Extropes,
Gene offered an extensive response to my post, and I wish to express my
appreciation. He wrote: (double indents ">>" are me, single indents ">"
are gene)
> > The predicted instability of the solid version of the dyson sphere has
been
> > a source of disappointment for moi ever since I heard about it. The idea
>
>While the idea of man-made celestial crystal sphere is rather neat, I
>do not see any essential features not covered by a cloud of orbiting
>objects, which can be made even optically dense. It would be
>interesting to see how we can use this to intercept the majority of
>the solar wind, too. Implanting them into periodically recycled
>sacrificial layers; using magnetic field traps?
>
>So, whence the disappointment?
My preference for the solid shell comes from the comparative simplicity of
that structure compared to the layer upon layer of orbiting rings of
satellites built up until they finally and fully shroud the central star.
The problematic stability of the solid shell is, in the latter case,
replaced by the (seemingly daunting--I confess, this is a function of my
mental laziness--) task of maintaining the cloud's vast multitude in
orderly lockstep. While I acknowledge the feasibility of the Dyson "cloud"
approach (the accepted teminology is as gene noted, "shell" for the solid
sphere, and "sphere" for the orbital cloud), my aged neural mitochondria
plead exhaustion in prospect of the required analysis, and retire from the
field of battle.
> > of the fully enclosed star was so delightful to me, and so elegant, that I
> > pondered long over the matter seeking some remedy, some bit of phenomenon
> > to rescue me from my despair. I considered the predicted instability, and
> > concluded that it was factually supported, but based on +considerations of
> > gravity and gravity alone. Gravity, to the exclusion of all other forces
> > in the universe, as if no other forces existed. Fortuitously, this is not
> > the case. Also fortuitously, the gravitational 'instability' is neutral.
> > That is, gravity does not actively drive the system to collapse, but
rather
> > stands by, having no effect at all. This is in contrast to orbital
>
>Hmm. Even if you could construct it, what about self gravitation
>(collapse is autocatalytic), and tidal forces?
I ran the risk of being misunderstood when I said that gravity "stands by,
having no effect at all." I needed the reader to understand the
gravitational dynamics at work in the solid-shell-encloses-central-star
configuration.
So, you are correct. The challenges of construction and the maintenance of
structural integrity against collapse--due to the combined gravitaional
contributions of both the star and the shell--are fundamental, crucial,
non-trivial considerations.
>Also, the thing as a
>whole is in orbit (since static sphere can only be too gossamer to be
>of much use), so the equator will be doing allright, but you're in
>deep shit as soon as you start deviating from the equatorial
>orbits.
Spinning the shell--in contrast to a static shell--is a standard
assumption. Centrifugal 'force' counters gravitational force; most
perfectly at the 'equator', less so as you deviate from the equitorial
plane. I'm not yet willing to characterized the non-equitorial
circumstance as "deep shit", nor the static shell as "gossamer". In the
former case the vectors, aided by the commendable structural
characteristics of the spherical shape seem quite promising, though
admittedly I'm still working the math looking for support. (Others have
said that they've done the math, and that the prognosis is bad. Ok. Call
me an untrusting fellow, but I'd just like to work it out for myself. I'm
sure you know how that is.)
>Orbital cloud solves this by changing height and inclination
>of the orbit, so stuff never collides (this won't work if the orbiting
>stuff is dumb and has no control over own orbit). With the cloud, you
>have only the fragmentation catastrophe, and you can lick that with
>active orbit control.
Orbital cloud good. Yes. Active control required, yes, agreed. The shell
too, even best case, will, I suspect, need active control. If the magnetic
field of the star makes it like a giant magnet, then, like a small magnet
in a metal box, what's it gonna do but seek to glom onto the side? Can't
have that.
> > Radiation pressure, baby. It ain't big, but it's real.
>
>If your orbit has no radial component (wheee!), you can't held
>anything relevant afloat, even if the floating stuff is heated to
>white incandescence. Do the math. Magnetic plasma sails do much
>better, but you can't operate another one within few 10 km radius.
I'm not entirely clear on what you mean here. But guessing that you're
referring to resistance to gravitational collapse, I repeat that I'm
holding off on a conclusion about the "gossamer" nature of the shell.
>
> > Solar radiation on the interior of the sphere will tend to drive the
sphere
> > so that the center of the sphere describes an 'orbit' around the star. To
> > see why this is so, consider what happens if the star is off-center. The
> > portion of the sphere nearest the star has the greater density of incident
> > photons to both absorb and reflect. It will reach a higher temperature
and
> > thus a higher albedo, so it will reflect (rather than absorb) a higher
> > percentage of the incident photons than the more distant (and thus cooler)
> > interior surface. These two factors, photon density and correlated albedo
> > values, should be the determining factors in the net thrust derived from
> > radiation pressure. Higher albedo means higher reflection, and a
reflected
> > photon delivers twice the thrust of an absorbed photon. This thrust
should
> > act as a restoring force driving the sphere back towards the concentric
> > position. There being no particular reason for it to stop once it reaches
> > the concentric position, it would tend to overshoot, decelerate and then
> > reverse direction, oscillating in a back and forth manner. This
> > oscillation, in two dimensions, and out of phase, would stabilize to a
> > condition where the sphere center describes an 'orbit' around the star.
>
>Have you considered that you're heating the star up, by reflecting the
>bulk of radiation back in in form of shortwave photons? (Instead of
>making the orbiting objects as black as possible). Whatever the star
>is going to do, this is not going to be sustainable. You're going to
>heat the star, which will bloat and emit bluer and bluer. You'll
>likely can boost mass loss that way, by making the surface hotter.
Ahh, this is the part I liked. This is something I had not considered.
This is a major point. Unlike the current situation, where the earth
intercepts a trivial amount of the sunlight and reradiation is beyond
trivial, the solid sphere contains one hundred percent of the energy. At
equilibrium, conservation considerations require that the outer surface
must re-emit that full one hundred percent. If the radius of the sphere is
one AU--the radius of the sphere is an unstated variable in all these
discussions; it could be anywhere from just above the solar corona
to,...?--then the solar radiation intensity is ~1300 watts per meter^2. So
1300 w/m^2 in means 1300 w/m^2 out. As you say, the interior is going to
heat up. The exact dynamics of the situation is uncertain (to me anyway).
How much the star will heat up seems at least partly contingent on the size
of the shell (thus my comment on the oft unspecified nature of that
variable). How hot the shell and the star get will affect the thermal
characteristics and dynamics of the entire affair. So, returning to the
original reasons for considering this vast project in the first
place--beyond the thrill of hubris and transhuman ambition-- we need to
know something about the design details of the shell with respect to the
critical energy harvesting, transport, and reemission. I must leave that
to others. So say my tired old mitochondria.
>
> > The exterior of the sphere will likewise have an asymmetric radiation
> > emission pattern resulting in a net thrust. A worst case scenario is that
>
>Why assymetric?
If the star is off center relative to the shell, such as when thay are in
constant motion relative to one another, then the interior of the shell is
hottest nearest the star. Assuming passive, or locally proportionate heat
transmission, the exterior of the shell will also be correspondingly
hotter. Thus the greater, ie assymetric, exterior emission centered about
that point.
>A fraction of the photons get absorbed, and is heating
>the envelope. The asymmetric thing is more interesting if you can make
>the cloud radiate anisotropically,
Just to keep things straight, I note that you appear to be applying the
discussion of radiation pressure issues to the dyson sphere/cloud case here.
>predominantly in one
>direction. However, this impacts recoil upon the cloud, shifting it's
>orbit. Will you cancel out the impulse impacted upon the residual
>system (minus the radiated photons), if you use photonic pressure to
>readjust the disturbed cloud component orbits? I have no idea. Someone
>please tell me, as it is rather significant.
>
<snipped section of my original post, which gene didn't comment on>
>>The devil is in the details.
>
>The devil is that you can't build a sphere of any relevant size
>without infinitely strong material (Unobtainium still isn't in the PSE
>last time I looked).
I acknowledge that others have run the numbers and claim that the strengths
of available materials are not up to the task of constructing a stable
dyson shell. On that basis I provisionally concede its infeasability.
Nevertheless, I'll want to run those numbers myself, to see if I come up
with the same answer.
>Denser objects are sufficiently small, but
>they're typically fierce and have literally astronomic gravitation
>gradients. You might construct you classical Dyson sphere around a
>very small singularity (a primeordial relict or a newly minted),
>though. If it has something like a few g/s mass loss as Hawking
>radiation, and has the right wavelength (I'm rather not warm my toes
>directly on a hard xRay source), you could build your sphere around
>it. Even fill the sphere envelope with some gas/dust, so that you can
>homeostate the hole in the right regime by feeding just as much as it
>evaporates, and attenuate the hard radiation from the eye of the
>dragon (hole and accretion disk). The whole thing doesn't have to have
>more than a few Gt mass, probably, and would be rather cold on the
>surface. Invisible unless there are lots of them, or you look at one
>from real close. And rather efficient, of course.
Thanks again, to gene, for the substantial effort and quality of his
response. And thanks as well to Adrian, Mike, Eliezer, John, Sterling,
Emlyn, and Damien.
And best wishes to all in this holiday season.
Best, Jeff Davis
"Everything's hard till you know how to do it."
Ray Charles
This archive was generated by hypermail 2b30 : Mon May 28 2001 - 09:50:34 MDT