>CurtAdams wrote:
>
>> >Cars are optimised.
>>
>> In no sense. They get substantially better every year even after you
>> discount for technological improvement. In what sense could they possibly
be
>> optimal at any point?
>
>(Fairly optimal.) In the sense that it would be *much* easier to
>build something on four wheels that moves by itself than it is to
>build a vehicle that can compete on the open market today. The
>context was this: Carl said that a simple self-replicator would
>contain about the same amount of information as a car. So some
>kind of analogy-inference might be made if we know how difficult is
>it to design a car. Well, how difficult is it? Many highly skilled
>people have been busy for many decades designing cars, so it seems
>very hard. But this would be to overlook the fact that the
>self-replicators we are trying to build need not be optimised in the
>sense that cars need to be, if they are to be acceptable to car
>designers. The relevant analogy (a weak one, to be sure) is rather to
>steerable automobiles on four wheels or something like that; not to
>car that could be sold today.
Any self-replicator has a much harder job than a steerable car. Based on Von
Neumann's estimates, even in a tank with semi-processed raw materials, you'll
need about 250,000 parts in a sophisticated (i.e., well-designed ) system.
Carl's point (I think) was that that is roughly comparable to a modern car -
an early car is much simpler. The analogy "simple car"="simple replicator"
is not correct, in the same way that "simple cart"!="simple car".
>> >> >To build an optimised nano
>> >> >self-reproducing device would be much harder than simply to make
>> >> >something useful that can replicate. For example, a universal Turing
>> >> >machine has been constructed in Conway's Life world. The entity is
>> >> >very big and it was hard, but nothing near a thousands of genius-year
>> >> >task, to do it.
>> >>
>> >> Nobody has presented a self-replicating Life system. All Conway did
was
>> >> produce a feasibility proof, so you know it *can* be done. Actually
>> >> designing such a system is still considered not yet possible.
>> >
>> >Really? I thought I've heard that the Universal Turing machine was
>> >actually designed, with streams of gliders serving as tape etc. But I
>> >may be wrong, in which case I'm glad you pointed it out. Do you have
>> >any references?
>>
>> "The Recursive Universe" by William Poundstone. It has a good layman's
>> description of Conway's proof, and some guesstimates of what it would take
to
>> actually make a self-replicating Life computer (conclusion: no time soon).
>
>I don't have that book handy, but now I'm beginning to suspect that
>what Poundstond makes guesstimates about is constructing such a
>gadget in the real world. What we were talking about was Conway's
>universal Turing machine in the Life world (a mathematical model).
>I think Carl Feynman said that he was aquainted with Conway's proof
>and thought that going to the detailed design was only a matter of
>filling in some details.
No. Poundstone was talking about programming a self-replicating Life system
in your typical Life simulator. He concluded that actually designing the
system - without even implementing it on a simulator - would be a whopper of
a task. Roughly speaking, the life computer has to generate billions of
precisely timed and spaced gliders at arbitrary times, places (within the
computer), and directions. It is a truly monumental task. Yes, it's all
details, but soooooo many ...
>(Did you really think that I believed that
>somebody had built a Life world replicator in the real world out of
>cogwheels or whatever?)
No. I thought you thought that somebody had programmed a self-replicating
Life entity.