Re: Brown dwarves, deep future

On 2008-04-24, sigidunum@[EMAIL PROTECTED]
 <sigidunum@[EMAIL PROTECTED]
> wrote:
> Say we have a civilization that wants to last forever. One caveat:
> they want to live as physical beings on a planet around a star, not as
> downloads or gas clouds or such.

Planet around a star?  Sounds grossly wasteful.  Almost all of the
useful power radiates into space.


> Say they can harvest a billion brown dwarves from the galaxy --
> surely a very low estimate.

I'm sure it is very low indeed, by a factor of at least ten and more
likely a hundred.


> Say further that they can build a small main sequence star by
> slamming ten dwarves together; and that this star (likely an M
> dwarf) will have a lifespan of a trillion years.

Stars (even cool M dwarves) probably put out vastly more power than
the civilization needs to survive.  Just putting very rough numbers to
it: suppose they need 10^16 W.  From the brown dwarf stars, they've
probably got reserves of something like 10^39 kg of hydrogen, with a
fusion energy density of rather more than 10^14 J/kg.  In theory
that's enough for 10^30 years - on the order of a billion times that
of the M star method.


Though it's actually even better than that!  There will be plenty of
cold neutron stars lying around.  Those have an escape energy of
something like 10^16 J/kg.  So that means if they trickle just one
kilogram of hydrogen onto it per second, it will radiate roughly
enough power to supply their civilization's needs.  Better yet, it
should radiate mostly in visible light wavelengths.

They would want to redirect a large fraction of the light to the
planet, since a bare neutron star at tidally safe distances is a tiny
object in the sky.

Every 10^23 years or so they'll need to fetch (or make) a new neutron
star because the previous one is getting massive enough to collapse
into a black hole.  That's OK, they'll run out of unfused hydrogen
long before they would run out of neutron stars.


This approach should be good for something approaching 10^32 years.


- Tim