From: Spike Jones (spike66@attglobal.net)
Date: Mon Dec 11 2000 - 22:31:51 MST
> >Spike Jones wrote: So that puts an upper bound
> >of about 200 nanograms of interstellarlopers per cubic
> >kilometer. Right? Amara? spike
>
> Amara Graps wrote: My dear Spike, you are the third person who has asked
> me that question in the last several months (the other two being Eric
> Drexler and
> Robert Bradbury).
WOW cooool, Drexler, Bradbury, spike. There are three names I
like to see mentioned in the same sentence.
> You all have the same goal in mind, I know. Your
> approach makes perfect sense to me ... but ..
Today I was doing some back of the envelope calcs and I think
I know why the other two wanted to know. If interstellar matter
is as dense as 200 nanograms per cubic kilometer, then crossing any
interstellar distance in one human lifetime will be extremely difficult
if not impossible.
Heres how I figure: an astronaut is usually at least 30 and human
life expectancy is about 75, so 45 years to cover... lets see, the
nearest star is about 4 light years and some change I think, so
about 0.1c to get there in a lifetime, neglecting acceleration and
deceleration phases.
Assuming away the inherent difficulties in actually pushing a
vehicle to that speed, the erosion as a function of interstellar
matter becomes a hell of a problem. I did a calc today assuming
an iceberg shield up front, as in Clarke's Songs of Distant Earth.
Assuming that we start with the iceberg at about 10 Kelvin
and that the energy of collision with interstellarlopers provides the
heat of fusion and vaporization to the water molecules, then
also provides the heat of dissociation for the hydrogen
molecules and the oxygen, plus complete ionization to plasma;
even assuming all that energy is taken from the transfer of
kinetic energy from the interstellar matter, if the overall density
is as high as 200 nanograms per cubic kilometer, we will still
need an iceberg shield many kilometers thick, maaaaany
kilometers, to erode away during the trip.
Before I post my calculations, I want to let others try the calcs
independently, and I do hope I did something extremely wrong.
Some of the oddities: I found that the thickness of the shield
required is directly proportional to the velocity and directly
proportional to the density of interstellar matter. Secondly and
even more disturbing, I calculated that the actual makeup of
the interstellar matter makes no difference: stray helium and
hydrogen are just as erosive as an equal mass of dust particles,
and I fear there is a lot more of that stuff than the
interstellarlopers. I do hope the overall density of stuff
between stars is way less than my estimated upper bound
of 200 nanograms per cubic kilometer.
The erosion rate I got for .1c was on the order of a cm a day,
using some admittedly rough estimates.
Nowthen, this whole exercise may explain some things,
specifically why humans have a history of low technology.
Ever wonder why we can pick up a history book and
read that a few centuries ago we didnt have machines?
Funny question?
Assume away all interstellar matter. Then intelligent life
would have evolved somewhere and built spaceships and
crossed the gulfs between stars and seeded itself everywhere
in the galaxy, either by going really fast or travelling really
long. Within a few short eons, the first intelligent
species would arrive on various planets, teraform
and start out with a culture of technology. Right?
Evidently humans did not start that way. We have a history
of low technology. Why? The dust calc would offer an
explanation: that interstellar space is *inherently* difficult
to cross. If there is no unknown technology {such as
opening a temporary wormhole}, then we are left with
two technologically daunting paths for crossing interstellar
space: either 1) multigeneration ships far from any energy
source, or 2) very high speeds, in which interstellar dust
and gas would grind our ship to powder. So then crossing
the starry gulf is *inherently difficult* even for the very
most advanced races, and consequently no one has done
it ever, and so humans have had time to evolve technology
the old fashioned way.
Amara thanks for the references, you are too kind. spike
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