Thanks. Basically I'm thinking of an all-terrain, omnivorous kinda
guy--water, rock, soil; wouldn't really matter. Could be a single design, or
something which produces environment-specific assemblers (water, rock, soil, etc.)
if that would reduce size/complexity and, thus, increase speed of
replication. Something stripped down for speed, but still adaptable. Larger
size/number of arms will at some point prove counterproductive in terms of replication
interval.
If I'm not mistaken, Drexler's logic (assembler arm can move back and
forth 50 million times faster than a human arm (approx 150 million times a
second))--times however many arms there are--has gotta yield an astonishingly
fast replication rate--much faster than Freitas' stated rate. Particularly as
Drexler envisioned a larger assembler (100-150 million atoms). But then
Freitas' work is more recent than EoC, which is where I read this.
If (for simplicity) each back-and-forth-cycle is moving only one atom,
aren't we talking replication of a 150 million-atom assembler in 1 second? And
9.3 replications of a 16 million-atom assembler?
I'm trying to get a handle on just how fast the things can reproduce
initially, i.e.--before their descendants can't get out of the way fast enough to
avoid slowing them down. (By which time the maximum replication rate would
only be attainable by the "leading edge" assemblers.
Any thoughts?
john marlow
--Hal Finney wrote:
John Marlow writes: > Maximum practical number of "arms" on a free-roaming assembler...
Maybe you could clarify what you mean by free-roaming. Roaming in what? An assembler that roams in water might be different than one which roams in air, and one that roams in vacuum would be more different yet.
As far as the number of arms, I suppose you could have larger or smaller assemblers with different number of arms. At a minimum you'd probably need two arms. But of course you could have a much larger assembler with more arms. I don't see why there would be any particular limit to the number of arms you could have on a large assembler.
> Approximate diameter (or largest cross-sectional measurement) of such an > assembler, including "arms..."
I can't find my copy of Nanosystems, but Nanomedicine describes a minimal two-armed assembler with an external power supply and control unit that would be about 200 nm long by about 50 nm wide.
> Approximate maximum replication rate for each individual assembler of the > type specified above, i.e.--how many copies of itself it can construct per > second, assuming an environment rich in raw materials...
Freitas estimates that the assembler can work at the rate of 1 million atoms per second (not to imply it necessarily places one atom at a time). The simple assembler above has about 16 million atoms so would take 16 seconds to replicate. A more complete assembler with nanocomputer and instructional tape would take more like 100 seconds.
> References to earlier speculations by others on these matters are also > welcome. I'm aware of some, but surely not all. I would guess the most > practical shape for a free-roaming assembler would be spheroid.
Again, it depends a great deal on what it is going to do. If it is supposed to engage in locomotion then we need to have room for propulsion units, etc. Similarly, what are the raw materials for the replication? If it is given a "molecular soup" of tagged reactants designed to be used by the replicator then its job is relatively easy. If it is sitting in the ocean then things are harder. If it is supposed to build copies of itself out of bare rock then it probably needs a lot of components dedicated to dissolving the rock and making it useful.
It may also be that the arms are not pointing outward, but rather inward so that the work area is clean (i.e. hard vacuum). In that case some other shape might be best.
As another data point you might look at Freitas' respirocyte design, http://www.foresight.org/Nanomedicine/Respirocytes.html. This does not replicate, it just stores and releases oxygen and CO2, but it is a one micron, spherical machine built from about 27 billion atoms. If you had an assembler+extras of similar size with only the two arms inside it could self-replicate in 27000 seconds or 8 hours. I suspect you could put many more arms in something of this size though and get replication time down to several minutes.
Hal
-- The strength to change what I can, the inability to accept what I can't, and the incapacity the tell the difference. --Calvin
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