Re: PHYS: dyson shell thermodynamics

Amara Graps (
Fri, 24 Oct 1997 22:36:03 -0800

From: Wei Dai <>:

>Anders Sandberg gives a calculation of the thermodynamic efficiency of the
>dyson shell in his (draft) Jupiter Brain paper, and concludes that a
>smaller dyson shell is more efficient than a larger one. But I think his
>reasoning is slightly flawed. I'll show that the reverse is true, and that
>dyson shells may have a much lower temperature than previously thought
>(see for example
> which may
>explain why they have not yet been observed. Previous temperature
>estimates assume that the energy captured by dyson shells is also used up
>(dissippated) by them.
>However it
>turns out that the effect of the cosmic background radiation is not
>noticeable until we get to a radius of about 100 AUs. Ignoring the cosmic
>background radiation gives us Tl~=(S/(4*pi*r^2*sigma*Th))^(1/3). At 1 AU
>this is 161 K with efficiency of 97%, and at 10 AUs this is 35 K with
>efficiency of 99.4%. At 100 AUs, and taking into account cosmic background
>radiation, Tl is 7.6 K with efficiency of 99.87%.

My (educated) guess is that a Dyson sphere would show up as a very strange
looking object in the sky surveys such as IRAS, COBE, DIRBE (Diffuse
Infrared Background Explorer) satellite datasets. It would be especially
strange signal in the infrared: if one knew the object's distance very
well based on visual magnitude, but then found that its IR flux gave a
whopping signal compared to what you would expect.

The new temperatures that you gave, based on thermodynamic efficiencies,
are actually terrestrial dust temperatures. Those are the sort of
temperatures you would find for dust around infrared excess stars.
(I have a little bit of text about infrared excess stars at

For example, the four brightest infrared excess stars are
alpha Lyr, alpha PsA, beta Pic, and eps Eri. Those four have
dust temperatures (micron-sized dust around the star) of 85K, 60K, 110K, and
50K, respectively.

So I don't think that something like this would be invisible to
our current instrumentation.

However, I hope that someone is working with some of the better
datasets to look further for Dyson spheres. For example, the
Hipparcos data is publically available now, so one can use that
for finding positions of stars much more accurately than known before.
And the ISO, IRTS and other infrared observatories should give one better
infrared fluxes than the early-80s IRAS data. People who are looking
for planets (there is a mission under study now called the "Terrestrial
Planet Finder"), are already considering many of these issues,
that is, how to detect signatures of cool dusty material and other unsual
things around a star.


Amara Graps email:
Computational Physics vita: finger
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