From: Spike Jones (spike66@attglobal.net)
Date: Sat Jan 12 2002 - 12:27:24 MST
"Robert J. Bradbury" wrote:
> The critical point Spike makes is the fact that stellar
> civilizations don't go anywhere "quickly". There is a
> big difference between "colonizing" at a milli-c vs. 0.1 c
> (a figure typically used for interstellar travel).
Right. But sci-fi writers want to spin yarns on time scales
that will hold the interest of non-uploaded humans. This
puts too many unnecessary restrictions on our thinking.
Last time Robert was here, we did a series of calculations
that I tried to recall last night, but I dropped three orders
of magnitude. I just redid those calcs. First I will go into
more detail on how an advanced civilization could drag
along its entire star and why that would be a good idea
to do so.
Imagine a star with a flat ring of material in orbit (a bit like
the planet Saturn) and assume the mass of the ring at one
milli-star. Now assume the ring made of individual particles
that are capable of reflecting the photons from the star in
any direction they want. Now imagine the star bisected
by a plane, such as the table top at which you may be sitting,
and the ring in that plane. If every particle in that ring reflects
the photons downward, then the entire ring is lifted slightly
above the plane of that table. Even tho the individual particles
no longer pass thru the equatorial plane of the star, the orbit is
stable. The mass of the ring exerts a slight gravitational
attraction on the star, gently tugging it in the direction opposite
the way the photons are deflected. The star/ring system becomes
an enormous photon driven spacecraft.
Next time Rob Bradbury suggests taking the solar system
apart and making an MBrain, recall that the mass of everything
in the solar system outside of the sun is not far from one
milli-sun, ignoring the hydrogen and helium, which likely
cannot be used.
Now realize that if the particles can reflect and can control
their attitude, they can adjust their orbits over time. This can
be done without violating any laws of conservation of angular
momentum. If this is not clear how to tip an orbit using only
light pressure, ask and I will go over the reasoning. The rings
that exist even in the plane of the direction of motion would
have an offset CG, and would contribute to tugging the star.
If the individual particles can adjust their orbits, both radius
and inclination, I can imagine no reason whatsoever why
in principle an MBrain could not be constructed from the
planets. Nearly every photon that leaves the sun could
be reflected in the same direction, making the star system
a maximum efficiency photon propelled rocket.
Now, here is where I dropped 3 orders of magnitude last
night. I am pulling all the following numbers out of biomemory,
so if any of them are way wrong, do correct me:
At one earth radius, 1.5E11m, the solar insolation is about 1340
watts per square meter, (I think) and the energy of each photon is
h*nu and the momentum of each photon is h*nu/c. So for each
photon deflected, the h*nu cancels, so we can take the total area
of the ring, estimate the energy that falls upon it and divide by c
and that is the momentum transfer. Assuming an MBrain covers
the entire surface of the star, everything cancels and we find that
the thrust available is the total energy the star emits over c. This
relation works regardless of the radius at which the rings of
material exist, so we could use nested rings of individual orbitting
particles, Bradbury style.
Examples of calcs on this:
One AU = 1.5E11 meters,
Area of a sphere at that radius = 2.8E23 m^2
Solar energy that fall on that sphere = 3.8E26 Watt
Force exerted by momentum of all those photons = 1.3E18 newtons
Mass of the sun = 2E30 kg
Maximum acceleration of the sun possible = 6.3E-13 m/sec^2
or about 2E-5 m/sec/yr
or 630 meters per square year, if you prefer to think in square years.
Last night I was thinking it was about 3 cm/sec per year, but turns out
its only 20 microns per second per year. But whats 3 orders of magnitude
between friends? Furthermore, whats the hurry? We need not suffer
any discomfort on this trip to the nearest star, which if I recall is about
4 light years away, or 2E16 m, so the trip will take about 11 million
years,
or about 16 million if you want to reverse thrust and actually stop once
you
get there. The nested shells of individual particles could be arranged
well
outside the earth's orbit, if we wanted to keep this planet for a wildlife
refuge, historical site, etc. The only change the earthbound observer
would notice would be a total absence of stars.
Does 16 million years seem too long for a trip? How old will we be
in 16 million years if we dont go?
> You can go quickly in a small ship --
But why suffer the discomfort of a small ship? Whats the hurry?
The sun wont significantly change in that much time.
> You have to keep in mind the problem of the lack of "current"
> information. ...We are ~24,000 l.y from the center,
> never mind the other side.
Patience, Grahsshoppah...
> You can never know what the "reality" is outside of a very
> local region of space. So chasing after a star 10 l.y. away
> may make sense.
Sure. Then when we get there, we take that star and
start chasing the next one.
> Chasing after a star 100 l.y. away is iffy.
> Chasing after a star 1000 l.y. away seems really silly --
> you have no way of assuring it will still be there when you
> finally arrive.
But new ones may be born by then. Each time you are
always chasing the nearest one.
> One point that wasn't clear to me from Spike's post was
> what the point was of going to another system. You can't
> harvest material from the stars quickly. So the only thing
> you can do is rip off the planetary material.
Or make new, by merging the two stars to catalyze a
supernova, which would create new metals. I can imagine
the universe has plenty of energy but is metal-starved.
> It looks to me like advanced civilizations should be able to
> detect MBrains. You can see them either by their microlensing
> signatures or slight variations in the background temperature.
But sufficiently advanced MBrains might not let very many
photons leak out, or may intentionally mask them for fear of
some crazy yahoo coming to merge her star with yours, hoping
to create a supernova.
> The maximum amount of material - millions of stars worth is
> going to be in large gas clouds (the question is is it cheaper
> to collect it there than extracting it from a stellar gravity well).
The problem I see is that those clouds are nearly all hydrogen
and we want metals. Now if you can propose a way to fuse
hydrogen and be able to keep the remaining material, without
using up too much of the scarce metals we already have...
> It seems to me that MBrains don't have to "fight" -- not at this stage
> of development in the Universe.
Lets hope not. {8-]
> There are lots of brown dwarfs to feed
> on. But if MBrains can detect other MBrains and if one subscribes to
> the "fighting" scenario, one would want to avoid any region of space
> another MBrain could reach by the time you could get there. As the
> MBrain population of a galaxy increases, it would seem that would
> require expanding outward occupying an increasing volume of space
> around the galaxy.
Thats right. I can imagine an entire galaxy winking out over a period
of a billion years, all from a single Singularity. Here one eon, gone the
next. Could be that we evolved just in time to see the final stages of
our own galaxy going dark.
Counter evidence: all galaxies are mostly dark matter? We couldn't
have caught *all* of them in that stage.
> Thus the really long term prospects for SIs do not appear
> to be very good to me at this point (unless you want to
> invoke magic physics). Robert
No magic physics. An MBrain could theoretically be constructed
using entirely that set of physical laws which are well understood today,
which we have understood since the time of Newton (with the
exception of how these individual particles in the MBrain would
be controlled. That breakthru insight came about only since
we developed microprocessors and lasers.)
Looks to me like we need only a Singularity to figure out how
to unify the existing earthbound intelligent nodes, organize the
collective will of those intelligences, and we are on our way,
perhaps within 50 yrs. Friends, we are standing on the
threshold of a dream. spike
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