Re: Possible solution to Fermi's Paradox?

Robin Hanson (
Wed, 03 Mar 1999 14:07:14 -0800

[Sorry for the late response to this and other threads. I was overwhelmed by the job market & allowed inboxes to pile up. Hope to catch up soon. RH]

James Annis' article, "An Astrophysical Explanation for the `Great Silence'" JBIS 52:19-22 1999, is thoughtful and interesting. Gamma ray bursts do seem a plausible component of the total
"great filter" that stands between dead matter and expanding
lasting life. Some critiques, however:

  1. It's a relatively weak effect. We're talking a typical impact across a galaxy of "over a few seconds, and all in 300 Kev Gamma-rays, the equivalent of 1/10 the solar flux." This won't directly hurt life under the sea or under rocks, or on the other side of a planet. Nor would it kill advanced space traveling civilizations. Speculations about "disruption of the ozone layer" are as specific as Annis gets about how this flux kills all land life.
  2. This model requires that evolution on land (under an atmosphere nearly as weak as ours) be required for evolving advanced civilizations, and that the time for land creatures to evolve to civilization be independent of how evolved sea creatures are.
  3. Annis gives us no calculations to help estimate how big a part of the great filter this could be. He assumes the burst rate has been falling exponentially with time, with a decay constant of 2.5 billion years and with the current rate averaging one every 220 million years per galaxy. And then he is content to just note that land evolution is about 270 million years old on Earth.

But clearly if life appeared quickly in most every system, and if civilizations expanded at .1c, then most of the universe would now be colonized if gamma ray bursts were the only problem. (A simulation could be constructed if anyone doubts this.) Thus this can't be the whole filter. So how much of the filter can it be?

Btw, I actually found Annis' *other* paper in the same issue to be more interesting. That is:

"Placing a Limit on Star-Fed Kardashev Type III Civilizations"

James Annis JBIS 52:33-36, 1999.

A civilization that uses the bulk of a galaxy's starlight for its own power requirements is a remarkably obvious entity. This is because galaxies naturally obey scaling laws between the surface brightness, the radius of the stellar distribution and the thermal velocities of the stars. The latter two quantities reflect the mass and its resulting gravitational potential, while the formal is the result of this same mass emitting light as stars. If civilization takes that light for its own purposes, the scaling laws are broken and the galaxy becomes an outlier on plots showing the scaling laws. For a sample of 137 galaxies, no such outliers are found.

Robin Hanson   
RWJF Health Policy Scholar             FAX: 510-643-8614 
140 Warren Hall, UC Berkeley, CA 94720-7360 510-643-1884 after 8/99: Assist. Prof. Economics, George Mason Univ.