Joel Dobrzelewski, <[EMAIL PROTECTED]>, writes:
> So the Universal Dovetailer simply enumerates all possible states for "me".
> (Whatever "me" is, is unimportant). And somehow, these states are joined by
> their similarity to one another, which allows (virtual) motion from one
> state to the next.
Let me explain in English what the Universal Dovetailer (UD) is.
I don't necessarily agree with all the conclusions which come from the
potential existence of such a program but it will be helpful if we are
all in agreement about it.
First, the word "dovetail" originally comes from carpentry and refers a
kind of wooden joint in which the pieces are joined in an interlocking way
that holds them together, which is supposed to resemble a dove's tail.
See http://www.kellerdovetail.com/ for a picture of wood joined in this
way. You can see that in this case the two pieces of wood alternate
along the joint.
The word then came to be used in cards. What is commonly called a
"riffle" shuffle is technically known as the dovetail shuffle. This is
the shuffle in which the deck is split into two parts which are each held
in a hand, and then the cards from the two halves are quickly sprung
together so that they interleave into a whole, a few cards alternately
from each hand.
Dovetailing thus came to be known as any system which interleaves two
or more independent processes in this way. This is the sense in which
we mean to refer to the Universal Dovetailer.
The idea of the UD is simply to run all possible computer programs on
some particular type of computer in an interleaved way, so that all of
them make progress. It's essentially no different from what is done on
every computer today, which runs multiple tasks at once via timesharing.
Each task gets a little slice of run time, and then the computer switches
to another task.
The UD has the additional burden that there are an infinite number of
possible programs. Unlike in Linux or Windows, it is not given a fixed
set of programs and asked to dovetail between them, it has instead a
virtual machine specification and needs to generate all possible programs,
and then make sure that all of them get run eventually.
Generating all the possible programs is easy; they are just all possible
bit strings. But if you did that first, you'd never get around to
running them. So you also have to interleave the program generation
with the running.
One approach is to generate all 1-bit programs, then run each of them
for one cycle. Then generate all 2-bit programs, and run each of the
1- and 2-bit programs for one cycle. Then generate all 3-bit programs,
and run each of the 1-, 2- and 3-bit programs for one cycle, and so on.
In this way you can show that every program of any number of bits will
eventually be run for as many cycles as you specify.
So that's the UD. In terms of your comments above, it does more than
enumerate all possible states for "you"; it runs all possible programs,
and therefore it runs all possible programs for universes which contain
you, including this one you're in right now. What about the fact that the
programs are interleaved, so they start and stop? Why are you not aware
of this, why does your consciousness stay stuck in this one program,
even though other programs are constantly being run? Is it Is it a
matter of similarity of state which allows "virtual motion" from one
state to the next, even though the UD was doing other things in between?
Here we appeal to our intuition from working with timeshare computers,
or to the logic of computation, and say that this doesn't matter.
I'm typing into a window here, talking to my word processor, and the fact
that there are 83 other things going on on my computer doesn't matter;
as far as my interactions with the word processor are concerned, it is
running as a logical unit. The only affect from the other parts of the
computer is that it might slow down.
I still think there is a philosophical issue about whether running *all
possible programs* has the same effect and implications as running a
single selected program. It's perhaps analogous to the question of
whether emitting white noise (which includes all sounds) has the same
effects as emitting a particular sound.
But I won't get into that here, I just wanted to explain the UD plainly,
in terms of what it means and where the term came from.