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First let me sound a quick note of technical confusion... Timothy's
response to my post on laser-plasma accelerators came through to my mail
system as a series of random characters. I assumed it happened to
everyone, but since Rex just quoted Timothy's response to me in his last
post I suppose the problem was more localized (I have to Telnet down to
UCLA to read my mail, although I'm not sure how that would mess things
up so badly in this isolated case; all Timothy's other posts come
through fine.) Could someone please re-post Timothy's 1/30 Re:
Relativistic Electric Thrusters so I could try to read it? Thanks!
As for the ongoing discussion on Relativistic Thrusters, I'm
slowly getting up to speed. I checked out Timothy's Web Page and saw why
a fusion spacecraft should limit the fuel speed to 0.1c; I didn't check
the numbers, but it sounds reasonable.
So for self-powered ships, that seems to leave relativistic
engines to the regime where you can transfer your ship mass to energy
with a near-unity efficiency. The most obvious way to do this is with
antimatter, of course, but here I'm a bit skeptical. Even if we're
assuming some pretty amazing technological advances, I find it hard to
imagine storing huge quantities of anti-matter on a ship. I don't know
if this has been discussed already, but I would guess there would be
some sort of theoretical minimum matter/antimatter ratio, just from
containment considerations. Anyone want to tackle that one?
So, keeping this in mind, here are another two ideas for
"antimatter-type" engines; ways to convert mass to pure energy--without
using antimatter. These are nearly as speculative as large-scale
antimatter containment, but I'd still argue that they're more probable.
The first is probably the most easily shot down: power the ship
from an enormous relativistic or near-relativistic flywheel. There's a
significant amount of research into flywheels for electric cars, but
there are many big disadvantages: they have to be kept in vacuum, they
need to always point in the same direction, and they need to be small
enough to fit in a car. All three of these problems, of course, aren't
big deals for an interstellar spacecraft. The big limiting factor is
the acceleration that the rim can handle before flying apart, but for a
given acceleration limit--with a hollow cylindrical flywheel--an
arbitrarily high Energy/Mass ratio is possible just by making the
flywheel larger. To be specific, the Energy/Mass (J/Kg) of the system
is simply half the acceleration of the rim (m/s^2) times the radius of
the wheel (m).
Current flywheels are made out of graphite and can handle rim
accelerations greater than 10^8 m/s^2. The actual limits are not public
knowledge, as far as I can tell; too much competition between the people
who make these things. So already, for a 200 meter radius flywheel,
we're talking about an energy storage of 10^10 J/Kg. We'd probably need
at least 10^14 J/Kg to make a decent spaceship. Making up four orders
of magnitude as well as structurally weakening the flywheel by making it
hollow seems quite a long-shot. But two possibilities that are at least
credible are fullerene and diamond flywheels. I'm not sure which would
be stronger; Diamond is a 3-D structure while buckytubes are 2-D, but
the crystal structure of a diamond magnifies small imperfections.
Perhaps a braid of buckytubes would be the best; on the other hand, if
nanotechnology allows the creation of a *perfect* diamond flywheel, it's
hard to see how anything could beat it.
Of course, a 200 meter flywheel with 10^14 J/Kg means the rim is
traveling at 10^7 m/s. Any bigger/faster and you get relativity playing
a big role. Probably some problems inherent in that situation... but it
might make a great gravity-wave generator!
Idea #2: Catalyze nucleon decay using captured magnetic
monopoles. Okay, okay... at least antimatter has been proven to exist,
right? Granted, magnetic monopoles are still hypothetical particles,
but on the plus side every single Grand Unified Theory predicts that
they should exist. Basically, if there IS a grand unified theory, then
magnetic monopoles are viable and stable particles. Unfortunately, most
of the GUT's predict HUGE masses; sometimes as big as 10^19 GeV! If
correct, this means we're not going to make a monopole in an
accelerator: they're only going to be left over from the Big Bang, and
we're going to have to find one and catch it.
As I mentioned, another predicted feature of monopoles is that
they should catalyze proton and neutron decay. The great thing about
this is that it doesn't destroy the monopole; it's still waiting around
to decay more nucleons. One can easily imagine what a great power
source this would be; feed a monopole protons and neutrons and it
continually converts them into pure energy. You'd probably need quite a
few monopoles to make a decent spaceship engine, but it'd be a whole lot
less than the needed amount of antimatter.
Well, this was a pretty big tangent off the subject here, but
hopefully it'll spark some ideas...