From: Hal Finney (hal@finney.org)
Date: Fri May 17 2002 - 11:31:00 MDT
I never understood thermodynamics that well, but something bothers me
about this stealthiness concept.
Ignoring black holes for the moment (which are super-cold), a star is
radiating a certain amount of power into an artificially constructed
shell which surrounds it. It seems to me that in equilibrium, the same
amount of power has to be radiated from the shell.
Equilibrium in this context means that the shell is just rearranging
matter. It is not absorbing energy by storing it into batteries or the
equivalent, because that obviously cannot go on forever. All the energy
the shell "uses" is ultimately dissipated as heat and therefore radiated.
So in practice the same amount of power comes from outside the shell
as from the star, and the only difference is that because the shell is
larger, the temperature is less. The outgoing shell temperature depends
solely on the temperature of the star, and the ratio of the size of the
shell to the size of the star.
I don't see where "cooling" comes into play here, except that you could
redistribute the outgoing radiation to be stronger in some directions
and weaker in others. But it seems like for a given star, the radiated
temperature will depend solely on the size of the shell, and you can't
cool it.
Now, adding black holes changes the equation, because they are effectively
black bodies at the Hawking temperature, which will probably be very low.
So if you can get your black hole to absorb large quantities of energy,
then you can reduce your radiation.
Beaming power back at the star, from this perspective, amounts to
reducing the net amount of power radiated by the star. It is equivalent
to the star being partially shut down and radiating less. But you could
accomplish the same thing by building your shell around a cooler star,
so it's not clear that this accomplishes anything.
One other idea, based on what I wrote above, is to store much of your
excess energy. I said it couldn't go on forever, but theoretically you
could convert the energy to matter/antimatter and it might be practical
to store it almost forever. So this is one other kind of cooling that
could work.
Hal
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