br>
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.<br>
<br>
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: <a class="moz-txt-link-freetext" href="https://bookfrom.net/greg-egan/55803-diaspora.html">https://bookfrom.net/greg-egan/55803-diaspora.html</a>) 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.<br>
<br>
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.<br>
<br>
<pre class="moz-signature" cols="72">
Ben Zaiboc</pre>
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