Re: Where are they?

carl feynman (
Tue, 25 Nov 1997 11:01:36 -0500

At 08:12 PM 11/21/97 -0800, Damien R Sullivan wrote:
>On Nov 21, 9:16am, Leevi Marttila wrote:
>} Where is missing mass in the universe?
>} If there are other intelligent creatures, where are they?
>} Maybe these two questions are related...
>I've wondered about the intergalatic bubbles: if you had a soup of
>galaxies, and randomly seeded it with rare expanding intelligence, would
>you get the bubbles we observe, with visible galaxies in the areas
>between colonized volumes? I have trouble checking the idea against the
>fact that we're seeing everything in the past, though, and against the
>flux in the old background radiation.

I spent a while looking into this last year, and, as is my wont, never
wrote anything down. Let's see what I can remember...

There are walls between the bubbles, whose thickness is small compared to
the size of the bubbles. This is inconsistent with the notion that life
originated at random times and places and spreads spherically, since in
that case we would observe that some bubbles had run into each other, while
others were small and in the middle of large 'uncolonized' areas. However,
if civilizations for some reason like to leave 'neutral zones' between
them, then this would explain the observed pattern. In this case, we are
in a neutral zone, and can expect to be swatted down as soon as we become
politically unacceptable (:-).

The bubbles are not completely empty. The contain the occasional galaxy.
This is a small problem for the theory.

The stars in the dark zones cannot simply have been Dysoned, since their
waste heat would be clearly visible to some kind of telescope, regardless
of its temperature. The bubbles contain 90% of the volume of the universe,
so if the universe was uniform, 90% of the stars would have been Dysoned.
Alas, the spectral region between 10 microns and 1000 microns (300K to 3K)
where the Dyson spheres would radiate does not contain 10 times as much
light as is emitted by all visible stars. Also, the bubbles should be
visible as large infrared blobs, and they're not.

If the Dyson sphere heat radiators are built of solid hydrogen, they could
be big enough to actually be opaque on a galctic scale, and would have a
radiation temperature on the order of 5K, which would be close to a
thermodynamic optimum. Unfortunately, the bubbles would have clearly
visible effects on the cosmic background radiation.

If the civilizations that built the bubbles wanted to conserve energy for
the longest possible time, they might have modified the stars in their
domains to make them burn more slowly. In this case, the objections of the
previous two paragraphs vanish.

If a civilization is expanding at a speed comparable to that of light, we
would see their frontier as an ellipsoid rather than as a sphere. The long
axis of the ellipsoid would be pointed toward us, and the closer the
conquest velocity was to lightspeed, the more elongated the ellipsoid.
This isn't really an issue, since the 'neutral zone' hypothesis requires
that all expansion was complete a long time ago.