# [extropy-chat] What if Dyson spheres are obsolete? (was the Drake Equation)

Eugen Leitl eugen at leitl.org
Thu Oct 26 06:55:59 UTC 2006

```On Wed, Oct 25, 2006 at 10:38:23PM -0700, The Avantguardian wrote:

> I did a back of the envelope calculation and given the
> approximate gravitational binding energy of the earth
> as (3/5)GM/r, it would take 2.745 E 32 joules of
> energy to form a Dyson at 1 AU out of the earth. This

I don't know how you did the calculation, but I would
account of lessened gravity with each incremental bucketload
lifted. Also, you need a lot less than Earth mass (I
haven't checked, but Robert thinks mass of Mercury is
enough), the lower cloud boundary could be easily in
800 K range (SiC does fine at that temperature), and
the cloud doesn't have to be completely opaque.

Also, check out the Moon, Pallas, Ceres and consorts.

> is very close to the energy it would take to scatter
> the atoms of the earth to infinity. (Actually some new
> math I have been working suggests it would take more
> than this but I don't have time go into this here.)

This result strikes me as somewhat implausible.

> This is equal to one whole week of total solar output.

Of course you do know that we've got a fully stocked asteroid belt,
right? The math works out very differently if you can jump
into orbit, even in a space suit. There, you don't launch.
You just extrude stuff into infinity.

> This suggests one would need a Dyson sphere in
> operation for a week to construct an equivalent Dyson
> sphere. This would seem to indicate that any
> civilization CAPABLE of building a Dyson Sphere would
> have little use for one.

The sun is dumping 2 MT of matterenergy/s into space.
Most of it goes down the drain, but orbiting material
can't help but intercept it. It's 1.3 kW/m^2 hereabouts,
and I'm sure you can work out the growth of the launch
rate (actually, simple aramide is good enough for a tether)
starting from a km^2 of lunar PV array driving a
mass driver launching PV few um thick.

Just for the same reason, you *can* make PV arrays on
earth, even though with current primitive technology it
takes several years to recover the energy for their
production.

> Just my two cents.

--
Eugen* Leitl <a href="http://leitl.org">leitl</a> http://leitl.org
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