Not bad exposition at all. Thanks for the remind, Richard.
Except that I am not french, it is much worse: I'm belgian :)
(but then we are all Löbian, isn't?)
On 27 Aug 2012, at 18:09, Richard Ruquist wrote:
CLUB OF SUPPER CLUB
COPIED FROM WHITE ON BLACK BACKGROUND
MONDAY, AUGUST 29, 2011
My topic - the Universal Dovetailer Argument
This month I'm breaking with tradition and actually posting early!
This is for two reasons, 1) because I have some time on my hands
being sick at home today and 2) because the topic may be more
challenging than the average so I'm giving you all some time to get
acquainted with the material before we discuss it. I'm going to try
to be as concise as possible but a certain level of verbosity is
First of all, let me make my own position on this theory clear. I
don't 'believe' it as such. In fact I actively disbelieve it, for
reasons I won't go into here - I'll leave that for the day.
Nevertheless it does open up some pretty fascinating philosophical
vistas, and ties into a whole bunch of stuff including the dreaded
Cryogenic Paradox, quantum theory, the objective existence of
numbers, the modern multiverse cosmology, and more. As a
backgrounder, I stumbled on this through a Google group called the
Everything List, a discussion group bound together by the concept
that 'everything exists', which I'd been referred to by someone who
left a comment on my Cryogenic Paradox post.
But I promised concision, so onwards! The idea I want to present is
called the Universal Dovetailer Argument (UDA), and was proposed by
a French academic by the name of Bruno Marchal, who is a research
fellow in AI at the Université Libre de Bruxelles. Here is Marchal's
original paper on the subject (from which I attempt to distill this
summary). Unfortunately Marchal's argumentation style is not exactly
crystalline in its clarity for a non-mathematician, a problem
exacerbated by his inconsistent conjugation of English verbs.
Nevertheless, I think I've succeeded in deciphering 90% of the gist.
I'll tell you the bit I don't follow (and have my doubts about) when
I get there.
So, the UDA proposes itself as a refutation of materialism, and
purports to "reduce physics to fundamental machine psychology", by
which I think he means the mathematically defined laws by which
computing devices (namely, us) compute their self-consistent states
(I'll explain). It relies on the following assumptions:
That a brain (and a consciousness) can be substituted by a 'Turing-
machine' type computational device, i.e., we could theoretically
have a brain transplant and not know the difference.
The 'Church Thesis', which basically says that one type of computer
can always perfectly emulate another (contradicting my suspicion
that my old Commodore 64 would struggle with Photoshop CS5, but if
you fed the data through in small enough blocks, and could maintain
state with enough memory...). We're talking mathematical inputs and
outputs here, not the time taken nor the form of presentation.
Arithmetical Realism - namely the idea that mathematical relations
and propositions "are true independently of me, you, humanity, the
physical universe" etc.
These assumptions sum to what Marchal calls Classical
Computationalism (or just "comp" for brevity's sake). I can
personally accept the second of these assumptions, C64
notwithstanding. The first and last are a lot more debatable, but as
I said, we're "running with it".
So here goes the argument in 8 steps:
"Comp" allows for teleportation, whereby a person is cut at one
place and pasted at another. (I can't resist relating a joke at this
point: Man standing in teleporter hears an announcement: "Your
duplicate has been created at the destination, but due to a
technical fault, there has been a delay in vaporization at this end.
Please stand by...") Now the person at the end of the teleportation
is a "consistent extension" of the one prior to teleportation, so
Marchal assumes it is the same individual/consciousness. All we have
done is add to the individual's belief set a new one relating to
location. This is in fact irresistible unless we try to "stick"
consciousness to the physical substrate of the "computation" that is
the mind, but that violates assumptions 1 (and 2) above.
Now let us imagine this person keeps a diary which gets teleported
along with him or her and records what happens. A third-person
account of the teleportation will at this stage be completely
consistent with the diary (first person) account. The pronouns will
be different, but the proposition will be logically identical
("Pierz was teleported from Melbourne to Sydney" vs "I was
teleported from Melb to Syd"). But imagine there is now inserted a
time delay between "departure" and "arrival" in the teleportation
process. If the teleportee is not allowed reference to any external
cues to the passage of time (eg, a non-teleported calendar), the
first person account will now differ from the third person account,
as the diary will not register any delay - the gap is invisible/non-
existent from the first person perspective.
Now imagine the person's data is transmitted to two destinations,
say Adelaide as well as Sydney. From the first person perspective,
what is the chance my diary will say I arrived in Sydney after the
experiment is complete? It has to be 50%. Even though there is no
uncertainty in the third-person account, there is an irreducible
uncertainty in the first person account (I'm going to use the
abbreviations 1-p and 3-p from now on for these perspectives because
I'm lazy). From 3-p, there's a fully determined, objective
situation; from 1-p, there's a 50% uncertainty. Objectively (3-p)
the person is going to end up in both places. But there is no diary
where someone is in both places at once, so you could say from the
point of view of the traveller, the chance of being in one place or
another post-teleport is 50% each way.
Now let us introduce a delay in one branch of the teleportation. The
internet connection to Adelaide is down and they can't email all the
attachments required for reconstitution for a couple of days, say.
In this scenario, the probability of ending up in Sydney or Adelaide
is still 50% - the delay is irrelevant because it does not affect
the 1-p account.
Now imagine we don't vaporize the person in Melbourne, but simply
email a copy to Sydney and duplicate them. What now is the
probability of ending up in Melbourne or Sydney from 1-p? It is
still 50%, since the "original" that stays at home can be considered
as having been "teleported" to the same place with a zero delay.
Next we can postulate the possibility of "teleporting" a subject
into a virtual reality rather than an actual one. All the preceding
steps can be performed with the reconstitution being virtual rather
than real, without any change in the 1-p account for as long as the
simulation is maintained and given an accurate enough virtual Sydney.
Now we come to the so-called Universal Dovetailer (UD), and now we
get contentious (if the above wasn't contentious enough). The UD is
basically an algorithm that executes all possible programs. It is
called a dovetailer because, in order not to "crash" on infinitely
executing programs (there is in theory no way to exclude such
algorithms), it executes one instruction from each program in
sequence then goes on to the next instruction in the next program,
and so on, thus doing what a multitasking OS with a single core
effectively does - simulating parallelism. Now suppose that the
universe is large enough and robust enough to allow such a device to
run forever. It follows that will generate all possible Turing
machine states infinitely often, including all possible virtual
reconstructions of yourself in all possible locally emulable
environments. And since we have established (or assumed) that your
mind/brain is emulable and any such emulation will in fact result in
a 1-p indeterminacy as to where 'you' end up - this indeterminacy
being the infinite union of all finite portions of the UD machine
where you are emulated - we can see that 'you' exist within the UD
as one trace through the all the existing computational states of
this "machine". So physics, as the science which gives correct
predictions about the probabilities of entities being in particular
states, can be reduced to "fundamental machine psychology", or "some
measure of consistent states" from a first-person point of view. It
becomes an "average of consistent histories". GEDDIT??
OK, but here's the obvious objection. What if the universe is not
big enough to support the UD? Then our normal physical predictions
would be safe from interference from all the virtual states of this
machine. According to Marchal, "such a move can be considered ad hoc
and disgraceful" (!), but he doesn't really explain the disgrace of
this obvious objection. Instead he goes on to argue that we cannot
associate inner experiences with physical processing (this was the
point I raised earlier when introducing teleportation), but instead
that inner states must correspond to a type or branch of computation
in the platonic mathematical space of all computation (Arithmetical
Realism). But I cannot frankly follow his reasoning why (it's on
page 11 of the paper if you want to have a crack). He draws the
conclusion that computation is not performed by a physical machine
(which the universe may not be able to support), but that these
computations exist in mathematical space, requiring no physical
OK, so the logic of the last part gets kinda weird and flaky, but
consider this: modern cosmology now suggests the existence of an
infinite universe, or in fact an infinity of universes separated by
space expanding faster than the speed of light and therefore unable
to communicate in a classical sense, but presumably still quantum-
entangled. If this picture is true, then the universe itself could
be the UD, creating all possible Turing states, and if it is the
case (solving the Cryogenic Paradox) that a sufficiently accurately
emulated state in effect "blurs into" all other equal states (i.e.,
another way of saying there is only one observer), then something
like the above becomes almost plausible. We have a version of
Everett's many universes interpretation of QT, in which the wave
function is the "presence" of all the other universes adjoining ours
in information space or, to use Marchal's lingo, it is the 1-p
indeterminacy of all consistent states in the infinite trace of the
UD. You can at this point dispense with the awkward terminology of
the "dovetailer" and simply consider the universe as a generator of
all possible configurations of information. If information rather
than matter is primary, then something along the lines outlined
above is not as preposterous as it at first sounds.
(I realize this was a mega-crash-course in a some kinda out-there
ideas, so apologies for that. I do recommend having a go at the
first 12 pages of Marchal's paper if anything is unclear.)
POSTED BY PIERZ NEWTON-JOHN AT 11:19 PM
Guy Montag said...
Thanks for this and the link to the paper. I've been lurking on the
everything list for several months now. needless to say Marchal's
ideas are fascinating, regardless of how you feel about them. I for
one am slowly coming to terms with his argument and am becoming
increasingly sympathetic to them, although i do have several
Does the UDA really require empirical confirmation from modern
cosmology? In the last step Marchal disposes of any concrete UD and
only requires a platonic one. Elsewhere he says
"Once you explain why arithmetical machines are statistically right
to believe in physical laws without any real universe, such a real
universe is redundant.
By Arithmetical Realism and OCCAM razor, there is no need
to run the concrete UD, nor is there any need for a real concrete
(Or you can use the movie graph argument to show that a first
person is not able to distinguish real/virtual/and *Arithmetical*
nature of his own implementations, and this eliminates OCCAM.)"
In any case, it is causing me to challenge many of my assumptions
OCTOBER 3, 2011 7:30 PM
C. Seligo said...
Forestalling an objection by calling an argument a disgrace is so
French. There are much clearer exponents of these ideas. David
Deutsch, in The Fabric of Reality, does a pretty good job of
explaining what it would mean to call the universe "computational" ,
without ever insisting that it is so (which is my sense of graceful
thinking.) Sci Fi author Greg Egan's short stories and novels
explore these questions, while retaining a sense of how human beings
and error in the system can be very consequential. Read "The
Kidnapping" or "Transition Dreams" or " The extra" to get an idea of
how virtualized consciousness is problematic. There are so many
unknowns, tenuous propositions, and we are generally very bad about
thinking about large numbers, to say nothing of infinity. Any
history of science will find great minds crashing at the cusp.
FEBRUARY 10, 2012 11:59 AM
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