[ExI] The Great Silence again

[Keith Henson]

On Sat, Apr 30, 2011 at 10:00 AM, Stefano Vaj <stefano.vaj at gmail.com> wrote:
> On 30 April 2011 17:38, Anders Sandberg <anders at aleph.se> wrote:
>> One problem I see with our starmoving scheme is keeping the Dyson cloud in
>> orbit. When it reflects the star's light it will also tend to expand/move
>> ahead, and we need to restore it to it's original position relative to the
>> star. That seems to require some work.
>
> Shouldn't a relatively rigid, highly symmetric Dyson sphere stay in
> position even without moving, since both gravity and light pressure
> would work equally in all directions?

For a non rotating Dyson sphere that's thin enough the light pressure
would balance the gravity from the sun.  The light pressure at the
distance of the earth's orbit is around 9.3 N per square km.

The force on this square km from solar gravity at the distance of
earth is (solar mass) x mass of a square km x G/distance to the sun
squared

Thus 9.3 N/square km = 1.9891×10^30 x 6.674×10^-11 x m / (1.496×10^11)^2

Solving for m, m = 9/(1.9891×10^30 x 6.674×10^-11 / (1.496×10^11)^2)

m = 9/(1.9891×10^30 x 6.674×10^-11 / (1.496×10^11)^2)

m - 9/10^-3 x (1.9891 x 6.674× / (1.496)^2

m = 1568 kg/square km.

or .001568 kg/square meter.

Aluminum has a density of 2700 kg/cubic meter so if the shell were
made of Al, it would be .001568 kg/m^2/2700 kg/m^3 or 5.8 x 10-7
meters thick.

Or 580 nm thick.

That's about the thickness of a soap bubble.

It checks with what I remember of Eric Drexler's work on aluminum high
performance light sails that were around 200 nm thick.

Keith



> I have always wondered, however, if such a sphere built by the
> re-arrangement of the non-stellar matter of our system would find its
> internal surface at a distance from the sun suitable to life... I have
> no idea, also because I expect that the internal temperature of a
> Dyson star would probably be fairly different from that of a planetary
> surface at the same distance...
>
> --
> Stefano Vaj
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